This study was designed to analyze the mechanism(s) underlying the positive inotropic effect (PIE) of endothelin-1 (ET-1) in the guinea pig left atrium. ET-1 exhibited a greater PIE at lower frequencies of pacing and potentiated significantly the postrest contraction similar to isoproterenol. However, ET-1 prolonged the duration of a single contraction, whereas isoproterenol shortened it. ET-1 was similar to methyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyridine-5- carboxylate in the prolonged duration of a single contraction but different from this drug in the force-frequency relationship. ET-1 at concentrations of 10 nM and higher caused a dual-component PIE composed of an initial increasing phase (early component) and a second greater positive inotropic phase (late component). The early component was correlated to the ET-1-induced prolongation of the duration of the action potential in the time course. Both nifedipine and ryanodine suppressed the late component much more than the early component. ET-1 (> or = 3 nM) produced significant stimulation of phosphoinositide hydrolysis as measured by [3H]inositol monophosphate accumulation. ET-1 was found to activate protein kinase C (PKC) instantaneously but transiently (evaluated by the translocation of PKC activity to the particulate fraction). Pretreatment with 1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine and staurosporine, PKC inhibitors, markedly inhibited the late component of the PIE of ET-1 without affecting the early component. These data indicate that the two components of the PIE induced by ET-1 in the guinea pig left atrium may be mediated by different mechanisms. The early component may be attributed to the increased Ca++ influx as a result of the prolongation of the duration of the action potential, whereas the late component may be linked to stimulation of phosphoinositide hydrolysis and subsequent PKC activation.