Dosage-dependent absorption of cadmium in the rat intestine measured in situ

Abstract

Previous studies have shown that the disposition of cadmium (Cd) following oral administration is dosage dependent and may possibly be due to dosage-dependent intestinal absorption of Cd. Though extensively studied, the precise nature of Cd absorption by the intestine remains unclear. Similarly, the role of metallothionein (MT) in the intestinal absorption of Cd remains equivocal. The present study was designed (1) to characterize the intestinal absorption of Cd in the rat, and (2) to determine the role of MT in intestinal Cd absorption. The study has been conducted with an isolated intestinal loop preparation in situ, which allows direct measurement of intestinal absorption under nearly physiological conditions. Under urethane-induced anesthesia, Cd (0.1, 10, 100, 1000, or 10,000 μg/kg) was injected intraluminally into the isolated intestinal loop in situ and all mesenteric venous (portal) blood exiting from the loop was collected for 90 min. Absorption of Cd into the portal circulation was low at all dosages studied. The percentage of the dosage absorbed ranged from 0.09% at the 0.1 μg/kg dosage to 3.4% at the 10,000 μg/kg dosage. At low dosages (0.1 and 10 μg/kg), little difference was noted in the fractional absorption of Cd (0.09 and 0.14% of the dosage, respectively). However, the fractional absorption of Cd was 10-fold greater in rats administered 100 μg Cd/kg (1.1% of the dosage). Administration of higher dosages of Cd (1000 and 10,000 μg/kg) further increased the percentage of the dosage absorbed (1.8 and 3.4%, respectively). To evaluate the role of MT in the intestinal absorption of Cd, rats were subcutaneously injected with zinc (Zn) for 4 days (30 mg/kg/day) and the absorption of an intermediate dosage of Cd (100 μg/kg) was subsequently assessed in situ. Zn pretreatment increased the endogenous concentration of MT in the intestine 25-fold. Following intraluminal administration, 93% of Cd in intestinal cytosol of Zn-treated rats was bound to MT whereas 40% of the cytosolic Cd was bound to MT in saline-treated (control) rats. Moreover, the amount of Cd in intestinal cytosol was 2-fold greater in Zn-treated rats than in control rats. However, the intestinal absorption of Cd in rats pretreated with Zn demonstrated no difference from that in saline-treated rats. These results indicate that the intestinal absorption of Cd is dosage independent at low dosages of Cd (<10 μg/kg) and dosage dependent at high dosages (> 10 μg/kg). Furthermore, saturation of intestinal MT is not a major determinant of the observed dosage-dependent absorption of Cd.