Regulation of calcium current by intracellular calcium in smooth muscle cells of rabbit portal vein

Abstract

Effects of concentrations of intracellular calcium, [Ca2+](i), on the voltage-dependent Ca2+ current (I(Ca)) recorded from dispersed single smooth muscle cells of the rabbit portal vein were studied, using a whole cell voltage clamp method combined with an intracellular perfusion technique. Outward currents were minimized by replacement of Cs+-rich solution in the pipette and 20 mM tetraethylammonium in the bath. The I(Ca) was evoked by command pulses of above -30 mV, and the maximum amplitude was obtained at about 0 mV. This I(Ca) was dose dependently inhibited by increases in the [Ca2+](i) above 30 nM. The K(d) value of the [Ca2+](i) required to inhibit the I(Ca) was about 100 nM. The Ba2+ current was also inhibited by increases in the [Ca2+](i). Conversely, perfusion of Ba2+ into the cell up to 100 μM did not suppress the I(Ca). Changes in the [Ca2+](i) did not modify the steady-state inactivation curve. The inhibition of the I(Ca) evoked by the test pulse is most prominent when the preceding influx of Ca2+ during the conditioning pulse was large, as estimated using a double pulse protocol. This inhibition was proportionally reduced by increases in the concentration of the Ca2+ chelator, ethylene glycol bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). Therefore, the Ca2+-dependent inactivation of the Ca2+ channel may contribute toward regulating [Ca2+](i) in smooth muscle cells of the rabbit portal vein.