Possible site of decreased intestinal zinc absorption in chronic uremic rats

A Guide to Human Zinc Absorption: General Overview and Recent Advances of In Vitro Intestinal Models

Zinc absorption in the small intestine is one of the main mechanisms regulating the systemic homeostasis of this essential trace element. This review summarizes the key aspects of human zinc homeostasis and distribution. In particular, current knowledge on human intestinal zinc absorption and the influence of diet-derived factors on bioaccessibility and bioavailability as well as intrinsic luminal and basolateral factors with an impact on zinc uptake are discussed. Their investigation is increasingly performed using in vitro cellular intestinal models, which are continually being refined and keep gaining importance for studying zinc uptake and transport via the human intestinal epithelium. The vast majority of these models is based on the human intestinal cell line Caco-2 in combination with other relevant components of the intestinal epithelium, such as mucin-secreting goblet cells and in vitro digestion models, and applying improved compositions of apical and basolateral media to mimic the in vivo situation as closely as possible. Particular emphasis is placed on summarizing previous applications as well as key results of these models, comparing their results to data obtained in humans, and discussing their advantages and limitations.

Zinc requirements of broiler breeder hens

One hundred and twenty Cobb 500 hens, 20 wk of age, were randomly allocated into individual cages with the objective of estimating their Zn requirements. The study was composed of 3 phases: adaptation to cages (basal diet), depletion (deficient diet containing 18.7 ± 0.47 ppm Zn) for 7 wk, and experimental phases. Hens were fed diets with graded increments of Zn sulfate heptahydrate (ZnSO4·7H2O), totaling 18.7 ± 0.47, 50.3 ± 10.6, 77.3.0 ± 11.0, 110.2 ± 12.8, 140 ± 12.2, and 170.6 ± 13.2 ppm analyzed Zn in feeds for 12 wk (experimental phase). Requirements of Zn were done using quadratic polynomial (QP), broken line quadratic (BLQ), and exponential asymptotic (EA) models. In general, the non-linear statistical models were the ones that best fit the results in this study. Requirements obtained for hen day egg production and settable egg production were 83.3, 78.6 ppm and 61.4, 65.4 ppm for period of 33 to 36 wk, and 63.3, 53.1 and 60.4, 46.1 ppm for period of 37 to 40 wk, and 62.8, 52.8, and 67.7, 62.1 ppm for period of 41 to 44 wk, respectively, using BLQ and EA models. Total eggs and total settable eggs produced per hen had Zn requirements estimated as 75.7, 64.7 ppm, and 56.5, 41.5 ppm, respectively, for BLQ and EA models, whereas for alkaline phosphatase and eggshell percentage were 161.8, 124.9 ppm and 126.1, 122.4 ppm, using QP and BLQ models. Maximum responses for Zn in yolk for periods of 37 to 40 and 41 to 44 wk were 71.0, 78.1 and 64.5, 59.6 ppm, respectively, using BLQ and EA models. Breaking strength had Zn requirements estimated at 68.0 and 96.7 ppm, whereas eggshell palisade layer and eggshell thickness were maximized with 67.9, 67.9 ppm, and 67.7, 64.4 ppm, respectively, for BLQ and EA models. The average of all Zn requirement estimates obtained by EA and BLQ models in the present study was 72.28 ppm or 11.1 mg/hen/d.

In vitro study on the transport of zinc across intestinal epithelial cells using Caco-2 monolayers and isolated rat intestinal membranes

The variety of physiologic and biologic functions of zinc is fascinating and could be applicable to medicine. Our previous studies demonstrated that the absorption of zinc after oral administration to rats is dose-dependent. In order to clarify the detailed mechanism of the dose-dependent in vivo absorption, the transport of zinc across intestinal epithelial cells was investigated using Caco-2 monolayers and isolated rat intestinal membranes. The permeation of zinc across Caco-2 monolayers is concentration-dependent, and both saturable and nonsaturable components are involved. The Michaelis constant and maximum transport rate for saturable transport are 11.7 μM and 31.8 pmol min(-1) cm(-2), respectively; the permeability coefficient for nonsaturable trasnport is 2.37×10(-6) cm s(-1). These parameters for permeation across membranes isolated from duodenum, ileum, and jejunum of rats are similar with those of Caco-2 cells. The comparison of the parameters for permeation across isolated intestinal membrane suggests that the major site of intestinal zinc absorption in rats is the duodenum. The maximum rate of zinc transport across the isolated intestinal membrane (V(max)) shows no correlation with mRNA expression of ZIP4, ZIP5 or ZnT1 in rats, but shows an inverse correlation with that of metallothioneins (MTs). This finding may be partly explained by the buffering role of metallothionein in intestinal absorption. The saturable transport of zinc is not simply determined only by the influx transporter, ZIP4, since three influx and efflux transporters are involved in the transport of zinc.

Kinetics of zinc absorption by in situ ligated intestinal loops of broilers involved in zinc transporters

Two experiments were conducted to study the kinetics of Zn absorption and Zn transporter mRNA levels in the duodenum, jejunum, and ileum of broilers to investigate the main site of Zn absorption in the small intestine, the absorption mechanisms, and whether those transporters were involved. In experiment 1, we compared Zn absorption in 3 segments and at different post-perfusion time points, and found that Zn absorption increased linearly within 30 min and was higher (P < 0.05) in the ileum than in the other 2 segments. In experiment 2, intestinal loops were perfused with solutions containing 0, 0.077, 0.154, 0.308, 0.616, 1.232, or 2.464 mmol/L of ZnSO(4) . 7H(2)O, and Zn concentrations in perfusates were determined at 30 min after perfusion. The mRNA levels of transporters in 3 intestinal loops from the control group and the 0.616 mmol of Zn/L group were analyzed. The kinetic curves showed that Zn transported to the duodenum and jejunum depended on a saturable carrier-mediated process. The Michaelis-Menten constant (K(m)) value was higher in the duodenum than in the jejunum (1.44 +/- 0.33 vs. 0.51 +/- 0.17 mM, P = 0.012). Similarly, the maximum absorption velocity (J(max)) value was greater in the duodenum than in the jejunum (5.32 +/- 1.46 vs. 2.57 +/- 0.39 nmol/min per cm, P = 0.069), whereas absorption in the ileum occurred with a nonsaturable diffusion process and had a diffusive constant (P) of 5.72 x 10(-3) +/- 1.1 x 10(-4) cm(2)/min. Moreover, the mRNA levels of metallothionein, zinc transporter 1, and Zn transporter 5 were lower in the ileum than in the duodenum or jejunum in the Zn-supplemented group, further indicating that Zn absorption in the ileum occurred mainly by a nonsaturable diffusive pathway. The Zn fluxes were significantly higher (P < 0.005) in the ileum than in the other 2 segments at different Zn concentrations. This research suggests that the ileum is the main site of Zn absorption and that the mechanism involved is nonsaturable diffusion, which is different from that in the other 2 segments, depending on the regulation of Zn transporter expression.

Effect of Hypothyroidism on Intestinal Zinc Absorption and Renal Zinc Disposal in Five-Sixths Nephrectomized Rats

Both hypothyroid (Hypo) and hypozincemia are commonly observed in patients and animals with chronic renal failure (CRF). In CRF whether the hypothyroid plays a role in the pathogenesis of hypozincemia is unclear. This study is designed to investigate the effects of hypothyroid on intestinal zinc absorption and urinary zinc excretion in 5/6 nephrectomized (Nx) rats, because plasma zinc balance is attained through a controlled rate of intestinal uptake as well as renal reabsorption. Intestinal zinc absorption was carried out in jejunum and ileum segments by an in vivo perfusion technique and the renal zinc disposal was evaluated by a conventional method using a standard formula to calculate the zinc tubular reabsorption and the excretion of urinary zinc in 5/6 Nx rats with hypothyroidism. The Hypo-NxT rats showed a significant decrease in the rate of intestinal zinc absorption and in the response of plasma zinc levels during intestinal zinc perfusion compared with Eu-NxT rats. They also had significantly lower levels of mucosal zinc and MT as well as lower content of liver zinc than Eu-NxT rats after intestinal zinc perfusion for 80 min. Hypo-NxT rats showed low plasma zinc levels, but had a similar output of pancreaticobiliary zinc and excretion of 24-h urine zinc compared with the Eu-NxT rats. When 2% alcohol intestinal perfusion was used to produce water diuresis, the Hypo-NxT rats presented a higher excretion of urinary zinc than the Eu-NxT rats did, especially during 2% alcohol intestinal zinc perfusion. In the Hypo-NxT rats, the lower plasma zinc levels may thus result from the hypothyroid because it reduces intestinal zinc absorption. Increasing the urine flow rate may aggravate the reduction of plasma zinc level in Hypo-NxT rats because of the increased excretion of urinary zinc.

Intestinal Absorption and Biliary Secretion of Zinc in Rats with Chronic Renal Failure

BACKGROUND/AIMS

In chronic renal failure (CRF), zinc deficiency is partially attributed to decreased intestinal zinc absorption, but the mechanism of this decrease in intestinal zinc absorption is obscure. Therefore, the objective of this study was to investigate the cause of decreased intestinal zinc absorption in a uremic rat model using an in vivo perfusion technique as well as to evaluate the effect of intestinal zinc perfusion on the secretion of biliary zinc in normal and CRF rats.

METHODS

CRF was induced by five-sixths nephrectomy (Nx). During zinc sulfate perfusion, absorption of zinc in the small intestine and the response of plasma zinc level were measured. After intestinal zinc perfusion for 80 min, the concentrations of zinc and metallothionein (MT) in the intestinal mucosal and liver tissue were assayed. The secretion of biliary zinc and the excretion of urinary zinc were also determined before and after zinc sulfate perfusion. The results were compared with those obtained from 10 sham-operated normal rats.

RESULTS

The CRF rats showed a significant decrease in the rate of intestinal zinc absorption and in the response of plasma zinc levels during intestinal zinc perfusion. They also had significantly higher levels of mucosal zinc and MT than sham-operated normal rats, but their contents of liver zinc were significantly lower than those of sham-operated normal rats after zinc sulfate perfusion. CRF rats showed a low plasma zinc level and a high urinary zinc excretion in baseline levels, but had similar output of basal biliary zinc as compared with sham-operated normal rats. Zinc sulfate perfusion in the small intestinal was not found to increase the secretion of biliary zinc and the excretion of urinary zinc, either in normal or CRF rats.

CONCLUSION

In the CRF rat, the reduction of intestinal zinc absorption may result from reduced mucosal zinc efflux from the basolateral membrane into plasma. These data also suggest that the absorbed zinc from the gastrointestinal tract is mostly taken up by the liver or other tissues, and is less excreted in bile juice and urine.