Progress and problems in understanding the involvement of calcium in heart function.

In this article we have briefly reviewed the role of Ca2+ in the excitation contraction coupling in the myocardium and have indicated that cardiac contraction and relaxation are initiated upon raising and lowering the intracellular concentration of free Ca2+, respectively. Different mechanisms for the entry of Ca2+ through sarcolemma as well as release of Ca2+ from sarcoplasmic reticulum and possibly mitochondria have been outlined for initiating cardiac contraction. Relaxation of the cardiac muscle appears to be intimately dependent upon efflux of Ca2+ through sarcolemma as well as sequestration of Ca2+ by the intracellular storage sites, particularly sarcoplasmic reticulum and possibly mitochondria. The actions of some pharmacological and pathophysiological interventions have been explained on the basis of changes in subcellular Ca2+ movements in myocardium. Quinidine, which produced an initial positive inotropic action on rat heart was also found to increase sarcolemmal Ca2+-ATPase activity without any changes in the Na+-K+ ATPase. Other antiarrhythmic agents, procainamide and lidocaine, also increased sarcolemmal Ca2+-ATPase activity without affecting the Na+-K+ ATPase. On the other hand, both Ca2+-ATPase and Na+-K+ ATPase activities were increased in heart sarcolemma obtained from cardiomyopathic hamsters. In this model the increased Ca2+-ATPase activity may promote the occurrence of intracellular Ca2+ overload in the cardiac cell whereas the increased Na+-K+ ATPase activity may increase Ca2+ efflux through Na+-Ca2+ exchange systems as an adaptive mechanism. It has been suggested that some caution should be exercised while interpreting the data from in vitro experiments in terms of functional changes in the myocardium. Furthermore, it has been proposed that the pathophysiology and pharmacology of Ca2+ movements at different membrane sites be understood fully in normal and diseased myocardium in order to improve the therapy of heart disease.

Energy production and utilization in contractile failure due to intracellular calcium overload.

Intracellular calcium overload was produced by perfusing the Ca2+- deprived rat hearts for 5 or 10 min with normal medium containing 1.25 mM Ca2+ for 10 min and changes in the myofibrillar and membrane ATPase, sarcolemmal adenylate cyclase, mitochondrial oxidative phosphorylation and high energy phosphate stores in failing hearts were examined. Myocardial creatine phosphate and ATP were decreased by the intracellular calcium overload whereas the myofibrillar, mitochondrial and microsomal ATPase activities were not altered. The intracellular calcium overload markedly depressed the mitochondrial oxidative phosphorylation as well as sarcolemmal Ca2+ ATPase, Mg2+ ATPase, Na+ - K+ ATPase and adenylate cyclase. These results suggest that abnormalities in the process of energy production rather than energy utilization may primarily account for the depressed energy state of hearts failing due to an intracellular calcium overload.

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.

Influence of quinidine on ATP-linked calcium binding by heart mitochondria and microsomes.

The action of quinidine on heart microsomal and mitochondrial calcium binding in the presence of MgATP was studied under different experimental conditions and compared with other antiarrhythmic agents such as procaine amide and lidocaine. Quinidine stimulated microsomal calcium binding but depressed mitochondrial calcium binding. Although procaine amide stimulated microsomal calcium binding, it did not affect mitochondrial calcium binding. On the other hand, lidocaine depressed calcium binding by mitochondria without affecting calcium binding by the microsomal fraction. The stimulation of microsomal calcium binding by quinidine was not apparent at high concentrations of Mg2+, low concentrations of ATP, or low concentrations of Ca2+. The depressant action of quinidine on mitochondrial calcium binding was not observed at low concentrations of Mg2+ or ATP but was more pronounced at low concentrations of Ca2+. These results suggest that the action of quinidine on mitochondria may play a major role in eliciting cardiodepressant effect.

Role of sarcolemmal changes in cardiac pathophysiology.

Sarcolemmal Ca++-ATPase, Mg++-ATPase, and (Na+-K+)-ATPase activities were increased in late stages of heart failure in myopathic hamsters (BIO 14.6) without any changes in the adenylate cyclase activity. On the other hand, these hamsters at early and moderate stages of heart failure showed depressions in mitochondrial calcium binding and uptake and microsomal calcium binding. Sarcolemmal (Na+-K+)-ATPase was decreased in failing hearts because of substrate lack, oxygen lack, and perfusion with Ca++-free, Na+-free, or K+-free medium. Both Mg++-ATPase and Ca++-ATPase activities of sarcolemma did not change on perfusing the hearts with substrate-free, hypoxic, Na+-free, or K+-free medium. Adenylate cyclase activity decreased on substrate-free or Ca++-free perfusion. Intracellular calcium overload produced by perfusing the hearts with medium containing calcium after Ca++-free perfusion was associated with decrease in all the sarcolemmal-bound enzyme activities. All types of failing hearts employed in this study showed a dramatic shift in the electrolyte composition. Failure of the cardiac muscle to generate contractile force on treatment with trypsin was associated with defects in the functions of sarcolemma, mitochondria, and sarcoplasmic reticulum, whereas such an effect on treatment with phospholipase C was limited to alterations in the activities of sarcolemma. The data suggest that abnormality at the level of sarcolemma plays an important role in the pathogenesis of heart dysfunction; however, the degree and direction of alterations in the sarcolemmal functions seem to be dependent upon the type of heart failure.

Subcellular and functional effects of quinidine, procaine amide, and lidocaine on rat myocardium.

Different antiarrhythmic agents such as quinidine, procaine amide, and lodocaine at 1 mM concentrations were found to depress the ability of an isolated perfused rat heart to generate contractile force. Quinidine, but not procaine amide or lidocaine, decreased calcium uptake by both mitochondrial and microsomal fractions at different concentrations of calcium. The mitochondrial phosphorylation rate, respiratory control index, and state 3 oxygen consumption, but not ADP:O ratio and state 4 oxygen consumption, were depressed by only quinidine. None of these agents had any effect on myofibrillar Mg2+-ATPase or Ca2+-stimulated ATPase activities. On the other hand, sarcolemmal Mg2+-ATPase and Ca2+-ATPase activities, but not Na+-K+-ATPase activity, were increased by all these drugs. The sarcolemmal adenylate cyclase (EC 4.6.1.1) activity was decreased by quinidine only. These results suggest some similarities and differences in the sites of action of quinidine, procaine amide, and lidocaine within the myocardium.

Studies on adenylate cyclase-cyclic AMP system of the myopathic hamster (UM-X7.1) skeletal and cardiac muscles.

Cyclic AMP content, adenylate cyclase (EC 4.6.1.1) activity and phosphodiesterase I (EC 3.1.4.1) activity of the hind leg skeletal muscle and cardiac muscle in 60- and 150-day-old normal and myopathic (UM-X7.1) hamsters were examined. In 60-day-old myopathic animals, cardiac cyclic AMP levels were higher and phosphodiesterase I activity was lower, without any changes in the basal adenylate cyclase activity, whereas in 150-day-old myopathic hamsters, cardiac cyclic AMP and basal adenylate cyclase activity were lower, without any changes in the homogenate phosphodiesterase I activity. On the other hand, basal adenylate cyclase and phosphodiesterase I activities in the skeletal muscle homogenate from 60- and 150-day-old myopathic animals were not different from the normal values but the skeletal muscle cyclic AMP levels were significantly less in 60-day-old myopathic hamsters only. The plasma cyclic AMP levels in 60-day-old myopathic hamsters, unlike 150-day-old myopathic animals, were higher than the normal. Although these results reveal differences in myopathic cardiac and skeletal muscles, it is concluded that changes in adenylate cyclase-cyclic AMP system in myopathy are dependent upon the degree of disease.