EXTRACELLULAR CALCIUM AND INTRINSIC TONE IN THE HUMAN UMBILICAL ARTERY

Previous in vitro studies of the human umbilical artery (HUA) have sug gested that this vessel exhibits significant intrinsic tone, even in t he absence of contractile agonists. Other investigators have found tha t these vessels are unresponsive to nitrovasodilators, suggesting that the guanylate cyclase-mediated relaxation mechanism may not be operat ive. To clarify these observations, human umbilical cords were obtaine d at the time of delivery and HUA rings were studied in organ baths. I n contrast to other systemic arteries, HUA rings exhibited unstable in itial tone in the absence of pharmacologic agonists. This intrinsic to ne was augmented when the extracellular ionized calcium concentration was increased and abolished when extracellular calcium was removed. Ri ngs maintained in the presence of extracellular calcium for 1 hr becam e less responsive to changes in extracellular calcium than did rings m aintained in calcium-free buffer. Serotonin produced oscillatory, quan tal contractions in the presence of calcium; in the absence of extrace llular calcium, it elicited only concentration-dependent graded contra ctions. In the presence of nifedipine or in the absence of extracellul ar calcium, nitroglycerin fully relaxed contractions elicited by serot onin. Thus, in HUA, extracellular calcium produces intrinsic tone that diminishes spontaneously with time.