Effects of potassium channel modulators on myotropic responses of aortic rings of pregnant rats

The contribution of potassium channels [ATP-sensitive potassium (K-ATP) and high-conductance calcium-activated potassium (BKCa) channels] in the resis tance of aortic rings of term pregnant rats to phenylephrine (Phe), arginin e vasopressin (AVP), and KCl was investigated. Concentration-response curve s to tetraethylammonium (TEA), a nonselective K+ channel inhibitor, were ob tained in the absence or presence of KCl. TEA induced by itself concentrati on-dependent responses only in aortic rings of nonpregnant rats. These resp onses to TEA could be modulated in both groups of rings by preincubation wi th different concentrations of KCl. Concentration-response curves to Phe, A VP, and KCl were obtained in the absence or presence of cromakalim or NS-16 19 (K-ATP and BKCa openers, respectively) and glibenclamide or iberiotoxin (K-ATP and BKCa inhibitors, respectively). Cromakalim significantly inhibit ed the responses to the three agonists in a concentration-dependent manner in both groups of rats. Alternatively, in the pregnant group of rats, glibe nclamide increased the sensitivity to all three agonists. NS-1619 also inhi bited the response to all agonists. With AVP and KCl, its effect was greate r in aortic rings of pregnant than nonpregnant rats, Finally, iberiotoxin i ncreased the sensitivity to all three agents. This effect was more importan t in aortic rings of nonpregnant rats and was accompanied by an increase of the maximal response to Phe and AVP. These results suggest that potassium channels are implicated in the control of basal membrane potential and in t he blunted responses to these agents during pregnancy.