Abstract
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1.
A quick release of an isometrically contracting cat papillary muscle results in a depression of the ability to redevelop tension (deactivation) and an increase in the duration of the accompanying action potential (prolonged depolarization). The nature of the mechanical perturbation influencing both phenomena was investigated.
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2.
The prolongation of the action potential depends on the amplitude of the release and the time it is applied and, provided quick release-quick restretch cycles of less than 50 ms are used, on the duration of the cycle.
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3.
No change in action potential duration is observed, if initial muscle length, or the velocity of shortening is altered, or if the muscle is stretched at any time during contraction.
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4.
Although stretches and releases both have a “deactivating” effect on the muscle the effect is more pronounced with releases. This difference in “deactivation” is related to the prolongation of the action potential in so far as it is also controlled by the time and extent of release and release-restretch cycle duration, and is independent of shortening velocity.
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5.
Caffeine (8 mmol/l) in the bathing solution prolongs isometric tension development whilst the duration of the action potential is relatively unchanged. Under these conditions release-restretch cycles applied at times when the membrane has apparently repolarized, produce a deactivation and an after depolarization which can reach threshold to elicite an action potential.
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6.
If the membrane is partially depolarized by increasing extracellular potassium to 20 mmol/l, release-restretch cycles still induce deactivation but no change in the action potential.
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7.
The results are in keeping with the hypothesis that shortening during contraction partly contributes to the deactivating effect by reducing the concentration of internal free ionic calcium. This change in [Ca]i decreases the outward potassium current to produce a prolongation of depolarization which can take the form of an increase in action potential duration or an afterdepolarization wave.
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This work has been supported in part by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 30, Kardiologie
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Hennekes, R., Kaufmann, R. & Lab, M. The dependence of cardiac membrane excitation and contractile ability on active muscle shortening (Cat papillary muscle). Pflugers Arch. 392, 22–28 (1981). https://doi.org/10.1007/BF00584577
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DOI: https://doi.org/10.1007/BF00584577