Augmented K(Ca)2.3 Channel Feedback Regulation of Oxytocin Stimulated Uterine Strips from Nonpregnant Mice

Uterine contractions prior to 37 weeks gestation can result in preterm labor with significant risk to the infant. Current tocolytic therapies aimed at suppressing premature uterine contractions are largely ineffective and cause serious side effects. Calcium (Ca(2+)) dependent contractions of uterine smooth muscle are physiologically limited by the opening of membrane potassium (K(+)) channels. Exploiting such inherent negative feedback mechanisms may offer new strategies to delay labor and reduce risk. Positive modulation of small conductance Ca(2+)-activated K(+) (K(Ca)2.3) channels with cyclohexyl-[2-(3,5-dimethyl-pyrazol-1-yl)-6-methyl-pyrimidin-4-yl]-amine (CyPPA), effectively decreases uterine contractions. This study investigates whether the receptor agonist oxytocin might solicit K(Ca)2.3 channel feedback that facilitates CyPPA suppression of uterine contractions. Using isometric force myography, we found that spontaneous phasic contractions of myometrial tissue from nonpregnant mice were suppressed by CyPPA and, in the presence of CyPPA, oxytocin failed to augment contractions. In tissues exposed to oxytocin, depletion of internal Ca(2+) stores with cyclopiazonic acid (CPA) impaired CyPPA relaxation, whereas blockade of nonselective cation channels (NSCC) using gadolinium (Gd(3+)) had no significant effect. Immunofluorescence revealed close proximity of K(Ca)2.3 channels and ER inositol trisphosphate receptors (IP(3)Rs) within myometrial smooth muscle cells. The findings suggest internal Ca(2+) stores play a role in K(Ca)2.3-dependent feedback control of uterine contraction and offer new insights for tocolytic therapies.