We studied the influence of altered ionic conditions on the recycling of synaptic vesicle membrane in frog retinal photoreceptors using horseradish peroxidase to monitor synaptic activity and trace the fate of internalized membrane. The addition of 1.2 mM barium or 20 mM tetraethylammonium to isolated retinas maintained in Ringer's solution, changes the usual balance of membrane circulation in the rod cells; the cone cells are much less affected. Retrieval of synaptic vesicle membrane in the rods, which normally regenerates small vesicles, becomes mediated predominantly by large sacs and vacuoles ("cisternae"). Because these cisternae can be labeled with peroxidase, they appear to arise from endocytized membrane. Morphometric analysis suggests strongly that the cisternae are formed of circulating synaptic vesicle membrane. The effects of barium and tetraethylammonium can be inhibited by high extracellular potassium, by high intensity light, and by 5 mM cobalt. They seem likely to depend on potassium channels, though additional more complex mediation may also be involved. The alterations in membrane retrieval that we find are of interest in terms of the multiple pathways of membrane cycling now being uncovered. They open potential experimental approaches to the controls of this circulation. In addition, the findings extend our previous ones demonstrating that rod cells and cone cells differ in their responses to divalent cations in ways that seem likely to be of physiological importance.

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