Muscarinic receptors of the albino rabbit ciliary process

Muscarinic receptor binding sites were identified in membranes prepared from albino rabbit ciliary processes, using the muscarinic antagonist [3H]L-quinuclidinyl benzylate as the radioligand. Analysis of saturation binding experiments demonstrated that [3H]L-quinuclidinyl benzylate bound to an apparent homogeneous population of binding sites with a Kd value of 6.4 pm and a Bmax value of 155 fmol mg-1 protein. Seventy percent (70%) of binding sites showed high affinity for pirenzepine, i.e. belonged to the M1 subtype. In contrast, AF-DX 116 was unable to discriminate between subtypes of muscarinic binding sites in this tissue. Carbachol caused a dose-dependent increase in phosphatidylinositol turnover (EC50 = 154 microM) in ciliary processes. A maximum stimulation of 652% of basal activity was obtained following a 45 min incubation with 10 mM carbachol. The potency of muscarinic antagonists to block the carbachol-induced response was comparable to that found for M1 receptors in other tissues. Oxotremorine and pilocarpine behaved like partial agonists in this assay. The carbachol-induced increase in phosphatidylinositol turnover was also observed in a suspension of epithelial cells from ciliary processes and it was blocked by atropine; thus, indicating the presence of muscarinic receptors functionally coupled to phosphatidylinositol turnover in these cells.