CALCIUM-ACTIVATED POTASSIUM CHANNELS FROM CORONARY SMOOTH-MUSCLE RECONSTITUTED IN LIPID BILAYERS

This work is the initial characterization of Ca2+-activated K+ (K(Ca)) channels from coronary smooth muscle reconstituted into lipid bilayers. The channels were obtained from a surface membrane preparation of porcine coronary smooth muscle. K(Ca) channels were the predominant K+ channels in this preparation. The conductance histogram (n = 137 channels) revealed two main populations of "maxi" K(Ca) channels with conductances of 245 and 295 pS. Each population could be subdivided in two "isoforms" or "isochannels" with different functional properties (voltage and Ca2+ sensitivities and kinetics). The analysis of "burst" probability of opening showed that at pCa 4 the two isochannels of 245 pS (K(Ca-1) and K(Ca-1')) had half-activation potentials (V1/2) of -80 and 6 mV, respectively. The isochannels of 295 pS (K(Ca-2) and K(Ca-2')) had V1/2 of -28 and -66 mV, respectively. K(Ca-1) had the highest Ca2+ sensitivity; at -60 mV, the concentration of half-activation value for Ca2+ was 1.2 +/- 0.3-mu-M (n = 5). External tetraethylammonium reduced channel amplitude in a voltage-dependent manner; dissociation constant was 180 +/- 6 and 466 +/- 41-mu-M at -40 and +80 mV, respectively (n = 5). Charybdotoxin (5-50 nM) produced typical long closings. These effects were similar in all the channels. We conclude that coronary smooth muscle possesses isoforms of maxi K(Ca) channels with Ca2+ and voltage sensors with different properties, which may confer to each channel a specific functional role.