The renal effects of arginine vasopressin and oxytocin were studied in the conscious unrestrained rat infused with 0·077 m NaCl. Peptides were infused at rates of 24 and 160 pmol/min (vasopressin) or 30 and 200 pmol/min (oxytocin) either alone or as a combination of the two lower or two higher doses. The rates of infusion were selected to give ratios of oxytocin:vasopressin similar to those seen in the plasma of euhydrated and dehydrated rats.
Vasopressin produced dose-dependent antidiuretic and natriuretic responses, the natriuresis commencing after 15–30 min infusion. Oxytocin produced dose-dependent diuretic and natriuretic responses, the natriuresis commencing within the first 15 min of infusion. Combined infusion of vasopressin and oxytocin produced dose-dependent antidiuretic responses which were comparable to those seen with vasopressin alone. The natriuretic response from combined infusion at the higher rate appeared to have the greater magnitude for individual 15-min periods of the vasopressin response combined with the longer duration of the oxytocin response. Although the total natriuretic response was therefore greater, this difference failed to reach significance.
Only the higher rates of infusion of vasopressin and oxytocin significantly increased the clearance of sodium, by 53 ± 23 and 62 ± 18% and glomerular filtration rate (GFR) by 23 ± 4 and 23 ± 4% respectively. The clearance of sodium during the combined hormone infusion was significantly greater (109 ± 21%), while the rise in GFR at 23 ± 5% was comparable to that seen when each hormone was given separately. Both fractional excretion of sodium and potassium excretion were also significantly elevated by this combined infusion, suggesting an additional tubular component to the response. Although no synergistic effect of neurohypophysial hormones on the antidiuresis was found in the conscious rat, they may act together to promote sodium excretion