Adenylate Cyclase and Specificity of the Histamine H2-Receptor in the Gastric Mucosa of the Guinea-Pig

Abstract

Experimental data indicate that the histamine activated gastric mucosal adenylate cyclase (AC) of the guinea-pig is inhibited by histamine H1 and H2-receptor antagonists and the question raises whether the activation of AC occurs via a specific H2-receptor mechanism as it is known for the histamine stimulated gastric secretion. Receptor specificity of AC-activation and inhibition was tested by selective H1 (PEA) and H2 (dimaprit)-receptor agonists and antagonists (mepyramine and cimetidine) in two broken cell preparations (P1 and P2) of the guinea-pig gastric mucosa. P1 consisted of the mucosal homogenate in 25 mM Tris-HCl buffer pH 7.4 containing 1 mM EDTA and 1 mM dithiotreithol, P2 of the 2000·g pellet 3 times resuspended in the homogenisation medium. When P1 was used as enzyme no difference could be detected between the histamine and dimaprit dose response curves. Both compounds (Ka=5·10−6 M) actived the basal AC maximally 8–10 fold. PEA (Ka=10−4 M) had only a 3–4 fold stimulating action. Maximal activation of AC by PEA and dimaprit was not additive. Cimetidine (10−6 M) reduced the histamine or dimaprit stimulated AC by 50%, whereas mepyramine (10−6M) was ineffective. To achieve the same degree of inhibition with this blocker a concentration of 10−3 M was necessary. When P2 was used as enzyme histamine and dimaprit (same Ka as above) activated the basal AC maximally only 5–6 fold. The stimulatory effect of PEA was not reduced. In this preparation the inhibitory profile of cimetidine did not differ from that of mepyramine, i.e. cimetidine (10−6 M) was ineffective and a concentration of 10−3 M had to be employed to block the histamine activated AC by 50%. Resuspension of P2 with the supernatant did not restore the original H2-receptor specificity. The data suggest: AC-activation by histamine is a H2-receptor mediated process and H2-receptor specificity is destroyed during the preparation of gastric mucosal membranes.