title: Action potential duration and contraction after rest at room temperature in guinea pig papillary muscle.
abstract: 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.
authors.affiliation: Departamento de Fisiología, Facultad de Medicina, Montevideo, Uruguay.
authors.forename: E R
authors.initials: ER
authors.lastname: Migliaro
authors.forename: M
authors.initials: M
authors.lastname: Michelini
authors.forename: H N
authors.initials: HN
authors.lastname: Durán
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D000818:Animals; D005260:Female; D006168:Guinea Pigs; D007035:Hypothermia; D066298:In Vitro Techniques; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D009200:Myocardial Contraction / Q000502:physiology*; D010210:Papillary Muscles / Q000187:drug effects / Q000502:physiology*; D013997:Time Factors