Phorbol ester, but not endotoxin, desensitizes mannan-induced glycogenolysis in the perfused rat liver

Mannan, a ligand for the mannose/N-acetylglucosamine (GlcNAc) receptor, induces suppression of oxygen consumption and increases glucose production in the perfused rat liver, and repeated infusion of mannan causes desensitization of the responses. In this study, we examined whether activation of Kupffer cells by endotoxin and phorbol ester alters the glycogenolytic responses to mannan. Infusion of lipopolysaccharide (LPS, 10 micrograms/ ml) in the perfusate failed to inhibit the responses to mannan. Intravenous administration of LPS (1 mg/kg) 6 and 24 h before perfusion did not desensitize the responses to mannan, suggesting that the responses through mannose/GlcNAc receptors in the liver are retained even after activation of Kupffer cells by LPS. In contrast, prior infusion of phorbol 12-myristate 13-acetate (PMA, 100 nM) in vitro abolished the glycogenolytic responses to subsequently infused mannan, but not that to norepinephrine (100 nM), while prior infusions of 4-alpha-phorbol 12,13-didecanoate (100 nM), A23187 (50 nM), or forskolin (1 microM) had no effect on the mannan-induced responses. H-7, an inhibitor of protein kinase C, reduced the glycogenolytic responses to mannan, while it failed to restore the desensitization. These results suggest that protein kinase C may be involved in the process of glycogenolysis by mannan, but is unlikely to be involved in the homologous desensitization of the responses.