Action of a semi-purified extract of Pseudophryne coriacea skin on the frog rectus abdominis muscle.

Extracts of the skin of the Australian frog Pseudophryne coriacea (PS) displayed a striking potentiating effect on contractions evoked in the isolated frog rectus abdominis muscle by direct electrical stimulation. There was both a conspicuous increase in twitch amplitude and a remarkable prolongation of twitch duration. High concentrations of PS also elicited an increase in tone. The effect on twitch was mainly due to direct stimulation of muscle fibres, the effect on tone to facilitation of acetylcholine release from the motor nerve terminals embedded in the musculature. Unlike the first effect, the second was blocked by tubocurarine and alpha-bungarotoxin. Tetrodotoxin inhibited both the basal and PS-stimulated twitch. Experiments with EGTA, a calcium chelating agent, suggest that activation of voltage-dependent calcium channels is involved in the response of the frog muscle to PS. Low concentrations of PS were ineffective on the non-stimulated muscle; high concentrations, on the contrary, generally caused an increase in tone. This was completely inhibited by the blockers of the nicotinic acetylcholine receptors, again suggesting a transmitter release from the nerve terminals.