An analysis of calcium-magnesium antagonism in contractility and ionic balance in isolated trabecular muscle of rat ventricle.

The influence of elevated Mg (20mM) on mechanical response fibre electrolyte composition and Ca-exchange, as a function of external Ca concentration, has been studied in an isolated trabecular muscle column of rat ventricle. At [Ca]o 2.5 mM, cellular Mg-content increased in 20 mM [Mg]o without effect on contractility or fibre content of other electrolytes. As [Ca]0 was reduced from 2.5 mM, 20 MM Mg caused progressive inhibition of contractile response of muscle to electrical stimulation. In both resting and stimulated trabeculae the intrafibre Mg-content rapidly increased and Ca-content fell in the initial 20 min incubation in reduced (1.5 or 0.75 mM) Ca, as the contractile response declined. Subsequent restoration of [Ca]0 to 2.5 mM restored contractile response in the presence of high Mg concentration. Exposure of trabeculae to high Mg also caused a significant decrease in 45-Ca exchange in a muscle calcium-pool exchanging with t1/2 7 min when [Ca]0 was 1.5 mM, but had no effect on Ca-exchange when [Ca]0 was 2.5 mM. The effect of high Mg on exchange of Ca indicated that displacement of a fraction of superficially-bound muscle Ca was responsible for the diminished contractile response in Mg-loaded trabeculae.