Effects of high levels of dietary zinc oxide on ex vivo epithelial histamine response and investigations on histamine receptor action in the proximal colon of weaned piglets

The aim of the study was to identify the effect of high dietary zinc oxide (ZnO) levels on the histamine-induced secretory-type response and histamine metabolism in the porcine proximal colon. After weaning at d 26, 3 diets with low (LZn), normal (NZn), and high (HZn) concentrations of zinc (57, 164, or 2,425 mg/kg) were fed to a total of 120 piglets. Digesta and tissue samples were taken from the ascending colon after 7 +/- 1, 14 +/- 1, 21 +/- 1, and 28 +/- 1 d. Partially stripped tissue was mounted in Ussing chambers, and histamine was applied either to the serosal or mucosal compartments. Tissue was pretreated with or without aminoguanidine and amodiaquine to block the histamine-degrading enzymes diamine oxidase (DAO) and histamine N-methyltransferase (HMT), respectively. Gene expression and catalytic activity of DAO and HMT in the tissue were analyzed. The numbers of mast cells were determined in tissue samples, and histamine concentration was measured in the colon digesta. Colon tissue from another 12 piglets was used for functional studies on histamine H-1 and H-2 receptors by using the neuronal conduction blocker tetrodotoxin (TTX) and the H-1 and H-2 receptor blocker chloropyramine and famotidine, respectively. After serosal histamine application to colonic tissue in Ussing chambers, the change of short-circuit current (Delta I-sc) was not affected by pretreatment and was not different between Zn feeding groups. The Delta I-sc after mucosal histamine application was numerically lower (P = 0.168) in HZn compared to LZn and NZn pigs. Mast cell numbers increased from 32 to 46 d of life (P < 0.05). Further studies elucidated that the serosal histamine response was partly inhibited by chloropyramine or famotidine (P < 0.01). The response to mucosal histamine tended to be decreased when chloropyramine but not famotidine was applied from either the serosal or the mucosal side (P = 0.055). Tetrodotoxin alone or in combination with chloropyramine resulted in a similar reduction in the mucosal histamine response (P < 0.01). In conclusion, the present study could not identify marked changes in colonic histamine metabolism on dietary ZnO oversupplementation. For the first time, however, H-1 receptors were functionally identified in the pig colon that are localized either on neurons or on cells that activate secretion via neurons. Luminal histamine can elicit a secretory-type response via these receptors.