Previous data from our laboratory indicated that the slow Ca2+ channel of vascular smooth muscle cells was regulated by cyclic nucleotides. In the present study, the effects of isoproterenol (ISO) on L-type calcium current (ICa(L)) were investigated in freshly-isolated single smooth-muscle cells from the rabbit portal vein using the whole-cell voltage-clamp technique. With high-Cs+ solution in the pipette and physiolocial salt solution (containing 2.0 mM Ca2+) in the bath, ICa(L) was recorded. At a holding potential of -80 mV, low concentrations of ISO (< or = 100 nM) increased ICa, whereas higher concentrations (1-100 microM) transiently increased ICa but then inhibited it persistently. At 10 microM ISO, ICa was initially increased by 44 +/- 9%, and was subsequently decreased by 24 +/- 3%. Pretreatment of cells with 30 microM H-7 [1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine dihydrochloride] caused the first phase to persist and the second inhibitory phase to disappear. Intracellular application of 1 mM GDP[beta S] (guanosine 5'-O-2-thiodiphosphate) abolished both phases of ISO action. In contrast, intracellular application of 100 microM GTP caused the initial stimulatory phase of ISO action to be significantly potentiated; the later inhibitory phase was slightly diminished. In addition, the activated G protein alpha subunit (Gs alpha) mimicked the stimulatory effect of ISO. Pertussis toxin had no effect on either phase of the ISO action. These results suggest that ISO modulates the Ca2+ channel through mechanisms that involve the pertussis-toxin-insensitive G protein(s).(ABSTRACT TRUNCATED AT 250 WORDS)