Myometrial Calcium and Potassium Channels Play a Pivotal Role in Chromium-Induced Relaxation in Rat Uterus: an In Vitro Study.

Hexavalent chromium, a well-known environmental toxicant, adversely affects female reproduction and results in abnormal implantation, fetal resorption, and reduction in litter size. Uterine myogenic activity is under control of number of receptors and ion channels, and it regulates fetal-implantation and feto-maternal communication. Despite several known adverse effects of chromium on female reproduction, direct action of chromium on myometrial activity is yet to be understood. In the present study, the effect of in vitro exposure of hexavalent chromium (Cr-VI) on the myogenic activity of isolated myometrial strips of rats was evaluated after mounting the tissue in thermostatically (37 ± 0.5 °C) controlled organ bath under a resting tension of 1 g. Chromium produced concentration-dependent (0.1 nM-0.1 mM) inhibitory effect on myometrial activity. Following pre-treatment of the myometrial strips with glibenclamide (a KATP channel blocker) and 4-aminopyridine (a Kv channel blocker), the concentration-response curve (CRC) of chromium was significantly (P < 0.05) shifted towards right with decrease in the maximum relaxant effect. Contractile effects of CaCl2 and BAY K-8644 (a selective opener of L-type Ca2+ channel) were significantly (P < 0.05) attenuated in the presence of chromium. Chromium-induced myometrial relaxation was also significantly (P < 0.05) reduced in the presence of ICI 118,551 (a selective ß2-antagonist) and SR 59230A (a selective ß3-antagonist). These findings evidently suggest that chromium produced relaxant effect on rat myometrium by interfering with Ca2+ entry through voltage-dependent Ca2+ channels, and by interacting with beta-adrenoceptors (ß2 and ß3) and potassium channels (especially KATP and Kv channels). Graphical Abstract Proposed signaling pathway(s) of chromium (VI)-induced myometrial relaxations in rats. KATP: ATP-sensitive K+ channel; KV: voltage-dependent K+ channel; VDCC: voltage-dependent Ca2+ channel; [Ca2+]i: intracellular calcium concentration, stimulatory mechanism, inhibitory mechanism.

Functional and molecular characterization of endothelium-dependent and endothelium-independent relaxant pathways in uterine artery of non-pregnant buffaloes.

Present study was undertaken to unravel the endothelium-dependent and endothelium-independent relaxant pathways in uterine artery of non-pregnant buffaloes. Isometric tension of arterial rings was recorded using data acquisition system based polyphysiograph. Acetylcholine (ACh) produced endothelium-dependent vasorelaxation by releasing nitric oxide (NO), and inhibition of nitric oxide synthase (NOS) by L-NAME (300 µM) significantly (P < 0.05) reduced the NO release and thereby the vasorelaxant effect of ACh. However, L-NMMA, another NOS inhibitor, and PTIO, a NO scavenger, did not have any additional inhibitory effect on NO and ACh-induced vasorelaxation. Cyclooxygenase (COX) inhibitor (indomethacin) alone did not have any inhibitory action on vasorelaxant response to ACh; however, simultaneous inhibition of COX and NOS enzymes significantly (P < 0.05) attenuated the relaxant response indicating the concurrent release of these two mediators in regulating ACh-induced relaxation. Besides NOS and COX-derived metabolites (EDRF), small (SKCa) and intermediate (IKCa) conductance K+ channels being the members of EDHF play predominant role in mediating ACh-induced vasorelaxation. Using different molecular tools, existence of eNOS, COX-1, and,IKCa in the endothelium, BKCa in vascular smooth muscle, and SKCa in both endothelium and vascular smooth muscle was demonstrated in buffalo uterine artery. Gene sequencing of COX-1 and SKCa genes in uterine artery of buffaloes showed more than 97% structural similarity with ovine (Ovis aries), caprine (Capra hircus), and Indian cow (Bos indicus). Endothelium-independent nitrovasodilator, sodium nitroprusside (SNP), produced vasorelaxation which was sensitive to blockade by soluble guanylate cyclase (sGC) inhibitor (ODQ), thus suggesting the important role of cGMP/PKG pathways in uterine vasorelaxation in buffaloes. Taken together, it is concluded that both endothelium-dependent (EDHF and EDRF) and endothelium-independent (sGC-cGMP) relaxant pathways are present in uterine arteries of non-pregnant buffaloes, and they differently contribute to vasorelaxation during non-pregnant state.