Evaluation of Internal Transmitter Candidates: Ca

Abstract

This paper reviews the evidence for and against the “calcium hypothesis,” which proposes that light releases Ca from the discs of vertebrate photoreceptors and that the light-sensitive channels are closed by combination with this Ca. In favor of this hypothesis are the observations that when Ca is applied either externally or internally the light-sensitive current is reduced, and that a net efflux of Ca from the cell occurs after a flash of light, as one would expect if Ca is released by light and is subsequently pumped from the cell. More recent observations show that an abrupt rise in internal Ca does not produce an abrupt closure of channels. The slow step intervening between a rise in internal Ca and channel closure is inconsistent with a simple form of the Ca hypothesis but is consistent with a model where channels are maintained open by cyclic GMP, and where Ca inhibits the cyclase responsible for producing cyclic GMP. In this model light closes channels by activating the phospho-diesterase responsible for breaking down cGMP, while internal Ca closes channels by inhibiting the production of cGMP.