Mechanical response of the frog's rectus to potassium. Two phases of the KCl contracture

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

The mechanical response of rectus abdominis muscles of Rana esculenta to potassium ions (5.6–23.5 mm in Ringer's solution) shows two distinct phases. The twitch fiber reaction, giving rise to the first phase of the response curve, does not last more than 1–2 minutes. It can be provoked with KCl concentrations of the order of 5–7 mm; its intensity is not much influenced by increasing KCl concentrations higher than 10 mm; its intensity is reduced by high CaCl₂ concentrations, and also by shortening the length of the interval between two exposures of the muscle to increased KCl concentrations. The slow fiber reaction, giving rise to the second phase, is a true tonic contracture which can be obtained only with concentrations of KCl larger than 10 mm. Changes in the CaCl₂ concentration and in the length of the interval do not affect it so much as the first phase.