The concentration dependence of sodium efflux from muscle.

Frog sartorius muscles subjected to overnight loading with Na(+) in K-free Ringer in the cold were subsequently labeled with Na(24) and then immersed in choline Ringer and the efflux of Na(24) followed for 4 hours. The initial efflux of Na(+) appeared to be 17 pmole/cm(2) sec.; this value was maintained for 20 minutes and was followed by an abrupt decline to about 9 pmole/cm(2) sec. This latter rate was maintained for the next 20 minutes of efflux. The efflux then declined gradually with time and reached values of the order of 0.1 pmole/cm(2) sec. The back addition of counts lost from muscles enabled one to calculate the relationship between efflux and [Na](i) for muscle. This roughly approximates an S-shaped curve with a value at half-saturation of about 17 mmole Na per liter of fiber water. The efflux-concentration curve is closely described by assuming that 3 Na(+) are transported per carrier cycle.