POTASSIUM INWARD RECTIFIER AND ACETYLCHOLINE-RECEPTOR CHANNELS IN EMBRYONIC XENOPUS MUSCLE-CELLS IN CULTURE

Embryonic muscle cells of the frog Xenopus laevis were isolated and gr own in culture and single-channel recordings of potassium inward recti fier and acetylcholine (ACh) receptor currents were obtained from cell -attached membrane patches. Two classes of inward rectifier channels, which differed in conductance, were apparent. With 140 mM potassium ch loride in the electrode, one channel class had a conductance of 28.8 /- 3.4 pS (n = 21), and, much more infrequently, a smaller channel cla ss with a conductance of 8.6 +/- 3.6 pS (n = 7) was recorded. Both cha nnel classes had relatively long mean channel open times, which decrea sed with membrane hyperpolarization. The probability of finding a patc h of membrane with an inward rectifier channel was high (66%) and many membrane patches contained more than one inward rectifier channel. Th e open state probability (with no applied potential) was high for both inward rectifier channel classes so that 70% of the time there was a channel open. Seventy-three percent of the membrane patches with ACh r eceptor channels (n = 11) also had at least one inward rectifier chann el present when the patch electrode contained 0.1 mu M ACh. Inward rec tifier channels were also found at 71% of the sites of high ACh recept or density (n = 14), which were identified with rhodamine-conjugated a lpha-bungarotoxin. The results indicate that the density of inward rec tifier channels in this embryonic skeletal muscle membrane was relativ ely high and includes sites of membrane that have synaptic specializat ions. (C) 1996 John Wiley & Sons, Inc.