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Carnitine octanoyltransferase (COT), which facilitates the transport of shortened fatty acyl-CoAs from peroxisomes to mitochondria, is expressed in the intestinal mucosa of suckling rats; its mRNA levels increase rapidly after birth, remain steady until day 15, and decrease until weaning, when basal, adult values are established, which remain unchanged thereafter. The process seems to be controlled at the transcriptional level since the developmental pattern of mRNA coincides with that of pre-mRNA values. Dam's milk may influence the intestinal expression of COT, since mRNA levels at birth are low and increase after the first lactation. Moreover, mRNA levels decrease in rats weaned on day 18 or 21. COT is also expressed in the liver of suckling rats. Hepatic COT mRNA is maximal at day 3, remains constant until day 9, and decreases thereafter; this pattern is also similar to that of pre-mRNA values. The profile of expression of COT in intestine and liver strongly resembles that of mitochondrial 3-hydroxy 3-methylglutaryl-coenzyme A synthase and carnitine palmitoyltransferase I, suggesting that analogous transcription factors modulate ketogenesis and mitochondrial and peroxisomal fatty acid oxidation.

The cytosolic enzyme phospho(enol)pyruvate carboxykinase (PEPCK) is markedly expressed in the intestinal mucosa of suckling rats. The expression is located in the small intestine, but there is no expression in stomach, colon, or cecum. The expression changes with age. The mRNA levels at birth are very low, increase after the first lactation, reach maximum levels between 3 and 9 days after birth, and then decrease smoothly. At weaning, when animals begin to feed on a solid chow diet, the expression falls to adult levels, which are hardly detectable. Mother's milk may influence the intestinal expression, since in rats weaned at Day 18, 3 days before normal weaning, the mRNA levels decreased dramatically. mRNA levels for PEPCK in liver present a rather different developmental pattern from that of intestine, remaining high at weaning and in adult rats. On the ninth day after birth, the mRNA levels are the same in intestine and liver.