(A) HDC, H1-H4HR expression increased in Mdr2−/− mice fed BAC compared to WT, whereas Mdr2−/− mice fed UDCA had decreased levels compared to Mdr2−/− mice fed BAC. (B) Histamine secretion significantly increased in Mdr2−/− mice fed BAC and secretion diminished in Mdr2−/− mice fed UDCA compared to BAC fed Mdr2−/− mice. (C) In human patients with PSC, H1-H4HR expression increased compared to controls and in PSC patients treated with UDCA, there was a decrease in the expression of H2HR, H3HR and H4HR. (D) Histamine serum levels were increased in some patients with PSC and levels were decreased in 1 patient treated with UDCA. Data are expressed as mean ± SEM of at least 12 experiments for real-time PCR and at least 15 experiments for EIA. *p<0.05 versus WT mice or control patients; #p<0.05 versus Mdr2−/− mice fed BAC.