The kidney plays an important role in vitamin A metabolism. Vitamin A is an essential micronutrient with several biological functions. Dogs and other carnivorous species transport vitamin A differently from other mammals. Dogs transport vitamin A in the plasma as retinol and as vitamin-A retinyl esters that are bound to three lipoproteins. The retinyl esters are dependent upon dietary vitamin A intake. Retinol is bound to retinol-binding protein (RBP). This study measured several variables linked to vitamin A status in the plasma and urine of normal dogs and dogs with chronic renal failure. Dogs with renal failure had higher levels of retinol and RBP in their urine than control dogs. Normal dogs do not excrete much RBP in urine, indicating that it is reabsorbed from the ultrafiltrate. Dogs with chronic renal disease had significant impairment of excretion of Tamm-Horsfall protein (THP). Measurement of urinary RBP and THP may be an appropriate marker for diagnosis of tubular proteinuria.

COMMENTARY: Proteinuria in dogs is usually detected by the use of urine dipsticks and urine protein-urine creatinine ratios. Measuring urinary RBP and THP may provide an earlier indication of renal function, allowing earlier intervention.

Effects of chronic renal disease on the transport of vitamin A in plasma and urine of dogs. Raila J, Forterre S, Kohn B, Brunnberg L, Schweigert FJ. AM J VET RES 63:874-879, 2003.