Action potential duration and contraction after rest at room temperature in guinea pig papillary muscle.

Simultaneous recordings of action potential and isometric tension of right papillary muscles were performed. After regular stimulation at 1 Hz, pauses of 10 min were allowed. In the first beat after rest, we measured action potential duration at the 90% of the repolarization (APD90), maximal twitch tension (T), time to peak of contraction (TTP), and rate of development of tension (+dT/dt) and relaxation (-dT/dt). Values were normalized against pre-rest ones. No significative changes were observed after rest at 35 degrees C. After rest at 25 degrees C APD90 and TTP were prolonged but T was reduced. Post-rest +dT/dt were slower, dT/dt did not show significative changes. Nifedipine 10 microM prevented post-rest APD90 lengthening, and produced a further reduction of mechanical response. Substitution of external Na+ by Li+ shortened APD90, increased T of either regular or post-rest beats and led to calcium overload signs. When pause were allowed during Na+ substitution, calcium overload signs were attenuated. We conclude that the combination of rest and room temperature diminished [Ca++]i mainly by Na+/Ca++ mechanism. The reduction of [Ca++]i in turn could delay the inactivation of iCa. As a consequence, longer APs were obtained, accompanied by weaker and slower mechanical responses. Changes in TTP and + dT/dt could suggest that post-rest contractions in room temperature, are dependent of extracellular Ca++ rather than a deplected RS.