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Vascular smooth muscle tone is an important determinant of peripheral vascular resistance and blood pressure and K+ channel activity is intimately involved in the regulation of this tone, through modulation of membrane potential. In this study, we have examined K+ currents in enzymatically isolated rat coronary artery smooth muscle cells, using the whole cell patch clamp technique. Isolated cells were relaxed and spindle-shaped (mean size, 78.5 x 16.9μm; capacitance, 9.81 ± 0.47pF (n=33)). Cells were maintained at 30oC and stimulated with 0.5s depolarising steps to +50mV from a holding potential of –70mV. This generated an outward current, whose density varied from 9 to 266 pA/pF. Pharmacological inhibitors of K+ channels were used to determine which channels are involved in the generation of this outward current. 1mM TEA, a concentration known to selectively block BKCa channels, produced a mean inhibition of 35.4 ± 8.1% (n=11), indicating the presence of BKCa channels in these cells, albeit at varying levels. 10mM TEA, a concentration that will block Kv channels in addition to BKCa channels, in all cases produced a further inhibition of the K+ current (mean 48.7 ± 8.1% inhibition, n=11), indicating that Kv channels are also implicated in the generation of this current. In addition, 2mM 4-AP produced an inhibition of the whole cell outward current (mean 56.3 ± 9.7% inhibition, n=6), indicating that the voltage-dependent, transient outward K+ channel also has an important role to play in the regulation of this tissue. In some cells, a small residual outward current remained despite the application of both 10mM TEA and 2mM 4-AP, which may be due to either an inward rectifier channel or the KATP channel. Preliminary experiments with methyl β cyclodextrin, to reduce cholesterol in the myocyte membrane, suggest that cholesterol extraction leads to an increase in total outward current and a decrease in capacitance.
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J. Vasc. Biol. 42, Sup:2 (2005) pp115-116