Effect of increased free [Mg2+]i with myocardial stunning on sarcoplasmic reticulum Ca(2+)-ATPase activity.

Intracellular free Mg2+ concentration [( Mg2+]i) has been shown to increase markedly during ischemia from 0.6 to 3.2 mM and remain elevated severalfold at 1.5 mM after reperfusion of the stunned heart. The significance of this rise in [Mg2+]i after reperfusion on cellular function is not well known. To determine whether this increase in free [Mg2+] would alter the function of the sarcoplasmic reticulum (SR), the effects of an increase in free [Mg2+] on the SR Ca(2+)-dependent Mg(2+)-adenosinetriphosphatase (ATPase) activity were examined in SR isolated from Langendorff-perfused, isovolumic rabbit hearts after 15 min of reversible ischemia (global stunning). Oxalate-supported Ca2+ transport, assessed under identical conditions (0.4 mM free Mg2+, 15 microM free Ca2+), was reduced from 495 +/- 29 to 395 +/- 27 nmol Ca2+.mg protein-1.min-1 in control and stunned hearts, respectively, indicating a defect in enzyme function. This defect was confirmed by a decrease in the maximal Ca(2+)-dependent Mg(2+)-ATPase activity. An increase in the free [Mg2+] to simulate conditions after reperfusion leads to a decrease in the Ca2+ sensitivity of the SR Mg(2+)-ATPase. Fifty percent activation was shifted from a control free [Ca2+] of 0.42 microM at 0.6 mM free [Mg2+] to 0.63 microM free [Ca2+] at 1.2 mM free [Mg2+], conditions that simulate the reperfused stunned myocardium. These results indicate that after stunning the observed decline in SR Ca2+ transport, determined under similar incubation conditions, may be further jeopardized by the sustained increase in free [Mg2+].(ABSTRACT TRUNCATED AT 250 WORDS)