BASAL ACTIVATION OF ATP-SENSITIVE POTASSIUM CHANNELS IN MURINE COLONIC SMOOTH-MUSCLE CELL

The function and molecular expression of ATP-sensitive potassium (K-AT P) channels in murine colonic smooth muscle was investigated by intrac ellular electrical recording from intact muscles, patch-clamp techniqu es on isolated smooth muscle myocytes, and reverse transcription polym erase chain reaction (RT-PCR) on isolated cells. Lemakalim (1 mu M) ca used hyperpolarization of intact muscles (17.2 +/- 3 mV). The hyperpol arization was blocked by glibenclamide (1-10 mu M) Addition of glibenc lamide (10 mu M) alone resulted in membrane depolarization (9.3 +/- 1. 7 mV). Lemakalim induced an outward current of 15 +/- 3 pA in isolated myocytes bathed in 5 mM external K+ solution. Application of lemakali m to cells in symmetrical K+ solutions (140/140 mM) resulted in a 97 /- 5 pA inward current. Both currents were blocked by glibenclamide (1 mu M). Pinacidil (1 mu M) also activated an inwardly rectifying curre nt that was insensitive to 4-aminopyridine and barium. In single-chann el studies, lemakalim (1 mu M) and diazoxide (300 mu M) increased the open probability of a 27-pS K+ channel. Openings of these channels dec reased with time after patch excision. Application of ADP (1 mM) or AT P (0.1 mM) to the inner surface of the patches reactivated channel ope nings. The conductance and characteristics of the channels activated b y lemakalim were consistent with the properties of K-ATP. RT-PCR demon strated the presence of K-ir 6.2 and SUR2B transcripts in colonic smoo th muscle cells; transcripts for K-ir 6.1, SUR1, and SUR2A were not de tected. These molecular studies are the first to identify the molecula r components of K-ATP in colonic smooth muscle cells. Together with th e electrophysiological experiments, we conclude that K-ATP channels ar e expressed in murine colonic smooth muscle cells and suggest that the se channels may be involved in dual regulation of resting membrane pot ential, excitability, and contractility.