Abstract
1. The effect of extracellular MgATP on cytosolic pH (pHi) was investigated in single rat cardiac cells loaded with the pH-sensitive probe Snarf-1. 2. Basal pHi in HEPES-buffered solution (containing 4.4 mM-NaHCO3) was 7.08. MgATP induced a transient acidification followed by an alkalinization. The latter is prevented by ethylisopropylamiloride (EIPA) and has been attributed to the activation of the Na+/H+ antiport. The MgATP-induced acidification reached a maximal value of 0.42 +/- 0.03 pH units (U pH). It was concentration dependent with a K0.5 of 2.6 microM-MgATP. This acidification was also observed with the same magnitude in the presence of the more physiological Krebs-bicarbonate buffer but was greatly reduced in nominally HCO3-free HEPES. 3. The MgATP-induced acidification was prevented by 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS), probenecid and ethacrynic acid but not by bumetanide. It was dependent upon the external chloride concentration. The K0.5[Cl-] was 9 mM and the maximal acidification required 60 mM-Cl-. 4. MgATP accelerated the recovery from an alkalinization triggered by a pulse of NH4Cl. The nucleotide also facilitated the efflux of HCO3- when the cell was switched from a Krebs-bicarbonate buffer gassed with 5% CO2 to an HEPES buffer. 5. The acidification was only evoked by MgATP and its poorly hydrolysable analogues but not by the other nucleotides (ADP, GTP (guanosine triphosphate), CTP (cytidine triphosphate) UTP (urodine triphosphate), ITP (inositol triphosphate) nor by adenosine. It required the presence of Mg2+ ions. 6. These results provide evidence that MgATP activates the Cl-/HCO3- exchanger and that this activation accounts for the acidification. Such an activation could not be related to the P1- or the P2-purinergic receptors since it requires triphosphate adenylic compounds and Mg2+ ions. This leads us to suggest the existence of a putative P3-type of purinergic receptor.