Characteristics of single, large-conductance calcium-dependent potassium channels (BKCa) from smooth muscle cells isolated from the rabbit mesenteric artery

Smooth muscle cells isolated from the secondary and tertiary branches of the rabbit mesenteric artery contain large Ca2+-dependent channels. In excised patches with symmetrical (140 mM) K+ solutions, these channels had an average slope conductance of 235 +/- 3 pS, and reversed in direction at -6.1 +/- 0.4 mV. The channel showed K+ selectivity and its open probability (P-o) was voltage-dependent. Iberiotoxin (50 nM) reversibly decreased P-o, whereas tetraethylammonium (TEA, at 1 mM) reduced the unitary current amplitude. Apamin (200 nM) had no effect. The channel displayed sublevels around 1/3 and 1/2 of the mainstate level. The effect of [Ca2+] on P-o was studied and data fitted to Boltzmann relationships. In 0.1, 0.3, 1.0 and 10 mu M Ca2+, V-1/2 was 77.1 +/- 5.3 (n = 18), 71.2 +/- 4.8 (n = 16), 47.3 +/- 10.1 (n = 11) and -14.9 +/- 10.1 mV (n = 6), respectively. Values of k obtained in 1 and 10 mu M [Ca2+] were significantly larger than that observed in 0.1 mu M [Ca2+]. With 30 mu M NS 1619 (a BKCa channel activator), V-1/2 values were shifted by 39 mV to the left (hyperpolarizing direction) and k values were not affected. TEA applied intracellularly, reduced the unitary current amplitude with a K-d of 59 mM. In summary, BKCa channels show a particularly weak sensitivity to intracellular TEA and they also display large variation in V-1/2 and k. These findings suggest the possibility that different types (isoforms) of BKCa channels may exist in this vascular tissue.