Long‐term cholesterol labeling as a convenient means for measuring ecdysteroid production and catabolism in vivo: Application to the last larval instar of Pieris brassicae

In vivo biosynthesis of ecdysteroids during the last larval instar of Pieris brassicae was investigated by administering [3H] cholesterol followed byhigh‐performance liquid chromatography analysis of the resulting [3H] ecdysteroids. The demonstration that the specific activity of the ecdysteroids synthesized at a given time is always identical with that of cholesterol indicates that the cholesterol pool is uniformly labeled, and this allows us to easily calculate the amounts of ecdysteroids produced by animals. The total amount of ecdysone produced throughout the last larval instar was measured as 1.17 nmol/insect. This quantity is more than three‐fold the maximal level of molting hormones (ecdysone +20‐hydroxyecdysone) reached during the instar (0.37 nmol/animal) because a high catabolic activity occurs at the beginning of the hormone production period. Larvae thus differ from pupae, where catabolism is minimal when ecdysone synthesis takes place, resulting in a more “economical” system.