Die Bedeutung der extracellulären Calciumkonzentration für die Spaltung von energiereichem Phosphat in ruhendem und tätigem Myokardgewebe

Summary

The effects of changes in external calcium concentration (0–34,6 mM Ca⁺⁺/l) on the intracellular levels of ATP, ADP, AMP, creatine phosphate and orthophosphate were analysed in 270 isolated rabbit's atria with the use of paperchromatography. During incubation in the special media the auricles either were kept at rest or were stimulated for 15 min at different frequencies at 30°C. The experiments revealed that the high levels of ATP and creatine phosphate in resting atria are rather resistent to changes in external calcium. In active auricles, on the other hand, the rate of splitting of energy-rich phosphates was found to be highly „calcium-sensitive“. Thus in the absence of extracellular calcium the breakdown of high-energy phosphates was minimized to-gether with the well-known decrease of contractility, whereas, at all stimulation frequencies, a significant extrasplitting of ATP and creatine phosphate took place if the external calcium rised above normal. Isometric tension changed in parallel with high-energy phosphate breakdown in the concentration range between 0 and 8,6 mM Ca⁺⁺/l. Calcium concentrations higher than 10 mM/l, however, further intensified splitting of ATP and creatine phosphate but failed to augment contractility or even depressed it. The results indicate that Ca⁺⁺-ions exert a mediator function in connecting excitation of the myocardial fibre surface with the fundamental process of utilisation of phosphatebond energy in contraction.