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Experimental Physiology 89.4 pp 363-371
DOI: 10.1113/expphysiol.2004.027334
© The Physiological Society 2004
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The effects of insulin on mesenteric blood flow in anaesthetized pigs

E. Grossini, A. Battaglia, G. Bona, D. A. S. G. Mary, C. Molinari and G. Vacca

Dipartimento di Scienze Mediche, Facoltà di Medicina e Chirurgia, Università del Piemonte Orientale ‘A. Avogadro’, via Solaroli 17, I-28100 Novara, Italy


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 References
 
Infusion of insulin in anaesthetized pigs has been shown to cause an increase in renal blood flow and a decrease in coronary blood flow, which were the net result of a vasoconstriction involving sympathetic {alpha}-adrenoceptor-mediated mechanisms and of a local vasodilatation involving the endothelial release of nitric oxide. In the present study, the effect of insulin on superior mesenteric blood flow was examined in pentobarbitone-anaesthetized pigs at constant heart rate, aortic blood pressure, left ventricular contractility and blood levels of glucose and potassium. In 10 pigs, infusion of 0.004 IU kg –1 min–1 of insulin increased mesenteric flow. In five of these pigs, intravenous phentolamine enhanced the increase in mesenteric flow elicited by insulin, a response which was abolished by the subsequent injection of N{omega}-nitro-L-arginine methyl ester (L-NAME) into the mesenteric artery. In the remaining five pigs, infusion of insulin after intramesenteric injection of L-NAME caused a decrease in mesenteric flow. This response was abolished by the subsequent intravenous administration of phentolamine. The present study showed that infusion of insulin in anaesthetized pigs primarily caused a mesenteric vasodilatation, which was the net result of two opposite effects, namely a predominant vasodilatation mediated by the endothelial release of nitric oxide and a sympathetic vasoconstrictor mechanism mediated by {alpha}-adrenoceptors.

(Received 2 February 2004; accepted after revision 18 March 2004; first published online 1 April 2004)
Corresponding author E. Grossini, Facoltà di Medicina e Chirurgia, via Solaroli 17, I-28100 Novara, Italy. Email: grossini{at}med.unipmn.it


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 References
 
It has previously been shown that insulin infusion in humans and experimental animals causes an activation of central sympathetic vasoconstrictor output and a vasodilatation partly attributed to direct effects on vascular smooth muscle and the release of nitric oxide from the endothelium (e.g. Baron, 1994; Scherrer & Sartori, 1997; Yki-Jarvinen & Utriainen, 1998). The vasodilatation caused by insulin has been reported to be widespread (Liang et al. 1982; Baron, 1994) and to affect mainly skeletal muscle, carotid and coronary vascular beds (Reikeras & Gunnes, 1986a,b; Ferrannini et al. 1993; Laine et al. 2000; McNulty et al. 2000; Scott et al. 2002), while the activation of vasoconstrictor sympathetic output has been reported to involve the lumbar more than adrenal or renal regions (Morgan et al. 1993; Muntzel et al. 1995; Scherrer & Sartori, 1997; Lu et al. 1998).

Recent studies of the effects of insulin infusion on regional vascular beds in anaesthetized pigs have shown that this hormone caused an increase in renal blood flow and a decrease in coronary blood flow, which were the net results of a vasoconstriction involving sympathetic {alpha}-adrenoceptor-mediated mechanisms and of a local vasodilatation involving the endothelial release of nitric oxide (Molinari et al. 2001a, 2002a). These findings of a predominant effect of renal vasodilatation and a predominant effect of coronary vasoconstriction caused by insulin were obtained whilst preventing changes in haemodynamic variables which could secondarily affect the primary responses of vascular beds to the hormone.

The reported effects of insulin on the mesenteric vascular bed in experimental animals and humans have not been consistent, possibly being influenced by concomitant changes in other haemodynamic variables. For instance, infusion of insulin in dogs has been reported to cause a dose-dependent increase in left ventricular contractility, stroke volume and cardiac output, which were accompanied by a proportional increase in mesenteric blood flow (Reikeras & Gunnes, 1986a). In rats, insulin infusion was shown to cause either no changes in superior mesenteric vascular conductance or, at high doses, a slight increase in blood pressure and mesenteric vasoconstriction (Pitre et al. 1996; Gaudreault et al. 2001). In another report in rats, short-term sustained euglycaemic hyperinsulinaemia was shown to increase blood flow to skeletal muscle without changes in splanchnic blood flow (Hilzenrat et al. 2001). In normal humans and in patients with chronic heart failure, insulin infusion caused a dose-dependent increase in forearm blood flow and a decrease in superior mesenteric blood flow (Parsonage et al. 2002).

The present investigation was planned to determine the effects of insulin on mesenteric blood flow in anaesthetized pigs and to study the mechanisms involved. For this purpose, the experiments were performed whilst preventing changes in heart rate, arterial blood pressure and left ventricular contractility to avoid interference with any responses of mesenteric blood flow to controlled insulin infusions.


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 References
 
Experiments were carried out in 10 pigs, weighing 70–75 kg and supplied by an accredited dealer (Azienda Cornelia srl, San Pietro Mosezzo, Novara, Italy). The animals, which were fasted overnight, were anaesthetized with ketamine (20 mg kg–1I.M., Parke-Davis, Milan, Italy) followed after about 15 min by intravenous sodium pentobarbitone (15 mg kg–1, Siegfried, Zofingen, Switzerland), and were artificially ventilated with oxygen-enriched air using a respiratory pump (Harvard 613, Harvard Apparatus, South Natick, MA, USA). Anaesthesia was maintained throughout the experiments by a continuous intravenous infusion of sodium pentobarbitone (7 mg kg–1 h–1) and assessed as previously reported (Linden & Mary, 1983), judging from responses of the animals to somatic stimuli. The experiments were carried out in accordance with national guidelines (DLGS 27/01/1992, no. 116).

Blood pressures in the ascending aorta and in the right atrium were recorded via catheters connected to pressure transducers (Statham P23 XL, Gould, Valley View, OH, USA) inserted into the right femoral artery and the right external jugular vein, respectively. The chest was opened in the left fourth intercostal space, the pericardium was cut and a catheter was inserted into the left ventricle through the left atrium to record left ventricular pressure. The frequency response of the catheter–manometer system was found to be flat (±5%) up to 40 Hz. To pace the heart, electrodes were sewn on the left atrial appendage and connected to a stimulator (Model S8800, Grass Instruments, Quincy, MA, USA) delivering pulses of 3–5 V with 2 ms duration at the required frequency. The abdomen was opened by a mid-line incision and an electromagnetic flowmeter probe (model BL 613, Biotronex Laboratory, Chester, MD, USA) was positioned near the origin of the superior mesenteric artery to record mesenteric blood flow. A plastic snare was placed distal to the probe for zero blood flow assessment.

To prevent changes in arterial blood pressure during the experiments, a large-bore cannula was introduced into the left femoral artery and connected to a reservoir containing Ringer solution kept at 38°C. The reservoir was pressurized using compressed air, which was controlled with a Starling resistance, and pressure within the reservoir was measured by a mercury manometer. This method has been shown in anaesthetized pigs to allow the aortic blood pressure to be maintained at steady levels without significant changes in right atrial and left ventricular pressures or the haematocrit (e.g. Molinari et al. 2002b, 2003). Blood coagulation was avoided by the intravenous injection of heparin (Parke-Davis; initial dose of 500 i.u. kg–1; subsequent doses of 50 i.u. kg–1 every 30 min).

Arterial blood pH, PaO2 and PaCO2 were measured using a blood gas analyser (IL 1304, IL Instrumentation Laboratory, Lexington, MA, USA). Haematocrit was also measured. The acid–base status of the animals was kept within normal limits during the experiments by the infusion of a solution of 2.8% sodium bicarbonate and by adjusting the respiratory stroke volume, when necessary (Linden & Mary, 1983). The rectal temperature of the pigs was monitored and kept between 38 and 40°C using an electric pad.

Aortic and right atrial pressures, left ventricular pressure, mean and phasic mesenteric blood flow were monitored and recorded together with heart rate and the maximum rate of change of left ventricular systolic pressure (dP/dtmax) on an electrostatic strip chart recorder (Gould ES 2000, Gould Instruments). The heart rate was obtained from the electrocardiogram. The frequency response of the differentiator used to obtain left ventricular dP/dtmax was flat (±5%) up to 150 Hz.

At the end of each experiment, each animal was killed by an intravenous injection of 90 mg kg–1 sodium pentobarbitone.

Experimental protocol

The experiments were begun after at least 30 min of steady-state conditions with respect to measured haemodynamic variables. To avoid interference of any possible changes in heart rate and aortic blood pressure during the experiments, the heart was paced to a frequency higher, by 20 beats min–1, than that observed during the steady state and the arterial system was connected to the pressurized reservoir. Changes in left ventricular contractility were prevented by the intravenous administration of 1 mg kg–1 of atenolol (Sigma), a dose which has previously been shown in anaesthetized pigs to prevent the increase in left ventricular dP/dtmax caused by insulin (Molinari et al. 2001a).

After at least 10 min of steady-state conditions, the experiments were carried out by intravenously infusing in a random order 0.004 IU kg–1 min–1 of insulin (Novo Nordisk, Bagsvaerd, Denmark) dissolved in saline, or saline only, for a period of 1 h using an infusion pump (Model 22, Harvard Apparatus) working at constant rate of 1 ml min–1. Similar doses of insulin have previously been used in euglycaemic infusion experiments in humans and experimental animals to examine the vascular effects of insulin (Rowe et al. 1981; Liang et al. 1982; Reikeras et al. 1985). The same dose has been shown in anaesthetized pigs to cause coronary vasoconstriction and renal vasodilatation (Molinari et al. 2001a, 2002a). During insulin infusion experiments, venous blood samples were used to measure blood levels of glucose (Blood Glucose Monitor, Lifescan, Milpitas, CA, USA) every 5 min. Euglycaemia was maintained with a variable infusion of 33% dextrose solution (DeFronzo et al. 1979) and hypokalaemia was prevented by administration of KCl infused at a rate of about 10 mEq h–1.

After haemodynamic variables and blood levels of glucose and potassium had returned to pre-infusion control values (usually within 1.5 h) and remained at steady levels for at least 20 min, a second infusion of insulin was performed once a steady state was attained following blockade of {alpha}-adrenoceptors with the intravenous injection of phentolamine (Ciba-Geigy, Varese, Italy; 5 pigs) or following blockade of mesenteric nitric oxide synthase with the intra-arterial administration of N{omega}-nitro-L-arginine methyl ester (L-NAME; Sigma; 5 pigs). After haemodynamic variables had returned to the pre-infusion control values, a third experiment was performed. In the 5 pigs which received phentolamine, the insulin infusion experiment was repeated after injection into the mesenteric artery of L-NAME. In the 5 pigs which received intra-arterial L-NAME, the insulin infusion experiment was repeated after the intravenous administration of phentolamine. Phentolamine was given intravenously at a dose of 1 mg kg–1. This dose has been shown in anaesthetized pigs to abolish the reflex coronary vasoconstriction caused by distension of the gallbladder (Vacca et al. 1996) and the mesenteric vasoconstriction caused by distension of the uterus (Vacca et al. 1997) and has been used previously to block sympathetic {alpha}-adrenergic effects (e.g. Molinari et al. 2003). Similar doses of the blocking agent have been used in anaesthetized pigs by other authors to block {alpha}-adrenoceptors (Gregory & Wotton, 1981). The dose of L-NAME injected into the mesenteric artery was 2 mg for each 1 ml min–1 of measured blood flow during the control period. This dose of the blocking agent was previously shown in anaesthetized pigs to abolish the mesenteric vasodilatation caused by intravenous infusion of 17ß-oestradiol or progesterone (Vacca et al. 1999; Molinari et al. 2001b) and by intra-arterial infusion of testosterone (Molinari et al. 2002b).

Recordings taken for 10 min during the pre-infusion steady state were used as controls. Measurements of haemodynamic variables were obtained during the last 10 min of infusion (test period) in the steady state and compared with values in the control period. Mesenteric vascular resistance was calculated as the ratio between mean aortic blood pressure and mean mesenteric blood flow. Student's parametric t test for paired data was used to analyse the statistical significance of the responses observed. Analysis of variance for repeated measurements and the Newman–Keuls test were used to examine the effects of subsequent infusion of insulin. A value of P < 0.05 was considered statistically significant. The results are expressed as means ±S.D. (range).


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 References
 
In the 10 pigs, recordings commenced approximately 5 h after the induction of anaesthesia. The mean arterial pH, PaO2 and PaCO2 of arterial blood were 7.40 ± 0.02 (7.38–7.43), 111.9 ± 6.5 (103–121) mmHg and 39.9 ± 1.3 (38–42) mmHg, respectively, and the haematocrit was 38 ± 1.8 (35–41)%.

Infusion of the vehicle (60 ml of saline) did not cause any changes in the control values of measured haemodynamic variables. Individual changes in mean mesenteric blood flow caused by infusion of insulin are shown in Fig. 1. Group changes in measured haemodynamic variables are shown in Table 1. Infusion of insulin caused an increase in mean mesenteric blood flow which amounted to 18.4 ± 4.2 (8.1–24.2, P < 0.0005)% of the control values in the absence of changes in the other haemodynamic variables (Table 1). This corresponded to a decrease in mesenteric vascular resistance of –15.4 ± 2.9 (– 19.8 to –8.7, P < 0.0005)% from a control value of 0.100 ± 0.014 (0.069–0.116) mmHg ml–1 min. An example of the above response in 1 pig is shown in Fig. 2.



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  		Figure 1.  The response of mean mesenteric blood flow (MBF) to the intravenous infusion of 0.004 IU kg–1 min–1 of insulin in 10 pigs
The values of blood flow obtained during the test period of measurement are plotted against the corresponding control values before infusion. The continuous line is the line of equality.

 

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  		Table 1.  Changes in haemodynamic variables caused by intravenous infusion of 0.004 IU kg –1 min–1 of insulin in 10 pigs
 


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  		Figure 2.  Example of experimental recordings showing the effects of the intravenous infusion of 0.004 IU kg–1 min–1 of insulin in an anaesthetized pig From top to bottom: heart rate (HR), mean aortic blood pressure (ABP), left ventricular pressure (LVP), maximum rate of change of left ventricular systolic pressure (dP/dtmax), mean right atrial pressure (RAP), mean and phasic mesenteric blood flow (MBF). C, control; T, test.

 
Experiments after intravenous phentolamine and after subsequent intramesenteric L-NAME

In the 5 pigs, the intravenous administration of phentolamine caused a decrease in mean aortic blood pressure of –14 ± 4.8 (– 22 to –10, P < 0.0025) mmHg from control values of 90.6 ± 6.3 (83–100 mmHg. This decrease was accompanied by an increase in heart rate of 12.8 ± 4.3 (8–19, P < 0.0025) beats min–1 from control values of 81.6 ± 4.7 (77–88) beats min–1 and by a small group decrease in left ventricular dP/dtmax of –35 ± 40 (–73 to +12, P > 0.05) mmHg s–1 from control values of 1944 ± 231 (1703–2310) mmHg s–1. Mesenteric blood flow decreased by –45 ± 89 (–144 to +54, P > 0.15) ml min–1 from control values of 876 ± 99 (745–972) ml min–1, with a decrease in mesenteric vascular resistance of –0.011 ± 0.009 (–0.021 to +0.003, P < 0.05) mmHg ml–1 min from a control value of 0.104 ± 0.009 (0.092–0.114) mmHg ml–1 min. The subsequent injection of L-NAME into the mesenteric artery caused an increase in mean aortic blood pressure of 12.2 ± 5.8 (7–22, P < 0.005) mmHg, which was accompanied by a group increase in heart rate of 3.6 ± 5.7 (–3 to +9, P > 0.10) beats min–1 and by an increase in left ventricular dP/dtmax of 83 ± 42 (43–153, P < 0.01) mmHg s–1. Mean mesenteric blood flow decreased by –16 ± 74 (–77 to +100, P > 0.30) ml min–1, with an increase in mesenteric vascular resistance of 0.018 ± 0.006 (0.010–0.025, P < 0.0025) mmHg ml–1 min. A comparison between individual responses of mean mesenteric blood flow to insulin infusion before and after the intravenous administration of phentolamine and the intramesenteric injection of L-NAME is shown in Fig. 3.



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  		Figure 3.  The response of mean mesenteric blood flow (MBF) to insulin in the 5 pigs (numbered 1 to 5)
The response of mean mesenteric blood flow (MBF) to the intravenous infusion of 0.004 IU kg–1 min–1 of insulin before blockade of {alpha}-adrenoceptors with phentolamine ({blacksquare}), after blockade of {alpha}-adrenoceptors ({square}) and after additional blockade of mesenteric nitric oxide synthase with L-NAME (hatched columns) in 5 pigs (numbered 1 to 5).

 
Responses to insulin after intravenous phentolamine.  Despite the decrease in mesenteric vascular resistance caused by blockade of {alpha}-adrenoceptors, infusion of insulin after the administration of phentolamine caused an increase in mesenteric blood flow which was greater than that previously obtained. This increase was of 222 ± 15 (203–240, P < 0.0005) ml min–1 and amounted to 26.8 ± 3.6 (22.4–31, P < 0.0005)% of the control values, with a decrease in mesenteric vascular resistance of –0.020 ± 0.004 (–0.024 to –0.015, P < 0.0005) mmHg ml–1 min. Changes in the other measured haemodynamic variables were small and insignificant (at least P > 0.15). In the same 5 pigs, the increase in mesenteric blood flow and the corresponding decrease in mesenteric vascular resistance elicited by infusion of insulin before the administration of phentolamine were, respectively, 175 ± 15 (156–195, P < 0.0005) ml min–1 and –0.017 ± 0.004 (–0.023 to –0.013, P < 0.0005) mmHg ml–1 min. The differences between these responses caused by insulin before and after blockade of {alpha}-adrenergic receptors were statistically significant (P < 0.0025 and P < 0.05, respectively).

Responses to insulin after subsequent intramesenteric L-NAME.  Blockade of mesenteric nitric oxide synthase completely abolished the response of mesenteric blood flow to infusion of insulin. After the intramesenteric injection of L-NAME, infusion of insulin did not cause any significant changes in mean mesenteric blood flow. These changes were of 2 ± 6 (–5 to +10, P > 0.20) ml min–1. Changes in the other measured haemodynamic variables were small and insignificant (at least P > 0.10). Analysis of variance for repeated measurements showed a significant difference in the responses of mean mesenteric blood flow to insulin infusion before and after intramesenteric injection of L-NAME (F= 446.8, P < 0.0005). The Newman–Keuls test indicated that the response obtained with insulin after the intramesenteric injection of L-NAME was significantly different from those obtained before and after giving phentolamine and that the response elicited by insulin after phentolamine was significantly greater than that obtained before the administration of the {alpha}-adrenoceptor blocking agent.

Experiments after intramesenteric L-NAME and after subsequent intravenous phentolamine

In the 5 pigs, the intramesenteric injection of L-NAME caused an increase in mean aortic blood pressure of 13.4 ± 4.5 (8–20, P < 0.0025) mmHg from a control value of 97.6 ± 4.3 (93–102) mmHg. This increase was accompanied by a group increase in heart rate of 3.6 ± 4.7 (–4 to +8, P > 0.05) beats min–1 from a control value of 82.6 ± 10.4 (73–95) beats min–1 and by an increase in left ventricular dP/dtmax of 85 ± 26 (54–115, P < 0.0025) mmHg s–1 from a control value of 1977 ± 166 (1843–2250) mmHg s–1. Mesenteric blood flow decreased by –26 ± 68 (–88 to +86, P > 0.20) ml min–1 from a control value of 1067 ± 199 (870–1371) ml min–1, with an increase in mesenteric vascular resistance of 0.015 ± 0.004 (0.011–0.022, P < 0.0025) mmHg ml–1 min from a control value of 0.094 ± 0.017 (0.068–0.115) mmHg ml–1 min. The subsequent intravenous administration of phentolamine caused a decrease in mean aortic blood pressure of –17.2 ± 3.7 (–21 to –12, P < 0.0005) mmHg, which was accompanied by an increase in heart rate of 12.4 ± 3 (9–17, P < 0.0005) beats min–1 and a group decrease in left ventricular dP/dtmax of –45 ± 51 (–112 to +5, P > 0.05) mmHg s–1. Mean mesenteric blood flow decreased by –30 ± 62 (–112–45, P > 0.15) ml min–1, with a decrease in mesenteric vascular resistance of –0.014 ± 0.005 (–0.019 to –0.009, P < 0.0025) mmHg ml–1 min. A comparison between individual responses of mean mesenteric blood flow to insulin infusion before and after the intramesenteric injection of L-NAME and the intravenous administration of phentolamine is shown in Fig. 4.



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  		Figure 4.  The response of mean mesenteric blood flow (MBF) to insulin in the 5 pigs (numbered 1 to 5)
The response of mean mesenteric blood flow (MBF) to the intravenous infusion of 0.004 IU kg–1 min–1 of insulin before blockade of mesenteric nitric oxide synthase with L-NAME ({blacksquare}), after blockade of mesenteric nitric oxide synthase ({square}) and after additional blockade of {alpha}-adrenoceptors with phentolamine (hatched columns) in 5 pigs (numbered 1 to 5).

 
Responses to insulin after intramesenteric L-NAME.  Despite the increase in mesenteric vascular resistance caused by L-NAME, infusion of insulin after blockade of mesenteric nitric oxide synthase caused a decrease in mean mesenteric blood flow of –84 ± 17 (–107 to –66, P < 0.0005) ml min–1. This decrease amounted to –8.2 ± 2.1 (–10.1 to –5, P < 0.0005)% of the control values and corresponded to an increase in mesenteric vascular resistance of 0.010 ± 0.004 (0.004–0.015, P < 0.005) mmHg ml–1 min. Changes in the other measured haemodynamic variables were small and insignificant (at least P > 0.25). In the same 5 pigs, the increase in mesenteric blood flow and the corresponding decrease in mesenteric vascular resistance elicited by infusion of insulin before the intramesenteric injection of L-NAME were, respectively, 172 ± 35 (112–201, P < 0.0005) ml min–1 and –0.014 ± 0.005 (–0.020 to –0.006, P < 0.0025) mmHg ml–1 min.

Responses to insulin after subsequent intravenous phentolamine.  Blockade of {alpha}-adrenoceptors completely abolished the response of mesenteric blood flow to infusion of insulin. After the intravenous administration of phentolamine, infusion of insulin did not cause any significant changes in mean mesenteric blood flow. These changes were of 1 ± 5 (–5 to +7, P > 0.30) ml min–1. Changes in the other measured haemodynamic variables were small and insignificant (at least P > 0.20). Analysis of variance for repeated measurements showed a significant difference in the responses of mean mesenteric blood flow to insulin infusion before and after administration of phentolamine (F= 140.4, P < 0.0001). The Newman–Keuls test indicated that the response obtained with insulin after giving phentolamine was significantly different from those obtained before and after giving L-NAME and that the response elicited by insulin after L-NAME was significantly different from that obtained before the intramesenteric injection of the nitric oxide synthase-blocking agent.


   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
References
 
The present study has shown that infusion of insulin in anaesthetized pigs primarily caused a mesenteric vasodilatation, which was the net result of two opposite effects, a predominant effect involving a vasodilatation mediated by the endothelial release of nitric oxide and another involving a vasoconstriction mediated by {alpha}-adrenoceptors.

The design of the present study ensured that the increase in mesenteric blood flow obtained with the infusion of insulin was a primary effect of the hormone. Firstly, the experiments involving infusion of insulin were performed under controlled conditions which included a euglycaemic clamp and prevention of changes in blood levels of potassium as previously reported (DeFronzo et al. 1979; Liang et al. 1982). Secondly, changes in heart rate and aortic blood pressure were prevented, thus avoiding any secondary interference from reflex and local physical effects which could secondarily interfere with the primary response of mesenteric circulation to infusion of insulin. Thirdly, ß1-adrenoceptors were blocked, to prevent changes in left ventricular contractility, as demonstrated by the absence of changes in left ventricular dP/dtmax during the experiments. It has previously been shown that infusion of insulin caused an increase in left ventricular contractility which could be abolished by blockade of ß-adrenoceptors with propranolol or atenolol (Liang et al. 1982; Reikeras & Gunnes, 1986a,b;. Law et al. 1988;. Molinari et al. 2001a, 2002a). Fourthly, the increase in mesenteric blood flow in response to infusion of insulin was obtained in the absence of changes in mean right atrial pressure and left ventricular end-diastolic pressure, thus excluding the secondary interference from reflexes related to cardiac receptors. In addition, infusion of the vehicle alone did not reproduce any of the effects observed in this study.

The present study showed for the first time that infusion of insulin caused an increase in mesenteric blood flow and a decrease in mesenteric vascular resistance. Previous reports concerning the effects of insulin on the mesenteric circulation are scarce and inconsistent. In dogs, insulin infusion was found to cause a dose-dependent increase in left ventricular contractility, stroke volume and cardiac output, which were accompanied by a proportional increase in mesenteric blood flow (Reikeras & Gunnes, 1986a). In conscious rats, infusion of insulin has been reported to cause vasodilatation in renal and hindquarter vascular beds but no changes in superior mesenteric conductance (Pitre et al. 1996) and to elicit a slight increase in arterial blood pressure and mesenteric vasoconstriction only at high doses (Gaudreault et al. 2001). In another report in rats, using the radioactive microsphere technique it was shown that short-term sustained euglycaemic hyperinsulinemia increased blood flow to skeletal muscle without changing splanchnic blood flow (Hilzenrat et al. 2001). In normal humans and in patients with chronic heart failure, insulin infusion caused a dose-dependent increase in forearm blood flow and a decrease in superior mesenteric blood flow, which were accompanied by a decrease in arterial pressure (Parsonage et al. 2002). In the present study in anaesthetized pigs, the increase in mesenteric blood flow and the decrease in mesenteric vascular resistance in response to insulin infusion were obtained in the absence of changes in haemodynamic variables which could secondarily affect the response of the mesenteric circulation to the hormone, thus demonstrating that infusion of insulin primarily caused a mesenteric vasodilatation.

The experiments performed to study the mechanisms involved in the insulin-induced increase in mesenteric blood flow showed that this effect of the hormone was the net result of a predominant vasodilatation mediated by the endothelial release of nitric oxide and of a vasoconstriction involving sympathetic effects mediated by {alpha}-adrenoceptors. In 5 pigs, it was shown that the insulin-induced mesenteric vasodilatation was enhanced by blockade of {alpha}-adrenoceptors, an effect which was not influenced by changes in baseline haemodynamic variables caused by the blocking agent. The increase in the response of vasodilatation occurred despite the decrease in mesenteric vascular resistance caused by phentolamine. This increased effect of vasodilatation after phentolamine suggested the presence of a concomitant vasoconstriction elicited by insulin through {alpha}-adrenoceptors. This was confirmed in the remaining 5 pigs by blocking the endothelial release of nitric oxide with the intramesenteric injection of L-NAME. Infusion of insulin after blockade of mesenteric nitric oxide synthase caused a decrease in mesenteric blood flow despite the increase in mesenteric vascular resistance caused by the blocking agent. The mechanisms involved in the effect of insulin on the mesenteric circulation were confirmed in the subsequent experiments of infusion. The mesenteric vasodilatation caused by insulin in the first 5 pigs after the administration of phentolamine was abolished by intramesenteric L-NAME. In addition, in the remaining 5 pigs, the mesenteric vasoconstriction caused by insulin after intramesenteric L-NAME was abolished by intravenous phentolamine.

The present results are consistent with previously reported findings showing that insulin caused an activation of central sympathetic vasoconstrictor output and a vasodilatation which was partly attributed to direct effects of the hormone on vascular smooth muscle and the endothelial release of nitric oxide (e.g. Baron, 1994; Scherrer & Sartori, 1997; Yki-Jarvinen & Utriainen, 1998). Together with our previous findings obtained in the same experimental model, they also confirm the reported regional variations in the overall effects of the hormone on resting blood flow. In our controlled experiments, infusion of insulin was shown to increase renal blood flow because of a predominant vasodilatation involving the endothelial release of nitric oxide and despite a vasoconstriction involving sympathetic {alpha}-adrenoceptor-mediated mechanisms (Molinari et al. 2001a) and to decrease coronary blood flow because of a predominant vasoconstriction involving {alpha}-adrenoceptor-mediated mechanisms and despite a vasodilatation involving the endothelial release of nitric oxide (Molinari et al. 2002a). Our results in anaesthetized pigs indicate that insulin infusion after blockade of sympathetic effects increased baseline coronary, renal and mesenteric blood flow by about 12, 18 and 27%, respectively, and that the same infusion after blockade of the local endothelial release of nitric oxide decreased baseline values of the same blood flows by about –20, –7 and –8%, respectively. These differing effects were obtained in the same anaesthetized preparation by using methods which allowed prevention of changes in confounding factors and are consistent with reports showing heterogeneity of central sympathetic activation by insulin (Morgan et al. 1993; Muntzel et al. 1995; Scherrer & Sartori, 1997; Lu et al. 1998) and with reports showing a widespread vasodilator effect of insulin (Reikeras & Gunnes, 1986a,b; Ferranini et al. 1993; Laine et al. 2000; McNulty et al. 2000; Scott et al. 2002). In particular, our results indicate that the increase in sympathetic drive caused by insulin was similar in renal and mesenteric vascular beds.

The present results add new information on the vascular effects of insulin and, together with the previously reported effects (Molinari et al. 2002a), can be argued to have important physiological and clinical implications. For instance, secretion of insulin stimulated by gastrointestinal hormones delivered early during digestion (e.g. Meier et al. 2002) can be suggested to contribute to the intestinal hyperaemia that occurs during digestion and absorption of nutrients and has been considered as a complex response mediated by neural, humoral and paracrine elements (e.g. Matheson et al. 2000). In addition, the increase in plasma levels of insulin during digestion and its effect of coronary vasoconstriction could be argued to contribute to the occurrence of postprandial worsening of angina pectoris in patients with coronary artery disease, variably attributed to increases in heart rate and arterial blood pressure or cardiac output (Grollman, 1929; Goldstein et al. 1971; Kelbaek et al. 1989; Cowley et al. 1991).

In conclusion, the present investigation has shown that insulin infusion in anaesthetized pigs caused a mesenteric vasodilatation. This effect was the net result of a predominant vasodilatation mediated by the endothelial release of nitric oxide and of a vasoconstriction which involved sympathetic vasoconstrictor mechanisms mediated by {alpha}-adrenoceptors.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
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   Acknowledgements
 
The research has received generous sponsorship from the Università del Piemonte Orientale ‘A. Avogadro’. We thank the Azienda Ospedaliera Maggiore della Carità di Novara for its help.





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