Studies of subcellular control factors in hearts of uremic rats

Subcellular fractions in hearts from rats with severe acute uremia (24 hours after total nephrectomy) and moderate chronic uremia (2 weeks after five sixths nephrectomy) were studied and compared with preparations from acute and chronic sham-operated rats, respectively. Calcium- and magnesium-sensitive actomyosin adenosine triphosphatase (ATPase) activities were normal in both groups. Acute uremia was associated with a significant depression of sarcolemmal Na+,K+ ATPase activity. Calcium transport by fragmented sarcoplasmic reticulum was also depressed in the presence and absence of oxalate in acute uremia. Mitochondrial calcium transport and adenosine triphosphate (ATP) and creatine phosphate (CP) concentrations were normal in these animals. Chronic uremic animals showed no abnormal subcellular mechanisms. These data suggest a direct effect of acute uremia on some membrane functions in myocardial cells. The discrepancies observed between acute and chronic uremic groups may be due to a different degree of uremic state. The observation of depressed calcium transport by fragmented sarcoplasmic reticulum (FSR) in acute uremic hearts which were previously shown to have increased contractile reserve suggests that studies of calcium transport in FSR may not always truly reflect the contractile capacity of the heart.