Synthesis and release of histamine studied on slices from rat hypothalamus.
In slices from rat hypothalamus, incubated in the presence of 3H-L-histidine (3H-L-His), the aminoacid was rapidly taken up by a saturable process, and partially converted into 3H-histamine (3H-HA). The overall conversion was prevented either by inhibitors of L-histidine decarboxylase or by aromatic aminoacids competing with L-His uptake. The synthesis process exhibited Michaelis--Menten kinetics with an affinity of the aminoacid not different from that for the decarboxylase in homogenates; however the Vmax in homogenates was more than 10 times higher than in slices. Depolarization of the slices by 50 mM potassium resulted in: (a) a calcium-dependent release of 3H-HA which was more marked than that previously reported for endogenous HA, (b) a significant acceleration in the rate of 3H-HA synthesis, which was characterized by an unchanged Km but a significantly elevated Vmax. The regulation of HA synthesis did not appear to depend on end-product inhibition since it was not midified by the addition of exogenous HA. The release of 3H-HA was followed by the accumulation of 3H-methylhistamine, which was enhanced by a monoamine oxidase inhibitor. Aminoguanidine, a diamine oxidase inhibitor, had no effect on catabolism. The involvement of mast-cells in the storage of a fraction of endogenous HA in hypothalamic slices was assessed by the significant releasing effect of compound 48/80. Hence, the data support the existence of two distinct HA stores in the brain: depolarization relases the amine and increases its synthesis, probably in neurones, whereas compound 48/80 releases it from a slowly turning-over store, probably in mast-cells.
D000594:Amino Acid Oxidoreductases / Q000037:antagonists & inhibitors; D000596:Amino Acids / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002262:Carboxy-Lyases / Q000037:antagonists & inhibitors; D006632:Histamine / Q000096:biosynthesis* / Q000378:metabolism; D006636:Histamine Release / Q000187:drug effects; D007031:Hypothalamus / Q000378:metabolism*; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D008407:Mast Cells / Q000378:metabolism; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D013997:Time Factors; D003189:p-Methoxy-N-methylphenethylamine / Q000494:pharmacology
D000596:Amino Acids; D003189:p-Methoxy-N-methylphenethylamine; D006632:Histamine; D000594:Amino Acid Oxidoreductases; D002262:Carboxy-Lyases; D011188:Potassium; D002118:Calcium