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

Abstract

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 Qγ component (delayed 'hump' component) of the intramembranous charge movement current (Icm) results from movement of these voltage sensors. Ca2+ release and Icm 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 (Qβ) and Qγ 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 (κ) and half-maximal voltage (V̄) for Qγ 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 κ and V̄ of 4.7 and -45.3 mV, respectively. The similarity of the values of κ for Qγ and Ca2+ release supports the idea that Qγ reflects movement of voltage sensors for Ca2+ release. The greater value of V̄ for Ca2+ release compared to Qγ is consistent with multi-state models of the voltage sensor involving movement of Qγ charge during non-activating transitions.