N,N-Diethyl-2-(1-pyridyl)ethylamine, a partial agonist for the histamine receptor in guinea pig ileum.

N,N-Diethyl-2-(1-pyridyl)ethylamine (E-2-P) produced 59 +/- 7% of the maximal response to histamine in guinea pig ileal longitudinal smooth muscle and antagonized the responses of this tissue to histamine. The estimated binding constant of E-2-P for the histamine receptor predicted a binding curve nearly coincident with the agonist concentration-response curve indicating no receptor reserve for this partial agonist. Diphenhydramine (30 nM) produced competitive antagonism of response to E-2-P (pKB = 8.3 +/- 0.13). Triprolidine (0.3 nM), a slower acting antihistamine, produced a depression of the maximal responses to E-2-P. This effect was analyzed in terms of a "hemi-equilibrium" hypothesis which approximates a pseudoirreversible antagonism of histamine receptors by triprolidine with respect to E-2-P. All data are consistent with the classification of E-2-P as a simple partial agonist for the histamine receptor of guinea pig ileum.