HISTAMINE-INDUCED INCREASES IN CYCLIC-AMP LEVELS IN BOVINE ADRENAL-MEDULLARY CELLS

1 The effect of histamine on cellular cyclic AMP levels in cultured bovine adrenal medullary cells has been studied. 2 Histamine (0.3-30-mu-M) increased cyclic AMP levels transiently, with a maximal response after 5 min, a smaller response after 20 min, and no increase seen after 80 or 180 min. The EC50 at 5 min was approximately 2-mu-M. Histamine had no effect on cyclic AMP release from the cells over 5 min, but increased it after 90 min. 3 The cyclic AMP response to 5-mu-M histamine was reduced by 45% by 1-mu-M mepyramine and by almost 30% by 1-mu-M cimetidine, and was abolished by the combination of both antagonists. Cimetidine at 100-mu-M did not inhibit the response to histamine more than 1-muM cimetidine. The H-3-receptor antagonist, thioperamide (1-mu-M), had no effect on the response to histamine. 4 The H-1-receptor agonist, 2-thiazolylethylamine (5-100-mu-M) and the H-2-receptor agonist, dimaprit (5-100-mu-M), each induced a cyclic AMP response, and gave more-than-additive responses when combined. The H-3 agonist (R)alpha-methylhistamine (100-mu-M) had no effect either on its own or in combination with either the H-1 or the H-2 agonist. The response to 100-mu-M 2-thiazolylethylamine was unaffected by cimetidine (100-mu-M). 5 The cyclic AMP responses to 5-mu-M histamine, 100-mu-M thiazolylethylamine and 100-mu-M dimaprit were each weakly enhanced in the presence of 1 mM 3-isobutyl-1-methylxanthine. The response to dimaprit was enhanced more than 10 fold in the presence of 0.3-mu-M forskolin, while the responses to histamine and thiazolylethylamine were weakly enhanced. 6 The cyclic AMP response to 5-mu-M histamine was partially reduced in the absence of extracellular Ca2+, and the residual response was fully antagonized by 1-mu-M cimetidine and was unaffected by 1-mu-M mepyramine. In the absence of Ca2+, the cyclic AMP response to 100-mu-M thiazolethylamine was abolished, while that to 100-mu-M dimaprit was unaffected. 7 Reincubation of 5-mu-M histamine solutions with a second set of chromaffin cells, following prior incubation with another set of cells, induced a cyclic AMP response in the fresh cells. This response was reduced by a combination of mepyramine and cimetidine to the same degree as the response to fresh 5-mu-M histamine solutions. 8 The results indicate that histamine increases cellular cyclic AMP levels in bovine chromaffin cells by three mechanisms: by acting on H-1 receptors, by acting on H-2 receptors, and by an interaction between H-1 and H-2 receptors. The H-1 response does not require concomitant activation of H-2 receptors, is fully dependent on extracellular Ca2+, does not depend on secreted chromaffin cell products, and is not due to reduced cyclic AMP degradation or export. The H-2 cyclic AMP response is the first functional response reported for H-2 receptors on chromaffin cells, is independent of Ca2+, is not due to reduced cyclic AMP export or degradation, and is likely to be mediated via a direct action through G(s). The role of these different mechanisms in the regulation of cyclic AMP-dependent processes in chromaffin cells by histamine is under investigation.