Concentrations of phosphatidylinositol 4,5-bisphosphate and inositol 1,4,5-trisphosphate within the distal segment of squid photoreceptors.
Although inositol trisphosphate (InsP3) is a key substance in phototransduction of invertebrate photoreceptors, its intracellular concentration remains unknown. The purpose of this study was to assay its concentration and the concentration of its precursor, phosphatidylinositol bisphosphate (PtdInsP2), within squid photoreceptors. Rhabdomeric membranes were purified and their PtdInsP2 content measured with a phosphate assay after the extracted phospholipids were deacylated and separated by ion-exchange chromatography. At least 75% of the total PtdInsP2 found in the retinal homogenate was associated with the plasma membranes of the rhabdomeric microvilli, where PtdInsP2 was 3.1 +/- 0.7% of the total phospholipids, a level comparable to values published for rat brain. In terms of rhodopsin, microvillar membranes contained 3.7 +/- 0.9 mol PtdInsP2/mol rho. The InsP3 content of living retinas was measured with a radioreceptor assay. The basal content of dark-adapted retinas was 0.15 +/- 0.05 InsP3/rho, equivalent to 30 +/- 9 nmol/g tissue that is about twice that of rat brains. Flash illumination (approximately 1 ms in duration) that photoactivated 1% of rhodopsin increased the level about fivefold to 0.68 +/- 0.22 InsP3/rho. Corresponding decrease in PtdInsP2 was undetectable as it was within measurement errors. For PtdInsP2, the measured content corresponds to 5.6 +/- 1.4 mM within the volume of rhabdomere. Maximal light-induced concentration of InsP3 is calculated to be 1.2 +/- 0.4 mM within the cytoplasm of the distal segment. Each photoactivated rhodopsin leads to the formation of < or = 500 InsP3 molecules when measured 15 s after the flash.(ABSTRACT TRUNCATED AT 250 WORDS)
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Laboratory of Sensory Physiology, Marine Biological Laboratory, Woods Hole, MA 02543.
D000818:Animals; D002462:Cell Membrane / Q000737:chemistry; D002852:Chromatography, Ion Exchange; D049832:Decapodiformes; D015544:Inositol 1,4,5-Trisphosphate / Q000032:analysis* / Q000302:isolation & purification; D008027:Light; D019269:Phosphatidylinositol 4,5-Diphosphate; D018129:Phosphatidylinositol Phosphates / Q000032:analysis* / Q000302:isolation & purification; D017956:Photoreceptor Cells, Invertebrate / Q000737:chemistry*; D012243:Rhodopsin / Q000032:analysis; D015398:Signal Transduction; D013347:Subcellular Fractions
D019269:Phosphatidylinositol 4,5-Diphosphate; D018129:Phosphatidylinositol Phosphates; D015544:Inositol 1,4,5-Trisphosphate; D012243:Rhodopsin