DO VASOPRESSIN AND OXYTOCIN HAVE SYNERGISTIC RENAL EFFECTS IN THE CONSCIOUS RAT

The renal effects of arginine vasopressin and oxytocin were studied in the conscious unrestrained rat infused with 0.077 M NaCl. Peptides we re 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 rati os of oxytocin:vasopressin similar to those seen in the plasma of euhy drated and dehydrated rats. Vasopressin produced dose-dependent antidi uretic and natriuretic responses, the natriuresis commencing after 15- 30 min infusion. Oxytocin produced dose-dependent diuretic and natriur etic 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 w ith vasopressin alone. The natriuretic response fr om combined infusio n at the higher rate appeared to have the greater magnitude for indivi dual 15-min periods of the vasopressin response combined with the long er duration of the oxytocin response. Although the total natriuretic r esponse was therefore greater, this difference failed to reach signifi cance. Only the higher rates of infusion of vasopressin and oxytocin s ignificantly increased the clearance of sodium, by 53 +/- 23 and 62 +/ - 18% and glomerular filtration rate (GFR) by 23 +/- 4 and 23 +/- 4% r espectively. The clearance of sodium during the combined hormone infus ion was significantly greater (109 +/- 21%), while the rise in GFR at 23 +/- 5% was comparable to that seen when each hormone was given sepa rately. Both fractional excretion of sodium and potassium excretion we re also significantly elevated by this combined infusion, suggesting a n additional tubular component to the response. Although no synergisti c effect of neurohypophysial hormones on the antidiuresis was found in the conscious rat, they may act together to promote sodium excretion.