Counter-current and high performance liquid chromatography techniques were used to purify an endogenous modulator for voltage-dependent Ca2+ channels from rat brain tissue. The nitrendipine binding inhibiting activity increased calcium current through L- and T-type channels in cardiac myocytes of rat, guinea pig, and frog in whole cell patch clamp studies. This result indicates that the endogenous modulator acts as an agonist for L- and T-type channels. Pretreatment of cardiac myocytes with 10 (mu)M phorbol 12,13-diacetate attenuated the enhancing effect of the endogenous modulator and thereby suppressed the calcium current through these channels. It can be inferred that the endogenous modulator may bind to an extracellular receptor of phorborol 12,13-diacetate.