Splanchnic Vascular Effects of Pharmacologic Doses of Oxytocin in the Canine 1

Abstract

Abstract

Effects of iv infusion of oxytocin in pharmacologic doses (0.95-7.6 U/min for 10 min) upon systemic arterial and portal venous pressures and on small intestinal and hepatic arterial resistances were observed in anesthetized male dogs. Arterial pressure underwent a biphasic response; transient decrease followed by an increase throughout the remainder of the infusion. With the largest dose pressure decreased 22% from 104 ± 10 to 81 & 12 after 3 min, but returned to 108 t 11 mm Hg after 10 min. One hour later, oxytocin infusion was repeated and no transient depressor response was seen, only a slight further 9% increase in pressure with 7.6 U/min. Small intestinal vasculature reflected a somewhat similar picture; a transient decrease in resistance, but followed by a rather prominent increase. During 7.6 U/min infusion it decreased 23%, from 3.5 ± 0.8 to 2.7 ± 0.8 after 3 min, but then increased 60% to 5.6 ± 0.9 mm Hg/ml · min · 100 g after 10 min. No dilation but rather only a further 57% resistance increase was seen during the second infusion, from 5.1 ± 1.0 to 8.0 ± 2.0 after 10 min. Hepatic arterial resistance fell during the first infusion reaching a minimum after 3 min (30% decrease with 3.8 U/min). This was partially reversed, but not converted to an increase over initial level, even after 10 min at which time it was still 20% below control level. No increase was seen during the second infusion either; only a slight 7% decrease. The vasculature may contain both dilator and constrictor receptors for oxytocin. The dilator response is either overcome by the constrictor moiety or develops a tachyphylaxis to oxytocin challenge. The constrictor component is more prominent, and dilation more transient, in small intestinal than it is in hepatic arterial vasculature.