Atropine and M
muscarinic agonists block carbachol-induced pigment granule dispersion. RPE were isolated from bluegill, and following treatment with forskolin were subjected to 100 nM carbachol and varying concentrations of muscarinic receptor antagonists (n = 4, except as noted for tropicamide). The general muscarinic receptor antagonist atropine completely blocked carbachol-induced dispersion at concentrations as low as 10 pM. The M1 antagonist pirenzepine and the M3 antagonist 4-DAMP also blocked dispersion at concentrations as low as 10 nM. In contrast, the M2 antagonist AF-DX 116 and the M4 antagonist tropicamide were ineffectual at blocking carbachol-induced pigment dispersion, even at concentrations as high as 1 μM.