INFLUENCE OF BISTRAMIDE-A ON THE TWITCH TENSION IN RAT ATRIAL HEART-MUSCLE

Bistramide A, a new toxin isolated from the Urochordate Lissoclinum bi stratum Sluiter, was applied to rat auricular heart muscle bundles. At a stimulation frequency of 0.2 Hz, the toxin induces a dose-dependent reduction of the stimulated twitch tension force; it decreases V-max and shortens the duration of the plateau and the slow repolarizing pha se of the action potential. In the control solution, switching from a stimulation frequency of 0.2 Hz to 1 Hz decreases the force with which a positive potentiation develops either at a maintained high frequenc y or after switching from 1 Hz to 0.2 Hz. Bistramide A reduces both th e force evoked at 1 Hz and the potentiation. The data suggest that Bis tramide A blocks Na+ conductance; inhibits Ca++ channels in a time-and frequency-dependent manner; reduces Na+-Ca++ exchange activity; but d oes not modify the ability of the sarcoplasmic reticulum to be refille d although the rate of Ca++ accumulation is decreased.