Radioactive calcium influx at rest and during potassium contracture in the T-disrupted, and the urea- or manganese-treated frog sartorius muscles.

45Ca influx in the intact, the T-disrupted, and the urea-treated or Mn++-treated frog sartorius muscles was measured at rest and during potassium contracture. In the intact muscle, Ca influx increased with the increasing of potassium concentration in the bathing solution, and the time course of the Ca influx showed considerable correspondence with that of the potassium contracture. This increased Ca influx was divided into two components: early rapid Ca influx and late slow Ca influx. The amount of early rapid Ca influx at higher concentrations of potassium occupied about 60% of the total Ca influx. The early rapid Ca influx markedly decreased in the T-disrupted and in the urea-treated or Mn++-treated muscles. On the other hand, the late slow Ca influx and the resting Ca influx in these muscles changed only slightly. These results suggest that the early rapid Ca influx occurs mainly through the membrane of the T-system, particularly at the triadic junctional area of muscle fiber, while the late slow Ca influx and the resting Ca influx occur mainly through the surface membrane.