Contraction of muscle in altered ionic environment

Abstract

This thesis contains three experimental sections concerned with changes that occur in the twitch response of mammalian muscle to alterations in the local environment of the cell. The isolated rat diaphragm bathed in Synthetic Interstitial Fluid was used as the test preparation. GENERAL INTRODUCTION: The processes involved in the elicitation of a mechanical response from a muscle stimulated electrically, are reviewed. The areas of this process - Excitation Contraction Coupling - that are of particular relevance to this study are defined as the sarcolemma membrane, and the sarcotubular system. SECTION A: The effects of temperature on the twitch response of the preparation to direct stimulation, have been observed. Parameters of the isometric myogram ie. Tension, Contraction Time and Half Relaxation Time were assessed at temperatures from 5 to 45°C and were compared with similar literature values. The twitch demonstrates increased speed of both contraction and relaxation as temperature rises while twitch tension displays peaks near 15 and 40 °C. The rat diaphragm is a muscle consisting of similar proportions of both slow and fast fibres which demonstrate differential temperature dependence. The peak in twitch tension near 15°C is associated with the fast muscle fibres, while the slow muscle component is responsible for the anomalous peak near 40 °C. SECTION B: The effects of some potential stabilising agents were assessed on the impermeant anion, 'chloride free’ treated preparation. This preparation simulates the responses of myotonic muscle. High K⁺ (25 mM), while it did not produce contracture, eliminated the delayed relaxation of mechanical activity normally observed when the impermeant anion was substituted for chloride ion, provided it was added prior to Cl⁻ replacement. γ-amino-butyric acid (GABA) and glycine were ineffective in opposing the chloride free effect which became apparent in this preparation when Cl⁻ concentration in the bathing solution was less than 42 mM. SECTION C: 2,4-Dichlorphenoxyacetate (2,4-D) and 20,25-Diazacholesterol (DAG) treated muscle was examined for evidence of in vitro myotonic mechanical activity. Interaction of these two drugs produced significant myotonia - delayed relaxation in response to single stimuli. Short trains of stimuli were effective in eliciting myotonic responses from 2,4-D treated muscle; these were opposed by addition of GABA to the bathing solution. Since one significant myotonic effect of 2,4-D (and DAG) is to cause a decrease in muscle membrane resting chloride conductance, the GABA effect may indicate a small specific action of this amino acid to increase chloride conductance in skeletal muscle, thereby acting as a weak though specific myotonic membrane, stabiliser.