Characterization of Cardiac Sarcoplasmic Reticulum Dysfunction during Short‐Term, Normothermic, Global Ischemia

It has been proposed that breakdown of the excitation-contraction coupling sytem plays a pivotal role in myocardial dysfunction during the course of acute ischemia. We tested this hypothesis by characterizing the function of the sarcoplasmic reticulum at pH 7.1 and 6.4 after 7.5, 15, and 30 minutes of canine normothermic global ischemia. At pH 7.1, whole heart homogenate sarcoplasmic reticulum demonstrated a 49% depression of oxalate-supported calcium uptake at 7.5 minutes of ischemia, which progressed to 85% at 30 minutes of ischemia. At pH 6.4, control homogenate calcium uptake rates were significantly depressed, accompanied by a further depression in the ischemic groups. Isolated sarcoplasmic reticulum calcium uptake mirrored the effects of the whole heart homogenate. Calcium-stimulated magnesium-dependent ATPase (calcium-ATPase) activity was significantly depressed by both ischemia and acidosis, with a decrease in the coupling ratio (/*mol calciumμmol ATP) at 15 and 30 minutes of ischemia. Acidosis (pH 6.4) significantly shifted the sarcoplasmic reticulum pCalcium-ATPase curve to the right, increasing 50% activation from pCalcium 6.0 to 5.5 and depressing the maximum velocity (pH 7.1 = 2.06 ± 0.14; pH 6.4 = 1.41 ± 0.05 μmol Pi/mg per min; P < 0.01). With ischemia, there was a progressive decrease in maximal activation of the calcium-ATPase enzyme and a progressive shift in calcium sensitivity to a higher concentration. Steady state calcium uptake, in the absence of oxalate, demonstrated a similar depression after 7.5 and 15 minutes of ischemia at pH 7.1 and 6.4, associated with a significant increase in the passive permeability coefficient for calcium. Sarcoplasmic reticulum isolated from the 30-minute ischemic groups could not support steady state calcium uptake. It is concluded that during short-term normothermic ischemia, there is significant and progressive sarcoplasmic reticulum dysfunction which is magnified at pH 6.4, characterized by a decrease in calcium uptake and ATPase activity. There is also an uncoupling of calcium transport from ATPase activity which may be the result in part of an increase in the calcium permeability of the sarcoplasmic reticulum membrane. It is postulated that, during primary myocardial ischemia, this breakdown in sarcoplasmic reticulum function may serve as the source of intracellular calcium overload.