THE M-2 MUSCARINIC RECEPTOR ANTAGONIST METHOCTRAMINE ACTIVATES MAST-CELLS VIA PERTUSSIS-TOXIN-SENSITIVE G-PROTEINS

Methoctramine, a selective M-2 muscarinic cholinergic receptor antagon ist, has been reported to activate phosphoinositide breakdown at high concentrations.;Its polyamine structure suggests a putative activation of guanine nucleotide-binding proteins (G proteins). Incubation of me thoctramine with rat peritoneal mast cells resulted in a dose-dependen t noncytotoxic histamine release, with an EC50 of 20 mu M and a maximu m effect at 1 mM. Atropine, pirenzepine and HHSiD neither inhibited me thoctramine-induced histamine release nor stimulated histamine release . Histamine release and inositol phosphates generation induced by meth octramine were both inhibited by pertussis toxin pretreatment. Benzalk onium chloride, a selective inhibitor of histamine secretion induced b y basic secretagogues, inhibited the secretory response to methoctrami ne. [p-Glu(5), D-Trp(7,9,10)]-SP5-11 (GPAnt-2), a well-characterized a ntagonist of G proteins, blocked the methoctramine-induced histamine r elease when the antagonist was allowed to reach its intracellular targ et by streptolysin O-permeabilization. The response to methoctramine w as prevented by the hydrolysis of sialic acid residues of the cell sur face by neuraminidase. The response of mast cells was restored by perm eabilization of the plasma membrane. These results demonstrate that me thoctramine, following its entry into the cell and the involvement of pertussis toxin-sensitive G proteins, activates phosphoinositide hydro lysis leading to mast cell exocytosis.