Transfer kinetics of 3,5,3'-triiodothyronine and thyroxine from rat blood to large and small intestines, liver, and kidneys in vivo.

Enterohepatic circulation of thyroid hormone in the rat involves transfer of hormone to intestines in bile from the liver and absorption of some intestinal hormone, via portal blood to liver. Transfer of hormone to intestines from mesenteric arterial blood has generally been considered minimal, although this alternate pathway has received little attention. We have measured uptake kinetics of iv trace doses of radioactivity labeled T3 (T3*) and T4* from blood to intestines, longitudinally in 14 segments of total intestines in situ, with bowel contents included and bile duct ligated, and also to liver and kidneys, for comparison. Both T3* and T4* entered the entire length of intestines from blood, and into contents as well as tissue. Tissue uptake of T3* and T4* were fairly uniform longitudinally, but uptake into contents and thus into total intestines (tissue + contents) decreased linearly from pylorus to anus. Unidirectional uptake rate constants (hours-1) or clearance rates (milliliters per h) of T3* were 13 times greater than corresponding T4* fractional uptake rates to intestines. In contrast, T4 mass fluxes (congruent to 38 ng/h.100 g body wt) exceed T3 fluxes (congruent to 6.3 ng/h.100 g body wt) about 6-fold, because plasma contains far more T4 than T3. Comparing these results with published biliary and fecal T3 and T4 flux data, it appears that mesenteric arterial mass influxes to intestines are 3-5 times greater than both biliary secretion and fecal excretion of T3 and T4. Intestinal kinetics, in the absence of biliary influx, are characterized by a moderately rapid uptake followed by a very slow washout phase, which fit into neither the fast nor slow compartment paradigms of a 3-compartment mammillary model, in contrast to liver and kidney kinetics, which fit well into a single fast compartment. Intestinal dynamics are consistent with an organ in which storage and exchange of thyroid hormone dominate over metabolic and excretory processes.