Significance of Changes in Intracellular Ca²⁺ for the Mechanism of Signal Transduction in Vertebrate Rod Cells

Abstract

Light and cyclic GMP stimulate the flux of Ca²⁺ ions across the plasma and disk membrane in rod cells of the vertebrate retina. Ca²⁺ enters the cytosol through the light-sensitive channels in the plasma membrane and a cyclic GMP-regulated conductance in the disk membrane. Ca²⁺ is extruded from the cell by an Na⁺/Ca²⁺ exchange mechanism. The existence of an active, ATP-dependent uptake of Ca²⁺ into disks is likely; a Ca²⁺-transport ATPase in the plasma membrane, however, has not yet been identified. Some of these transport systems may be directly or indirectly regulated by light. The individual contributions of each transport system to the maintenance of the cytosolic Ca²⁺ concentration in the dark and its change by light have not yet been delineated. In particular, attempts to detect the rapid injection of Ca²⁺ (<100 ms) from inside disks into the cytosol — a crucial component of the “Ca²⁺ hypothesis” — have been unsuccessful. A final decision on the relevance of changes in Ca²⁺ _i for the generation of the electrical signal must be postponed until the much needed measurements of intracellular Ca²⁺ in these cells are available.