Potassium contracture and calcium influx in frog's skeletal muscle

Hrvoje Lorkovi ¹

¹ Institute for Medical Research, Yugoslav Academy of Sciences and Arts, Zagreb, Yugoslavia

The height and the duration of the K contracture of frog's sartorii (Rana esculenta) is a function of extracellular calcium. The rate of inhibition of the K contracture in a Ca-free solution and the rate of recovery from such inhibition in a Ca-containing solution measured by the percentage increase and decrease of the height of the K contracture are not alike, the recovery being quicker. The apparent diffusion constant of Ca as calculated from recovery data is about ten times, in Na-free solutions about 100 times, smaller than that calculated from inhibition data. The additional Ca influx connected with contractures, measured by Ca⁴⁵ entry, is almost proportional to the extracellular Ca concentration. However, the size of K contractures of and the Ca influx into muscles soaked in Ca-free nitrate solutions suggests that both the "removable" as well as the "more firmly surface-bound" calcium take part in the K contracture.

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