Abstract

Perfusion experiments on the frog's heart show that small concentrations of muscarine lessen the duration of the electrical disturbance in the ventricle without appreciably altering the rate of propagation of the excited state in the heart. The reduction in duration of the electrical disturbance ("excited state") is accompanied by a reduction in the duration of the refractory phase towards strong induction shocks ("absolute" refractory phase) and by a reduction in the strength of the mechanical response. The removal of the drug is followed by very prompt and complete recovery. Muscarine can produce its characteristic effect on the electric response of the heart when, owing to removal of calcium from the perfusion liquid, inhibition of the heart by electric stimulation of the pre- or post-ganglionic fibers of the vagus is impossible. The effect is, as usual, antagonised by atropine.

The localised application of muscarine to the sinus venosus causes no changes in the ventricular electrogram except those consequent on simple slowing. If muscarine is applied locally to the base or apex of the ventricle, changes in the form of the electrogram are produced due to the alteration in the relative duration of the excited state in the different regions; these are comparable to the effects produced by local warming. Qualitatively, synthetic muscarine, natural muscarine and arecoline act alike on the electrical response of the heart.