H+-activated Na+ influx in the ventricular myocyte couples Ca2Â +-signalling to intracellular pH

Acid extrusion on Na+-coupled pH-regulatory proteins (pH-transporters), Na+/H+ exchange (NHE1) and Na+-HCO3− co-transport (NBC), drives Na+ influx into the ventricular myocyte. This H+-activated Na+-influx is acutely up-regulated at pHi < 7.2, greatly exceeding Na+-efflux on the Na+/K+ ATPase. It is spatially heterogeneous, due to the co-localisation of NHE1 protein (the dominant pH-transporter) with gap-junctions at intercalated discs. Overall Na+-influx via NBC is considerably lower, but much is co-localised with L-type Ca2 +-channels in transverse-tubules. Through a functional coupling with Na+/Ca2 + exchange (NCX), H+-activated Na+-influx increases sarcoplasmic-reticular Ca2 +-loading and release during intracellular acidosis. This raises Ca2 +-transient amplitude, rescuing it from direct H+-inhibition. Functional coupling is biochemically regulated and linked to membrane receptors, through effects on NHE1 and NBC. It requires adequate cytoplasmic Na+-mobility, as NHE1 and NCX are spatially separated (up to 60 μm). The relevant functional NCX activity must be close to dyads, as it exerts no effect on bulk diastolic Ca2 +. H+-activated Na+-influx is up-regulated during ischaemia-reperfusion and some forms of maladaptive hypertrophy and heart failure. It is thus an attractive system for therapeutic manipulation. This article is part of a Special Issue entitled “Na + Regulation in Cardiac Myocytes”.