External ATP induced increase in intracellular free calcium in mammalian atrial myocytes.

Extracellularly applied ATP elicited a transient increase in intracellular free calcium concentration ([Ca2+]i) measured by fura-2 fluorescence digital imaging microscopy in isolated rabbit and guinea-pig atrial myocytes. In the electrophysiological study, ATP induced a transient depolarization of the membrane potential due to activation of a rapidly desensitizing inward current. Our results indicate that in mammalian atrial myocytes, ATP evoked transient increase in [Ca2+]i through release of internally stored Ca2+ associated with inflow of external Ca2+, mainly through L-type Ca channels which were voltage-activated secondarily to ATP-induced conductance changes.