Some Observations on Cardiac Automatism in Certain Animals

Abstract

Certain aspects of the acetylcholine hypothesis of cardiac automatidty have been tested in dtro with spontaneously beating cardiac tissue from rabbits, rats, dams, and hagfish. The beat of atria from rabbits and rats may be depressed or excited by acetylcholine, depending upon the state of the tissue. Proguanil and cocaine inhibition of the beat in the rat may be antagonized by acetylcholine so that reversal of the depression occurs. The action of acetylcholine on the hearts of dams was found to be strictly inhibitory. Proguanil and cocaine, in contrast to their action on mammalian atria, exert a stimulatory effect on the heart of the molluscs studied. In fact, cocaine stimulated these hearts when they were inhibited by acetylcholine.Studies on the non-innervated hagtish heart revealed that this tissue is completely insensitive to the action of acetylcholine. Extracts prepared from beating hearts of this species will accelerate hypodynamic hearts of the hagiish as well as of the mussel. An extract of the neurogenic lobster heart was without effect on the hagiish heart. Proguanil was likewise ineffective in concentrations which produced inhibition and excitation in rat and clam hearts respectively.It was concluded that acetylcholine does not play a role in the myogenic automatism of all species, and that another mechanism is responsible is suggested on the basis of results obtained in the hagfish hearts.

TheTale‐TellHeart: Evolutionary tetrapod shift from aquatic to terrestrial life‐style reflected in heart changes in axolotl (Ambystoma mexicanum)

“…In case of the heart rate, there is a different approach in our study compared to Bullfrog ( Lithobates catesbeianus ), 57 where changes in resting heart rate after metamorphosis were reported. While the study of measured the heart rate in vivo, 57 we used ex vivo approach, where the heart rates ex vivo reflects automaticity of the unregulated pacemaker 58,59 . In the amphibians, already at the larval stage, the resting heart rate is affected by vegetative nervous system 57,60 …”

“…While the study of measured the heart rate in vivo, 57 we used ex vivo approach, where the heart rates ex vivo reflects automaticity of the unregulated pacemaker. 58,59 In the amphibians, already at the larval stage, the resting heart rate is affected by vegetative nervous system. 57,60 Morphological changes showed differences in trabeculation between the P and the M axolotl groups, which are more likely connected to terrestrial life-style.…”

Performance of the heat of the hagfish,Eptatretus cirrhatus

“…The preparation used here did not include the sinus venosus, which acts as the pacemaker in vivo (Davie et al 1987). However, bearing in mind the relative constancy of heart rates at rest and when swimming (Forster et al 1987), it is likely that the marked changes in heart rates of isolated hagfish hearts reported earlier (Jensen 1958) represent an artefact of the recording conditions and perhaps inadequate perfusion volumes. For example, Chapman et al (1963) reported an A-V block in experiments on the isolated heart of E. stouti, which was not seen in these experiments.…”