Abstract
The effects of retinoic acid (RA) (50 μg/100 g body wt. per day) on hepativ heme oxygenase activty, δ-aminolevulinate synthase (ALAS) activity and on cytochrome P-450 content were determined in thyroidectomized rats treated with T 3 (10 μg/100 g body wt. per day) or diluent. RA, when administered for 3 days, failed to influence significantly the activity of either heme oxygenase or ALAS, however, the retinoid depleted hepatic cytochrome P-450 content by 17% ( P < 0.01) and microsomal heme content by 47% ( P < 0.001). T 3 administration enhanced heme oxygenase activity by 72% ( P < 0.001) and ALAS activity by 251% ( P < 0.001) above levels in diluent treated controls and depleted cytochrome P-450 levels by 55% ( P < 0.001) and heme levels by 75% ( P < 0.001). When RA and T 3 were administered together, the retinoid markedly enhanced the T 3 stimulation of heme oxygenase activity; 173% above controls ( P < 0.001), and 61% above T 3 alone ( P < 0.001). However, RA failed to influence the effect of T 3 on ALAS activity or cytochrome P-450 depletion. The results indicate that RA can influence the levels of hepatic cytochrome P-450 and can modulate the stimulation of heme oxygenase activity by thyroid hormone in vivo.
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