Differential effects of reoxygenation on intracellular calcium and isometric tension

We used the bioluminescent Ca²⁺ indicator, aequorin to record intracellular calcium transients during reoxygenation of hypoxic ferret ventricular muscle in order to determine whether alterations in the amplitude and time course of isometric contraction are emdiated by changes in [Ca²⁺]_i. Papillary muscles 1 mm in diameter were removed from the hearts of male ferrets and perfused with a bicarbonate-buffered physiologic salt solution at 30°C. Muscles were stimulated to contract isometrically at 0.33 Hz and were loaded with aequorin by a chemical procedure. Hypoxia was induced by changing the gas mixture bubbling the perfusate to 95% N₂, 5% CO₂; reoxygenation was accomplished by switching the gas mixture to 95% O₂, 5% CO₂. Hypoxia produced a decrease in peak Ca²⁺ and tension that was reversed by reoxygenation. However, the effects on tension of changes in oxygenation were greater than expected from the degree of change in [Ca²⁺]_i. The time courses of the Ca²⁺ transient and isometric twitch moved in opposite directions and were respectively prolonged/abbreviated by hypoxia and abbreviated/prolonged by reoxygenation. These results indicate that changes in the amplitude and time course of the isometric twitch induced by hypoxia and reoxygenation cannot be attributed alone to changes in intracellular Ca²⁺ availability and are caused in part by a significant decrease in the calcium sensitivity of the contractile apparatus.