According to the results of many investigations ATP has a three phasic positive inotropic effect on isolated hypodynamie trog hearts (for referenees seeS). Immediately after the addition of ATP to the perfusion fluid an initial inerease in the amplitude of cardiae eontrae~ion oeours during a few seconds, followed by a phase (~: 1 min) of gradual deerease of the amplitude, after whieh a seeond inerease appears, reaehing a maximum after ehe er two minutes.
The aufhor also investigated the effeet of I T P 4. The same positive inotropie effeet as wich ATP was observed, exeept for the short during initial phase whieh is rauch sma]ler in the ease of ITP.
In 1960 To~oMu~A and KITAOAWA a deseribed a differenee in initial rapid spli~ting of ATP and I T P by myosin-ATP-ase. Aeeording to these authors the initial rapid splitting of I T P is very small, as compared with that of ATP. The initial rapid splitting of ATP by myosin-ATP-ase is eompletely suppressed by addition of EDTA (ethylenediaminetetraaeetic acid), while the steady phase of phosphate liberation is almost unaffeeted.
I f the aetion of ATP on the trog heart is eaused by the splitting of ATP into ADP and AMP, whieh splitting is observed by FLEOKENSTEIN and Li~BB~~ ~, than it is possible that the initial positive inotropie phase of the ATP effeet is due to the initial rapid splitting of ATP by myosin-ATPa s e .
To furnish proof of this suggestion, whieh we made in a preeeding paper 5, we deeided to study the effeet of ATP on trog hearts during perfusion with Ringer solution to whieh EDTA was added.
Method. For these experiments we used heurts of l~ana esculenta and the same perfusion apparatus as described earlier 5. After preparation the trog hearts were used immediately. First the hearts were perfused with normal Ringer so]ution