Optical probes of intradiskal processes in rod photoreceptors I: Light-scattering study of ATP-dependent dark reactions

Abstract

ATP can cause dramatic structural changes in the outer segment of rod photoreceptors. These changes can be visualized by means of a concomitant light-scattering signal A D, a decrease in scattered light intensity of over 20%. The large size of the signal suggests that the major structural changes occur. The underlying molecular events may reflect an important, yet still unknown, part of the photoreceptor machinery. A D signals reflect ATPase-driven transmembrane events which occur in and at the disk membrane. Their only structural prerequisite is the structural integrity of the disk compartment. The angular dependence of A D, which can be mimicked by an osmotically-induced disk-swelling, suggests that the disk compartment swells during the production of the A D signal. A D signals proceed with first-order kinetics (half-life = 1 min at 20 °C and ATP concentrations of greater than 100 /gmM) and are accompanied by the hydrolysis of approximately 4 mol ATP (mol rhodopsin) −1. The A D signal is inhibited by a number of transport ATPase inhibitors (quercetin, NBD·Cl, vanadate, DCCD), but not by oligomycin, azide and ouabain. The sensitivity to DCCD, together with the fact that except magnesium no other cation has to be present, points to a proton translocation. This proton transport appears to be electrogenic, since A D signals require the presence of a permeant anion. In physiological saline this is chloride, and the chloride flux is facilitated by a DIDS-sensitive anion transport unit in the disk membrane.