Effects of Cadmium on the Contractility of a Frog Cardiac Muscle in Relation to pH of External Solution

Abstract

In order to examine the pH dependency of Cd effects on cardiac muscle, the electrical and mechanical activities in the bullfrog heart were investigated in relation to various external pHs and concentrations of Cd. The amplitude and duration of the action potential or the spontaneously beating frequency in the atrium were not significantly affected during the 3-min administration of various concentrations of Cd in a range from pH 6 to 10, but the isotonic contraction at the end of the same period was greatly altered: the lower the external pH the more Cd decreased the contractility of the atrium. This pH dependency of Cd effects was also observed in SO4-Ringer's solution whose anions, SO4, were more impermeable than Cl. Decrease in contractility in CdRinger's solution was counteracted by excess Ca. The lower the pH of the Cd-Ringer's solution, the more Ca was necessary to counteract an equal amount of Cd. The amount of Cd-uptake into the atrium was analyzed after soaking the atrium in various concentrations of Cd-Ringer's solution. The higher the concentration of external Cd, the larger the Cd-uptake into the atrium. No pH dependency, however, was observed in this relationship. This suggests that probably only a small fraction of total Cd-uptake interacts with Ca-binding sites which is specific to contraction, and causes the Cd-induced decrease in contractility. This process is considered to be pH-dependent. However, most Cd-uptake into cardiac muscle is pH-independent and may be nonspecific to contraction.Effects of cadmium (Cd) on the electrical and mechanical activities of cardiac muscle in normal pH solution have been studied (KLEINFELD et al., 1955;TAKAHASHI et al., 1956;KLEINFELD and STEIN, 1968;HAYASHI and HORIUCHI, 1971a;TODA, 1973). It was found that a low concentration of Cd (about 10 /AM) brought about a decrease in contractility without an appreciable change in the action potential, and this Cd effect on contractility was completely antagonized by excess Ca (HAYASHI and HORIUCHI, 1971a). It was also reported

Effects of cadmium on the slow inward current of frog heart muscle in relation to a lowering of pH in external solution

“…Moreover, we observed that lowering the external pH altered neither the efficiency of Cd inhibition on Ij nor the affinity of Cd for the site. The present results might account for the increase in the negative inotropic effect of Cd in low pH solution reported by Horiuchi & Hayashi (1979), since low pH media is shown to reduce markedly the maximal Ca entry while the ability of Cd to block I~j is unchanged. This might also suggest that the blocking effect of H and Cd ions on Isj was additive.…”

“…Cd ions have been found to be a selective blocker of the slow inward current (Is;) in various species (Kostyuk et al, 1977;Lee & Tsien, 1982; Giles et al, 1983;Josephson et al, 1984;Gautier et al, 1987). In addition, H ions, known to depress ISi in heart muscle (Chesnais et al, 1975), have also been found to cause a dramatic enhancement of the negative inotropic effect of Cd in frog cardiac muscle mechanical activity (Horiuchi & Hayashi, 1979). These findings prompted us to investigate the combinative effect of H and Cd ions on Is; in frog cardiac tissue.…”