Blunted Responses to Vasoconstrictors in Mesenteric Vasculature but not in Portal Vein of Spontaneously Hypertensive Rats Treated with Relaxin

Abstract

Relaxin (RLX), an ovarian polypeptide hormone that is particularly assoicated wth gestation in viviparous species, has recently been shown to decrease blood pressure in virgin spontaneously hypertensive rats (SHR) upon chronic infusion. In this investigation, vascular reactivity to angiotensin II, arginine-vasopressin, and norepinephrine was studied in the perfused mesenteric artery and isolated portal vein of control and RLX-treated virgin spontaneously hypertensive rats. The latter received an intravenous infusion of 75 ng/hr purified rat RLX for 2 days, whereas the controls were given an equal infusion of saline. All of the animals were then killed and their tissues processed for in vitro study. In the perfused mesenteric artery, the concentration-response curves for arginine-vasopressin and norpeinephrine were shifted to the right by a factor of about 2 (P < 0.05 and P < 0.005, respectively) after RLX treatment. In the isolated portal vein, the response to angiotensin II was not affected; the effect of norepinephrine was slightly displaced to the right (increase in EC50) and the maximum response remained unchanged. These results demonstrate that RLX treatment for 42 hr blunted vascular response to vasoconstrictor agents in the mesenteric vasculature and are consistent with similar observations reported previously in the same tissue of 20-day-old pregnant rats. It is concluded that RLX may be involved in the blunted response to vasoconstrictor agents during gestation in the rat.