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 μm 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 μm [Ca2+] were significantly larger than that observed in 0.1 μm [Ca2+]. With 30 μm 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.
The Journal of Membrane Biology – Springer Journals
Published: Jul 15, 1998
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