MITOCHONDRIAL CREATINE-KINASE ISOFORM EXPRESSION DOES NOT CORRELATE WITH ITS MODE OF ACTION

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 mu M to 111 +/- 40 mu M (P < 0.001) on the addition of 25 mM creati ne, due to a local accumulation of ADP close to the ATP/ADP translocat or. 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 coupl ing in this tissue, as has previously been observed in smooth muscle. mi-CK is encoded by two different genes, which are expressed in a tiss ue-specific manner: the sarcomeric isoform is expressed in ventricular and skeletal muscles, while the ubiquitous isoform is expressed in sm ooth muscle, brain and other tissues. In order to determine whether a specific function can be attributed to the expression of a specific is oform, we investigated mi-CK mRNA expression by Northern blot analysis . Hybridization with synthetic oligonucleotides specific for each mi-C K isoform showed the expression of only the sarcomeric isoform in rat atria. This result was confirmed by PCR using primers specific for eac h isoform. In addition, electrophoretic analysis of CK isoforms showed no difference in the octamer/dimer ratio of mi-CK in the atria and ve ntricles. In atria, unlike the soleus or ventricles, the maximum poten tial rate of mitochondrial phosphocreatine synthesis was lower than th e maximal rate of ATP production by the mitochondria. The total CK/ade nylate kinase ratio was also lower in atria than in the other tissues, suggesting a greater contribution of adenylate kinase to adenine nucl eotide compartmentation in this tissue. The functional differences bet ween mi-CK in the two cardiac tissues seem to imply a specific arrange ment of the proteins in the intermembrane space rather than the expres sion of specific isoforms.