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Extra activation component of calcium release in frog muscle fibres

Paul C. Pape, Karine Fénelon and Nicole Carrier

In addition to activating more Ca²⁺ release sites via voltage sensors in the t-tubular membranes, it has been proposed that more depolarised voltages enhance activation of Ca²⁺ release channels via a voltage-dependent increase in Ca-induced Ca²⁺ release (CICR). To test this, release permeability signals in response to voltage-clamp pulses to two voltages, -60 and -45 mV, were compared when [Ca²⁺] was decreased in two kinds of experiments. (1) Addition of 8 mM of the fast Ca²⁺ buffer BAPTA to the internal solution decreased release permeability at -45 mV by > 2-fold and did not significantly affect Ca²⁺ release at -60 mV. Although some of this decrease may have been due to a decrease in voltage activation at -45 mV - as assessed from measurements of intramembranous charge movement - the results do tend to support a Ca-dependent enhancement with greater depolarisations. (2) Decreasing SR (sarcoplasmic reticulum) Ca content ([Ca_SR]) should decrease the Ca²⁺ flux through an open channel and thereby [Ca²⁺]. Decreasing [Ca_SR] from > 1000 µM (the physiological range) to < 200 µM decreased release permeability at -45 mV relative to that at -60 mV by > 6-fold, an effect shown to be reversible and not attributable to a decrease in voltage activation at -45 mV. These results indicate a Ca-dependent triggering of Ca²⁺ release at more depolarised voltages in addition to that expected by voltage control alone. The enhanced release probably involves CICR and appears to involve another positive feedback mechanism in which Ca²⁺ release speeds up the activation of voltage sensors.

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