Ca2+-dependent membrane currents in vascular smooth muscle cells of the rabbit

The membrane potential in vascular smooth muscle cells contributes to the regulation of cytosolic [Ca2+], which in turn regulates membrane potential by means of Ca2+i-dependent ionic currents. We investigated the characteristics of Ca2+i-dependent currents in rabbit coronary and pulmonary arterial smooth muscle cells. Ca2+i-dependent currents were recorded using the whole-cell patch-clamp technique while cytosolic [Ca2+] was increased by caffeine. The reversal potentials of caffeine-induced currents were between -80 and -10 mV under normal ionic conditions, whereas they were about 0 mV when K+-free NaCl solutions were used both in pipette and bath. The total substitution of extracellular Na+ with membrane-impermeable cation N-Methyl-D-glucamine did not affect caffeine-induced currents, implying no significant contribution of Na+ as a permeant ion to the currents. The substitution of extracellular NaCl with sucrose reduced outward component of the currents and shifted the reversal potentials according to the change in Cl- equilibrium potential. Upon application of the niflumic acid under K+-free conditions, most of the current induced by caffeine was inhibited. Taken together, the results of the present study indicate that K+ and Cl- currents are major components of Ca2+i-dependent currents in vascular smooth muscles isolated from coronary and pulmonary arteries of the rabbit, and the relative contribution of each type of current to total currents are not different between the two arteries.