Pierre Cuchet

Do iodinated fatty acids undergo a nonspecific deiodination in the myocardium

The intracellular and subcellular distribution of 16-(123I)-iodo-9-hexadecenoic acid were studied in isolated rat hearts, perfused with or without glucose. At various time intervals after injection, cardiac lipids were extracted and the activity was determined for all fractions and all lipid calsses. The total cardiac activity was maximal within 1 min postinjection and most of the activity was in the aqueous phase. The presence of glucose in the perfusion medium induced an increase of total cardiac and organic fraction activities. In the latter fraction, activity was very low for FFA, but high for triglycerides (TG), and especially polar lipids. The presence of an exogenous substrate, led to a more active esterification of fatty acids. Coronary effluent analysis showed, in the hydrophilic phase, a lower activity spike in the presence than in the absence of glucose. In the mitochondrial fraction most activity occurred in the organic phase, especially as polar lipids. In the nonmitochondrial fraction, activity was much higher in the aqueous phase. At 90 s postinjection of 1-14C-palmitic acid, over 80% of the myocardial activity was found in the hydrophilic fraction, which indicates, as for the iodo-fatty acid (IFA), an immediate and important oxidation, especially without glucose. These data seem to prove that IFA is taken up by the myocardial cell, subsequently enters the mitochondria and, without an early deiodination, is oxidized with iodide release. Changes in IFA metabolism, consecutive to modifications of glucose concetration in the perfusion medium can be observed by external detection of the myocardial activity curve. ω-Iodinated fatty acids do not undergo a nonspecific deiodination and are therefore well suited for an external study of myocardial metabolism.

Kinetics of iodomethylated hexadecanoic acid metabolism in the rat myocardium: influence of the number and the position of methyl radicals

The methyl-branched fatty acids, if radioiodine labelled in alpha position, are potentially adapted to a selective study of FA myocardial uptake. To determine the position and the number of methyl radicals that are necessary to obtain a maximal uptake and a minimal degradation, we measured time-activity evolution of isolated and perfused rat hearts after an injection of iodinated fatty acids which are mono- or dimethylated in alpha or beta position. Except for dimethyl fatty acid, the uptake is similar for all fatty acids studied to that of the straight chain analogue; beta mono- or dimethyl fatty acids seem best adapted to a study of the uptake because alpha monomethyl fatty acids undergo a metabolic degradation and alpha mono- and dimethyl fatty acids induce ventricular fibrillations.

Iodomethylated fatty acid metabolism in mice and dogs

The myocardial uptake of fatty acids labeled with radioactive iodine and injected i.v. can only be evaluated with SPECT if their oxidation kinetics is slow enough. For this reason, we evaluated different iodomethylated fatty acids in mice and dogs to determine which of them shows the highest myocardial uptake and the slowest oxidation. The most suitable was found to be 16-iodo-3-methyl hexadecanoic acid (mono β) since its myocardial fixation was the same as that of the reference, i.e. 16-iodo-9-hexadecenoic acid (IHA), whereas it was degraded more slowly. Thirty min after injection of mono β into dogs, the decrease in myocardial activity with respect to the maximum was two fold less than after IHA injection. The myocardial uptake of the two dimethylated fatty acids studied, i.e. 16-iodo-2,2-methyl hexadecanoic acid and 16-iodo-3,3-methyl hexadecanoic acid, was less than that of IHA in mice and dogs. In the latter, the myocardial uptake was so small that we were unable to study the time course of its activity. Consequently, these dimethylated fatty acids are not suitable for the study of the myocardial uptake of fatty acids in man.

Mathematical model of the metabolism of 123I-16-iodo-9-hexadecenoic acid in an isolated rat heart. Validation by comparison with experimental measurements

The aim of the present study was to demonstrate that it is possible to estimate the intracellular metabolism of a fatty acid labelled with iodine using external radioactivity measurements. 123I-16-iodo-9-hexadecenoic acid (IHA) was injected close to the coronary arteries of isolated rat hearts perfused according to the Langendorff technique. The time course of the cardiac radioactivity was measured using an INa crystal coupled to an analyser. The obtained curves were analysed using a four-compartment mathematical model, with the compartments corresponding to the vascular-IHA (O), intramyocardial free-IHA (1), esterified-IHA (2) and iodide (3) pools. Curve analysis using this model demonstrated that, as compared to substrate-free perfusion, the presence of glucose (11 mM) increased IHA storage and decreased its oxidation. These changes were enhanced by the presence of insulin. A comparison of these results with measurements of the radioactivity levels within the various cellular fractions validated our proposed mathematical model. Thus, using only a mathematical analysis of a cardiac time-activity curve, it is possible to obtain quantitative information about IHA distribution in the different intracellular metabolic pathways. This technique is potentially useful for the study of metabolic effects of ischaemia or anoxia, as well as for the study of the influence of various substrates or drugs on IHA metabolism in isolated rat hearts.

A new experimental model for studies of drug actions on myocardial metabolism. Application to a study of the influence of POCA

In order to study myocardial metabolism by external detection, quantitative information on the metabolism of a gamma-emitting iodinated fatty acid (IHA) was obtained from time-activity curves of radioactivity in different compartments. A 4-compartment mathematical model was then developed; compartments 0, 1, 2, and 3 correspond respectively to vascular IHA, intracellular IHA, esterified forms, and iodide resulting from mitochondrial oxidation of IHA. We applied this model to a study of the influence of an inhibitor of fatty acid oxidation, POCA (2-[5(4 chlorophenyl) pentyl]-oxirane-2-carboxylate). Isolated rat hearts were perfused for 20 min with Krebs liquid containing increasing concentrations of POCA. IHA was then injected as a bolus at the entrance of the coronary network. The level of cardiac radioactivity was recorded for 30 min and the division into the 4 compartments was simulated at different concentrations of POCA. The drug appeared to increase the myocardial retention of IHA and slow down the speed of degradation and storage; the variations were dose-dependent. These results correspond to those obtained by intracellular analysis. The proposed method, which is reliable and sensitive, is an interesting experiment for pharmacological studies of cardiac metabolism.

Effect of short-term fasting on [123I] iodohexadecenoic acid metabolism in the isolated perfused rat heart. Validation of mathematical model by comparison with experimental measurements

In order to study metabolic modifications induced by short term fasting and their consequences on the uptake and intracellular fate of fatty acids iodine labelled in omega position, rats undergo a 36h fasting. Hearts are perfused in a Langendorff system with a glucose (11 mM) perfusion medium; [123I] hexadecenoic acid (IHA) is injected as a bolus. A comparison between time-activity curves p.i. demonstrates a much faster activity decrease for the hearts fasted animals. The intracellular analysis shows that short fasting did not significantly increase the myocardial uptake of fatty acids, but decreased the storage and increased the degradation of the fatty acids taken up. Mathematical analysis of the myocardial time-activity curves obtained by external detection provided results comparable to those of intracellular analysis. The coefficients of correlation between the values of the aqueous phases, organic phases and free fatty acids measured by intracellular analysis and calculated with the compartmental model are consistently higher than 0.97. Consequently, this experimental model combined with mathematical analysis of the time-course of myocardial radioactivity after 123IHA administration appears to be very promising method for studying the effects of drugs or variations of energy substrate availability on myocardial fatty-acid metabolism.

Influence of the presence of a methyl group on the myocardial metabolism of 15-(paraiodophenyl)-3 methyl pentadecanoic acid (IMPPA)

The objective of the present study was to determine the mechanism of accumulation of myocardial activity following i.v. injection of 15-(paraiodophenyl)-3 methyl pentadecanoic acid (IMPPA). IMPPA and 15 phenyl-3 methyl pentadecanoic acid (MPPA) were labeled with 14C at position 1 and used to perfuse isolated rat hearts in a closed system. After 5 min of perfusion, IMPPA reached 2/3 of its value at 45 min. 14CO2 production was low. Most of the myocardial activity was in the form of free IMPPA. Analysis of IMPPA activation by CoA SH revealed that it was very strongly inhibited. The retention of myocardial activity is thus due to intracellular accumulation of free IMPPA following inhibition of activation. Comparison of results obtained with IMPPA and MPPA showed that the presence of iodine in the molecule accentuates the inhibition of activation.