Role of a Ca2+-activated K+ current in the maintenance of resting membrane potential of isolated, human, saphenous vein smooth muscle cells

Calcium-activated potassium currents were studied in dissociated smooth muscle cells from human saphenous vein (HSV) using the patch-clamp technique in the whole-cell configuration. The average measured resting membrane potential (V(m)) was -41±2 mV (n=39), when the cells were dialyzed with an intracellular pipette solution (IPS) containing 0.1 mM ethyleneglycolbis(β- aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA) (IPS-0.1 mM EGTA). When the EGTA concentration was increased to 10 mM (IPS-10 mM EGTA) V(m) became significantly less negative: -13±2 mV (n=23, P<0.05). These results suggest that 10 mM EGTA reduces a calcium-dependent current involved in the maintenance of V(m). Depolarizing voltage steps up to +60 mV from holding potentials of -60 mV resulted in large (1-10 nA) time- and voltage-dependent outward currents. The amplitudes of total whole-cell current densities measured at voltages above -20 mV were significantly greater in the cells dialyzed with IPS-0.1 mM EGTA than in those dialyzed with IPS-10 mM EGTA. In the cells dialyzed with IPS-0.1 mM EGTA, 0.1 mM tetraethylammonium chloride (TEA) and 50 nM iberiotoxin (IBTX), which selectively block large conductance Ca2+-activated potassium channels (BK(Ca)), diminished the total current recorded at +60 mV by 45±14% (P<0.05, n=5) and 50±6% (n=8, P<0.05), respectively. These blockers at the same concentrations did not affect the total current in cells dialyzed with IPS-10 mM EGTA. When tested on intact HSV rings, both 0.1 mM TEA and 50 nM IBTX elicited vessel contraction. We conclude that BK(Ca) channels present in HSV smooth muscle cells contribute to the maintenance of the V(m) and sustain a significant portion of the total voltage-activated, outward current. Finally, BK(Ca) channels appear to play a significant role in the regulation of HSV smooth muscle contractile activity.