The chlorinated insecticide 1,1-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 oscillatory contractions to a similar extent in isolated rat uterine segments. Because regulation of intracellular calcium is fundamental for the development of oscillatory contractions, the present study examined the effects of p, p′-DDD on intracellular free calcium concentration ([Ca²⁺]_i) in individual rat myometrial smooth muscle cells loaded with the fluorescent Ca²⁺ indicator fura 2. In the presence of extracellular calcium, 50 and 100 μM p, p′-DDD significantly increased peak [Ca²⁺]_i 586 and 921%, respectively, over basal [Ca²⁺]_i. No significant effect was observed with 10 μM p, p′-DDD. In the absence of extracellular calcium, the response to 100 μM p, p′-DDD was significantly attenuated, with cells averaging a 108% increase in peak [Ca²⁺]_i over basal levels, presumably through Ca²⁺ release from intracellular stores. Nifedipine and cadmium chloride, blockers of voltage-dependent calcium channels, inhibited 100 μM p, p′-DDD-stimulated increases in [Ca²⁺]_i such that peak [Ca²⁺]_i was increased 250% and 259%, respectively. Because of the prominent inhibition observed with the voltage-dependent calcium channel blockers, the effect of p, p′-DDD on membrane depolarization was examined using a cationic fluorescent indicator of membrane potential, [diS-C₂(5)]. A concentration of 50 μM 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 increase was largely dependent on influx of extracellular calcium through dihydropyridine-sensitive, voltage-dependent calcium channels. The data further show that p, p′-DDD depolarized the plasma membrane, providing a possible mechanism for activation of voltage-dependent calcium channels. Additionally, another calcium source, perhaps an intracellular pool, 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.