The influence of vitamin a on the susceptibility of the rat lung to 3‐methylcholanthrene

The susceptibility of rats to pulmonary carcinogens is increased in the absence of vitamin A intake even when considerable amounts (0.4 nmole/mg) of vitamin A are stored in the liver and deficiency symptoms are absent. This was demonstrated in studies on the induction of metaplastic lung nodules by intratracheally administered 3-methylcholanthrene. Only moderate amounts of all-trans-retinyl acetate in the diet (7.6 nmole/g of diet) were required to prevent the development of this state of enhanced susceptibility. A further increase in intake of retinoids (by intragastric administration of either all-trans-retinyl acetate or 13-cis-retinoic acid) provided no additional protection against the effects of carcinogen.