CONTRACTION AND ENDOTHELIUM-DEPENDENT RELAXATION IN MESENTERIC MICROVESSELS FROM PREGNANT RATS

We assessed KCl- and phenylephrine (PE)-induced vasoconstriction as we ll as acetylcholine (ACh)-induced endothelium-dependent vasodilation i n small, isometrically mounted mesenteric arteries from virgin and gra vid rats, studied in the absence and presence of N-G-nitro-L-arginine (L-NNA). Neither maximal vasoconstriction nor PE potency differed sign ificantly between vessels from virgin and pregnant rats, either in the absence or presence of L-NNA. L-NNA resulted in similar twofold leftw ard shifts in the PE dose-response curves for both groups. ACh-induced relaxation was potentiated in vessels from gravid rats (half-maximum effective concentration = 0.25 vs. 0.04 mu M, virgin and gravid rats, respectively). After L-NNA, maximal relaxation was inhibited significa ntly more in vessels from gravid rats (62 vs. 31%). Likewise, maximal slope of ACh dose-response curves and ACh potency were decreased in th is group so that values no longer differed from those in virgins. We c onclude that pregnancy does not alter basal nitric oxide (NO) synthesi s in these isolated microvessels, but it does enhance ACh-induced NO r elease, while apparently inhibiting the action of a NO-independent, en dothelium-derived vasodilator.