The effect of phosphate and calcium on force generation in glycerinated rabbit skeletal muscle fibers. A steady-state and transient kinetic study

N C Millar,E Homsher

doi:10.1016/s0021-9258(17)30494-5

Abstract

The effect of varying concentrations of Pi and Ca2+ on isometric force and on the rate of force development in skinned rabbit psoas muscle fibers has been investigated. Steady-state results show that the three parameters that define the force-pCa relation (Po, pK, and n) all vary linearly with log [Pi]. As [Pi] increases, Po and pK decrease while n increases. The kinetics of force generation in isometrically contracting fibers were studied by laser flash photolysis of caged phosphate. The observed rate of the resulting tension transient, kPi, is 23.5 +/- 1.7 s-1 at 10 degrees C, 0.7 mM Pi, and is independent of [Ca2+] over the range pCa 4.5-7.2. By contrast, kTR, the rate of tension redevelopment following a period of isotonic shortening, is sensitive to [Ca2+] and is slower than kPi (kTR = 13.6 +/- 0.2 s-1 at pCa 4.5, 0.7 mM Pi). The results show that [Ca2+] does not directly affect the Pi release or force-generating steps of the cross-bridge cycle and show that the observed rate of force development depends on how the measurement is made. The data can be interpreted in terms of a model in which strong cross-bridges activate the thin filament, this activation being modulated by Ca2+ binding to troponin.

Highlights

IntroductionThe terms strong cross-bridge state and switched on state can often be considered interchangeable, as can weak cross-bridge state and switched off state
The kinetics of force generation in isometrically contracting fibers were studied by laser flash photolysis of caged phosphate
Kawai et al (1981) found no change in the apparent rate constants of the three exponential processes which correspond to phases 2-4 of a Huxley-Simmons tension transient with [Ca”] and so concluded that Ca*+ acts as a switch, recruiting more active cross-bridges

Summary

Introduction

The terms strong cross-bridge state and switched on state can often be considered interchangeable, as can weak cross-bridge state and switched off state This terminology emphasizes the idea that these properties are not those of any individual myofibrillar protein but those of the contractile assembly as a whole. Steady-state measurements such as ATPase or the force-velocity relation (Podolsky and Teicholz, 1970; Julian, 1971) cannot resolve a graded or all-or-none effect on an individual step on the pathway, nor can they identify the step being controlled. To resolve this question one must use a transient kinetic approach, and so far two such techniques have been used. In contrast Brenner (1988) found that /+a,’ the rate of tension redevelopment following a period of rapid isotonic shortening with immediate restretch, slowed at low [Ca”‘]. Brenner (1988) concluded that Ca2+ controlled the force-generating step in a graded way

Methods

Results

Conclusion