Is staurosporine a specific inhibitor of protein kinase C in intact porcine coronary arteries?

We assessed the specificity of staurosporine, a putative protein kinase C inhibitor, for the enzyme in intact porcine coronary arteries by examining its effects on changes in intracellular free calcium level ([Ca⁺⁺](i)) and force induced by a phorbol ester, high KCl and caffeine. [Ca⁺⁺](i) was measured simultaneously with force by the fura-2 microfluorimetric method. Phorbol 12,13-dibutyrate (PDBu, 10^-6 M) produced a slowly developing and sustained contraction; however, [Ca⁺⁺](i) only increased slightly and transiently in the initial phase and then decreased gradually. On the other hand, 90 mM KCl elicited a contraction with a sustained increase in [Ca⁺⁺](i). Staurosporine (10^-10 to 10^-7 M) applied 20 min before the addition of PDBu inhibited the PDBu-induced changes in [Ca⁺⁺](i) and force in a concentration-dependent manner. In contrast, staurosporine only at 10^-7 M reduced an increase in [Ca⁺⁺](i) and contractile force produced by 90 mM KCl. The inhibitory effects of staurosporine on PDBu- and KCl-induced contractions depended on exposure time. The [Ca⁺⁺](i)-force relationship obtained with variable concentrations of KCl was shifted slightly to the right by staurosporine at 10^-7 M. Furthermore, staurosporine even at 10^-7 M did not affect an increase in [Ca⁺⁺](i) by caffine (25 mM), although it slightly attenuated a caffeine-induced contraction. These results suggest that staurosporine is a relatively specific inhibitor of protein kinase C in intact arteries at lower concentrations. At higher concentrations it may have actions unrelated to its inhibitory effect of protein kinase C, which include the inhibition of calcium influx into smooth muscle cells through the voltage-dependent Ca⁺⁺ channel.