Ionic permeabilities of the plasma membrane of isolated intact bovine rod outer segments as studied with a novel optical probe

Summary

The permeability properties of the plasma membrane of intact rod outer segments purified from bovine retinas (ROS) were studied with the aid of the optical probe neutral red as described in the companion paper. The following observations were made: (1) Electrical shunting of ROS membranes greatly stimulated Na⁺ and K⁺ transport, suggesting that this transport reflects Na⁺ and K⁺ currents, respectively. The dissipation of a Na⁺ gradient across the plasma membrane occurred with a half-time of 30 sec at 25°C. (2) The Na⁺ permeability was progressively inhibited when the external Ca²⁺ concentration was raised from 1 μm to 20mm. A similar Ca²⁺ dependence was observed for H⁺ and Li⁺ transport. The Na⁺ permeability was not affected when the total internal Ca²⁺ content of ROS was varied between 0.1 mol Ca²⁺/mol rhodopsin and 7 mol Ca²⁺/mol rhodopsin, or when the free internal Ca²⁺ concentration was varied between 0.1 and 50 μm. (3) The K⁺ permeability was progressively stimulated when the external Ca²⁺ concentration was raised from 0.001 to 1 μm, whereas a further increase to 20mm was without effect. A similar Ca²⁺ dependence was observed for Rb⁺ and Cs⁺ transport. (4) At an external Ca²⁺ concentration in the micromolar range the rate of transport decreased in the order: Na⁺>K⁺=H⁺>Cs⁺>Li⁺. (5) Na⁺ fluxes depended in a sigmoidal way on the external Na⁺ concentration, suggesting that sodium ions move in pairs. The concentration dependence of uniport Na⁺ transport and that of Na⁺-stimulated Ca²⁺ efflux (exchange or antiport transport) were very similar.