EFFECT OF INCREASED FREE [MG2+]I WITH MYOCARDIAL STUNNING ON SARCOPLASMIC-RETICULUM CA2+-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 Ca2+-dependent Mg2+-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-mu-M 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 Ca2+-dependent Mg2+-ATPase activity. An increase in the free [Mg2+] to simulate conditions after reperfusion leads to a decrease in the Ca2+ sensitivity of the SR Mg2+-ATPase. Fifty percent activation was shifted from a control free [Ca2+] of 0.42-mu-M at 0.6 mM free [Mg2+] to 0.63-mu-M 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+]. Thus the increase in intracellular free [Mg2+] combined with the depression in enzyme function may contribute significantly to the increase in diastolic tone and decline in systolic function with stunning.