RECEPTOR-COUPLED PHOSPHOINOSITIDE HYDROLYSIS IN HUMAN RETINAL-PIGMENT EPITHELIUM

Carbachol and histamine stimulated phosphoinositide (PPI) hydrolysis in cultured human retinal pigment epithelium (RPE), as reflected by an accumulation of H-3-inositol phosphates in the presence of 10 mM Li+. Carbachol increased PPI hydrolysis to greater than 600% of basal with an EC50 of 60-mu-M; stimulation was linear up to 60 min. This activation likely occurred via the M3 muscarinic cholinergic receptor based on the IC50 values for 4-diphenylacetoxy-N-methylpiperidine methiodide (0.47 nM), pirenzepine (280 nM), and 11-[[2-[(diethylamino)methyl]-1-piperidinyl]-acetyl]-5,11-dihydro-6H-pyrido[2,3-b][1,4]benzodiazepin-6-one (1.4-mu-M). Carbachol-mediated PPI hydrolysis was decreased by 80% in the absence of extracellular Ca2+. Histamine stimulated PPI turnover in a linear manner by 180% with an EC50 of 20-mu-M by the H1 histaminergic receptor. Serotonin, glutamate, norepinephrine, and dopamine were inactive. In human RPE, the resting cytoplasmic Ca2+ concentration, as determined by fura-2 fluorescence, was 138 +/- 24 nM. On the addition of carbachol, there was a 180% increase in peak intracellular Ca2+; addition of histamine increased intracellular Ca2+ by 187%. These results suggest receptor-mediated, inositol lipid hydrolysis is coupled to intracellular Ca2+ flux in human RPE.