Apparent activity of 5-methylfurmethide at muscarinic receptors.

Apparent activity of 5-methylfurfuryltrimethylammonium iodide (5-methylfurmethide, 5-MFT) at muscarinic receptors of the ileal longitudinal muscle of the guinea-pig was investigated. These parameters were determined by the analysis of concentration-response data before and after fractional inactivation of receptors by phenoxybenzamine, an irreversible receptor blocker. The apparent ED50, dissociation constant (KA), and relative affinity (in comparison to acetylcholine, ACh) were 2.75 +/- 0.22 X 10(-8) M, 7.22 +/- 0.15 X 10(-7) M and 1.33 respectively. The relative intrinsic efficacy compared to ACh was 1.54.