In vivo optophysiology reveals that G-protein activation triggers osmotic swelling and increased light scattering of rod photoreceptors

Significance Complete activation of the phototransduction G-protein cascade of dark-adapted rod photoreceptors causes outer segments to undergo 10% elongation and large local increases in backscattering, as measured in vivo with noninvasive, high-resolution optical coherence tomography. Maximal elongation is caused by a potentially harmful 20% increase in internal osmotic pressure generated by excess osmolytes arising from phototransduction. The light-stimulated elongation and backscattering responses can be explained by an osmo-elastic model of cytoplasmic swelling, combined with changes in refractive index consequent to the swelling and translocation of the G-protein subunits into the cytosol. Disease conditions that affect the structural integrity of rods may cause them to be especially vulnerable to osmotic stress caused by bright light.