In vascular smooth muscle (VSM) cells, the slow inward calcium current (ICa) may be regulated by phosphorylation of the calcium channel protein or of associated regulatory proteins. We investigated the role of several protein kinase systems in the regulation of ICa in cultured A7r5 cells, a clonal cell line derived from rat aorta. The perforated-patch voltage-clamp technique was used to record whole cell ICa. To isolate the ICa, the pipette contained high Cs+ and the bath contained 140 mM tetraethylammonium to block potassium currents. Ba2+ was used as the charge carrier. In control experiments, ICa was stable for at least 15 min. Compared with 23 +/- 3% in the time-control group (i.e., run-down; n = 10), 3 mM 8-bromo-adenosine 3',5'-cyclic monophosphate (8-BrcAMP) inhibited peak ICa by 53 +/- 3% (n = 9) within 15 min. Similarly, 3 mM 8-bromo-guanosine 3',5'-cyclic monophosphate (8-BrcGMP) inhibited ICa by 59 +/- 4 (n = 11). Application of 30 microM forskolin inhibited ICa by 58 +/- 9% (n = 6) within 5 min (compared with 4 +/- 3% for the 5-min time control). Forskolin also shifted the reversal potential to the left, suggesting a stimulation of an outward current. In the presence of the protein kinase inhibitor, 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine, the same dose of forskolin had no effect (n = 7). The water-soluble analogue of forskolin (L-858051, 30 microM) decreased ICa by 72 +/- 11% (n = 9) and reduced the outward current component.(ABSTRACT TRUNCATED AT 250 WORDS)