Abstract
1. An electron microscopic method has been developed and used to study cation binding sites in smooth muscle.2. Uranyl cations normally bind to the external surface of the plasma membrane. However, uranyl also binds to the inner surface when it is accessible. Similar binding has been observed in skeletal muscle, nerve and red blood cells.3. Uranyl binds electrostatically, and the binding can be competitively reversed by other cations. By a quantitative procedure the relative affinities Ca(2+) approximately Mg(2+) >> K(+) > Na(+) for the membrane sites have been determined. This sequence is in agreement with previous values determined analytically.4. The results support a counter-cation hypothesis for the plasma membrane surfaces of the taenia coli, and may explain features of the electrical activity in smooth muscle.