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This Article Abstract Full Text (PDF) All Versions of this Article: 89/3/287    most recent expphysiol.2003.026682v1 Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Leach, R. M. Articles by Forsling, M. L. Search for Related Content PubMed PubMed Citation Articles by Leach, R. M. Articles by Forsling, M. L. Related Collections Neuroendocrinology/Endocrinology

Department of Respiratory Medicine and Allergy, St Thomas' Hospital Centre for Neuroscience Research, Guy's, King's and St Thomas' School of Medicine, Guy's Campus, London SE1 1UL, UK

	Abstract

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There is evidence that changes in arterial P_CO2 (P_a,CO2), as well as P_O2, influence neuroendocrine function. The hyponatraemia and fluid retention (cor pumonale) seen in chronic obstructive pulmonary disease (COPD) and type II respiratory failure is associated with increased vasopressin release. This study examines the specific effects of altered P_a,CO2 on hormone release from the posterior and anterior pituitary. The study was performed in 20 ventilated ICU patients in the late recovery phase of their illness. None had primary respiratory disease. Control blood samples were taken and the alveolar ventilation was then adjusted to allow the P_a,CO2 increase or decrease for a period of 3 h, during which time further blood samples were taken for the determination, by radioimmmunoassy of vasopressin, oxytocin, growth hormone and cortisol. Urine output and electrolyte concentrations were also measured. Circulating concentrations of growth hormone and oxytocin increased with increasing P_a,CO2. Vasopressin release showed a similar pattern up to a P_a,CO2 of approximately 6.0 kPa, above which vasopressin concentrations were inversely related to P_a,CO2. There was no significant effect on cortisol concentrations. No significant effects were established in urinary parameters during the short period of this study. Thus an increase in CO₂ is associated with stimulated pituitary hormone release. The effect on the neurohypophysial hormones may account for the fluid retention and hyponatraemia seen in COPD and hence provide a rationale for treatment.

(Received 22 October 2003; accepted after revision 18 February 2004; first published online 17 February 2004)
Corresponding author M. L. Forsling: 2-38A Neuroendocrine Laboratories, New Hunt's House, GKT School of Medicine, Guy's Campus, London Bridge, London, SE1 1UL UK. Email: mary.forsling{at}kcl.ac.uk

	Introduction

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Altered P_a,CO2 is observed in a variety of conditions. Hypercapnia is not only associated with lung disease, but also with conditions affecting neural, muscular and circulatory components of the respiratory system. Hypocapnia is also a common clinical event and may have pulmonary as well as non-pulmonary causes. Changes in P_CO2 affect a variety of systems including the endocrine system. A proportion of patients suffering from pulmonary disease are known to retain fluid and although classically this is attributed to right heart failure, more recent evidence has not supported this theory (MacNee, 1988) and there is now evidence to suggest this fluid retention could result from elevated plasma vasopressin concentrations associated with hypoxaemia or hypercapnia (Kelestimur et al. 1997). Farber et al. (1982) showed that vasopressin was elevated in patients with chronic obstructive pulmonary disease and that this was correlated with the inability to excrete a water load. Altered renal responses could also contribute to the fluid retention, which would be consistent with the observation of Stewart et al. (1995) that lowering aldosterone concentrations in patients with COPD did not affect sodium excretion.

These and other studies on man did not distinguish between the effects of hypoxaemia and hypercapnia. However, this has been possible in experiments on animals. Early studies by Forsling & Rees (1975) showed that hypercapnia stimulated vasopressin in the cat. This was confirmed by Rose et al. (1984) working on conscious dogs. Coren et al. (1986), who performed studies on paralysed dogs to avoid any effects of hyperventilation, found that hypercapnic acidosis produced a significant increase in vasopressin over the first 90 min followed by a progressive reduction in plasma concentrations. There have been relatively few studies on the effect of hypocapnia on vasopressin release. Philbin et al. (1970) reported a fall in plasma vasopressin concentrations during a 50% reduction of P_a,CO2 in dogs, although more recent work of Krapf et al. (1995) found no change in hormone concentrations. There is evidence to suggest that release of the anterior pituitary hormones growth hormone and cortisol may be stimulated during hypercapnia, while there have been no studies on the posterior pituitary hormone oxytocin, although this has been shown to be stimulated by hypoxaemia in the rat (Kelestimur et al. 1997). While studies have been performed on animal models, there are few data on the effect of altered P_a,CO2 and neurohypohysial function in man. A study has now been performed on the neuroendcrine response to changes in P_a,CO2 in man.

	Methods

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The study was performed on 20 patients in the Intensive Care Unit at St Thomas' Hospital with the approval of the Ethics Committee of Guy's and St Thomas' Hospital Trust and informed assent . All patients had been on the Unit for more than 3 days and were not receiving hormone replacement therapy, inotropic support or dopamine at the time of the study. The patients were male with a mean age of 61.5 ± 2.9 years. All were haemodynamically stable with normal urine output at the time of the study. None of these patients had suffered with significant respiratory impairment before ICU admission (although most were current or previous smokers) . All patients were extubated within 48 h of the study. One patient required re-intubation after 48 h but was successfully extubated 3 days later. All patients made a full recovery and were discharged . All patients were still sedated at the time of the study. This was necessary to avoid excessive hyper- or hypoventilation and to allow changes in P_a,CO2 to be achieved. The 10 patients in whom hypercapnia was induced included five patients who had undergone previous cardiothoracic surgery, one of whom had briefly developed renal impairment postoperatively, which had fully recovered at the time of the study (i.e. normal renal biochemistry and urine output). Two patients were studied following recovery from road traffic accidents, two after single vessel coronary artery stenting and one patient following recovery from surgery. The 10 patients in whom hypocapnia was induced included four patients who had undergone cardiothoracic surgery, one post surgical patient, one patient following trauma and one following collapse due to a transient cerebrovascular event. Three patients suffered with ischaemic heart disease; one presented with left ventricular failure, one following a myocardial infarction and one with a suspected chest infection. With appropriate treatment, cardiac and respiratory function had returned to normal in these patients at the time of study.

All observations were performed between 10.00 h and 16.00 h and heart rate, blood pressure, central venous pressure and respiratory rate were monitored throughout. Control blood samples were taken at 11.00 h and 12.00 h and one aliquot centrifuged at 4°C to obtain plasma for the determination of osmolality and oxytocin, vasopressin, growth hormone and cortisol concentrations. The ventilation settings were adjusted to change minute alveolar ventilation to produce carefully monitored permissive changes in P_a,CO2. Patients were randomly allocated to one of two groups. In the first (n= 10) a stepwise increase in P_CO2 was produced, and in the second a fall (n= 10) (respectively hypercapnic and hypocapnic groups). Further blood samples were taken at 12.30 h, 13.00 h, 14.00 h and 15.00 h, after which time the P_a,CO2 was corrected . Blood samples were analysed for packed cell volume, P_a,CO2, P_a,O2, pH, sodium, potassium, glucose and lactate concentrations as well as the total haemoglobin content. An initial timed urine sample was collected to determine flow rate and hourly collections made during the three hour period of altered P_a,CO2 with a final sample collection two hours later. The study conditions did not affect the recovery or well-being of the patients.

Determinations

Blood gas analysis and measurement of electrolytes, glucose, lactate and total haemoglobin were carried out immediately on a blood gas machine (ABL625, Radiometer TM, Copenhagen, Denmark). Packed cell volume was determined using microhaematocrit tubes (Hawksley and Sons Ltd, Lancing, Sussex, England). Osmolality was determined by the depression of freezing point (Osmomat 030, Genotec, Berlin, Germany).

Plasma vasopressin concentrations were determined by radioimmunoassay (Forsling & Peysner, 1988) after prior extraction using Sep Pak C18 cartridges (Water Associates Inc., Milford, MA USA). The standard employed was the first International Standard for arginine vasopressin (77/501). The lower limit of detection of the assay was 0.08 pmol l^–1 and the intra and interassay coefficients of variation were 5.0 and 8.9%, respectively, at 2.5 pmol l^–1 . The cross reactivity of the antiserum with oxytocin was less than 1%.

Plasma oxytocin concentrations were similarly determined by radioimmunoassay (Balment et al. 1986) after extraction on Sep Pak C18 cartridges. The standard employed was the first International Standard for oxytocin (76/575), while the lower limit of detection of the assay was 0.1 pmol l^–1 and the intra and interassay coefficients of variation were 5.1 and 7.8%, respectively, at 2.5 pmol l^–1. The cross reactivity of the antiserum with vasopressin was less than 0.1%.

Cortisol was measured by ELISA, Ezymin-Test Cortisol (Boehringer Mannheim Immunodiagnostics, East Sussex, UK) using the ES automated immunoassay analyser. The sensitivity of the assay was 27.6 nmol l^–1. The intra-assay coefficient of variation of the assay was 5.6% at 67 nmol l^–1 and 2.9% at 800 nmol l^–1 with an interassay coefficient of variation of 11.5% at 67 nmol l^–1 and 2.9% at 800 nmol l^–1.

Plasma growth hormone was measured by a two site immunoradiometric assay with a sensitivity of 0.2 mU l^–1, an intra-assay coefficient of variation of 3% at 1.0 mU l^–1 and an interassay coefficient of variation of 10,04% at 1.68 mU l^–1, 4.9% at 12.12 mU l^–1 and 5.4% at 22.24 mU l^–1.

Statistics

Data are expressed as means ±S.E.M., with the values for hormone concentrations being taken in 6 blocks, according to P_CO2 (3–3.99 kPa, 4–4.99 kPa, etc.). The variance between two different groups in a set of data was determined by a single factor ANOVA. Where appropriate, two sided t tests were used for comparison between two means with equal or unequal variance, with the Bonneferroni correction for multiple comparisons. Correlation was determined using regression analysis. Statistical significance was taken as P < 0.05.

	Results

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The mean arterial blood pressure which initially was 77 ± 2.1 mmHg did not change significantly during the course of the study, neither did central venous pressure. Similarly, plasma sodium which was 142.9 ± 1.4 mmol l^–1 in the initial sample remained unchanged, as did plasma osmolality (300.2 ± 3.7 mOsm/kg) and packed cell volume (34.5 ± 1.3%).

Arterial P_O2 was maintained at over 11 kPa throughout the study period with saturation of greater than 95% in all patients. While there was overlap in the values for CO₂ in the two experiments, a clear stepwise increase or decrease in CO₂ was achieved (Fig. 1). The time course for the changes in P_a,CO2 was similar in all patients. Overall the initial P_a,CO2 was 5.2 ± 0.3 kPa with a pH of 7.42 ± 0.01. The lowest mean P_a,CO2 was 4.32 ± 0.2 kPa with a pH of 7.55 ± 0.01 while the highest CO₂ was 7.2 ± 0.4 kPa with a pH of 7.28 ± 0.03. The corresponding lactate concentrations were 1.69 ± 0.4 and 0.97 ± 0.07 mmol l^–1, respectively. Initial blood glucose was 8.02 ± 0.64 mmol l^–1 and was maintained for the duration of the study.

View larger version (11K): [in this window] [in a new window]   Figure 1.  Percent change in Pa,CO2 during (A) a stepwise increase (n= 10) and (B) a stepwise decrease (n= 10). The second control value represents that at 12.00 h, followed by 3 h altered Pa,CO2 and finally correction of the Pa,CO2. Overall the initial PCO2 was 5.2 ± 0.3 kPa. The changes produced were significant (P < 0.01).

The changes in vasopressin concentrations with P_a,CO2 are shown in Fig. 2, which includes data from both the hypocapnic and the hypercapnic group, since there was overlap in the P_a,CO2 in the two groups and for any given P_a,CO2 there was no significant difference in the vasopressin concentrations between the two groups. Data for the other hormones studied have been similarly grouped. As P_a,CO2 increased from 3.0 to 4.0 kPa, there was a significant increase in the plasma vasopressin concentrations (P < 0.05), which reached a plateau at 6.0–8.0 kPa (Fig. 2). Vasopressin concentrations were correlated with P_a,CO2 (r= 0.68, P < 0.001). As P_a,CO2 increased further plasma vasopressin concentrations fell to values not significantly different from those observed at 3.0–4.0 kPa. Over this higher range, vasopressin showed a negative correlation with P_a,CO2. Vasopressin was similarly correlated with plasma pH. Following the increase seen when P_a,CO2 was elevated there was a fall in urine followed by an increase as plasma vasopressin fell. Urine flow initially fell on reduction of P_a,CO2, but there was an increase following the fall in vasopressin concentrations. The fall in plasma oxytocin concentrations after 60 and 120 min of reduced CO₂ was statistically significant. Overall oxytocin showed a progressive increase up to a P_a,CO2 of 7.0–8.0 kPa (P < 0.01), as shown in Fig. 2.

View larger version (20K): [in this window] [in a new window]   Figure 2.  Plasma vasopressin (A) and oxytocin (B) concentrations with increasing Pa,CO2. Values are given as mean ±S.E.M.; *P < 0.05, **P < 0.01 compared to Pa,CO2 3–3.99, +P < 0.05 compared to Pa,CO2 7–7.99.

The initial growth hormone concentration prior to increasing P_a,CO2 was 6.6 ± 3.1 mU l^–1, and 13.2 ± 4,7 mU l^–1 after 120 min, falling significantly to 3.2 ± 0.9 mU l^–1 when P_CO2 values returned to the initial values (Fig. 3). The fall in GH from 7.25 ± 0.91 mU l^–1 to 5.9 ± 0.49 U l^–1 at 180 min hypocapnia was not statisically significant. Over the range of CO₂ pressures studied, growth hormone concentrations increased with increasing P_a,CO2 levelling off at 6.0–7.0 kPa (P < 0.05). There was no significant correlation between P_a,CO2 and plasma cortisol concentrations. Cortisol concentrations showed a tendency to increase with increasing P_a,CO2, but this was not statistically significant.

View larger version (19K): [in this window] [in a new window]   Figure 3.  Plasma growth hormone (A) and cortisol (B) concentrations with increasing Pa,CO2. Values are given as mean ±S.E.M.; *P < 0.05, compared to Pa,CO2 3–3.99.

	Discussion

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Hypocapnia as well as hypercapnia is a common clinical event and may have pulmonary as well as non-pulmonary causes. The decrease in vasopressin concentrations as the P_a,CO2 fell below the normal range is consistent with the data of Philbin et al. (1970) also studied dogs . Krapf et al. (1995), however, found no significant fall in vasopressin concentrations during acute respiratory alkalosis. Hypocapnia, however, was produced by voluntary hyperventilation, so that there was relatively little control over the degree of hypocapnia induced. The mechanisms underlying the response in the present study are not clear, although possible causes include altered P_a,CO2 or pH acting centrally on the neurohypophysial system or via altered chemoreceptor activity. The effect of decreased P_a,CO2 on systemic vasoconstrictor tone must also be considered, so the response could result from reduced perfusion of the hypothalamic neurones. It is unlikely to be due to an effect at the level of the neural lobe as Bryan et al. (1988) showed that blood flow in the nerve terminals is regulated differently from that in the hypothalamic nuclei during hypercapnia. There is increased venous tone during hypocapnic alkalosis (Weil et al. 1971), which would result in an increase in central venous volume and a fall in vasopressin. The change in vasopressin concentrations was not accompanied by a change in plasma osmolality, so that it is possible that there was increased renal responsiveness to vasopressin which in turn led to a fall in vasopressin.

As P_a,CO2 increased above the physiological range there was a continued increase in vasopressin, which is consistent with the study of Rose et al. (1984) showing increased vasopressin concentrations in acute hypercapnic acidosis and also with the study of Farber et al. (1982) on hypercapnic patients with COPD. Vasopressin release in pre-term infants was also found to be positively correlated with P_CO2 (Rees et al. 1984). Kc et al. (2002), studying activation of the hypothalamic vasopressin containing neurones in terms of immediate early gene expression, showed a marked increase following hypercapnia in both the supraoptic and paraventricular nuclei. By contrast Chabot et al. (1995) found no effect on vasopressin release, but their observations were for only one hour and over a narrower range of P_a,CO2 of 4.8–6.9 kPa, which spans the region in which vasopressin ceases to be stimulated and shows a decrease. Interestingly they noted a fall in vasopressin over the period of observation, but it was not statistically significant. Overall the fall in vasopressin concentrations at the highest P_a,CO2 observed could explain why some authors report no change with altered blood gas tensions (Farber et al. 1977). As for reduced P_a,CO2, the responses could represent a direct effect of pH or P_CO2, an indirect effect via chemoreceptors or via altered renal responsiveness to vasopressin. A role for pH would be consistent with the observations of Gardner et al. (2000), who found that vasopressin release when hypoxaemia was induced was signficantly higher in fetuses with acidaemia. Hypercapnic vasodilation and low mean arterial blood pressure could be the stimulus for neurohumoral activation, but monitored haemodynamic parameters, including blood pressure, did not change significantly during the course of the study. The constancy of central venous pressure would argue against the involvement of atrial receptors. Furthermore vasopressin concentrations fell when the highest values for P_a,CO2 were achieved. The fact that there was no significant change in the cortisol concentrations would indicate that the release was not a stress response.

The different profile of oxytocin release from that for vasopressin suggests that different mechanisms are involved or that the vasopressinergic neurones are more sensitive. With the exception of a study showing oxytocin concentrations were elevated during chronic hypoxaemia in the rat (Kelestimur et al. 1997), the effect of blood gas tensions on the release of this hormone has not been studied. Similarly there are very little data available on the effects of altered carbon dioxide tensions on circulating growth hormone concentrations. In a study on hypercapnic oedematous COPD, Anand et al. (1992) found that growth hormone concentrations were 9-fold higher in unrecovered as compared to recovered patients. This could have implications for fluid balance, since growth hormone is known to affect salt and water excretion (Hansen et al. 2001). Cortisol concentrations were also found to be higher in the unrecovered patients. In the same study, while there was a tendency for cortisol concentrations to increase with moderate hypercapnia, the increase was not statistically significant.

It is well established that there may be disturbed fluid balance in chronic obstructive airway disease. Disturbance of the hormonal systems controlling fluid balance is also seen, including increased vasopressin secretion (Stewart et al. 1995). The present investigation confirms that an altered P_a,CO2 and associated pH changes affect vasopressin and growth hormone secretion and demonstrates that oxytocin release is also affected. Vasopressin release inappropriate for fluid balance status will result in water retention and hypontraemia. In addition, there is evidence, mainly from the rat but also man, that oxytocin may increase sodium excretion (Forsling, 1986; Verbalis et al. 1991). The availability of oral antagonists of the renal actions of vasopressin (Yamamura et al. 1992; Gross, 2001) may therefore facilitate the treatment of the hyponatremia and fluid retention.

	References

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This Article Abstract Full Text (PDF) All Versions of this Article: 89/3/287    most recent expphysiol.2003.026682v1 Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Leach, R. M. Articles by Forsling, M. L. Search for Related Content PubMed PubMed Citation Articles by Leach, R. M. Articles by Forsling, M. L. Related Collections Neuroendocrinology/Endocrinology