Human retinal pigment epithelial cells possess muscarinic receptors coupled to calcium mobilization

Human retinal pigment epithelial (RPE) cells in culture demonstrated saturable specific binding of [³H]quinuclindinyl benzilate (QNB). Specific binding represents about 75% of total binding. Scatchard analysis yields a K_d of 0.178 nM and B_max of 42 fmol/mg protein. Atropine and carbachol show typical displacement curves, and a Hill plot has a slope of 0.96, suggesting a homogeneous population of receptors. Muscarinic agonists have no effect on intracellular cyclic adenosine monophosphate levels in RPE cells measured by radioimmunoassay, nor do they alter the isoproterenol-induced stimulation of adenylate cyclase. However, both acetylcholine and carbachol cause a rapid increase in intracellular calcium concentration measured by the fluorescent indicator quin 2. Atropine reverses the calcium rise when added after agonist and prevents the rise when added prior to agonist. These data suggest that human RPE cells possess muscarinic receptors coupled to calcium mobilization.