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

Calcium-activated potassium currents were studied in dissociated smooth mus cle cells from human saphenous vein (HSV) using the patch-clamp technique i n the whole-cell configuration. The average measured resting membrane poten tial (V-m) was -41+/-2 mV (n=39), when the cells were dialysed with an intr acellular pipette solution (IPS) containing 0.1 mM ethyleneglycol-bis(beta- 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 s ignificantly less negative: -13+/-2 mV (n=23, P<0.05). These results sugges t that 10 mM EGTA reduces a calcium-dependent current involved in the maint enance of V-m. Depolarizing voltage steps up to +60 mV from holding potenti als of -60 mV resulted in large (1-10 nA) time- and voltage-dependent outwa rd currents. The amplitudes of total whole-cell current densities measured at voltages above -20 mV were significantly greater in the cells dialysed w ith IPS-0.1 mM EGTA than in those dialysed with IFS-10 mM EGTA. In the cell s dialysed with IPS-0.1 mM EGTA, 0.1 mM tetraethylammonium chloride (TEA) a nd 50 nM iberiotoxin (IBTX), which selectively block large conductance Ca2-activated potassium channels (BKCa), diminished the total current recorded at +60 mV by 45+/-14% (P<0.05, n=5) and 50+/-6% (n=8, P<0.05), respectivel y. These blockers at the same concentrations did not affect the total curre nt in cells dialysed with IFS-10 mM EGTA. When tested on intact HSV rings, both 0.1 mM TEA and 50 nM IBTX elicited vessel contraction. We conclude tha t BKCa channels present in HSV smooth muscle cells contribute to the mainte nance of the V-m and sustain a significant portion of the total voltage-act ivated, outward current. Finally, BKCa channels appear to play a significan t role in the regulation of HSV smooth muscle contractile activity.