The effects of toxin Ts-gamma and tityustoxin purified from Tityus serrulatus scorpion venom were investigated on isolated rat atria. Rat atria were placed in an organ bath containing Krebs-Ringer solution, 30 degrees C, pH 7.4, and bubbled with a gas mixture of 95% O2 and 5% CO2. The atrial rate and contractile force were simultaneously recorded. Addition of toxin Ts-gamma to the bath (0.14 microM) evoked an initial reduction of both atrial rate and contractile force, followed by a small increase in force and a decrease in rate, and finally a long reduction of rate and force. Addition of an identical dose of Ts-gamma 30 or 60 min later did not evoke any effect. Addition of tityustoxin to the bath (0.14 microM) induced an increase of atrial rate and force. Addition of an identical dose of tityustoxin 30 min later evoked similar effects. The negative chronotropic and inotropic effects induced by Ts-gamma were abolished by tetrodotoxin (TTX, 1 microM) or atropine (1.5 microM), whereas the positive effects observed in the presence of atropine were prevented by TTX (1 microM) or alprenolol (10 microM). The negative chronotropic effect of 0.14 microM tityustoxin was only observed in the presence of physostigmine (0.3 microM). This negative effect was abolished by TTX (1 microM) or atropine (1.5 microM). The positive inotropic effect of tityustoxin was decreased by TTX (1 microM and 10 microM), but was totally prevented by guanethidine (10 microM) or alprenolol (10 microM).(ABSTRACT TRUNCATED AT 250 WORDS)