The effects of changes in external calcium concentration (0 34,6 mM Ca++/1) 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 extracelhilar calcium the breakdown of high-energy phosphates was minimized to-gerber with the wcll-know~ 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++/1. Calcium concentrations higher than 10 mM/1, 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.
Nach friiheren Ergebnissen yon FLECKEZ~STE~Z~, Sc~wo~ u. JA:NiKE (1961), FLECKENSTEI:N U. SC~IWOEtCEI¢ (1961) sowie FLEC~ZEZ~ST]~I~ (1963) spielen Ca++-Ionen bei den verschiedensten Kontrakturen des * tterrn Professor Dr. MAx SCHI~-EIDEI% zum 60. Geburtstag gewidmet. ** Eine vorl~ufige Mitteilung der Ergebnisse ist bereits anl~l~lich der 70. Tagung der Deutsch. Ges. Inn. Med. am 6.4. 1964 in Wiesbaden sowie auf der 29. Tagung (Friihjahrstagung) der Deutschen Physiologischen Gesellsehaft am 1.5. 1964 in Bad iNauheim gegeben worden [s. Verh. dtsch. Ges. inn. Med. 70, 81 (1964); Pfliigers Arch. ges. Physiol. 279, 1% 9 (1964)]. Wesent]iche Teile der vorliegenden Arbeit wurden bei der Medizinischen Faku]t~t Freiburg i. Br. als Dissertation yon Herrn cand. reed. J. Kr,:EK eingereicht.
*** Die Untersuehungen wurden mit dankenswerter Unterstfitzung der Deutschen Forschungsgemeinschaft durchgefiihrt. Fiir technische Assistenz sind wir Frl. tI. RVZ~BE~G sehr verbunden.