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+]iusing 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 N2and adding 10 m 2-deoxy-D-glucose to the Tyrode or (3) using 0.1–1.0 m sodium dithionite (sodium hyposulphite, Na2S2O4) in glucose-free Tyrode caused a significant decline in the rates of increase in [free Ca2+]iduring Na2+-free superfusion. The decreases in [free Ca2+]ion 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.