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

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 γ component (delayed [hump] component) of the intramembranous charge movement current ( I cm ) results from movement of these voltage sensors. Ca 2+ 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 Ca 2+ feedback mechanisms, the calcium content of the sarcoplasmic reticulum (SR) during rest was reduced to < 250 μ m (referred to volume of myoplasm) and maintained approximately constant. The early ( Q β ) and Q γ 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 γ were 4.3 and −57.4 mV ( n = 6 ), respectively. The SR membrane permeability for Ca 2+ release was assessed when a constant amount of calcium remained in the SR (usually about 60 μ m ). 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 γ and Ca 2+ release supports the idea that Q γ reflects movement of voltage sensors for Ca 2+ release. The greater value of V‐ for Ca 2+ release compared to Q γ is consistent with multi‐state models of the voltage sensor involving movement of Q γ charge during non‐activating transitions.