Renal excretion of riboflavin in the rat

Riboflavin transport by isolated perfused rabbit renal proximal tubules

Rabbit renal proximal tubular transport of riboflavin (RF) was examined by using the in vitro isolated tubule perfusion technique. We found that proximal tubules actively reabsorbed (J(lb)) and secreted (J(bl)) RF. At 0.1 microM RF concentration, J(bl) was significantly higher than J(lb), resulting in a net secretion. This net secretion of RF was decreased at 0.01 microM RF concentration and increased at 1 microM RF concentration. Both J(lb) and J(bl) were inhibited by lowering temperature or by adding iodoacetate, a metabolic inhibitor, and lumichrome, an RF analog, suggesting the involvement of carrier-mediated transport mechanisms. J(bl) was inhibited by probenecid, an anion transport inhibitor, and by para-aminohippuric acid, an organic anion, suggesting the relevance of RF secretion to renal organic anion transport. J(bl) was also inhibited by alkaline pH (8.0) and by the calmodulin inhibitor trifluoperazine, indicating the influence of pH and Ca(2+)/calmodulin-dependent pathway on RF secretion. Finally, we found that addition of chlorpromazine, a phenothiazine derivative, inhibited both J(lb) and J(bl), raising the concern about the nutritional status in patients receiving such a type of medication.

Plasma and urine riboflavin during riboflavin-free nutrition in very-low-birth-weight infants

BACKGROUND

Very-low-birth-weight (VLBW; birth weight <1500 g) infants receive enteral and parenteral nutriture that provides greater daily riboflavin (vitamin B2) than does term infant nutriture, and elevated plasma riboflavin develops in these infants after birth. The purpose of this study was to measure plasma and urine riboflavin concentrations in VLBW infants during riboflavin-free nutrition. Our hypothesis was that elevated plasma riboflavin develops in VLBW infants because of high daily intake and immature renal riboflavin elimination.

METHODS

Eighteen clinically healthy VLBW infants received parenteral nutrition and preterm infant formula during the first postnatal month. On postnatal days 10 and 28, the infants received specially prepared riboflavin-free enteral and parenteral nutrition for the 24-hour study period. Serial collections of plasma were made at time 0 and at 12 and 24 hours. Urine was collected continuously for the 24-hour period in 4-hour aliquots. Samples were analyzed for riboflavin concentration.

RESULTS

During the 24-hour riboflavin-free study period on postnatal day 10, plasma riboflavin decreased 56% from 185 +/- 37 ng/mL (mean +/- SEM), and urine riboflavin decreased 75% from 3112 +/- 960 mg/mL. Similarly, on postnatal day 28, plasma riboflavin decreased 79% from 184 +/- 32 ng/mL, and urine riboflavin concentration decreased 91% from 5092 +/- 743 ng/mL during the 24-hour riboflavin-free study period. Riboflavin half-life (t(1/2)) was 18.5 hours on postnatal day 10 and decreased 48% by postnatal day 28. Riboflavin elimination was 145.1 +/- 20.6 mg/kg per day on postnatal day 10 and increased 40% by postnatal day 28.

CONCLUSION

The VLBW infants who received parenteral nutrition and preterm infant formula had elevated plasma riboflavin on postnatal days 10 and 28. Plasma riboflavin t(1,2) was shorter and renal riboflavin elimination was greater on postnatal day 28 than on postnatal day 10. Plasma riboflavin was normal after 24 hours of riboflavin-free nutrition. The pattern of plasma and urine riboflavin in VLBW infants suggests a lower daily intake would maintain plasma riboflavin close to normal.

Riboflavin transport by rabbit renal basolateral membrane vesicles

The present study examined riboflavin (RF) uptake by isolated rabbit renal basolateral membrane (BLM). RF uptake was linear during the initial 10 seconds and leveled off thereafter with longer incubation. Studies on RF uptake as a function of incubation medium osmolarity indicated that the BLM RF uptake was the results of transport (approximately 45%) into the intravesicular space as well as binding (approximately 55%) to membrane surfaces. The RF binding to BLM was Na+-dependent so that replacement of Na+ by other cations eliminated the binding component of RF uptake. The process of BLM RF uptake was saturable as a function of substrate concentration and was significantly inhibited by cis-addition of its structural analogs, lumiflavin and lumichrome, indicating the involvement of a carrier-mediated process. The BLM RF uptake was affected by changes in extravesicular pH so that, as compared to pH 7.5, RF uptake was lower at pH 6.5 and higher at pH 8.5. The effect of extravesicular pH persisted when the transmembrane H+ gradient was dissipated by FCCP, indicating the direct effect of pH on BLM RF uptake. The BLM RF uptake was not affected by alterations of the transmembrane electrical potential, induced by either the presence of anions with different membrane permeability (Cl-=NO-3>SO-4>gluconate-) or using nigericin (10 microg/mg protein) with an outwardly or inwardly directed transmembrane K+ gradient. The BLM RF uptake was, however, inhibited by probenecid and p-aminohippurate, and was enhanced by trans-RF. In summary, these results demonstrate the existence of a Na+-dependent BLM binding of RF and a membrane-associated carrier system for RF uptake by renal BLM.

Vitamin concentrations in very low birth weight infants given vitamins intravenously in a lipid emulsion: measurement of vitamins A, D, and E and riboflavin

Because total parenteral nutrition with vitamins added to the glucose-amino acid mixture is often associated with a reduction in blood levels of vitamin A (retinol) during the routine treatment of many very low birth weight (VLBW) infants (less than 1500 gm), and because retinol losses in the plastic delivery system can be prevented by adding the vitamins to an intravenous lipid emulsion, seven VLBW infants with a mean birth weight of 900 gm (range 450 to 1360 gm) were given 40% of a unit dose vial, per kilogram of body weight, of a multivitamin preparation (M.V.I. Pediatric) (280 micrograms retinol; 160 IU vitamin D; 2.8 mg tocopherol; 0.68 mg riboflavin) in a lipid emulsion, Intralipid. After treatment with the intralipid-vitamin mixture for 19 to 28 days, plasma vitamin A (retinol) concentrations increased significantly from 11.0 +/- 0.76 (mean +/- SEM) before intralipid to 19.2 +/- 0.97 micrograms/dl after the intralipid-vitamin mixture (p less than 0.01); 25-hydroxyvitamin D concentrations increased from an initial value of 12.6 +/- 2.6 to 20.2 +/- 1.9 mg/dl (p less than 0.01); alpha-tocopherol concentrations increased from an initial value of 0.31 +/- 0.06 to 2.44 +/- 0.13 mg/dl (p less than 0.01); and riboflavin levels increased from 64.1 +/- 7.8 ng/ml to concentrations between 20 and 100 times the initial level. Erythrocyte riboflavin levels increased from 71.8 +/- 14 initially to 166 +/- 41 ng/gm hemoglobin, and erythrocyte flavin-adenine dinucleotide levels increased similarly from 972 +/- 112 initially to 2005 +/- 294 ng/gm hemoglobin. These results show that the addition of M.V.I. Pediatric to Intralipid decreases the extensive in vivo loss of retinol and is associated with an increase in plasma retinol concentrations in VLBW infants. The daily doses of vitamins D (160 IU/kg) and E (2.8 mg/kg) appear sufficient, but the dose of vitamin A (280 micrograms/kg) is insufficient to raise blood levels of all infants into the normal range. The current dose of riboflavin is excessive and may be harmful.