This paper summarizes the data concerning the role of the creatine phosphokinase system in muscle cells with main attention to the cardiac muscle. Creatine phosphokinase isoenzymes play a key role in the intracellular energy transport from mitochondria to myofibrils and other sites of energy utilization. Due to the existence of the creatine phosphate pathway for energy transport, intracellular creatine phosphate concentration is apparently an important regulatory factor for muscle contraction which influences the contractile force by determining the rate of regeneration of ATP directly available for myosin ATPase, and at the same time controls the activator calcium entry into the myoplasm across the surface membrane of the cells.

Skinned rat papillary muscles and purified preparations of rat cardiac myofibrils were used to study the nature of the interaction of creatine kinase with cardiac myofibrils. High activity of creatine kinase (2 IU/mg protein in fibers and 0.9 IU/mg in purified myofibrils) was due mostly to reversibly bound enzyme. This activity could be removed and rebound. The process of creatine kinase rebinding was characterized by apparent Km value of 0.14 mg/ml (approximately equal to 2 X 10(6) M). Rebinding of creatine kinase to cardiac myofibrils restored the phenomenon of functional compartmentation of adenine nucleotides in myofibrillar space and restored the ability of phosphocreatine to decrease the rigor tension in the presence of MgADP. The physiological experiments with quick length changes showed that rebinding of creatine kinase to skinned papillary muscle also restored Ca sensitivity, increased maximal tension development, decreased stiffness, and restored the tension recovery after quick length changes in muscle under condition of inhibition of endogenous creatine kinase by 1-fluoro-2,4-dinitrobenzene. It is concluded that creatine kinase reversibly bound to cardiac myofibrils is involved in the energy supply for cardiac contraction.

An investigation of isolated and purified heart sarcoplasmic reticulum performed in the current study indicates the presence of significant creatine phosphokinase (CPK) activity in this preparation. The localization of CPK on the membrane of sarcoplasmic reticulum has been revealed also by an electron microscopic histochemical method. Under the conditions of the Ca(2+)-ATPase reaction in the presence of creatine phosphate, the release of creatine into the reaction medium is observed, the rate of the latter process being dependent on the MgATP concentration in accordance with the kinetic parameters of the Ca2+-ATPase reaction. CPK localized on the reticular membrane is able to maintain the high rate of calcium consumption by the sarcoplasmic reticulum vesicles. The results obtained demonstrate the close functional coupling between CPK and Ca2+-ATPase in the membrane of sarcoplasmic reticulum and indicate the important functional role of CPK in supplying energy for the Ca(2+)-ATPase reaction and ion transport across the membrane of heart sarcoplasmic reticulum.

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In adult mammalian ventricular tissue, mitochondrial creatine kinase (mi-CK), which is bound to the outer surface of the mitochondrial inner membrane, is functionally coupled to oxidative phosphorylation. This is shown, in saponin-permeabilized rat ventricular fibres, by a shift in the apparent K(m) of mitochondrial respiration for ADP from 300 +/- 56 microM to 111 +/- 40 microM (P < 0.001) on the addition of 25 mM creatine, due to a local accumulation of ADP close to the ATP/ADP translocator. We have found that in atrial fibres, the apparent K(m) for ADP is high, but is not decreased by creatine, suggesting an absence of coupling in this tissue, as has previously been observed in smooth muscle. mi-CK is encoded by two different genes, which are expressed in a tissue-specific manner: the sarcomeric isoform is expressed in ventricular and skeletal muscles, while the ubiquitous isoform is expressed in smooth muscle, brain and other tissues. In order to determine whether a specific function can be attributed to the expression of a specific isoform, we investigated mi-CK mRNA expression by Northern blot analysis. Hybridization with synthetic oligonucleotides specific for each mi-CK isoform showed the expression of only the sarcomeric isoform in rat atria. This result was confirmed by PCR using primers specific for each isoform. In addition, electrophoretic analysis of CK isoforms showed no difference in the octamer/dimer ratio of mi-CK in the atria and ventricles. In atria, unlike the soleus or ventricles, the maximum potential rate of mitochondrial phosphocreatine synthesis was lower than the maximal rate of ATP production by the mitochondria. The total CK/adenylate kinase ratio was also lower in atria than in the other tissues, suggesting a greater contribution of adenylate kinase to adenine nucleotide compartmentation in this tissue. The functional differences between mi CK in the two cardiac tissues seem to imply a specific arrangement of the proteins in the intermembrane space rather than the expression of specific isoforms.