title: Ecdysteroids affect in vivo protein metabolism of the flight muscle of the tobacco hornworm (Manduca sexta).
abstract: Ecdysteroid growth promotion of the dorsolongitudinal flight muscle of Manduca sexta was studied by measuring in vivo protein metabolism using both "flooding-dose" and "non-carrier" techniques.  These procedures differ in that the former method includes injection of non-labelled phenylalanine (30 micromoles/insect) together with the [3H]amino acid.  Injected radioactivity plateaued in the haemolymph within 7 min.  With the flooding-dose method, haemolymph and intramuscular specific radioactivities were similar between 15 min and 2 h.  Incorporation of [3H]phenylalanine into muscle protein was linear with either method between 30 and 120 min.  Fractional rates (%/12 h) of synthesis with the flooding-dose technique were best measured after 1 h because of the initial delay in radioactivity equilibration.  Estimation of body phenylalanine turnover with the non-carrier method showed 24-53%/h which was negligible with the flooding-dose method.  Since the two methods yielded similar rates of protein synthesis, the large injection of non-labelled amino acid did not alter the rate of synthesis.  Because the flooding-dose technique requires only a single time point measurement, it is the preferred method.  The decline and eventual cessation of flight-muscle growth was mostly a consequence of declining protein synthesis though degradation increased between 76-86 h before eclosion and was relatively rapid.  This decline in muscle growth could be prevented by treating pupae with 20-hydroxyecdysone (10 micrograms/insect).  Protein accretion was promoted by a decline of up to 80% in protein breakdown, which was offset in part by a concurrent though much smaller decrease in protein synthesis.  Therefore, ecdysteroids may increase flight-muscle growth by inhibiting proteolysis.
authors.affiliation: Department of Biochemistry, University of Arizona, Tucson 85724, USA.
authors.forename: M E
authors.initials: ME
authors.lastname: Tischler
authors.forename: M
authors.initials: M
authors.lastname: Wu
authors.forename: P
authors.initials: P
authors.lastname: Cook
authors.forename: S
authors.initials: S
authors.lastname: Hodsden
mesh_terms: D000818:Animals; D004441:Ecdysterone / Q000494:pharmacology* / Q000502:physiology; D005260:Female; D005426:Flight, Animal / Q000502:physiology; D008297:Male; D018402:Manduca / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D009124:Muscle Proteins / Q000096:biosynthesis / Q000187:drug effects* / Q000378:metabolism*; D011679:Pupa / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D013997:Time Factors
chemical_list: D009124:Muscle Proteins; D004441:Ecdysterone