Nuclear magnetic resonance evaluation of metabolic and respiratory support of work load in intact rabbit hearts.

Pre-steady-state 13C nuclear magnetic resonance (NMR) spectra can provide a nondestructive probe of metabolic events associated with the physiology of intact organs. Therefore, the relation between phosphorylation state and intermediary metabolism in rabbit hearts, oxidizing [2-13C]acetate, was examined with a combination of 31P and 13C NMR. Multiple enrichment of the tissue glutamate pool with 13C as an index of metabolic turnover within the tricarboxylic acid cycle was readily observed as a function of work load. Dynamic changes in pre-steady-state 13C spectra evolved according to work load and correlated closely to respiratory rate in rabbit hearts perfused 1) under normal conditions (n = 7), 2) at basal metabolic rates (20 mM KCl arrest, n = 5), 3) and at heightened contractile state (10(-7) M isoproterenol, n = 7). The ratio of signal intensity arising from the secondary labeling sites within glutamate (C-2 and C-3) to that of the initial labeling site (C-4) reached steady state within 8.5 minutes in isoproterenol-treated hearts versus 18.5 minutes in control hearts. Work load did not affect glutamate concentration or fractional enrichment at the C-4 position, although an unlabeled fraction of glutamate persisted. Arrested hearts displayed slowed evolution of steady-state 13C enrichment with increased contributions from anaplerotic sources for tricarboxylic acid intermediate formation (32%) as compared with control (9%). Thus, the response of mitochondrial dehydrogenase activity to the demands of cardiac performance is likely to influence the recruitment of anabolic sources supplying the tricarboxylic acid cycle.(ABSTRACT TRUNCATED AT 250 WORDS)