Increase of Labeled Calcium Uptake in Heart Muscle during Potassium Lack Contracture

Analyses of ashed muscle tissue show that the uptake of Ca(45) by isolated frog heart ventricles from normal Ringer's solution containing 1 mM Ca reaches a maximum value in about 30 minutes of perfusion which is not exceeded after 3 hours of perfusion. The average amount of this labeled Ca taken up from normal Ringer's is 0.7 mM/kg. wet weight of muscle. In contrast to this, the amount of labeled Ca taken up by ventricles perfused with K-free Ringer's increases at a linear rate over a 60 minute period to twice the normal value coinciding with the gradual development of contracture and coinciding with a cellular K loss and Na gain of about 30 mM/kg. How much of the extra labeled Ca taken up from K-free Ringer's represents a net gain in cellular Ca content is not known. However, evidence has been obtained that some of this labeled Ca enters an intracellular compartment. EDTA in K-free Ringer's solution causes relaxation of ventricles in contracture and also renders the muscle fibers indiscriminately permeable. This indicates that a combination of Ca with sensitive intracellular sites is probably the cause of the K lack contracture.