Extracellular ATP induces Ca2+ transients in cardiac myocytes which are potentiated by norepinephrine

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Abstract
Isolated rat ventricular cardiac myocytes loaded with the fluorescent calcium indicator fura2 showed significant changes in intracellular calcium concentrations upon exposure to > 1 μM ATP (EC50 = 7.4 ± 1.3 μM, n = 4, SE), suggesting that extracellular ATP may have an important influence on myocardial contractility. The response was found to be highly ATP specific and required extracellular calcium. Furthermore, 30 s pretreatment of the cells with 0.2–1 μM norepinephrine decreased the concentration of ATP required for the Ca2+ transient, shifting the EC50 for ATP to 1.7 ± 0.1 μM (n = 3, SE). β-Propranolol (a β1-receptor antagonist) prevented potentiation, whereas phentolamine (an α1-receptor antagonist) did not, indicating that regulation is through the β1-adrenergic receptor. ATP and norepinephrine released locally from sympathetic neurons may act in concert through the ATP and β1-adrenergic receptors to regulate myocardial calcium homeostasis.