Dose/response curves of lipoic acid R-and S-forms in the working rat heart during reoxygenation: superiority of the R-enantiomer in enhancement of aortic flow.

Micromolar concentrations of lipoic acid racemate added to a working rat heart during hypoxia have been previously found to improve aortic flow during subsequent reoxygenation. Since the R-form represents the naturally occurring form of lipoic acid, and the S-form does not reveal a positive influence on ATP synthesis in isolated mitoplasts, a dose/response curve of both enantiomers has been performed in working rat hearts. After the end of perfusion mitochondria were isolated and further analysed. At a concentration of 0.05-0.1 mumol of the R-enantiomer, aortic flow rises precipitously during reoxygenation, reaching over 70% of normoxic values compared to 50% of the controls. By contrast, with the S-enantiomer a value of about 60% is attained at 1 mumol, only. Accordingly, ATPase activity in mitochondria isolated from rat hearts previously treated with 0.05-0.1 mumol of the R- or S-enantiomer was significantly decreased or increased respectively. Consequently, whereas mitochondrial ATP synthesis was increased when the R-enantiomer was previously added to the working heart at 0.05-0.1 mumol concentration, with the S-enantiomer ATP synthesis remained within the control range. Mitochondrial membrane fluidity, measured with diphenylhexatriene, revealed a trend towards increase with the R- and decrease with the S-enantiomer. The total amount of thiol added at 0.1 mumol concentration is consistent with a value of 2 nmol/mg mitochondrial protein. This value has previously been found to be connected with -SH groups which add oligomycin-sensitivity to the ATPase complex. It is suggested that oligomycin-sensitive mitochondrial -SH groups contribute to the overall efficiency of low concentrations of lipoic acid R-enantiomer to enhance aortic flow.

Generation of free radicals in Langendorff and working hearts during normoxia, hypoxia, and reoxygenation.

The release of .OH and alkyl free radicals into the coronary flow were compared in Langendorff perfused and working rat hearts during normoxia (30 min), hypoxia (30 min) and reoxygenation (60 min) by means of spin-trapping techniques using 5,5-dimethyl-1-pyrroline-1-oxide (DMPO). In Langendorff hearts, there was a small but steady increase in the radical concentration during the course of hypoxia and reoxygenation. At the start of reoxygenation, only small initial peaks of hydroxyl and alkyl radicals occurred. After a general decrease of free radical production during hypoxia, working hearts produced nearly double the amount of free radicals during reoxygenation as Langendorff hearts. After an initial large increase during early reoxygenation, the amount of free radicals produced fluctuated on a high level during the remaining reoxygenation period. Heart work is thus correlated with an increased production of free radicals, possibly due to an increase in oxygen consumption by the heart.