Extracellular ATP-induced Acidification Leads to Cytosolic Calcium Transient Rise in Single Rat Cardiac Myocytes

The origin of the increase in cytosolic free Ca2+ concentration ([Ca2+]i) induced by extracellular ATP was investigated in single isolated cardiac myocytes loaded with indo-1. The nucleotide added at a concentration of 10 microM triggers a few Ca2+ spikes, followed by a cluster of Ca2+ oscillations, increasing [Ca2+]i to around 200 nM from a basal value of 70 nM. Neither caffeine nor ryanodine affects the magnitude of the Ca2+ transient, but both shorten it by preventing the Ca2+ oscillations. This indicates that the latter must be related to the release of Ca2+ from the sarcoplasmic reticulum. Since ATP also induces cell depolarization (as shown by experiments using the potential sensitive dye bis-oxonol), the initial Ca2+ spikes were attributed to the opening of voltage-dependent Ca2+ channels. A small Ca2+ transient still remains under experimental conditions designed to prevent Ca2+ influx from external medium (low-Ca2+ high-Mg2+ medium containing La3+) and after depletion of the sarcoplasmic-reticulum Ca2+ load with caffeine. Under these conditions, when this Ca2+ transient was buffered by 1,2-bis-(O-aminophenoxy)ethane-NNN'N'-tetra-acetic acid, ATP was unable to trigger the initial Ca2+ spikes. These results indicate that ATP mobilizes Ca2+ ions from an intracellular pool other than the sarcoplasmic reticulum and that this Ca2+ release is responsible for the depolarization. The effects of ATP on [Ca2+]i share the same characteristics as the acidification simultaneously induced by the nucleotide (as shown by experiments using the pH-sensitive probe snarf-1). These ionic variations are highly specific to ATP and its hydrolysis-resistant analogues. They both require the presence of Mg2+ and Cl- ions in the extracellular medium, and they are prevented by pretreatment of the cells with 4,4'-di-isothiocyanostilbene or probenecid. These results suggest that: (1) the ATP-induced acidification leads to displacement of Ca2+ ions from or close to the internal face of sarcolemma; (2) the Ca2+ ions activate a non-specific membrane conductance responsible for the depolarization of the cells; (3) the depolarization leads to a Ca2+ influx, owing to the opening of the voltage-dependent Ca2+ channels; (4) this increase in Ca2+ triggers the release of Ca2+ from the sarcoplasmic reticulum, which is facilitated by the increase in inositol trisphosphate following P2-purinergic stimulation.