THE EFFECTS OF CHANGES OF THE TONICITY OF THE BATHING FLUID UPON THE TENSION GENERATED BY ATRIAL TRABECULAE ISOLATED FROM THE HEART OF THE FROG,RANA PIPIENS

Abstract

Raising the tonicity of the fluid bathing frog [R. pipiens] atrial trabeculae has 3 effects: an initial sustained relaxation, which depends on muscle length and probably originates from structures other than the contractile apparatus; an increase in contractility, which takes the form of a transient contracture if the muscle has previously undergone a high-k or a low Na contracture and a further rise in contractility on return to isotonic fluid (off response). The hypertonic contractures, in high K or Na-free fluids, are antagonized by local anesthetics and in Na-free media they are unaffected by removal of extracellular Ca2+. Off responses are insensitive to both experimental maneuvers. Hypotonic fluids applied in Na-free solutions evoke a phasic and a tonic contracture, neither of which are sensitive to local anesthetics. The tonic response is reduced by lowering the [Ca]o and occurs at tonicities where cell membrane permeability is likely to have increased. The phasic part of the hypotonic contracture resembles the off response which follows exposure to hypertonic solution. The effects of hypertonic fluids and caffeine on frog heart are alike and similar to the responses induced by the same experimental maneuvers in skeletal muscle. The intracellular relaxing system in frog heart appears sensitive to changes in tonicity, and may be functionally divided.