DIETARY VITAMIN-D DEPENDENCE OF CAT AND DOG DUE TO INADEQUATE CUTANEOUS SYNTHESIS OF VITAMIN-D

As in herbivores and omnivores, the biosynthesis of vitamin D-3 in the skin exposed to ultraviolet (uv) light is generally expected to also occur in the dog and the cat. The purpose of this in vitro study was t o measure the concentrations of vitamin D-3 and its precursor 7-dehydr ocholesterol (7DHC) in dog and cat skin before and after a quantitativ ely and qualitatively standardized exposure to uv light. The results a re compared to those obtained by the same method in the skin of the ra t. The efficiency of extracting 7DHC and vitamin D-3 from skin was 72 +/- 8% and 67 +/- 3%, respectively. In dog and cat skin the concentrat ions of nonesterified 7DHC were below the detection limit of the HPLC system, Therefore, skin extracts were saponified and total 7DHC and vi tamin D-3 concentrations were measured by normal-phase HPLC. Before ir radiation with uv-B light the total concentrations of 7DHC were 1858 /- 183, 1958 +/- 204, and 17,620 +/- 2345 ng/cm(2) skin (mean +/- SEM; n = 5) for the dog, the cat, and the rat, respectively. The correspon ding concentrations of vitamin D-3 were 211 +/- 44, 193 +/- 18, and 16 1 +/- 32 ng/cm(2) skin for the dog, the cat, and the rat, respectively . Irradiation of standard solutions of 7DHC with 0.15 J uv-B light/min resulted in a time-dependent decrease in 7DHC and a concomitant incre ase in previtamin D-3. After exposure of skin to a total of 2.25 J uv- B light no significant changes in concentrations in vitamin D-3 were f ound in extracts of the skin of the dog and the cat, whereas a 10-fold increase in the vitamin D-3 concentration occurred in the skin of the rat. It is concluded that in the skin of the dog and the cat only low concentrations of esterified 7DHC are present and that this 7DHC is a lso inadequately converted to vitamin D-3. As shown previously there i s no detectable increase in vitamin D-3 in the dog exposed to uv irrad iation in vivo. Therefore, these low 7DHC concentrations are not cause d by high turnover of 7DHC but are due to restricted availability of t his vitamin D-3 precursor in the skin of the dog. Thus, the dog and th e cat are, unlike herbivores and omnivores, not able to synthesize vit amin D-3 adequately in the skin and are mainly dependent on its dietar y intake, i.e., vitamin D-3 is an essential vitamin for the dog and ca t. (C) 1994 Academic Press, Inc.