Calcium binding and ATPase activities of heart sarcolemma.

Rat heart sarcolemma prepared by the hypotonic shock-LiBr treatment method was found to bind calcium by a concentration-dependent and saturable process. The calcium binding values at 50 muM and 1.25 mM Ca2+ concentrations were about 30 and 250 nmoles/mg protein, respectively. Both Mg2+ and ATP inhibited calcium binding and no evidence for energy-linked calcium binding with sarcolemmn was found. z sn the other hand, maximal ATP hydrolysis by heart sarcolemma was seen at 4 mM Mg2+ or Ca2+. The Ca2+-ATPase LEO) of Ca2+ failed to stimulate ATP hydrolysis in the presence of various concentrations of Mg-ATP. These results indicate the absence of a "calcium pump" mechanism in the heart sarcolemmal membrane preparation employed in this study.

Heart sarcolemma as a dynamic excitable membrane.

Sarcolemma consists of plasma and basement membranes and constitutes the real permeability barrier to the heart cell. It is considered to provide high electrical resistance and capacitance to heart cell and its properties are essentially similar to those of the other excitable membranes. Methods are now available for isolating heart sarcolemma with high specific activities of adenylate cyclase, (Na(+)-K(+) ATPase, Ca(++) ATPase, and Mg(++) APTase. These enzymes are considered to play an important role in heart function by regulating ion movements across sarcolemma as well as by providing signals for various metabolic processes. Sarcolemma possesses different hormone and drug receptors and any alteration in its composition could result in abnormal responses of the myocardium. We believe that heart failure is associated with sarcolemmal defects which can be detected by monitoring the activities of different membrane-bound enzymes and other related processes.

Defective membrane systems in dystrophic skeletal muscle of the UM-X7.1 strain of genetically myopathic hamster.

1. The function of mitochondria, sarcotubular membranes (heavy microsomes), sarcolemma and myofibrils from the hind-leg skeletal muscle of about 60- and 150-day-old normal and myopathic (UM-X7.1) hamsters was examined. 2. The mitochondrial calcium uptake as well as mitochondrial phosphorylation and respiratory rates were lower in 60-day-old myopathic skeletal muscle, unlike 150-day-old myopathic animals, when pyruvate-malate and glutamate-malate were used as substrates. However, mitochondria from 150-day-old myopathic animals showed depressed glutamate-dependent respiratory and phosphorylation rates and succinate-supported initial rate of calcium uptake. 3. The microsomal calcium-uptake, but not calcium-binding, and Ca2+-stimulated adenosine triphosphatase (ATPase) activity of the 150-day-old myopathic skeletal muscle were lower than the control values. Although microsomal calcium-binding, calcium-uptake and ATPase activities of the 60-day-old myopathic muscle were not depressed significantly, the initial rate of calcium uptake was less than the control. 4. The sarcolemmal Ca2+-ATPase, but not Mg2+-ATPase or Na+ +K+-ATPase, activity was higher in 60-day-old myopathic muscle whereas the activities of all these enzymes from 150-day-old myopathic animals were higher than the control. On the other hand, the Na+ +K+-ATPase activities from 60- and 150-day-old myopathic animals were inhibited by ouabain to a lesser extent in comparison with the respective control values. 5. The myofibrillar Ca2+-ATPase and Mg2+-ATPase activities as well as inhibition of Mg2+-ATPase due to Na+ and K+ in myopathic muscle were no different from the control values. 6. The results reported here give further support to the view that different membrane systems of the dystrophic muscle are defective.