Adenosine 3':5'-cyclic monophosphate phosphodiesterase activity in the dystrophic rat retina.

265 lysophosphoglyceride in response to neurotransmitter release. Analyses of other synapto-soma1 subfractions have given similar results. These experiments have revealed some features of interest in the incorporation of these labelled compounds into synaptosomal subfractions. Generally the relative order of labelling as expressed by specific radioactivities was phosphatidylinositol >phospha-tidylcholine > phosphatidylserine > phosphatidylethanolamine. The specific radio-activities of phosphatidylcholine and phosphatidylethanolamine in the vesicle fraction were higher with each labelled compound used than the corresponding values in the microsomal (E) fraction. The relation for phosphatidylserine and phosphatidylinositol varied with the tracer used. Analyses of marker enzymes indicated that fraction E had a higher specific content of microsomal membranes, outer mitochondria1 membrane and plasma membrane than did the vesicle fraction. Thus it was considered that the enhanced labelling of the phospholipids in the vesicle fraction was attributable to the vesicles. Although synaptic vesicles are capable of incorporating CDP-choline into phospholipid (Miller & Dawson, 1972), it was considered possible, particularly as radioactivity derived from [l-3H]glucose was localized in the glycerol backbone of the labelled phospholipid, that there exists a small amount of intrasynaptosomal endo-plasmic reticulum with the possibility of intrasynaptosomal phospholipid transport [as suggested by Miller & Dawson (1972)J. The enzymes involved in the independent acylations of lysophosphoglycerides may exist in synaptic vesicles, but the incorporation of labelled fatty acids with respect to radioactivity derived from glucose in the vesicle and microsomal phospholipids does not lend support to such a possibility. It is probable that retinal degeneration in inherited retinal dystrophy is the result of a number of defects af€ecting different enzyme systems. The pigment epithelium of dys-trophic rats has lost the ability to phagocytosize spent photoreceptor-cell outer segments (Herron et a/., 1971). This may be due either to an inherited defect in the enzymes or membrane structures involved in phagocytosis in the pigment epithelium or to some change in the properties of the outer segments which interferes with the ability of the pigment epithelium to engulf them. The failure to remove spent outer segments results in a build-up of visual pigment between the retina and pigment epithelium. This accumulation could result in retinal degeneration either by interfering with the nutritional supply of the retina or by an accumulation of its breakdown product, retinol, labilizing retinal VOl. 3