Characteristics of Single, Large-Conductance Calcium-Dependent Potassium Channels (BK Ca ) 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 (Po) was voltage-dependent. Iberiotoxin (50 nm) reversibly decreased Po, 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 Po was studied and data fitted to Boltzmann relationships. In 0.1, 0.3, 1.0 and 10 microM Ca2+, V1/2 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 microM [Ca2+] were significantly larger than that observed in 0.1 microM [Ca2+]. With 30 microM NS 1619 (a BKCa channel activator), V1/2 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 Kd of 59 mm. In summary, BKCa channels show a particularly weak sensitivity to intracellular TEA and they also display large variation in V1/2 and k. These findings suggest the possibility that different types (isoforms) of BKCa channels may exist in this vascular tissue.