Spatial regulation of intracellular pH in multicellular strands of neonatal rat cardiomyocytes.

AIMS Intracellular pH (pHi), an important modulator of cardiac function, is normally regulated to within narrow limits (7.1-7.2). In adult ventricular cell pairs, localized cellular pHi disturbances are removed by sarcolemmal acid/base transporters, but can also be dissipated (diluted) across gap junctions, aboard mobile buffers such as CO2/HCO3- and histidine-containing dipeptides (HCDPs). In the present work, we test this model of spatial pHi regulation in multicellular strands of neonatal rat ventricular myocytes. METHODS AND RESULTS We confocally image pHi (intracellular fluorescence emitted from the pH dye carboxy-SNARF-1) in multicellular (>500 microm long, approximately 30 microm wide) cultured strands of electrically coupled, neonatal rat ventricular myocytes. Activity of sarcolemmal Na+/H+ exchange and Na+-HCO3- co-transport resembles that in adult cells. Localized photolytic H+ uncaging from intracellular 2-nitrobenzaldehyde, in the presence of CO2/HCO3- buffer, triggers considerable passive H+ spread along a strand, thus helping to dissipate the acid load. Inhibition of gap junctions (with alpha-glycyrrhetinic acid) truncates the spread, indicating they are conduits for local intracellular H+ flux. Without CO2/HCO3- buffer, longitudinal H+ mobility is reduced by approximately 90%, indicating that intracellular and cell-to-cell H+ flux relies far less on intrinsic mobile buffers (e.g. HCDPs) in neonates than in adults. This is consistent with five-fold lower HCDP levels in neonatal, compared to adult, ventricular tissue, and also with measurements of a lower intrinsic (non-CO2/HCO3-) H+ buffering capacity in neonatal strands compared with freshly isolated adult cells. CONCLUSION We conclude that mobile buffers and gap junctions are key spatial controllers of pHi in cardiac tissue, helping to maintain a myocardial pHi syncitium. In neonatal tissue, intracellular H+ movement is CO2/HCO3- dependent, while adult tissue relies increasingly on intrinsic dipeptides that provide additional spatial pHi control, appropriate for the developmental increase in myocyte size.