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.

Raising the tonicity of the fluid bathing frog atrial trabeculae has three 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-potassium or a low-sodium contracture, and a further rise in contractility on return to isotonic fluid (off response). The hypertonic contractures, in high-potassium or sodium-free fluids, are antagonized by local anaesthetics and in Na-free media they are unaffected by removal of extracellular coat, whereas the 'off responses' are insensitive to both experimental manoeuvres. Hypotonic fluids applied in Na-free solutions evoke a phasic and a tonic contracture, neither of which are sensitive to local anaesthetics. The tonic response is reduced by lowering the [Ca]o, and occurs at tonicities where the permeability of the cell membrane 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 of caffeine on frog heart are alike, and are also similar to the responses induced by the same experimental manoeuvres in skeletal muscle. The results can be interpreted by assuming that the intracellular relaxing system in frog heart is sensitive to changes in tonicity, and could be functionally divided.