Responses in the absorptive phase in muscle and liver protein synthesis rates of growing rats

The effect of time after beginning of a meal (30, 60, 90 and 120 min) on li ver and gastrocnemius muscle protein synthesis was tested in growing male r ats using the large dose technique, based on a 10 min exposure to [N-15]phe nylalanine. The Fractional synthesis rate was estimated from the ratio betw een the atom percent excess of tissue protein-bound and free labelled pheny lalanine. The latter was measured by gas chromatography mass spectrometry u sing the tertiary-butyldimethylsilyl amino acid derivatives. The protein-bo und phenylalanine of gastrocnemius muscle was separated from the other amin o acids using preparative amino acid chromatography and then oxidised to N- 2 in an automated carbon-nitrogen Roboprep (CN) combustion module attached to a continuous flow isotope ratio mass spectrometer (IRMS), with m/z ions 28 and 29 monitored. The protein-bound phenylalanine from liver was separat ed by a gas chromatograph attached to a sample preparation module and an is otope ratio mass spectrometer (GC-C-IRMS), with again m/z ions of 28 and 29 monitored. The following results were obtained: the daily fractional protein synthesis rates (k(s)) in gastrocnemius muscle and liver were 13.9% and 65.6% respec tively, in 12 h fasted 145 g rats. These k(s) increased within 30 min after ingestion of meal to 14.9% and 91.8% for muscle and liver, respectively, a nd remained at these values for the next 90 min (14.6% and 87.4% at 60 min, and 14.3% and 88.6% at 120 min after the beginning of feeding). It was con cluded that measurement of protein synthesis rates characteristic for the a bsorptive phase can be undertaken in a period from thirty minutes to two ho urs after start of a meal, without significant changes in the k(s) values.