In vascular smooth muscle, phorbol esters cause a slowly developing contraction and an associated transmembrane calcium flux, both of which are inhibited by dihydropyridine calcium channel antagonists. In the A7r5 cultured vascular cell line, we used the whole-cell voltage-clamp technique to identify voltage-dependent calcium conductances and investigate the effect of phorbol esters on that conductance having characteristic dihydropyridine sensitivity (slowly inactivating, high-threshold, "L-type"). With barium as the charge carrier, large-amplitude (100-800 pA) inward currents of two types were characterized by their kinetics and voltage dependence. With holding potential--80 mV, a rapidly inactivating, low-threshold current ("T-type") was activated by depolarizations above-40 mV and was maximal at -10 mV. With holding potential -30 mV, this component was inactivated, and a second slowly inactivating, high-threshold current was activated above -10 mV and was maximal at +10 to +20 mV. These currents are similar to the T-type and L-type currents previously described in vascular smooth muscle cells. When added to the bath, the active phorbol ester, 12-O-tetradecanoyl phorbol-13-acetate (100 nM) increased the slowly inactivating (L-type) current by 32 +/- 20% (n = 8, +/- SD). Phorbol-12,13-dibutyrate (100 nM) caused a similar effect, but the inactive phorbol, 4-alpha-phorbol (100 nM), did not. We conclude that at least two distinct calcium conductances are expressed in A7r5 vascular smooth muscle cells, and that the dihydropyridine-sensitive calcium conductance is acutely modulated by phorbol esters, presumably acting through stimulation of protein kinase C. Such modulation may play a role in increasing transmembrane calcium influx mediated by agonist-receptor interactions that lead to activation of protein kinase C and may help to sustain or amplify calcium-dependent cell responses.