Effects of calcium on the intensity–response curve of toad rods

Abstract

THERE is evidence that Ca²⁺ is stored in the saccules in the outer segments of vertebrate rods, and released by light (for review, see ref. 1). Thus, light apparently increases [Ca²⁺]_i, the cytoplasmic Ca²⁺ concentration. It now seems unlikely that this increase of [Ca²⁺]_i serves as an internal transmitter in generating the light response^2–4. However, there is compelling evidence in the ventral photoreceptors of Limulus^5,6 that it reduces photoreceptor sensitivity during light adaptation. In vertebrate rods an alteration of [Ca²⁺]_o, the external Ca²⁺ concentration, alters [Ca²⁺]_i, and the primary effects of changing [Ca²⁺]_o seem to be exerted inside the cell^1,7,8. Increased [Ca²⁺]_o mimics certain aspects of light adaptation, including hyperpolarisation of the resting membrane potential and reduction of light responses^9–11. Light adaptation also shifts the entire intensity–response function (the V-log I curve) towards higher intensities on the intensity axis, thus reducing the sensitivity of toad rods¹¹. We report here that increased [Ca²⁺]_o can mimic this important aspect of light adaptation, whereas lowered [Ca²⁺]_o mimics dark adaptation. Control experiments indicate why altered [Ca²⁺]_o did not shift the V-log I curve in the recent work of Lipton, Ostroy and Dowling¹¹, who thus concluded that Ca²⁺ does not control this major aspect of photoreceptor sensitivity.