PHOTORECEPTOR DEGENERATION ASSOCIATED WITH A PHOTOTRANSDUCTION MUTANT OF DROSOPHILA AND THE USE OF CALCIUM AND BUFFER TO CONTROL THE CONCENTRATION OF CYCLIC-GMP IN TOAD PHOTORECEPTORS

EDWARD P MEYERTHOLEN, Purdue University

Abstract

A light-dependent degeneration of the photoreceptors of the norpa('H44) phototransduction mutant has been studied. Flies maintained on a light-dark cycle (19(DEGREES)C) from eclosion exhibit retinula cell degeneration. The extent of degeneration was enhanced by increasing the environmental temperature to 24(DEGREES)C. Under both of these environmental conditions, the time-course of the degeneration was found to be similar to the time-course of the reduction of the rhodopsin concentration as reported by Ostroy, 1978. However, by delaying the onset of the light-dark cycle by 10 days, it was possible to separate the time-course of these two defects. The rhodopsin was found to decrease substantially before any anatomical indications of degeneration could be detected. It is concluded that the rhodopsin decrease and the degeneration are due to alterations of separate regulatory processes, both of which are apparently regulated by the phototransduction process. This suggests that a possible link between the phototransduction process and light-regulated rhodopsin and/or membrane turnover processes of the photoreceptor cell may involve the wild-type gene product of the norpA gene locus. It was found that reducing the bicarbonate or HEPES concentration of the incubation media to 1-3 mM reduces the cGMP concentration of ROS isolated from intact retinas to the level normally observed upon a complete bleach of visual pigment in high bicarbonate or HEPES. This effect is reversible and is relatively stable over a 30 minute incubation period. This effect of bicarbonate or HEPES appears to be the result of an effect on the photoreceptor and does not occur using isolated ROS alone. The effect of the bicarbonate and HEPES concentration on the cGMP levels in the ROS can be altered by several procedures. These procedures all have the common effect of altering the internal Ca('++) levels, suggesting that the effect of bicarbonate and HEPES on cGMP levels may be mediated by Ca('++). Electrophysiological experiments using bicarbonate or HEPES to alter the cGMP concentration of the photoreceptor did not mimic the effect of light. Although other interpretations are possible these data suggest that the overall concentrations of cGMP in the photoreceptor may not be a critical factor in the transduction or light-adaptation processes.

Degree

Ph.D.

Subject Area

Anatomy & physiology|Animals

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