IN-VIVO MUSCLE PROTEIN-SYNTHESIS RATES IN THE AMERICAN LOBSTER HOMARUS-AMERICANUS DURING THE MOLT CYCLE AND IN RESPONSE TO 20-HYDROXYECDYSONE

Simultaneous measurements of in vivo rates of protein synthesis (Ks) i n claw, leg and abdominal muscles were made in the American lobster Ho marus americanus at three stages of the moult cycle, Ks values are sig nificantly elevated during the premoult (stage D-2-D-3) and fall durin g the intermoult (stage C-4) periods in all three muscles, Postmoult ( stage A/B) levels are not significantly elevated above intermoult leve ls. Intermoult levels are between 0.3 and 0.4 % protein synthesized pe r day. In the premoult animals, the ribosomal activity (milligrams pro tein synthesized per microgram RNA per day) of the claw, abdominal and leg muscles is elevated three- to fivefold. The claw muscle maintains an elevated ribosomal activity into the postmoult stage whereas, by t his stage, that of the other muscle tissues has fallen to intermoult l evels. The RNA/protein ratios of the three muscle groups from intermou lt, premoult and postmoult animals do not show any significant differe nces. 18S ribosomal RNA levels fluctuate slightly,,vith no consistent pattern over the moult cycle. In vivo injection of premoult concentrat ions of 20-hydroxyecdysone (20-HE) into intermoult lobsters results in elevated K-s values and ribosomal activity for the muscles after 3 da ys. RNA/protein ratios remain constant in the muscles in response to i njections of 20-HE in vivo. In vitro preparations of leg muscle treate d with 20-HE did not show similar elevated rates of protein synthesis.