Arachidonic acid enhances contraction and intracellular Ca2+ transients in individual rat ventricular myocytes

Modulation of intracellular free Ca2+ concentration ([Ca2+](i)) by inotropic stimuli alters contractility in cardiac muscle. Arachidonic acid (AA), a precursor for eicosanoid formation, is released in response to receptor activation and myocardial ischemia and has been demonstrated to alter K+ and Ca2+ channel activity. We investigated the effects of AA on contractility by simultaneously measuring [Ca2+](i) and shortening in single field-stimulated rat ventricular myocytes. [Ca2+](i) transients were measured using fura 2, and myocyte shortening was assessed using video edge detection. AA stimulated a doubling in the amplitude of the [Ca2+](i) transient and a twofold increase in myocyte shortening. In addition, AA stimulated a 30% in crease in the time to 50% diastolic [Ca2+](i) and a 35% increase in the time to 50% relengthening. These effects of AA were mediated by AA itself (56 +/- 5%) and by cyclooxygenase metabolites. Pretreatment with the protein kinase C inhibitors staurosporine and chelerythrine nearly abolished (>90% inhibition) these AA-induced effects. Inhibition of voltage-gated K+ channels with 4-aminopyridine mimicked the effects of AA. Addition of AA to the 4-aminopyridine-treated myocyte had no additional effect on parameters of contractile function. These data indicate that AA alters the amplitude and duration of Ca2+ transients and myocyte shortening via protein kinase C-dependent inhibition of voltage-gated K+ channels. Release of AA by phospholipases in response to receptor activation by endogenous mediators or pathological stimuli may be involved in mediating inotropic responses in cardiac muscle.