UBC Theses and Dissertations
The relationship between sinoaortic baroreceptors, atrial receptors and the release of vasopressin in the anaesthetized rabbit Courneya, Carol Ann Margaret
Vasopressin, a hormone released from the neurohypophysis, contributes to the regulation of body fluid balance through its known actions on the kidney and the vasculature. Release of vasopressin is influenced by plasma osmolality and by afferent activity from sensory receptors in the high and low pressure vascular systems. Previous studies have not defined the relative importance of the carotid sinus baroreceptors, aortic baroreceptors and atrial receptors in the control of the plasma concentration of vasopressin in the rabbit. Experiments were carried out in anaesthetized rabbits to define the quantitative relationship between stimulation of the carotid sinus baroreceptors and the plasma concentration of vasopressin. This relationship was examined in the presence and absence of afferent input from the aortic and atrial receptors. Changes in blood volume were induced to produce a change in the stimulus to the aortic baroreceptors and atrial receptors at high or low, constant carotid sinus pressure. Section, of the aortic depressor nerves and the vagus nerves allowed examination of the individual contributions of atrial receptors or aortic baroreceptors on the plasma concentration of vasopressin. It was also possible to examine the interaction between the carotid sinus baroreceptors and the aortic and atrial receptors. The results showed that plasma concentration of vasopressin was reduced by minimal stimulation of carotid sinus baroreceptors and that maximal inhibition of the release of vasopressin was achieved with a relatively low total arterial baroreceptor input. No influence of carotid sinus baroreceptors on vasopressin release was seen in the presence of intact aortic baroreceptors demonstrating the important interaction between the effects of stimulation of these two sets of receptors. It was not possible to demonstrate, in the rabbits used in this study, a significant contribution of atrial receptors to the control of vasopressin release either in response to changes in carotid sinus pressure or in response to changes in blood volume. To minimize the inhibitory effect of arterial baroreceptors on the release of vasopressin the aortic depressor nerves were cut and carotid sinus pressure was set at a low level. It was still not possible to demonstrate an effect of a reduction in blood volume on vasopressin release, confirming the absence of a contribution from atrial receptors in the anaesthetized rabbit. There appears to be considerable variation between species in the contribution of the different receptor groups to the release of vasopressin. The results suggest that in the normal rabbit there is likely to be significant tonic inhibition of the release of vasopressin by stimuli arising from arterial baroreceptors. The absence of a demonstrable influence of atrial receptors in these rabbits is consistent with findings in primates but differs from those in dogs. It is unlikely that changes in plasma vasopressin concentration induced by small changes in blood volume contribute to the control of arterial pressure through direct effects on vascular resistance and capacitance.