title: Effect of prolonged depolarizations on twitch tension and intracellular sodium activity in sheep cardiac Purkinje fibres.
abstract: 1. Twitch tension and intracellular Na+ activity (aiNa) were measured in voltage-clamped sheep cardiac Purkinje fibres. aiNa was measured using Na+-sensitive micro-electrodes filled with the liquid ion exchange resin. ETH 227. The stimulus for contraction was a constant 200 ms depolarizing pulse to 0 mV from a holding potential of -80 mV delivered at 0.25 Hz. Prolonged test pulses for 1.8 s (post-pulses) were applied at the end of the stimulus pulse. The effects of post-pulses on twitch tension and aiNa were examined. 2. Post-pulses in the range of -40 mV reduced twitch tension below control force produced without post-pulse. Progressively more positive post-pulses to levels above 0 mV profoundly increased twitch tension, with a greater than 400% rise in tension at +50 to +60 mV compared to control tension. aiNa declined at positive post-pulse potentials by more than 2 mM at +30 to +40 mV. 3. Tetrodotoxin (100 microM) did not affect the post-pulse voltage-tension or voltage-aiNa relation. Ca2+ channel modulation with nitrendipine (1 microM) similarly did not alter the post-pulse voltage-tension relation. 4. Removal of extracellular Na+ eliminated the nadir in tension at post-pulses to -40 mV and the augmentation of tension at post-pulses above 0 mV. 5. We interpret these findings as evidence of voltage-sensitive Na-Ca exchange promoting net Ca2+ influx and net Na+ efflux during positive post-pulses. The unusual shape of the post-pulse voltage-tension relation curve can be accounted for by a charged-carrier model of electrogenic Na-Ca exchange. The inverse relation between aiNa and twitch tension probably reflects the combined effects of reduced aiNa leak and changes in Na+ and Ca2+ flux via voltage-sensitive Na-Ca exchange.
mesh_terms: D000818:Animals; D002118:Calcium / Q000502:physiology; D006329:Heart Conduction System / Q000502:physiology*; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism*; D008564:Membrane Potentials / Q000187:drug effects; D009200:Myocardial Contraction / Q000187:drug effects*; D009568:Nitrendipine / Q000494:pharmacology; D011690:Purkinje Fibers / Q000378:metabolism / Q000502:physiology*; D012756:Sheep; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology
chemical_list: D007473:Ion Channels; D013779:Tetrodotoxin; D009568:Nitrendipine; D012964:Sodium; D002118:Calcium