The Light-induced Conductance Change in the Vertebrate Rod

Abstract

The molecular structure of the light-modulated channel in the vertebrate rod is unknown and we remain far from being able even to postulate what it might be. By discussing what is presently known about the electrical properties and ion selectivity of the channel and what they might imply for channel structure, this paper attempts to narrow the range of possibilities.

Several factors suggest that the structure of the light-modulated channel must be quite different from that of commonly studied ionic pores such as the Na⁺ channels of nerve membranes. Thus, although its ion selectivity falls in the same sequence as that of the Na⁺ channel in nerve, it is only poorly selective for Na⁺ over K⁺. Electrical measurements have revealed little evidence of any intrinsic, voltage-dependent channel gating. Moreover, the single-channel conductance, estimated on the basis of patch-clamp measurements of membrane noise, appears to be two orders or magnitude lower than that of the nerve Na⁺ channel, and estimated single-channel ion fluxes are similarly very small.

Of course, ions can also cross biological membranes via membrane-bound carrier molecules. The properties of such molecules are discussed in some detail and compared with those of the light-modulated channel. It is concluded that if the light-modulated channel is an ion carrier, it is probably not a mobile carrier like Valinomycin. A hybrid structure, having some of the features of a pore, seems more likely.