In vivo protein turnover and the influence of ecdysteroids in flight muscle of the tobacco hornworm, Manduca sexta

Abstract

In vivo protein turnover was measured during the last 4 days of flight muscle development in tobacco hornworm pupa. Linear synthesis rates were measured up to 2 hours after injection of 30 μmol (3H) phenylalanine. Since the results with this technique did not differ from another established method, the large bolus injection of phenylalanine did not affect protein synthesis. The former method is advantageous because only a single time point is required. Flight muscle growth and protein synthesis decreased in parallel between 100 and 24 hours. During this time free phenylalanine turnover decreased, and the total pool diminished, indicating that this pool could be a major sink for muscle protein synthesis. Proteolysis was rapid even in the growing muscle. 20-Hydroxyecdysone increased muscle growth at certain times by inhibiting proteolysis. Protein synthesis either decreased or was unchanged after injection of the hormone. Therefore ecdysteroids may play a role in controlling growth of the dorsolongitudinal flight muscle during adult development, especially by retarding proteolysis.