[Characteristics of the phase-dependent vagus effects in the heart of the frog Rana temporaria and of the cod Gadus morhua].

In experiments on the heart of the cod Gadus morhua and frog Rana temporaria in situ, studies have been made of changes in the heart rate induced by stimulation of the vagal nerve by single brief bursts delivered at various intervals after P wave of the ECG. Certain differences were found in changes of the heart rate between these animals. In the cod, maximum chronotropic effect was equal to 65% of the duration of initial cardiac cycle, the latency of this effect being equal to 290 ms; in the frog, corresponding figures were 12-13% and approximately 940 ms. The duration of negative chronotropic effect in the heart of the cod was equal to 700 ms, that of the frog--to 2.700 ms. Functional role of these differences is discussed in relation to the problem of the development of parasympathetic regulation of the heart rate in phylogenesis of vertebrates.