Synergism between histamine H1- and H2-receptors in the cAMP response in guinea pig brain slices: effects of phorbol esters and calcium.

The synergism between H1- and H2-receptors in the histamine-induced stimulation of cAMP accumulation was studied in slices from guinea pig hippocampus. Since H1-receptors appear to be coupled to the phosphatidylinositol cycle, the participation of the two branches of the cycle in this synergism was assessed by using phorbol esters and/or by removing Ca2+ from the external medium. The protein kinase C activator, 4 beta-phorbol 12,13-dibutyrate (4 beta-PDB), strongly potentiated, with an EC50 of 0.2 microM, the accumulation of cAMP elicited by dimaprit, an H2-receptor agonist used at supramaximal concentration (0.3 mM). The effect of 4 beta-PDB was also observed in the presence of impromidine, an H2-receptor agonist, and histamine. 4 beta-Phorbol 12-myristate, 13-acetate, another protein kinase C activator, also potentiated the effect of dimaprit in a concentration-dependent manner although less potently than 4 beta-PDB. In contrast, 4 alpha-phorbol or the phorbol esters, 4 alpha-phorbol 12,13-didecanoate or 4-O-methylphorbol 12-myristate, 13-acetate, all inactive on protein kinase C, had no potentiating effect. 2-Thiazolylethylamine (2-TEA), a predominantly H1-receptor agonist, increased the stimulation induced by dimaprit (0.3 mM), and this response was further enhanced in the presence of 4 beta-PDB in maximal concentration (1 microM). Mepyramine (0.1 microM) antagonized the H1-receptor-mediated effect in the absence as well as the presence of 4 beta-PDB. The phorbol ester did not significantly alter the EC50 of 2-TEA or the magnitude of its effect. In the absence of phorbol esters, removal of Ca2+ from the incubation medium did not change the response elicited by 0.3 mM dimaprit but reduced by 50% the response to a supramaximal concentration of 2-TEA. This effect was more marked when EGTA was added in the Ca2+-free medium. The EC50 value of 2-TEA was only slightly modified in the absence of Ca2+ (180 +/- 20 microM as compared with 70 +/- 4 microM in the presence of 2.6 mM Ca2+). In the presence of 4 beta-PDB (1 microM), removal of Ca2+, particularly in the presence of EGTA, did not affect or slightly increased the response to dimaprit, but still strongly reduced the response to 2-TEA. The Ca2+ ionophore A 23187 (10 microM) showed a tendency to mimic the potentiating effect of 2-TEA.(ABSTRACT TRUNCATED AT 400 WORDS)