Effects of hypoxia and metabolic inhibition on increases in intracellular Ca2+ concentration induced by Na+/Ca2+ exchange in isolated guinea-pig cardiac myocytes.

We have assessed if the sarcolemmal Na+/Ca2+ exchange of isolated cardiac myocytes becomes inhibited when the cells are subjected to hypoxia and metabolic inhibition. Function of the exchange was assessed by exposing single cardiac myocytes to Na+-free solutions and measuring the increases in [free Ca2+]i using the fluorescent indicator Fura-2. Removal of extracellular Na+ reverses the Na+ gradient and causes Ca2+ influx via Na+/Ca2+ exchange. This influx may trigger further Ca2+ release from the sarcoplasmic reticulum. Hypoxia and metabolic inhibition produced either by (1) vigorously bubbling glucose-free Tyrode with N2, or (2) bubbling with N2 and adding 10 mm 2-deoxy-D-glucose to the Tyrode or (3) using 0.1-1.0 mm sodium dithionite (sodium hyposulphite, Na2S2O4) in glucose-free Tyrode caused a significant decline in the rates of increase in [free Ca2+]i during Na2+-free superfusion. The decreases in [free Ca2+]i on re-application of Na+ were also significantly slowed. This suggests that reverse-mode Na+/Ca2+ exchange and the processes leading to release of Ca2+ from the sarcoplasmic reticulum are inhibited in hypoxia and metabolic inhibition.