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-type voltage-dependent calcium currents (I-Ca) in single smooth muscle cells from rabbit portal veins using a whole cell voltage clamp technique. The threshold of I-Ca was -30 mV and the peak current was reached at 0 mV. Halothane (0.25, 0.5, 1.0, 1.5 and 2.0%) decreased I-Ca in a concentration-dependent manner and shifted the I-Ca activation threshold to the depolarizing side. Halothane 2.0% abolished I-Ca. Diltiazem 10(-8)-10(-6) mol litre(-1), a calcium channel antagonist, also depressed I-Ca in a concentration-dependent manner. Administration of both 0.5% halothane and diltiazem 10(-7) mol litre(-1) (concentrations lower than the clinical therapeutic range) abolished I-Ca; however, halothane did not exhibit use-dependent inhibition of I-Ca whereas diltiazem showed partial use-dependency. We conclude that the decrease in I-Ca produced by halothane is associated with a direct vasodilator effect of this anaesthetic, but is not explained by block of Ca2+ channels similar to the action of diltiazem. Furthermore, administration of low concentrations of both halothane and diltiazem decreased I-Ca and may reduce the contractility of the vascular smooth muscle cells.