The role of the kidney in the biosynthesis of carnitine in the rat

Effects of a Dietary L-Carnitine Supplementation on Performance, Energy Metabolism and Recovery from Calving in Dairy Cows

Simple Summary

Dairy cows develop metabolic diseases especially in the transition period due to high energy requirements for the process of calving, beginning milk production and, simultaneously, restricted feed intake capacity. L-carnitine is endogenously synthesised as an obligatory, quaternary amine for the initial step of ß-oxidation, but with the onset of lactation it is also excreted with milk, whereby its availability for other metabolic pathways might be limited. Supplemental L-carnitine might be able to fill in this apparent gap and to enhance the efficiency of ß-oxidation, whereby the magnitude of negative energy balance would be decreased. The present experiment mainly focused on the energy-consuming process of calving itself and on the energy metabolism during the first weeks of lactation.

Abstract

Dairy cows are metabolically challenged during the transition period. Furthermore, the process of parturition represents an energy-consuming process. The degree of negative energy balance and recovery from calving also depends on the efficiency of mitochondrial energy generation. At this point, L-carnitine plays an important role for the transfer of fatty acids to the site of their mitochondrial utilisation. A control (n = 30) and an L-carnitine group (n = 29, 25 g rumen-protected L-carnitine per cow and day) were created and blood samples were taken from day 42 ante partum (ap) until day 110 post-partum (pp) to clarify the impact of L-carnitine supplementation on dairy cows, especially during the transition period and early puerperium. Blood and clinical parameters were recorded in high resolution from 0.5 h to 72 h pp. L-carnitine-supplemented cows had higher amounts of milk fat in early lactation and higher triacylglyceride concentrations in plasma ap, indicating increased efficiency of fat oxidation. However, neither recovery from calving nor energy balance and lipomobilisation were influenced by L-carnitine.

PPAR alpha-activation results in enhanced carnitine biosynthesis and OCTN2-mediated hepatic carnitine accumulation

In fasted rodents hepatic carnitine concentration increases considerably which is not observed in PPAR alpha-/- mice, indicating that PPAR alpha is involved in carnitine homeostasis. To investigate the mechanisms underlying the PPAR alpha-dependent hepatic carnitine accumulation we measured carnitine biosynthesis enzyme activities, levels of carnitine biosynthesis intermediates, acyl-carnitines and OCTN2 mRNA levels in tissues of untreated, fasted or Wy-14643-treated wild type and PPAR alpha-/- mice. Here we show that both enhancement of carnitine biosynthesis (due to increased gamma-butyrobetaine dioxygenase activity), extra-hepatic gamma-butyrobetaine synthesis and increased hepatic carnitine import (OCTN2 expression) contributes to the increased hepatic carnitine levels after fasting and that these processes are PPAR alpha-dependent.

Decreased carnitine biosynthesis in rats with secondary biliary cirrhosis

Carnitine biosynthesis was investigated in rats with secondary biliary cirrhosis induced by bile duct ligation (BDL) for 4 weeks (n = 5) and in pair-fed, sham-operated control rats (n = 4). Control rats were pair-fed to BDL rats, and all rats were fed an artificial diet with negligible contents of carnitine, butyrobetaine, or trimethyllysine. Biosynthesis of carnitine and its precursors was determined by measuring their excretion in urine and accumulation in the body of the animals. Four weeks after BDL, total carnitine content was increased by 33% in livers from BDL rats when compared with control rats, but was unchanged in skeletal muscle and whole carcass. The plasma total carnitine concentration averaged 29.0 +/- 4.1 vs. 46.4 +/- 7.3 micromol/L in BDL rats and control rats, respectively. Urinary total carnitine excretion was reduced by 56% in BDL rats as compared with control rats. Carnitine biosynthesis was significantly decreased in BDL rats (0.45 +/- 0.19 vs. 0.93 +/- 0.08 micromol/100 g body weight/d in BDL and control rats, respectively). The tissue content of free and protein-linked trimethyllysine, a carnitine precursor, and trimethyllysine plasma concentrations were not different between BDL and control rats. However, urinary trimethyllysine excretion was increased 5-fold in BDL rats and approximated glomerular filtration. In contrast, urinary excretion of butyrobetaine, the direct carnitine precursor, was decreased by 40% in BDL rats as compared with control rats. Trimethyllysine biosynthesis was not different, but butyrobetaine biosynthesis was decreased by 51% in BDL as compared with control rats. In conclusion, carnitine biosynthesis is decreased in BDL rats as a result of a defect in the conversion of trimethyllysine to butyrobetaine.

Mechanism of increased hepatic concentration of carnitine by clofibrate

Our previous studies have shown that treatment of rats with clofibrate, a hypolipidemic drug, greatly increases the total concentration of carnitine in the liver (H. S. Paul and S. A. Adibi, J. Clin. Invest. 64: 405-412, 1979). In the present experiment we have investigated some possible mechanisms to account for this increase. Clofibrate treatment (30 mg/100 g rat/day for 2 wk) increased significantly the concentration (nmol/g, mean +/- SE, 6 rats) of both free (289 +/- 21 vs. 1,747 +/- 131) and acylcarnitine (87 +/- 11 vs. 412 +/- 42). These increases were not the result of redistribution of carnitine among tissues or due to a decrease in urinary excretion. In view of previous observations that thyroid hormones increase the hepatic concentrations of carnitine, and clofibrate treatment causes a hyperthyroid state in the liver, we investigated the effect of clofibrate in thyroidectomized rats. Clofibrate treatment of thyroidectomized rats also increased the concentration of free (423 +/- 25 vs. 1,460 +/- 123) and acylcarnitine (35 +/- 6 vs. 305 +/- 31) in the liver. Finally, clofibrate treatment significantly increased the urinary excretion of trimethyllysine, a precursor of carnitine (31 +/- 3 vs. 47 +/- 4 nmol/mg creatinine, mean +/- SE, 5 rats). Our data suggest that clofibrate treatment stimulates hepatic synthesis of carnitine by increasing the availability of its precursor, trimethyllysine. This effect of clofibrate is independent of thyroid hormone.

[Serum levels and urine excretion of L-carnitine in patients with normal and impaired kidney function]

The influence of age, sex, and renal function on serum levels and urinary excretion of free carnitine was studied in 187 subjects. Sixty-one subjects with normal renal function (creatinine clearance greater than 100 ml/min) showed a serum carnitine level of 72.2 +/- 23.2 mumol/l. The carnitine values of males (76.8 +/- 23.3 mumol/l, n = 39) were higher (p less than 0.05) than those of females (64.0 +/- 21.0 mumol/l, n = 22). Carnitine levels did not correlate with age. Values in patients with normal renal function did not differ from serum carnitine levels in healthy controls (74.7 +/- 17.5 mumol/l, n = 49). The mean urinary carnitine excretion per day was 163.5 mumol (range 63.7-419.6 mumol) in patients with intact renal function. Extreme impairment of glomerular filtration rate (creatinine clearance less than 20 ml/min) resulted in higher carnitine concentrations in serum (108.9 +/- 39.4 mumol/l, n = 18, p less than 0.05), lower carnitine elimination per day (78.5 mumol, range 14.5 - 424.3 mumol, n = 18, p less than 0.05) and a decreased carnitine clearance (0.8 ml/min, range 0.2 - 3.8 ml/min). These data together with earlier results obtained in dialysis patients suggest that carnitine metabolism in renal failure is altered by reduction of both endogenous carnitine biosynthesis and renal carnitine clearance.

Effects of dietary carnitine on myocardial palmitate oxidation in the aging rat

Aging is associated with decreases in myocardial fatty acid oxidation and carnitine concentration. The purpose of this study was to investigate the effects of dietary carnitine manipulation on myocardial palmitate oxidation and carnitine content in young adult and middle-aged rats. Rats were fed either a carnitine-free or a carnitine-supplemented diet for nine weeks and killed at ages 6.5 and 18 months. Myocardial carnitine content was unaffected by age or diet. However, in 18 months rats fed a carnitine-free diet, myocardial palmitate oxidation was 77% higher, carnitine palmitoyltransferase activity 39% higher, and lipid droplet volume density 55% higher compared to 18 month rats fed a carnitine-supplemented diet. In 6.5 month rats, dietary carnitine had no effect on these variables. These results indicate that dietary carnitine restriction increases myocardial fatty acid metabolism in middle-aged but not in young adult rats.

Carnitine improves lipid anomalies in haemodialysis patients

51 chronic haemodialysis patients with hypertriglyceridaemia were given a daily oral dose of 2.4 g D,L-carnitine for 30 days to investigate a possible hypolipaemic effect. After 30 days' D,L-carnitine treatment the mean (+/- SEM) serum triglyceride concentration had decreased significantly from 3.50 +/- 0.39 to 2.87 +/- 0.27 mmol/l. Serum total cholesterol did not change. However, HDL cholesterol increased significantly from 0.89 +/- 0.05 to 1.35 +/- 0.07 mmol/l. This decrease in serum triglycerides and return of HDL cholesterol to normal levels in haemodialysis patients may be the result of correction of carnitine deficiency. Such treatment could reduce the risk factors for atherosclerosis and coronary-artery disease in uraemic patients.

Transport and metabolism of carnitine precursors in various organs of the rat

Isolated, vascularly perfused small intestine, liver, and kidney were used to investigate their interdependence in the absorption and metabolism of carnitine precursors in the rat. During 30 min of recirculating perfusion, the small intestine absorbed trimethyllysine, hydroxytrimethyllysine, and trimethylaminobutyrate fairly well when they were administered via the lumen or the perfusate. Trimethylaminobutyrate was synthesized from either trimethyllysine or hydroxytrimethyllysine by the small intestine, but further hydroxylation of trimethylaminobutyrate to carnitine did not occur. Trimethyllysine and hydroxytrimethyllysine were not readily absorbed by the liver. In contrast, trimethylaminobutyrate and trimethylaminobutyraldehyde were rapidly absorbed from the perfusate and readily incorporated into carnitine by the liver. Trimethyllysine and hydroxytrimethyllysine were taken up slowly by the kiodney and partially converted to trimethylaminobutyrate during 3409 min of perfusion. Trimethylaminobutyrate was neither absorbed readily by the kidney nor was it hydroxylated to carnitine. These results were compared to whole animal studies performed over an equivalent time period. The data suggest that the isolted small intestine absorbs trimethyllysine well, but it probably plays a minor role in metabolizing physiological quantities of this compound in the whole animal where other organs are competing for the same substrate. In both the isolated organ and in the whole animal, the kidney absorbs and metabolizes trimethyllysine more readily than the liver; whereas the liver absorbs trimethylaminobutyrate more rapidly than either the kidney or the small intestine and, unlike these organs, converts it to carnitine.