CA-2+ ACTIVATED K+ CHANNELS IN PREGNANT RAT MYOMETRIUM - MODULATION BY A BETA-ADRENERGIC AGENT

The properties of Ca2+-activated K+ currents and channels were characterized in pregnant rat myometrium in whole cell and cell-attached patches and in lipid bilayers. Membrane depolarization of cultured myometrial cells from a holding potential of -50 to +70 mV in 10-mV steps under voltage-clamp conditions (whole cell mode) activated K+ outward currents (I(K)). At +70 mV, in the presence of 0.2 mM external Ca2+, the amplitude and activation time constant of I(K) were 15.0 +/- 2.1 muA/muF and 1.5 +/- 0.2 ms, respectively. Addition of 1 muM A23187 to the external solution increased the current from a control value of 16.0 +/- 2.0 to 67.9 +/- 9.1 muA/muF. Charybdotoxin, a blocker of Ca2+-activated K (K(Ca)) channels, and a low concentration of tetraethylammonium chloride (TEA; 1 mM) decreased the amplitude of I(K) by 47 and 62%, respectively. In cell-attached patches from these cells, 1 muM A23187 increased the open time probability of a 143 +/- 6.0 pS K+ channel. Incorporation of plasma membrane vesicles from pregnant myometrium into lipid bilayers resulted in one predominant type of K+ channel. The unitary conductance of the K+ channel was 326 +/- 9.0 pS in symmetrical 450 mM KCl. The channel activation was both voltage and Ca2+ dependent. TEA inhibited the channel activity with a dissociation constant (K(d)) of 378 +/- 10 muM at -60 mV or 1,477 +/- 80 muM at +60 mV. The whole cell currents were found to be stimulated by isoproterenol, a beta-adrenergic agent. Isoproterenol (1 muM), when added to the bath, increased the whole cell I(K) from a control value of 9.1 +/- 2.0 to 18.7 +/- 3.9 muA/muF. The beta-antagonist propranolol (1 muM) reversed the effect of isoproterenol. These results demonstrate that channels with the properties of K(Ca). channels are present in pregnant myometrium and are regulated by beta-adrenergic stimulation.