Effects of ischemia on the isolation and function of canine cardiac sarcoplasmic reticulum

Normothermic global ischemia of 7, 10, 15 and 60 min was found to depress oxalate supported calcium uptake rate measured either in unfractionated homogenates or isolated sarcoplasmic reticulum. The degree of depression increased with the duration of ischemia. Comparison of the isolated sarcoplasmic reticulum with unfractionated homogenates showed that the isolated sarcoplasmic reticulum was more damaged by ischemia than the unfractionated homogenate. The cause of this discrepancy was not due to inactivation of sarcoplasmic reticulum during isolation but was due to the discard of greater portions of undamaged sarcoplasmic reticulum as the ischemic period increased. Ischemia preferentially affected that sarcoplasmic reticulum most easily fragmented by homogenization. To determine if the depression of sarcoplasmic reticulum function is uniform throughout the isolated fraction, we compared several properties of the isolated fractions. After 10 min of ischemia, extensive properties such as calcium oxalate uptake rate, calcium ATPase rate, calcium oxalate capacity and steady-state calcium loading were depressed 50, 41, 48 and 24% respectively. In contrast, intensive properties such as permeability, calcium-ATPase turnover rate, and ratio of forward nucleotide flux to reverse nucleotide flux were unaffected by ischemia. However, one intensive property, the coupling ratio, was depressed 20%. We conclude from this difference in the effects of ischemia on extensive and intensive properties that the major effect of ischemia is to inactivate the Ca-ATPase.