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

E R Migliaro, M Michelini, H N Durán
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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.

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豚鼠乳头肌在室温下静息后动作电位持续时间和收缩。
同时记录右乳头肌的动作电位和等长张力。在1 Hz的常规刺激后,允许暂停10分钟。在休息后的第一次搏动中,我们测量了90%复极时的动作电位持续时间(APD90)、最大抽搐张力(T)、收缩峰值时间(TTP)和张力(+dT/ dT)和松弛(-dT/ dT)的发展速度。根据休息前的值进行归一化。35℃休息后APD90和TTP延长,T降低,25℃休息后无明显变化。休息后+dT/ dT较慢,dT/ dT无明显变化。硝苯地平10微米阻止休息后APD90延长,并产生进一步降低机械反应。用Li+替代外部Na+缩短APD90,增加正常或休息后心跳的T,导致钙超载征象。当钠离子置换过程暂停时,钙超载迹象减弱。我们认为,休息和室温联合作用主要通过Na+/Ca++机制来降低[Ca++]i。[ca++]i的减少反过来又可以延缓iCa的失活。结果,获得较长的ap,伴随着较弱和较慢的机械反应。TTP和+ dT/ dT的变化可能表明室温下休息后收缩依赖于细胞外Ca++,而不是RS的变化。
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