The role of calcium in phorbol-12,13-dibutyrate (PDB)-induced contractions of the canine saphenous vein (CSV) was examined. Phorbol-12,13-dibutyrate elicited concentration-dependent contractions in CSV (EC50 = 1.6 +/- 0.2 X 10(-7) M) which were not affected by atropine (10(-6) M), pyrilamine (10(-6) M), and phentolamine (10(-5) M). The maximum contraction induced by PDB (10(-6) M) was slightly greater than that elicited by phenylephrine (PE; 10(-4) M). Phorbol-12,13-dibutyrate produced maximal 45Ca2+ uptake (0.80 +/- 0.07 mmol/kg wet wt) comparable with that induced by PE (0.90 +/- 0.04 mmol/kg wet wt) which was approximately fourfold above basal 45Ca2+ uptake (0.21 +/- 0.02 mmol/kg wet wt). The increase in 45Ca2+ uptake stimulated by PDB and PE was completely abolished by La3+ (5 mM). In the absence of Ca2+ entry, the contractions to PDB were reduced by only 29 +/- 3.4%. Substantial responses to PDB (51.3 +/- 4.8% of control) remained after reduction of intracellular Ca2+ store by repeated challenges with PE (10(-4) M) in the presence of La3+. Similar results were obtained when the contractions of CSV to PDB were determined in zero external Ca2+ medium. The data suggest that PDB utilizes both extracellular and intracellular Ca2+ for contractions of CSV.