Membrane maintenance and electrical properties of photoreceptors of wild-type andrpa (receptor potential absent) mutant blowflies (Calliphora erythrocephala)

Abstract

The maintenance of photoreceptor cell membranes in the blowfly was investigated in relation to the diurnal cycle, age, and therpa (receptor potential absent) phototransduction mutation. The effect of disturbed membrane assembly on the electrical membrane properties was examined using single-electrode discontinuous current-clamp techniques. In wild-type flies the cross-sectional dimensions of the rhabdomeres were markedly reduced with age, and the quantity of synthetic organelles decreased concurrently, whereas no correlation was found between the diurnal cycle and membrane turnover. Therpa mutation is thought to block the visual transduction cascade in photoreceptor cells and to lead to degeneration of the photoreceptor cell bodies. The volume of rhabdomeres decreased markedly inrpa mutants and the quantity of synthetic organelles was reduced significantly, indicating an imbalance between photoreceptive membrane renewal and degradation. Also, the plasma membrane underwent degenerative changes. The passive electrical properties of photoreceptor cells — resting membrane voltages and input resistances — were only slightly changed from those of wild-type flies, although the photoreceptive membrane did not depolarize in response to light. This indicates no apparent disturbance in the function of the ionic channels in these membranes. Taken together, these results suggest that the photoreceptor cells need a functional phototransduction cascade with its feedback controls to maintain continuous renewal of rhabdomeres, but that the plasma membrane maintains its normal electrochemical properties despite extreme morphological degeneration of photoreceptor cell.