A PHOSPHOLIPASE INHIBITOR, MANOALIDE, AND A G-PROTEIN ACTIVATOR, MAS-7, BOTH AFFECT THE TURNOVER OF PHOTOTRANSDUCTIVE MEMBRANES BY CRAB RETINAS IN DARKNESS - WITH A MODEL FOR THE REGULATION OF RHABDOMERAL MEMBRANE TURNOVER

(1) In vitro retinas of a crab, Leptograpsus, were treated with a phos pholipase inhibitor, manoalide, or a G-protein activator, Mas-7. Both drugs address early stages of the phototransduction cascade. (2) Manoa lide inhibited the light-dependent reduction of rhabdoms during the 'd ay' phase of the light cycle, but did not induce rhabdom overgrowth. F ollowing a period of darkness manoalide failed to affect the diminutio n of illuminated rhabdoms. (3) The diminution of rhabdoms that follows photoreceptor depolarisation induced by 100 mmol . l(-1) K+ in darkne ss was not affected by 2 mu mol . l(-1) manoalide. (4) When retinas in the 'night' phase were treated with Mas-7 in darkness, rhabdom diamet ers were augmented, concurrently with endocytosis of photoreceptor pla sma membranes. (5) The results of combining manoalide and Mas-7 with a ctinomycin D, U-57908 or okadaic acid, drugs used in previous studies to manipulate steps notionally lower in the transduction cascade, lead to a hypothetical model for the regulation of phototransductive membr ane turnover by arthropods.