Carnitine and carnitine transferases in the intestinal mucosa of suckling rats

Control of mitochondrial beta-oxidation flux

The control of mitochondrial beta-oxidation, including the delivery of acyl moieties from the plasma membrane to the mitochondrion, is reviewed. Control of beta-oxidation flux appears to be largely at the level of entry of acyl groups to mitochondria, but is also dependent on substrate supply. CPTI has much of the control of hepatic beta-oxidation flux, and probably exerts high control in intact muscle because of the high concentration of malonyl-CoA in vivo. beta-Oxidation flux can also be controlled by the redox state of NAD/NADH and ETF/ETFH(2). Control by [acetyl-CoA]/[CoASH] may also be significant, but it is probably via export of acyl groups by carnitine acylcarnitine translocase and CPT II rather than via accumulation of 3-ketoacyl-CoA esters. The sharing of control between CPTI and other enzymes allows for flexible regulation of metabolism and the ability to rapidly adapt beta-oxidation flux to differing requirements in different tissues.

Developmental Changes in Carnitine Octanoyltransferase Gene Expression in Intestine and Liver of Suckling Rats

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 effect of dexamethasone treatment on the expression of the regulatory genes of ketogenesis in intestine and liver of suckling rats

The influence of the injection of dexamethasone on ketogenesis in 12 day old suckling rats was studied in intestine and liver by determining mRNA levels and enzyme activity of the two genes responsible for regulation of ketogenesis: carnitine palmitoyl transferase I (CPT I) and mitochondrial HMG-CoA synthase. Dexamethasone produced a 2 fold increase in mRNA and activity of CPT I in intestine, but led to a decrease in mit. HMG-CoA synthase. In liver the mRNA levels and activity of both CPT I and mit. HMG-CoA synthase decreased. Comparison of these values with the ketogenic rate of both tissues following dexamethasone treatment suggests that mit. HMG-CoA synthase could be the main gene responsible for the regulation of ketogenesis in suckling rats. The changes produced in serum ketone bodies by dexamethasone, with a profile that is more similar to the ketogenic rate in the liver than that in the intestine, indicate that liver contributes more to ketone body synthesis in suckling rats. Two day treatment with dexamethasone produced no change in mRNA or activity levels for CPT I in liver or intestine. While mRNA levels for mit. HMG-CoA synthase changed little, the enzyme activity is decreased in both tissues.

The effect of fasting/refeeding and insulin treatment on the expression of the regulatory genes of ketogenesis in intestine and liver of suckling rats

The influence of fasting/refeeding and insulin treatment on ketogenesis in 12-day-old suckling rats was studied in intestine and liver by determining mRNA levels and enzyme activity of the two genes responsible for regulation of ketogenesis: carnitine palmitoyl transferase I (CPT I) and mitochondrial HMG-CoA synthase. Fasting produced hardly any change in mRNA or activity of CPT 1 in intestine, but led to a decrease in mitochondrial (mit.) HMG-CoA synthase. In liver, while mRNA levels and activity for CPT I increased, neither parameter was changed in HMG-CoA synthase. The comparison of these values with the ketogenic rate of both tissues under the fasting/refeeding treatment shows that HMG-CoA synthase could be the main gene responsible for regulation of ketogenesis in suckling rats. The small changes produced in serum ketone bodies in fasting/refeeding, with a profile similar to the ketogenic rate of the liver, indicate that liver contributes most to ketone body synthesis in suckling rats under these experimental conditions. Short-term insulin treatment produced increases in mRNA levels and activity in CPT I in intestine, but it also decreased both parameters in mit. HMG-CoA synthase. In liver, graphs of mRNA and activity were nearly identical in both genes. There was a marked decrease in mRNA levels and activity, resembling those values observed in adult rats. As in fasting/refeeding, the ketogenic rate correlated better to mit. HMG-CoA synthase than CPT I, and liver was the main organ regulating ketogenesis after insulin treatment. Serum ketone body concentrations were decreased by insulin but recovered after the second hour. Long-term insulin treatment had little effect on the mRNA levels for CPT I or mit. HMG-CoA synthase, but both the expressed and total activities of mit. HMG-CoA synthase were reduced by half in both intestine and liver. The ketogenic rate of both organs was decreased to 40% by long-term insulin treatment. The different effects of refeeding and insulin treatment on the expression of both genes, on the ketogenic rate, and on ketone body concentrations are discussed.

The expression of mitochondrial 3-hydroxy-3-methylglutaryl-coenzyme-A synthase in neonatal rat intestine and liver is under transcriptional control

Mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HOMeGlt-CoA) synthase regulates ketogenesis in the liver of adult rat and in the intestine and liver of neonatal animals but whose mechanisms of regulation have not been fully defined. To investigate transcriptional control of this gene in intestine and liver of suckling rats a quantitative PCR amplification of the pre-mRNA (heteronuclear RNA), compose of part of the first exon and of the first intron, was carried out. Results show that the intestinal pre-mRNA for mitochondrial HOMeGlt-CoA synthase from suckling rats follows a pattern that is nearly identical to that of mature mRNA, with maximum levels on the ninth postnatal day then decreasing smoothly so that at weaning there is no transcriptional activity. Mitochondrial HOMeGlt-CoA synthase protein follows a pattern that is identical to the pre-mRNA and mature mRNA, suggesting no translational regulation. The changes in transcriptional activity are not produced by the presence of an alternative promoter, since the transcription-initiation site is identical in several tissues assayed, including intestine and liver. Enterocytes are the only intestinal cells that express this ketogenic enzyme, as deduced from immunolocalization experiments. The mature intestinal protein is located in mitochondria and not in the cytosol, which coincides with what is found in liver. By using analogous techniques we conclude that hepatic pre-mRNA of mitochondrial HOMeGlt-CoA synthase from suckling rats follows a pattern of expression identical to that of mature hepatic mRNA, which also suggests a transcriptional modulation of this gene in the liver of neonatal rats.

Developmental changes in carnitine palmitoyltransferases I and II gene expression in intestine and liver of suckling rats

Carnitine palmitoyltransferase (CPT) I is expressed in the intestine of suckling rats; its mRNA increases very rapidly after birth, remains on a plateau until day 18 and decreases until weaning, when basal (adult) values are reached, which remain unchanged thereafter. CPT II mRNA values do not show any appreciable change in this period. CPT I and CPT II are expressed mainly in mucosa and, to a lesser extent, in the muscular part of the intestine. Intestinal expression of CPT I is maximal in duodenum and jejunum, whereas CPT II is expressed in a similar pattern throughout the whole intestine. Dam's milk may influence the intestinal expression of CPT I, since mRNA levels at birth are low but increase after the first lactation. Moreover, rats weaned at either day 18 or 21 decrease their mRNA levels. Apparently, CPT II gene expression is not influenced by the mother's milk. CPT I and CPT II are also expressed in the liver of suckling rats. Hepatic CPT I is maximal at day 3, and levels of CPT II mRNA do not change, in a similar fashion to that in intestine. The profile of expression of CPT I in liver and intestine strongly resembles that previously reported for mitochondrial 3-hydroxy-3-methyl-glutaryl-CoA synthase.

Effect of development and nutritional state on the uptake, metabolism and release of free and acetyl-L-carnitine by the rodent small intestine

Intestinal carnitine levels and the incorporation and release of exogenous, [14C]carnitine were compared in intestine from adult rat and guinea pig. Total carnitine levels were 4-fold higher in rat as compared to guinea pig intestine. Retention of label was also 4-fold greater, 4 h after placing carnitine (7 nmol) in the lumen. Carnitine was detected in rat chow (64 nmol/g) but not in guinea pig chow. Intestinal carnitine was reduced 2-fold in rats fed a carnitine-free diet for 2 weeks, suggesting the importance of dietary carnitine in determining intestinal carnitine levels. Two conditions where fatty acid oxidation is increased (fasting and suckling) resulted in elevated carnitine levels and retention. In the 3-day fasted guinea pig, intestinal carnitine increased by 40% and retention of a lumenal dose of [14C]carnitine increased about 7-fold after 4 h. During suckling, carnitine levels peaked after 3 days (792 nmol/g) and decreased to near adult levels after 7 days (108 nmol/g). Retention of a lumenal dose of carnitine was greater after 4 h in 1-day old neonatal, than in adult intestine (82% vs. 7% of a 7 nmol dose, respectively). This reflects, in part, the larger intestinal carnitine pool on day 1 (352 nmol/g) than on day 29 (91 nmol/g). The calculated efflux of total intestinal carnitine after 4 h was similar for adults and neonates (72 vs. 58 nmol/g) suggesting that efflux relative to pool size was greater in the adult than in the neonate. Uptake of [14C]acetylcarnitine was similar to [14C]carnitine in 1-day old animals, but was retained to a lesser extent (36% vs. 82%, respectively) after 4 h. The calculated efflux of total intestinal carnitine when acetylcarnitine was the substrate was about 4-fold that when carnitine was the substrate. Incorporation of [14C]carnitine into enterocytes isolated from 3-day old animals was 4-fold greater than into enterocytes isolated from adults (152 vs. 36 pmol/mg protein after 60 min). Active transport of carnitine into enterocytes from neonates, but not from adults is suggested, since labeled free intracellular carnitine reached 4-fold the calculated equilibrium value in neonatal enterocytes, but did not exceed the equilibrium value in adult enterocytes.

Ketone formation in the intestinal mucosa of infant rats

The intestinal mucosa of infant rats was found to produce ketones when incubated in Krebs-Ringer-Bicarbonate solution. No production was found in weaned rats. Ketogenesis could be inhibited by D-carnitine or tetradecylglycidic acid (TDGA) an inhibitor of long-chain acylcarnitine transferase, suggesting that ketone production is due to a large extent to break-down of long-chain fatty acids. It is considered possible that both ketones and glucose (also produced by the infant mucosa) serve as substrates for the muscular part of the intestine.

Does carnitine have a role in fat absorption?

The effect of D-carnitine and tetradecylglycidic acid (TDGA), an inhibitor of carnitine palmitoyltransferase, on intestinal absorption of palmitic acid was determined. The proximal intestinal segment was ligated in adult male rats and filled with 0.5 microCi of 14C-palmitic acid alone or with either D-carnitine or TDGA. Thirty minutes later the radioactivity was determined in the intestinal lumen, intestinal wall and plasma. The absorption of palmitic acid was decreased in the presence of D-carnitine (10 mg/ml) as evidenced by significantly lower levels of radioactivity in the gut wall and the plasma and by significantly greater residual radioactivity in the lumenal contents. L-carnitine had no effect on plasma radioactivity but if D- and L-carnitine were given together the effect of D-carnitine was still in evidence. TDGA also inhibited intestinal absorption of palmitic acid.

Uptake of L-carnitine, D-carnitine and acetyl-L-carnitine by isolated guinea-pig enterocytes

Uptake and metabolism of L-carnitine, D-carnitine and acetyl-L-carnitine were studied utilizing isolated guinea-pig enterocytes. Uptake of the D- and L-isomers of carnitine was temperature dependent. Uptake of L-[14C]carnitine by jejunal cells was sodium dependent since replacement by lithium, potassium or choline greatly reduced uptake. L- and D-carnitine developed intracellular to extracellular concentration gradients for total carnitine (free plus acetylated) of 2.7 and 1.4, respectively. However, acetylation of L-carnitine accounted almost entirely for the difference between uptake of L- and D-carnitine. About 60% of the intracellular label was acetyl-L-carnitine after 30 min, and the remainder was free L-carnitine. No other products were observed. D-Carnitine was not metabolized. Acetyl-L-carnitine was deacetylated during or immediately after uptake into intestinal cells and a portion of this newly formed intracellular free carnitine was apparently reacetylated. L-Carnitine and D-carnitine transport (after adjustment for metabolism and diffusion) were evaluated over a concentration range of 2-1000 microM. Km values of 6-7 microM and 5 microM, were estimated for L- and D-carnitine, respectively. Ileal-cell uptake was about half that found for jejunal cells, but the labeled intracellular acetylcarnitine-to-carnitine ratios were similar for both cell populations. Carnitine transport by guinea-pig enterocytes demonstrate characteristics of a carrier-mediated process since it was inhibited by D-carnitine and trimethylaminobutyrate, as well as being temperature and concentration dependent. The process appears to be facilitated diffusion rather than active transport since L-carnitine did not develop a significant concentration gradient, and was unaffected by ouabain or actinomycin A.