HEXOPRENALINE ACTIVATES POTASSIUM CHANNELS OF HUMAN MYOMETRIAL MYOCYTES

Hexoprenaline is a beta-adrenergic agent used for tocolysis after the 26th week of pregnancy. The purpose of the present study was to demons trate the site of action of hexoprenaline on the membrane of single is olated smooth muscle cells. The main action of beta-mimetics on the ce ll is hyperpolarization of the cellular membrane, i.e. beta-mimetics h ave similar effects as K+-ions (Standen et al., 1989). Our results ind icate a prolonged and significantly enhanced activity of K+-channels i n the cell membrane, as may also be demonstrated by the use of the K+- channel activator Calcitonin-gene related peptide (CGRP). In control e xperiments under physiological conditions, we observed a large conduct ance K+-channel with 158 pS. The channel was voltage dependent and Ca+ sensitive indicating that it belongs to the class of big conductance Ca++-activated K+-channels (BK(Ca)). Hexoprenaline and CGRP both incr eased the open probability (P(o)) of the channel measured with the pat ch clamp system in the cell attached configuration. Hexoprenaline was also an activator of the BK(Ca) in the presence of Nitrendipine, indic ating that the activation of the Ca++ sensitive channel is not an indi rect effect of Ca++ currents via L-type Ca++ channels.