Abstract
Genetic linkage studies implicated deficiency of CD36, a membrane fatty acid (FA) transporter, in the hypertriglyceridemia and hyperinsulinemia of the spontaneously hypertensive rat (SHR). In this study we determined whether loss of CD36 function in FA uptake is a primary determinant of the SHR phenotype. In vivo, tissue distribution of iodinated, poorly oxidized beta-methyliodophenyl pentadecanoic acid (BMIPP) was examined 2 h after its intravenous injection. Fatty acid transport was also measured in vitro over 20 to 120 s in isolated adipocytes and cardiomyocytes obtained from SHR and from a congenic line (SHRchr4) that incorporates a piece of chromosome 4 containing wild-type CD36. SHR heart and adipose tissue exhibited defects in FA uptake and in conversion of diglycerides to triglycerides that are similar to those observed in the CD36 null mouse. However, a key difference in SHR tissues is that fatty acid oxidation is much more severely impaired than fatty acid esterification, which may underlie the 4-5-fold accumulation of free BMIPP measured in SHR muscle. Studies with isolated adipocytes and cardiomyocytes directly confirmed both the defect in FA transport and the fact that it is underestimated by BMIPP. Heart, oxidative muscle, and adipose tissue in the SHR exhibited a large increase in glucose uptake measured in vivo using [(18)F]fluorodeoxyglucose. Supplementation of the diet with short-chain fatty acids, which do not require CD36-facilitated transport, eliminated the increase in glucose uptake, the hyperinsulinemia, and the heart hypertrophy in the SHR. This indicated that lack of metabolic energy consequent to deficient FA uptake is the primary defect responsible for these abnormalities. Hypertension was not alleviated by the supplemented diet suggesting it is unrelated to fuel supply and any contribution of CD36 deficiency to this trait may be more complex to determine. It may be worth exploring whether short-chain FA supplementation can reverse some of the deleterious effects of CD36 deficiency in humans, which may include hypertrophic cardiomyopathy.
Highlights
The spontaneously hypertensive rat (SHR)1 is a widely studied rodent model of human metabolic Syndrome X, in which hypertension is associated with dyslipidemia and with insulin resistance of glucose metabolism [1]
Since the ability to detect metabolic defects in the SHR may depend on the type of wild-type strain used for comparison, we examined BMIPP and 2-FDG uptake in tissues from SHR and a congenic line (SHRchr4) that incorporates a piece of chromosome 4 containing wild-type CD36
SHR tissues deficient in FA uptake exhibit a large increase in basal glucose uptake, which is eliminated by provision of short-chain FA that do not require CD36 for transport (Fig. 6 and Table IV)
Summary
Animals and Diets—Breeding pairs for SHR (NCrlBR) and WKY (NCrlBR) controls were purchased from the Charles River Company. Tissue Distribution of BMIPP and Fluorodeoxyglucose (2-FDG)— Each rat was injected in a lateral tail vein with 200 l of the radioisotope solution (14 –25 Ci). A 10-l aliquot of washed platelets (about 3 ϫ 107 cells) suspended in 0.02 M phosphate-buffered saline with 9 mM EDTA and 0.1% bovine serum albumin (PEB) was incubated with a 1:100 dilution of anti-CD36 antibody and with 1:150 fluorescein isothiocyanate-labeled secondary antibody. The band at the interphase was harvested, diluted (1:7) in TES buffer, and centrifuged at 350,000 ϫ g to yield a total microsomal pellet (P3) Samples from both P1 and P3 (20 –50 g of protein) were subjected to electrophoresis according to Laemmli [18] followed by transfer to a nylon-supported nitrocellulose membrane. Suspensions were kept at about 25% cell density and were assayed within 2 h after isolation using the same assay conditions as for adipocytes
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