Effects of halothane and diltiazem on L-type calcium currents in single smooth muscle cells from rabbit portal veins

We have studied the effects of halothane and diltiazem on L-typevoltage-dependent calcium currents (I_Ca) in single smooth musclecells from rabbit portal veins using a whole cell voltage clamptechnique. The threshold of I_Ca was –30 mV and the peakcurrent was reached at 0 mV. Halothane (0.25, 0.5, 1.0, 1.5and 2.07%) decreased I_Ca in a concentration-dependent mannerand shifted the I_Ca activation threshold to the depolarizingside. Halothane 2.0% abolished Diltiazem 10^–8–10^–6mol litre^–1, a calcium channel antagonist, also depressedI_Ca in a concentration-dependent manner. Administration of both0.5% halothane and diltiazem 10 mol litre^–1 (concentrationslower than the clinical therapeutic range) abolished I_Ca however,halothane did not exhibit use-dependent inhibition of I_Ca whereasdiltiazem showed partial use-dependency. We conclude that thedecrease in I_Ca produced by halothane is associated with a directvasodilator effect of this anaesthetic, but is not explainedby block of Ca²⁺ channels similar to the action of diltiazem.Furthermore, administration of low concentrations of both halothaneand diltiazem decreased I_Ca and may reduce the contractilityof the vascular smooth muscle cells.