Radioiodinated free fatty acids; can we measure myocardial metabolism?

Fatty acids for myocardial imaging

Radioiodinated free fatty acids are tracers that can be used to assess both myocardial perfusion and metabolism. There have been several fatty acids and structurally modified fatty acids studied since Evans' initial report of radiolabeled I-123 oleic acid in 1965. The radiolabeling of a phenyl group added to the long chain fatty acids in the omega-terminal position opposite the carboxyl terminal group prevents nonspecific deiodination and the rapid release of free iodine as the tracer undergoes beta-oxidation. The additional inclusion of a methyl or dimethyl group to the chain slows oxidation resulting in prolonged myocardial retention. The longer retention of the radiolabel permits longer image acquisitions more compatible with single photon emission computed tomography (SPECT) imaging, especially with single-detector imaging systems. Several protocols have been implemented using these compounds, particularly 15-(para-iodophenyl)-3-R,S-methyl pentadecanoic BMIPP, to detect abnormal fatty acid metabolism in ischemic heart disease as well as in nonischemic and hypertrophic cardiomyopathies. Successful management of patients with ischemic cardiomyopathies depends on the accurate identification of hibernating myocardium. The studies covered in this review suggest that both IPPA and BMIPP, especially when combined with markers of myocardial perfusion, may be excellent tracers of viable and potentially functional myocardium. Future studies with larger numbers of patients are needed to confirm the results of these studies and to compare their efficacy with that of other available imaging modalities. Cost and distribution issues will have to be resolved for these metabolic tracers to compete in the commercial marketplace. Otherwise they will likely be available only on a limited basis for research use. As progress is made with these issues and with the development of newer imaging systems, the use of radioiodinated and fluorinated fatty acids is likely to be increasingly attractive.

Myocardial fatty acid metabolism during acute cardiac allograft rejection

Fatty acids are promptly taken up, metabolised and eliminated by healthy cardiomyocytes. Cardiomyopathy, coronary heart disease and chronic rejection are known to be associated with an impaired fatty acid metabolism. It was the aim of this study to investigate fatty acid metabolism in a rat heart transplant model and to correlate scintigraphic findings with histological changes. After right-side nephrectomy of Lewis recipients Brown Norway cardiac allografts were anastomosed to the renal vessels. Animals were given no immunosuppression. The metabolism of carrier-free 17-123 jodo-heptadecanoic acid (123J-HDA) with a specific activity of > 2 x 10(17) Bq/ml was scintigraphically measured between days 1 and 11. An increase in the grade of rejection was observed over time. Fifty-six frames of 30 s duration each were recorded. For the region of interest (native heart, transplanted heart, left kidney) frames 10-56 were superimposed, time-activity curves generated and monoexponentially fitted. Furthermore, elimination half-life and intercepts were calculated. Following scintigraphic evaluation the animals were killed and graft as well as native hearts excised for histological examination. The uptake of the tracer identified severe grades of rejection. Elimination half-life of the tracer was twice as long from hearts with mild rejection and more than 14 times as long in severe rejection compared with no rejection. Elimination half-life and amplitude did not permit discrimination between grades 1, 2 and 3 a, but significantly decreased in groups 3 b and 4. This method therefore seems to be a valuable tool for the noninvasive detection of severe acute cardiac allograft rejection. Since fatty acid metabolism is clearly stress-dependent it remains to be seen whether this method allows detection of earlier rejection in loaded hearts.

Interrelationship between lactate and cardiac fatty acid metabolism

This overview is presented, in the main, to summarize the following aspects of lactate and cardiac fatty acid metabolism: 1. The utilization of exogenous carbohydrates and fatty acids by the heart. 2. The competition between lactate and fatty acids in cardiac energy metabolism. 3. The effect of lactate on endogenous triacylglycerol homeostasis. 4. Lactate-induced impairment of functional recovery of the post-ischemic heart. 5. The effect of lactate on lipid metabolism in the ischemic and post-ischemic heart. 6. The consequences of hyperlactaemia for cardiac imaging.

Incorporation of radioiodinated fatty acids into cardiac phospholipids of normoxic canine myocardium

The aim of this study was to assess the phospholipid distribution of radioiodinated 17-iodoheptadecanoic acid (IHDA), 15-(p-iodophenyl)pentadecanoic acid (p-IPPA) and 15-(p-iodophenyl)-3,3-dimethylpentadecanoic acid (DMIPPA) under normoxic conditions and to compare these data with the fatty acid composition of the phospholipid classes. After simultaneous i.v. injection of the radioiodinated fatty acids (I-123-IHDA; I-131-p-IPPA; I-125-DMIPPA) in open-chest dogs seven myocardial biopsies were taken over 40 min (n = 26). After lipid extraction of the biopsies the organic phase was analyzed for both neutral and polar lipids by two different TLC systems. The following polar lipid fractions were analyzed: lysophosphatidylcholine (LPC), sphingomyelin (SPH), phosphatidylcholine (PC; lecithin), phosphatidylinositol (PI), phosphatidylserine (PS), phosphatidylethanolamine (PE), diphosphatidylglycerol (DPG; cardiolipin) and neutral lipids. Fractions were counted in a gamma well counter and corrected for cross-over and recovery. Results of the polar phospholipids analysis showed that IHDA has the highest incorporation into the phospholipids. The IHDA was mainly incorporated into PI (45.6%) followed by PC (30.9%), PE (14.0%) and PS (5.6%). The p-IPPA was predominantly incorporated incorporated into PC (37.2%), followed by PS (20.1%) and PE (13.7%). In contrast to IHDA, incorporation of p-IPPA into PI was small (6.4%). The DMIPPA analogue was incorporated into phospholipids to only a very small degree, compared to IHDA and p-IPPA.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced lipolysis of myocardial triglycerides during low-flow ischemia and anoxia in the isolated rat heart

We studied lipolysis in the isolated rat heart, measured as glycerol release during anoxia, low-flow ischemia and subsequent reperfusion. It was found that the rate of lipolysis was enhanced during ischemia/anoxia while the lipase activities in tissue extracts involved in the myocardial lipolysis and the amount of triglycerides were not affected. This indicates the dominant occurrence of a lipolysis-reesterification principle in ischemic and anoxic tissue. A common observation of ischemia/anoxia is an increase in the tissue NADH/NAD+ ratio. Therefore we investigated the effect of lactate and malate, both of which enhance the tissue redox state on myocardial lipolysis. Perfusion in the presence of lactate (10 mM) and malate (10 mM) both stimulated myocardial lipolysis by about five times. This suggests that the rate of reesterification of product fatty acids to triglycerides, which is determined by the NADH/NAD+ ratio, because of the increased formation of glycerol 3-phosphate from dihydroxy acetone phosphate, plays an important role in the regulation of lipolysis. The existence of triglyceride-fatty acid-triglyceride cycle is discussed.