Ecdysteroids influence growth of the dorsolongitudinal flight muscle in the tobacco hornworm (Manduca sexta)

We have considered whether the decline of the ecdysteroid titre in the latter stages of adult Manduca development may induce cessation of growth of the flight muscle. Specific stages [Stages 6-13; Schwartz and Truman, Dev. Biol. 99, 103-114 (1983)] during the last 100 h of development were timed to allow construction of a growth curve for the dorsolongitudinal flight muscle. Pupa weight and muscle mass were unchanged in successive stages. The muscle ratio of wet weight to dry weight and the extracellular volume declined during these stages, reflecting loss of fluids. A growth curve plotted as total muscle protein content against the time before eclosion showed a gradual decline in the growth rate from 150 μg protein/h during Stage 6 to 50 μg protein/h during Stage 11. Growth ceased during Stage 12. Injection of 20-hydroxyecdysone (10 μg/insect) at Stage 10 or Stage 11 significantly increased the total muscle protein accretion by 6- and 3-fold, respectively, during these stages. However, 20-hydroxyecdysone failed to promote growth during Stage 12. A dose curve showed that 5 μg ecdysteroid/g insect produced a near maximal response. These results suggest that the fall of ecdysteroid during adult development leads to reduced growth of the flight muscle and that shortly before eclosion this muscle becomes unresponsive to these hormones.