Na+ ions as spatial intracellular messengers for co-ordinating Ca2+ signals during pH heterogeneity in cardiomyocytes

Aims Contraction of the heart is regulated by electrically evoked Ca2+ transients (CaTs). H+ ions, the end products of metabolism, modulate CaTs through direct interactions with Ca2+-handling proteins and via Na+-mediated coupling between acid-extruding proteins (e.g. Na+/H+ exchange, NHE1) and Na+/Ca2+ exchange. Restricted H+ diffusivity in cytoplasm predisposes pH-sensitive Ca2+ signalling to becoming non-uniform, but the involvement of readily diffusible intracellular Na+ ions may provide a means for combatting this. Methods and results CaTs were imaged in fluo3-loaded rat ventricular myocytes paced at 2 Hz. Cytoplasmic [Na+] ([Na+](i)) was imaged using SBFI. Intracellular acidification by acetate exposure raised diastolic and systolic [Ca2+] (also observed with acid-loading by ammonium prepulse or CO2 exposure). The systolic [Ca2+] response correlated with a rise in [Na+](i) and sarcoplasmic reticulum Ca2+ load, and was blocked by the NHE1 inhibitor cariporide (CO2/HCO3--free media). Exposure of one half of a myocyte to acetate using dual microperfusion(CO2/HCO3--free media) raised diastolic [Ca2+] locally in the acidified region. Systolic [Ca2+] and CaT amplitude increased more uniformly along the length of the cell, but only when NHE1 was functional. Cytoplasmic Na+ diffusivity (D-Na) was measured in quiescent cells, with strophanthidin present to inhibit the Na+/K+ pump. With regional acetate exposure to activate a local NHE-driven Na+-influx, D-Na was found to be sufficiently fast (680 mu m(2)/s) for transmitting the pH-systolic Ca2+ interaction over long distances. Conclusions Na+ ions are rapidly diffusible messengers that expand the spatial scale of cytoplasmic pH-CaT interactions, helping to co-ordinate global Ca2+ signalling during conditions of intracellular pH non-uniformity.