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Myonemal contraction of Spirostomum. III. The thermal dependence of contraction, relaxation and excitation-contraction coupling

✍ Scribed by R. B. Hawkes; D. V. Holberton

Publisher
John Wiley and Sons
Year
1976
Tongue
English
Weight
678 KB
Volume
87
Category
Article
ISSN
0021-9541

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✦ Synopsis

Abstract

A microphotometric technique that displays rapid length changes of Spirostomum has been used to follow the variation with temperature of three kinetic parameters of myonemal contraction: contraction rate, relaxation rate and stimulus duration at threshold. In each case the exponential form of the relationship indicated that the gross rate constant might be equated with the limiting rate constant, k, of a driving chemical reaction, and from standard expressions of chemical kinetics the change in activation free energy appropriate to this reaction has been computed.

The thermal dependence of contraction is described by an activation enthalpy (ΔL__H__‡) of 21.7 kcal mol^−1^, and the activation entropy (ΔL__S__‡) of 26.8 e.u. is consistent with a model of contraction requiring neutralization of fixed myonemal charges by divalent cations.

The analysis of thermal dependence of relaxation gives a negative activation entropy, a result predicted for a rate‐limiting reaction involving dissociation of a neutral molecule. On the other hand, values of ΔL__S__‡ and ΔL__H__‡ for relaxation fall close to an isokinetic correlation drawn in the literature from analysis of the thermal dependence of ciliary beat frequency in different organisms, and for which breakdown of an ATP‐ATPase complex could be the common rate‐limiting reaction.

ΔL__S__‡ for stimulus duration suggests that the rate‐limiting step in excitation‐contraction coupling is a reaction between ions of like charge, or ion pair formation from a neutral molecule.

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