Characterization of contractile and non-contractile protein synthesis in the stomach, small and large intestine and caecum of the rat, and response to acute ethanol dosage.

An investigation was made into the relative composition and synthesis rates of smooth muscle contractile proteins in vivo in different regions of the rat gastrointestinal tract. There was considerable homogeneity in the composition of sarcoplasmic proteins in the small bowel (i.e. 54-58 mg/g) but considerable variability in the large bowel, i.e. highest in the caecum (97 mg/g) and lowest in the colon (21 mg/g). The myofibrillary protein concentration was constant throughout the gastrointestinal tract, i.e. 20-34 mg/g. Stromal fractions varied from 6 to 39 mg/g and was highest in the cardiac region of the stomach and lowest in the duodenum. Fractional rates of protein synthesis were measured with a flooding dose of L-[4-(3)H]phenylalanine. In control rats synthesis rates in sarcoplasmic protein fractions were relatively much higher (43-107%/day) than myofibrillar (27-52%/day) or stromal (6-26%/day) proteins. Fractional rates of stromal protein synthesis showed the greatest variability while myofibrillar synthesis rates the least, throughout the alimentary tract. Jejunal seromuscular layer myofibrillar proteins had the highest synthesis rates (49%/day). In response to acute ethanol injection, protein synthesis in all jejunal fractions fell by 20-30%. Contractile and non-contractile proteins from the cardiac region of the stomach, duodenal seromuscular layer and large bowel seromuscular layer were insensitive to ethanol administration. Protein synthesis of sarcoplasmic proteins from the antrum, ileum seromuscular layer and myofibrillar proteins from the ileum seromuscular layer and caecum were also significantly depressed as a result of ethanol treatment.