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Calcium release and intramembranous charge movement in frog skeletal muscle fibres with reduced (< 250 µM) calcium content

Paul C. Pape and Nicole Carrier

It is generally accepted that activation of voltage sensors in the T-tubular membranes is a critical step of excitation-contraction coupling in skeletal muscle. The purpose of this study was to evaluate further whether the Q component (delayed 'hump' component) of the intramembranous charge movement current (I_cm) results from movement of these voltage sensors. Ca²⁺ release and I_cm were measured in voltage-clamped frog cut fibres mounted in a double Vaseline-gap chamber. In order to reduce effects of Ca²⁺ feedback mechanisms, the calcium content of the sarcoplasmic reticulum (SR) during rest was reduced to < 250 µM (referred to volume of myoplasm) and maintained approximately constant. The early (Q) and Q components of charge movement were estimated by fitting the sum of two Boltzmann functions to the total steady-state intramembranous charge vs. voltage data. The average voltage steepness factor (k) and half-maximal voltage (V-) for Q were 4.3 and -57.4 mV (n = 6), respectively. The SR membrane permeability for Ca²⁺ release was assessed when a constant amount of calcium remained in the SR (usually about 60 µM). A single Boltzmann function fitted to these data gave values on average for k and V- of 4.7 and -45.3 mV, respectively. The similarity of the values of k for Q and Ca²⁺ release supports the idea that Q reflects movement of voltage sensors for Ca²⁺ release. The greater value of V- for Ca²⁺ release compared to Q is consistent with multi-state models of the voltage sensor involving movement of Q charge during non-activating transitions.

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