Mercury affects uterine myogenic activity even without producing any apparent toxicity in rats: Involvement of calcium-signaling cascades

Background: Mercury is an established environmental toxicant reported to cause reproductive disorders in women, however, its direct action on myometrial activity is yet to be understood. Earlier we have reported the underlying mechanism of mercury-induced myometrial contractions following in vitro exposure; however, no such information on the effect of mercury on myometrial activity following in vivo exposure is available, therefore, the present study was undertaken. Objective: Present study was designed to evaluate the effect of mercury on myometrial activity following in vivo exposure of rats and unravel the possible underlying mechanism. Methods: Female Wistar rats were orally exposed to mercury (5, 50 and 500 mu g/L in drinking water) for 28 days to investigate the toxicodynamics of mercuric chloride (HgCl2)-induced alterations in myometrial activity. Response of the isolated myometrial strips to different spasmogens was recorded using polyphysiograph. Blood and uterine calcium, mercury, iron and zinc levels were estimated by atomic absorption spectrophotometry. Blood biochemicals and serum hormonal profiles (estradiol, progesterone) were also determined. Results: No systemic toxicity of mercury was observed in any of the treatment groups (5, 50 and 500 mu g/L) in terms of alterations in body weight, organ weights, blood biochemical parameters including hormonal profile. Interestingly, mercury at 5 mu g/L concentration significantly increased the receptor-dependent (PGF(2 alpha)-induced) and receptor-independent (CaCl2-induced and high K+-depolarizing solution-induced) myometrial contractions and it was coupled with corresponding increase in the uterine calcium levels. However, mercury at higher dose levels (50 and 500 mu g/L) did not significantly alter the myometrial response. Conclusion: Our results evidently suggest that mercury at low level (5 mu g/L) produced detrimental effect on myometrial activity by altering calcium entry into the smooth muscle and/or the release of calcium from intracellular stores without causing any apparent systemic toxicity in rats.