MERCURY EFFECTS ON THE CONTRACTILE ACTIVITY OF ISOLATED HEART-MUSCLE

The toxic effects of HgCl2(1, 2.5, 5, and 10 mu M) were studied in iso lated, isometrically contracting rat papillary muscles and frog ventri cular strips. In rat papillary muscles 1 mu M Hg2+ produced a small in crease in the force of contraction. Higher concentrations of HgCl2 pro duced a dose-dependent decrease in contractile force. The rate of forc e development was affected differently, increasing at 1 and 2.5 mu M H g2+ and decreasing to control levels at 5 and 10 mu M Hg2+. This was t he result of a progressive reduction in the time to peak tension obser ved when HgCl2 concentrations increased. This effect probably reflects the binding of Hg2+ to Sc groups inducing Ca2+ release from the sarco plasmic reticulum. The relative potentiation of postrest contractions was used as an index of sarcoplasmic reticulum activity. It was measur ed after pauses of increasing duration and was reduced at concentratio ns of 1 mu M Hg2+ when compared to that of the control. A further decr ement in the relative potentiation was observed with higher Hg2+ conce ntrations, indicating that the activity of the sarcoplasmic reticulum was depressed by mercury in a dose-dependent manner. Tetanic contracti ons were also studied in the rat myocardium. The tetanic tension did n ot change during treatment with 1 mu M Hg2+ hut decreased with 5 mu M Hg2+, suggesting a toxic effect on the contractile proteins only at hi gh Hg2+ concentrations. Frog ventricu; lar strips were studied using t he same HgCl2 concentrations and no effects on either force or relativ e potentiation were observed. These findings suggest that Hg2+ promote s dose-dependent toxic effects on heart muscle via actions on the sarc olemma, the sarcoplasmic reticulum, and contractile proteins. (C) 1994 Academic Press, Inc.