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 Na²⁴ and then immersed in choline Ringer and the efflux of Na²⁴ followed for 4 hours. The initial efflux of Na⁺ appeared to be 17 pmole/cm² sec.; this value was maintained for 20 minutes and was followed by an abrupt decline to about 9 pmole/cm² 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/cm² 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.