Received March 15, 2006
Revised March 29, 2006
Accepted after revision May 26, 2006

¹ University of Auckland
² University of Otago

^* To whom correspondence should be addressed. E-mail: chris.bolter{at}stonebow.otago.ac.nz.

	Abstract

The arterial baroreceptors and their afferent fibres provide the sensory arm of the reflex regulating systemic arterial pressure. We have examined whether the relationship between mean baroreceptor discharge and mean arterial pressure is altered when heart rate changes. Experiments were performed on pentobarbitone anaesthetised rabbits. We recorded the activity of single and multi-fibre preparations of the carotid sinus (CSN) and aortic depressor nerves (ADN). Data were collected under control conditions and while heart rate was increased by ~30-35% by right atrial pacing. Baroreceptor regions were exposed to ramps of pressure (~25 to 140 mmHg, at ~0.5 to 1 mmHg s-1) generated by inflation and deflation of cuffs placed around the inferior vena cava and descending thoracic aorta. Response curves relating baroreceptor discharge to mean pressure were constructed and fitted with 3rd order polynomial expressions. To provide a measure of an effect of an increase in heart rate on the response curve in the region of the normal operating pressure, we calculated position of the test response curve relative to the position of the control curve at 90 mmHg (BP90). For the ADN the activity of single fibres (presumptive myelinated fibres) was unaffected by increasing heart rate (BP90 = +0.1 ± 1.0 mmHg), while single fibres in the CSN showed a small increase in activity (BP90 = 1.5 ± 0.3 mmHg). In multifibre preparations there was a small increase in activity that may be attributable to additional activity in unmyelinated fibres (ADN, BP90 = 3.4 ± 1.2 mmHg, CSN, BP90 = 5.2 ± 0.9 mmHg,). We conclude that the mean discharge of arterial baroreceptors remains a reliable index of mean arterial pressure in the presence of substantial changes in heart rate.

Key Words: Arterial baroreflex, Baroreceptor

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