Characterization of histamine‐H3 receptors controlling non‐adrenergic non‐cholinergic contractions of the guinea‐pig isolated ileum
Authors: Taylor S.J. , Kilpatrick G.J. 
Published: July 2012

In the presence of atropine, mepyramine and ranitidine, electric field stimulation of the guinea‐pig isolated ileum longitudinal muscle‐myenteric plexus preparation resulted in a two component non‐adrenergic non‐cholinergic contraction. The initial contraction had a duration of approximately 1 s whereas the second contraction lasted approximately 10 s. The second contraction was completely inhibited by tetrodotoxin (0.2 × 10−6 m) with minimal effect on the initial contraction. Phentolamine (3 × 10−6 m), propranolol (3 × 10−6 m) and hexamethonium (10−4 m), did not significantly reduce either component of the contractile response.

The neurokinin NK1 receptor antagonists, GR82334 and GR71251, produced concentration‐related (EC50 = 564 and 173 nm respectively) inhibitions of the second contraction with no effect on the initial contraction. The neurokinin NK2 receptor antagonists MEN 10207 and Ac‐Leu‐Asp‐Gln‐Trp‐Phe‐Gly‐NH2 (R 396), 1 × 10−9‐10−5 m, were without effect on either component of the contractile response.

Concentration‐related inhibitions of the second contraction, with no effect on the initial contraction, were observed after inclusion of the histamine H3 receptor agonists (R)‐α‐methylhistamine (pD2 = 7.6), Nα‐methylhistamine (pD2 = 7.7) and Nα,Nα‐dimethylhistamine (pD2 = 6.3). Histamine also inhibited the second contraction (pD2 = 6.2) in a concentration‐related manner but produced a lower maximum inhibitory effect than the other agonists tested.

Inclusion of the H3 receptor antagonists, thioperamide, burimamide, impromidine and phenylbutanoylhistamine, caused parallel concentration‐related rightward shifts in the concentration‐response curve to (R)‐α‐methylhistamine. In each case, Schild analysis of these data gave slopes not significantly different from unity. Antagonist affinity values for thioperamide (pA2 = 8.2), burimamide (pA2 = 7.0) and impromidine (pA2 = 7.0) were consistent with values obtained in other assays of the H3 receptor. However, phenylbutanoylhistamine (pA2 = 5.8) and betahistine (pKB ≤ 4) had affinities more than ten fold lower than values obtained in other assays of the H3 receptor.

Exposure of the tissues to N‐ethoxycarbonyl‐2‐ethoxy‐1,2‐dihydroquinoline (10−6 m) for 7–30 min followed by extensive washing, had no effect on basal contractions, but produced a rightward shift in the concentration‐response curves to (R)‐α‐methylhistamine, Nα‐methylhistamine, Nα,Nα‐dimethylhistamine and histamine. This treatment also resulted in a decrease in the maximum inhibitory response obtainable. Apparent agonist affinity (pKD) values of 7.01, 7.06, 6.09 and 6.13 were estimated for (R)‐α‐methylhistamine, Nα‐methylhistamine, Nα,Nα‐dimethylhistamine and histamine respectively.

In conclusion, pharmacological analysis has revealed that histamine H3 receptors in the guinea‐pig ileum modulate the release of non‐adrenergic non‐cholinergic neurotransmitters, one of which is probably substance P. In addition we have identified N‐ethoxycarbonyl‐2‐ethoxy‐1,2‐dihydroquinoline as an irreversible antagonist at H3 receptors and have used this compound to estimate apparent affinity values of agonists at H3 receptors in this preparation.