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Oxytocin is known to play important roles in uterine contractions, mediated at least in part by increasing intracellular Ca(2+) concentration ([Ca(2+)](i)), through enhancing extracellular Ca(2+) entry and Ca(2+) release from the sarcoplasmic reticulum, processes that are intimately linked with mitochondria. This study examined the effects of oxytocin on mitochondrial function. This was achieved by measuring the ratiometric JC-1 fluorescence signal in isolated myometrial cells, which provides a relative measure of the mitochondrial membrane potential (ψ(m)), and also by loading the cells with Oregon Green BAPTA-AM to examine changes in [Ca(2+)](i). Oxytocin (1 nm) depolarized the ψ(m) to 73.8 ± 3.7% of the control value (P < 0.05; perfused for 11 min) and also caused a transient increase in [Ca(2+)](i). The depolarization of mitochondrial membrane potential was effectively reversed by 2-aminoethoxydiphenyl borate, nifedipine, Ca(2+)-free solution or oligomycin, with the ratiometric JC-1 fluorescence signal becoming no different from the control value in all cases (i.e. P > 0.05). The reduction in ψ(m) is likely to occur at least in part through the oxytocin-induced increase in [Ca(2+)](i), causing enhanced mitochondrial uptake of Ca(2+) and resultant dissipation of the mitochondrial electrochemical gradient. ATP synthase is also stimulated, which would further contribute to a decrease in ψ(m).

Background and Purpose: Uterine spontaneous contraction and pacemaking are poorly understood. This study investigates the role of the mitochondrial Ca(2+) store in uterine activity.Experimental Approach: We investigated the effects of mitochondrial and sarco-endoplasmic reticulum (SER) inhibitors on contraction, membrane potential (Vm) and cytosolic Ca(2+) concentration ([Ca(2+) ](c) ) in longitudinal smooth muscle of the mouse uterus.Key Results: The mitochondrial agents rotenone, carbonylcyanide-3-chlorophenylhydrazone (CCCP), 7-chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one (CGP37157) and kaempferol decreased the force of contractions. The ATP synthase inhibitor oligomycin had no significant effect. The effects of these agents were compared with those of SER inhibitors cyclopiazonic acid (CPA), 2-amino ethoxyphenylborate (2-APB) and caffeine. All agents, except CPA and oligomycin, decreased contractile force. CPA and CCCP transiently increased contraction frequency, which returned to control levels, whereas rotenone, CGP37157, kaempferol and 2-APB decreased frequency and caffeine had no significant effect. Application of the mitochondrial agents when CPA functionally inhibited stores did not change contraction frequency but, with the exception of kaempferol, decreased force. CCCP caused depolarization and maintained increase in [Ca(2+) ](c) or depolarization/transient hyperpolarization and transient increase in [Ca(2+) ](c) for oestrus and di-oestrus tissues respectively. Rotenone caused hyperpolarization and maintained increase in [Ca(2+) ](c) . CGP37157 and kaempferol caused hyperpolarization but no measurable change in [Ca(2+) ](c) . Application of a range of K(+) channel blockers indicated a role of Ca(2+) -activated K(+) (K(Ca) ) channels in the CCCP- and CGP37157-induced actions.Conclusions and Implications: Mitochondria have a modulatory role on uterine contractions, with mitochondrial inhibition reducing contraction amplitude and pacemaker frequency by changes in Vm, [Ca(2+) ](c) and/or Ca(2+) influx. [ABSTRACT FROM AUTHOR]