MYOCARDIAL CALCIUM CYCLING DEFECT IN FURAZOLIDONE CARDIOMYOPATHY

We have previously demonstrated that in furazolidone-induced congestive heart failure in turkeys the specific Ca2+-ATPase activity of myocardial sarcoplasmic reticulum (SR) is 60% increased in compensation for a 50% depression in net Ca2+-sequestration activity. This study tested the hypothesis that SR Ca2+-uptake and Ca2+-ATPase activities were uncoupled in this cardiomyopathy because of increased Ca2+-release channel activity. A novel microassay was used to monitor Ca2+ transport by myocardial homogenates using the fluorescent Ca2+ dye indo 1 to indicate extravesicular ionized Ca2+. The method is applied to cryopreserved biopsy specimens of myocardium and requires only 50 mg tissue. Both SR Ca2+-pump and SR Ca2+-channel activity were estimated using the channel-inhibitor ruthenium red (RR) and the mitochondrial inhibitor sodium azide. The specificity of the RR inhibition was confirmed using ryanodine. Cardiomyopathy was induced in 2-week-old turkey poults by the addition of 0.07% furazolidone to their feed for 4 weeks. Compared with controls, myocardial maximal Ca2+-channel activity relative to maximal Ca2+-pump activity was 22% greater and duration of Ca2+-channel activity was 100% increased. However, the heart failure birds had 43 and 53% decreases in absolute maximal Ca2+-pumping and Ca2+-channel activities, respectively. The abnormal Ca2+-channel activity resulted in 200% greater time before initiation of net Ca 2 +sequestration and 700% greater final myocardial Ca2+ concentrations. For all birds, the Ca2+-accumulating activity was highly correlated with Ca2+-release activity (all p < 0.05). These data indicate that in this animal model of congestive heart failure there is defective SR Ca2+-channel function resulting in abnormal Ca2+ homeostasis. However, this defect can only partially explain our previous finding of furazolidone-induced uncoupling of Ca2+ uptake from Ca2+-ATPase activities. The consequent myocardial Ca2+ overload predisposes the heart to fatigue and irreversible failure.