Characterization of a Newly Found Stretch-activated KCa,ATP Channel in Cultured Chick Ventricular Myocytes

With the use of the patch-clamp technique, five kinds of stretch-activated (SA) ion channels were identified on the basis of their single-channel conductances and ion selectivities in cultured chick ventricular myocytes. Because a high-conductance K+-selective channel predominated among these channels, we concentrated on characterizing its properties mostly using excised inside-out patches. With 145 mM KCl solution in the pipette and the bath, the channel had a conductance of 199.8 +/- 8.2 pS (n = 22). The ion selectivities among K+, Na+, Ca2+, and Cl- as estimated from their permeability ratios were PNa/PK = 0.03, PCa/PK = 0.025, and PCl/PK = 0.026. The probability of the channel being open (Po) increased with the Ca2+ concentration in the bath ([Ca2+]b; dissociation constant Kd = 0.51 microM at +30 mV) and membrane potential (voltage at half-maximal Po = 39.4 mV at 0.35 microM [Ca2+]b). The channel was blocked by gadolinium, tetraethylammonium, and charybdotoxin from the extracellular surface and, consequently, was identified as a Ca2+-activated K+ (KCa) channel type. The channel was also reversibly activated by ATP applied to the intracellular surface (Kd = 0.74 mM at 0.10 microM [Ca2+]b at +30 mV). From these data taken together, we concluded that the channel is a new type of KCa channel that could be designated as an "SA KCa,ATP channel." To our knowledge, this is the first report of KCa channel in heart cells.