Contribution of the biliary pathway to the homeostasis of vitamin D3 and of 1,25-dihydroxyvitamin D3.

The role of the biliary pathway in the homeostasis of the vitamin D3 (D3) group of compounds is poorly understood. The purpose of the studies was to investigate the biliary excretion pattern of materials derived from the parent compound D3 and from the hormone 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) during constant iv infusion, and to probe the influence of 1,25(OH)2D3 pretreatment on the excretion into the bile of D3-derived materials. Under anesthesia, the bile duct, duodenum (for bile replacement), and jugular veins were cannulated. The experiments were then carried out in fully awake rats with access to food and water ad libitum during a period of 6 h. Data indicate that before steady state was reached, biliary excretion of 1,25(OH)2[3H]D3 was much more important than that of [14C]D3 with bile/plasma concentration ratios above 1 in 1,25(OH)2[3H]D3-infused animals from the 15th-60th min of infusion compared to ratios between 0.12-0.40 in [14C]D3-infused rats (P less than 0.0001); this led to a cumulative excretion 6.3-fold higher after 1,25(OH)2[3H]D3 than after [14C]D3 administration, with 3.9 +/- 0.4% and 0.6 +/- 0.1% of the dose being recovered into the bile during the first hour of excretion. However, once stable plasma concentrations were reached, the rate of excretion of the two compounds became similar, with bile/plasma concentration ratios of 0.64 +/- 0.02 and 0.70 +/- 0.02 (P greater than 0.05), and plasma bile clearance of 26.4 +/- 1.0 and 25.7 +/- 1.7 microliters/min.kg for 1,25(OH)2[3H]D3 and [14C]D3, respectively (P greater than 0.05). During that period, the MCR of 1,25(OH)2D3 was estimated to be 118.3 +/- 10.5 microliters/min.kg. On the other hand, 1,25(OH)2D3 pretreatment as constant ip infusion (14 pmol/24 h for 6 days) significantly increased bile flow [1,25(OH)2D3 treated, 44.9 +/- 1.6 microliters/min.kg; untreated, 36.6 +/- 0.5 microliters/min.kg, P less than 0.01)], leading to significant increases in the plasma bile clearance of [14C]D3-derived compounds early in the course of the study (P less than 0.004) in the presence of similar bile/plasma concentration ratios in the two groups. During the steady state phase of investigation, however, bile/plasma concentration ratios became lower in 1,25(OH)2D3-than in placebo-treated animals (P less than 0.05), but due to the 1,25(OH)2D3-mediated increase in bile flow, similar plasma bile clearances were observed in both groups.(ABSTRACT TRUNCATED AT 400 WORDS)