Effects of extracellular calcium concentrations on cardiac muscle in selected vertebrates.

Hypercalcaemia was induced artificially in American eels, Anguilla rostrata LeSueur, by infusing CaCl$\sb2.$ The treatment did not have any significant effect on recorded blood pressure or heart rate. Considering these data I concluded that, at least in vivo, the eel heart is apparently insensitive to physiological increases in plasma ultrafiltrable calcium concentration. The effects of changes in external calcium concentrations on the strength of contraction of isolated electrically-paced heart strips were also measured in rats, frogs, rainbowtrout and American eels. This was done to test the hypothesis that eel cardiac tissue demonstrates a relatively reduced sensitivity to changes in extracellular calcium concentration. The data obtained, however, showed that contrary to the hypothesis, the eel cardiac strips were generally more sensitive than those from the other animals tested. As a result the null hypothesis that eel hearts do not have reduced sensitivity to changes in extracellular calcium concentrations in vitro was accepted. In fact, on the basis of the in vitro data, one should conclude that eel cardiac tissue is very sensitive to extracellular calcium concentrations. This conclusion is confounded by the observation that the uptake rate of $\sp{45}$Ca measured in contracting and non-contracting eel heart muscle strips did not differ, an observation that suggests that the eel heart relies more on internal than on external calcium stores for the development of tension. We are thus left with an indeterminate situation. That is, whereas eel cardiac tissue is very sensitive to changes in extracellular calcium concentrations in vitro, in vivo the heart is able to mitigate the effects of alterations in extracellular (plasma) calcium concentrations. Future studies should focus on how this mitigation. or at least the attenuation, of the effect of extracellular calcium on cardiac function in vivo is obtained