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 liver and gastrocnemius muscle protein synthesis was tested in growing male rats using the large dose technique, based on a 10 min exposure to [¹⁵N]phenylalanine. The fractional synthesis rate was estimated from the ratio between the atom percent excess of tissue protein‐bound and free labelled phenylalanine. The latter was measured by gas chromatography mass spectrometry using the tertiary‐butyldimethylsilyl amino acid derivatives. The protein‐bound phenylalanine of gastrocnemius muscle was separated from the other amino acids using preparative amino acid chromatography and then oxidised to N₂ 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 separated by a gas Chromatograph attached to a sample preparation module and an isotope 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% respectively, in 12h 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, and 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 concluded that measurement of protein synthesis rates characteristic for the absorptive phase can be undertaken in a period from thirty minutes to two hours after start of a meal, without significant changes in the k_s values.

Key words:

• Rat
• Protein synthesis
• Liver
• Muscle
• Postprandial response
• Stable isotopes