Effects of trachynilysin, a protein isolated from stonefish (Synanceia trachynis) venom, on frog atrial heart muscle

The effects of trachynilysin (TLY), a protein toxin isolated from stonefish (Synanceia trachynis) venom, were studied on the electrical and mechanical activities of frog atrial fibres. TLY (1 mu g/ml) hyperpolarized the membr ane, shortened the action potential (AP) duration (APD), exerted a negative inotropic effect and elicited contracture. These effects did not develop i n the presence of atropine. TLY shortened the APD of fibres isolated from a frog completely paralyzed with botulinum type A toxin, in the presence of Ca2+ but not when Ca2+ was replaced by Sr2+. TLY increased the basal and th e peak of the fluorescence ratio of stimulated fibres loaded with fura-2. C onfocal laser scanning microscopy revealed the existence of a diffuse inner vation in atrial tissue. Our results suggest that TLY enhances the release of acetylcholine from atrial cholinergic nerve terminals and activates indi rectly muscarinic receptors leading to a shortening of APD. They also show that the mechanical effects induced by TLY are due to an increase of the Ca 2+ influx and to a rise in intracellular Ca2+ levels which leads to (i) a s lowing of the Na+/Ca2+ exchange activity, which accounts for the contractur e and (ii) the activation of a Ca2+-dependent K+ current involved in the AD P shortening. (C) 2000 Elsevier Science Ltd. All rights reserved.