ZINC-INDUCED ALTERATIONS IN CONTRACTILE PROPERTIES OF RAT DIAPHRAGM MUSCLE IN-VITRO

The effect of zinc ions on the isometric contraction of rat diaphragm muscles in the presence and in the absence of external calcium was stu died. Using a transducer, the isometric force was measured as a functi on of supramaximal electrical stimulation, either directly or indirect ly applied to the muscle. The following parameters were measured: peak twitch tension, PT, twitch contraction time, CT, relaxation half-time , RT-1/2, and peak rates of tension increase and decrease, +dP/dt and -dP/dt. The following zinc-induced alterations were observed: an incre ase of the PT; a decrease of the RT-1/2; an increase in the +dP/dt and -dP/dt. The CT was not changed significantly. Our results suggest tha t zinc ions have a positive inotropic effect on isolated diaphragm mus cle. The increase in PT may be explained by a zinc-activated Ca2+ upta ke by sarcoplasmic reticulum. This was followed by an increase in the rate of rise of tension development, which was secondary to increased -dP/dt. The mechanism(s) by which extracellular Ca2+ contributes to th is action of zinc is not known.