Afferent Control of Vasopressin and Renin Release During Haemorrhage in Normal and Autonomically Blocked Rabbits

The role of the arterial and cardiac baroreceptors on the arginine vasopressin (AVP) and plasma renin activity (PRA) responses to haemorrhage was studied in conscious rabbits. They were bled at a rate of approximately 3% of their blood volume (BV)/min, both when the autonomic nervous system (ANS) was intact and during ANS blockade, which markedly enhances the AVP response due to the much greater haemodynamic disturbance. Under each condition of ANS function 2 x 2 factorial analysis was performed, each with four groups of rabbits, including animals with both sets of baroreceptors working, one or other set working and neither set working. 2. With intact ANS, haemorrhage had to be terminated at different times in the four groups. This presents problems for factorial analysis due to differences in the relationship between plasma AVP (or PRA) and release rate. A method for overcoming this was developed by extrapolating the BV-log AVP curves to a common time from the start of bleeding. 3. Under both conditions of ANS function the arterial and cardiac baroreceptors together accounted for 90-95% of the rise in AVP during haemorrhage. With normal ANS function, the rise in AVP was about 70% through cardiac (probably ventricular) baroreceptors (P = 0.01) and about 30% through arterial baroreceptors (P = 0.08). This compares with an earlier study at a rate of bleeding of 1.8% BV/min, where the entire drive came from the cardiac receptors. During ANS blockade, plasma AVP was enhanced approximately five-fold, which was mostly mediated through the arterial baroreceptors, but the cardiac baroreceptor component was also greater; arterial/cardiac baroreceptor drive was 2/1. 4. Baroreflexes played no role in renin release during haemorrhage, but the experiments with ANS blockade suggest that a hormonal factor, which was related to the cardiac innervation, may limit the rise in PRA in the latter part of haemorrhage.