Abstract
Ca 2+ entry under resting conditions may be important for contraction of vascular smooth muscle, but little is known about the mechanisms involved. Ca 2+ leakage was studied in the A7r5 smooth muscle-derived cell line by patch-clamp techniques. Two channels that could mediate calcium influx at resting membrane potentials were characterized. In 110 mM Ba 2+, one channel had a slope conductance of 6.0 ± 0.6 pS and an extrapolated reversal potential of +41 ± 13 mV (mean ± SD, n = 8). The current rectified strongly, with no detectable outward current, even at +90 mV. Channel gating was voltage independent. A second type of channel had a linear current-voltage relationship, a slope conductance of 17.0 ± 3.2 pS, and a reversal potential of +7 ± 4 mV ( n = 9). The open probability increased e-fold per 44 ± 10 mV depolarization ( n = 5). Both channels were also observed in 110 mM Ca 2+. Noise analysis of whole-cell currents indicates that ∼100 6-pS channels and 30 17-pS channels are open per cell. These 6-pS and 17-pS channels may contribute to resting calcium entry in vascular smooth muscle cells.
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