Alterations o f M embrane Potential and Ca2+ Flux of Sarcoplasmic Reticulum Vesicles in Ischemic Myocardium*f

The sequence of contraction-relaxation for myocardial cells is believed to be linked to Ca2+ flux across the sarcoplasmic reticulum. Alterations of sarcoplasmic reticulum function during ischemia may result in depressing the myocardial contraction-relaxation sequence. This study examines the relationship betw een the m em brane potential and Ca2+ flux across sar­ coplasm ic reticu lum vesicles isolated from non-ischem ic and ischem ic myocardium. Ischemic myocardium was produced by ligating the coronary artery of swine hearts for 15 and 30 minutes. M em brane potential was determ ined by use of the fluorescence-sensitive dye, 3,3'-diethylthiadicarbocyanine, and Ca2+ uptake was studied spectrophotometrically with the use of murexide. Results are as follows: (1) m em brane potential and Ca2+ uptake by sarcoplasmic reticulum from ischemic myocardium pro­ gressively decreased with the length of ischemia; and (2) preincubation of sarcoplasmic reticulum from non-ischemic myocardium with deoxycholate (0.01 ~ 0.09 percent) resulted in progressively decreasing m em brane po­ tential and Ca2+ uptake. Apparently a correlation exists betw een m em ­ brane potential and the rate of Ca2+ uptake. These results suggest that m em brane characteristics of sarcoplasmic reticulum are altered within as early as 15 m inutes of the onset of ischemia. Alteration of m em brane perm eability in sarcoplasmic reticulum from ischemic myocardium may be responsible for the observed decrease in m em brane potential and Ca2+ uptake. * This work was supported by the Veterans Ad­ ministration and the American Heart Association, Ar­ kansas Chapter. t Please send reprint requests to Chun Fu Peng, Ph.D., VA Medical Center (111B), 300 E. Roosevelt Road, Little Rock, AR 72206. Introduction The use of fluorescent probes to study m em brane potentials has been reported in intact cell preparations such as squid