A rapid transient rise in the free cytosolic Ca2+ concentration is one of the earliest events in smooth muscle cell activation and is involved in the opening of several classes of Ca2(+)-sensitive ion channels such as K+ channels and Cl- channels. In portal vein smooth muscle cells, in addition to the Ca(2+)-dependent Cl-current, single-channel activities were recorded in response to external application of 10 mM caffeine, in the absence of EGTA in the pipette solution. The conductance of this novel type of channel was around 200 pS for membrane potentials ranging between -100 to +60 mV. The single-channel activities were also induced by external application of noradrenaline (10(-5) M) or acetylcholine (10(-5) M), by Ca2+ entry through voltage-dependent Ca(2+)-channels, and by intracellular application of ryanodine (10(-5) M). The caffeine-activated single-channel currents disappeared when 10 mM EGTA were added to the pipette solution or after replacement of external Ca2+ with Ba2+. These results show that these channels are Ca(2+)-dependent. Alteration of the Cl- equilibrium potential did not produce any change in the reversal potential of the caffeine-activated single-channel current, indicating that it was not carried by Cl- ions. The value of the reversal potential was about + 10 mV, irrespective of the CsCl-, KCl- or NaCl-containing solutions used to fill the pipette. Caffeine activated single-channel currents when cells were bathed in 90 mM Ba2+ plus 1 mM Ca(2+)- or 91 mM Ca(2+)-containing solutions, showing that divalent cations permeate the channels.(ABSTRACT TRUNCATED AT 250 WORDS)