Proceedings of the Physiological Society, 12‐14 September 1985, Cambridge Meeting: Communications

Abstract

The effects of intravenous infusion of arginine vasopressin (AVP) are buffered by baroreceptor reflexes on which a central potentiating action of AVP is postulated (Matsuguchi & Schmid, 1982). This corresponds with the finding that AVP elicits a greater baroreceptor response for a given pressor stimulus than other vasoconstrictor agents. In our study we examined the effect on carotid sinus baroreflex sensitivity of infusing AVP at a dosage likely to result in plasma AVP levels within the normal daily range.Six healthy male volunteers (21-25 yr) attended the laboratory on two separate occasions. Institutional ethics committee approval was obtained and each subject gave written informed consent. Endogenous AVP secretion was suppressed by drinking 1-5 1 of water. This water load was maintained for the duration of the experiment by drinking water equal in volume to the amount of urine collected, usually at 20 min intervals. After ninety minutes of water loading, either synthetic AVP (5 pg kg-' min-') in 5 % dextrose or dextrose alone was infused intravenously.Infusions were administered in random order. One hour after the start of infusion, transmural pressure in the carotid sinus was increased by brief bursts of neck suction at -20, -30, -40 and -50 mmHg using the method described by Eckberg, Cavanaugh, Mark & Abboud (1975). The resultant bradycardia (prolongation of R-R interval of ECG) was plotted against the change in carotid sinus pressure to give a baroreflex function curve.Infusion of AVP caused a mean fall in urine flow rate from 13l9 to 1X3 ml min-(P < 0-001) and an increase in mean urine osmolality from 68 to 656 mosmole kg-' H20 (P < 0001). Mean prolongation of the R-R interval at each suction level did not differ significantly as a result of AVP infusion. Accordingly, the slope of the baroreflex function curve, an index of baroreflex sensitivity, is unchanged by AVP infusion at this dosage (-14-1 ms mmHg-' vs. -133 ms mmHg-', P > 0 8). There was no change in mean resting heart rate between test and control days (57 vs. 59 beats min', P > 0 4).Based on the changes observed in urine osmolality, the levels of plasma AVP in our subjects were likely to lie in the daily physiological range (Robertson, Shelton & Athar, 1976). In this range carotid sinus baroreflex sensitivity appeared to be unaltered by plasma AVP.