EFFECTS OF AN INCREASE IN INTRACELLULAR FREE [MG2+] AFTER MYOCARDIAL STUNNING ON SARCOPLASMIC-RETICULUM CA2+ TRANSPORT

Background. Myocardial stunning has been associated with a greater than twofold increase in intracellular free [Mg2+] from 0.6 to 1.5 mM. The effect of this increase in free [Mg2+] on the function of the sarcoplasmic reticulum (SR) Ca2+ pump was assessed in SR isolated from Langendorff perfused, isovolumic rabbit hearts after 15 minutes of global ischemia. Methods and Results. Our results indicate that myocardial stunning results in a shift in the Ca2+ sensitivity of oxalate-supported, Ca2+ transport over the entire range of free [Ca2+] associated with the cardiac cycle. Using 0.6 mM free Mg2+ as control, maximal rates of Ca2+ transport occurred at 1-mu-M free Ca2+ (control, 519 +/- 32; stunned, 337 +/- 37 nmol Ca2+ . min-1 . mg-1). At 0.56-mu-M free Ca2+, SR Ca2+ transport was reduced from a control of 351 +/- 49 to 263 +/- 12 nmol Ca2+ . min-1 . mg-1 at 0.6 mM free [Mg2+]. Moreover, an increase in the free [Mg2+] from 0.6 to 1.5 mM results in a greater shift in the Ca2+ activation curve with no change in the level of maximal activation. Ca2+ transport at 0.56-mu-M free Ca2+ was shifted in the stunned SR from 263 +/- 12 to 138 +/- 29 nmol Ca2+ . min-1 . mg-1 at 0.6 and 1.5 mM free Mg2+, respectively. Conclusions. These results indicate that an increase in free [Mg2+] after stunning in combination with the inherent defect in the SR Ca2+ ATPase may reduce the ability of the cell to regulate Ca2+ to a greater extent than previously observed. This impairment in Ca2+ regulatory function may contribute directly to the increase in diastolic tone and indirectly to the reduced systolic function characteristic of the stunned myocardium.