THE CHLORINATED INSECTICIDE 1-DICHLORO-2,2-BIS(4-CHLOROPHENYL)ETHANE(P,P'-DDD) INCREASES INTRACELLULAR CALCIUM IN RAT MYOMETRIAL SMOOTH-MUSCLE CELLS

A previous report from this laboratory showed that two DDT isomers, o, p'-DDT and p,p'-DDD, increased the frequency of spontaneous oscillator y contractions to a similar extent in isolated rat uterine segments. B ecause regulation of intracellular calcium is fundamental for the deve lopment of oscillatory contractions, the present study examined the ef fects of p,p'-DDD on intracellular free calcium concentration ([Ca2+]( i)) in individual rat myometrial smooth muscle cells loaded with the f luorescent Ca2+ indicator fura 2. In the presence of extracellular cal cium, 50 and 100 mu M p,p'-DDD significantly increased peak [Ca2+](i) 586 and 921%, respectively, over basal [Ca2+](i). No significant effec t was observed with 10 mu M p,p'-DDD In the absence of extracellular c alcium, the response to 100 mu M p,p'-DDD was significantly attenuated , with cells averaging a 108% increase in peak [Ca2+](i) over basal le vels, presumably through Ca2+ release from intracellular stores. Nifed ipine and cadmium chloride, blockers of voltage-dependent calcium chan nels, inhibited 100 mu M p,p'-DDD-stimulated increases in [Ca2+](i) su ch that peak [Ca2+](i) was increased 250% and 259%, respectively. Beca use of the prominent inhibition observed with the voltage-dependent ca lcium channel blockers, the effect of p,p'-DDD on membrane depolarizat ion was examined using a cationic fluorescent indicator of membrane po tential, [diS-C-2(5)]. A concentration of 50 mu M p,p'-DDD depolarized the cells by 35% of maximum during treatment with p,p'-DDD. The data demonstrate that p,p'-DDD increased [Ca2+](i) in rat myometrial smooth muscle cells in a concentration-related manner, and that this increas e was largely dependent on influx of extracellular calcium through dih ydropyridine-sensitive, voltage-dependent calcium channels. The data f urther show that p,p'-DDD depolarized the plasma membrane, providing a possible mechanism for activation of voltage-dependent calcium channe ls. Additionally, another calcium source, perhaps an intracellular poo l, contributes significantly less to the rise of [Ca2+](i). Whether p, p'-DDD initiates the calcium response by direct actions on the plasma membrane or by other means remains to be determined. (C) 1995 Academic Press, Inc.