Coronary autoregulation and purine release in normoxic heart at various cytoplasmic phosphorylation potentials: disparate effects of adenosine

The impacts of energy-yielding substrates on coronary flow autoregulation, cytoplasmic phosphorylation potential {[ATP]/([ADP][P_i])} and purine nucleoside production were studied in Langendorff-perfused guinea pig hearts. The perfusion medium was substrate-free or contained glucose alone or in combination with pyruvate, lactate, acetate, or octanoate as fatty acid. When coronary flow was adjusted for myocardial oxygen consumption, only pyruvate supported near-perfect intrinsic autoregulation at highly sustained [ATP]/([ADP][P_i]) and low interstitial adenosine concentrations ([Ado]). In contrast, hearts perfused with substrate-free medium were deenergized at very high [Ado], especially at supraphysiological pressures, which markedly impaired autoregulatory vasoconstriction. Thus, efficient autoregulatory vasoconstriction was associated with high [ATP]/ ([ADP][P_i]) at low [Ado]. On the other hand, autoregulatory vasodilation at subphysiological pressures was associated with increased [Ado] and partially blocked by 28 μM theophylline demonstrating (partial) adenosine mediation. Massive accumulation of IMP, especially relative to free cytoplasmic AMP, occurred at normal intracellular pH during myocyte deenergization by substratefree perfusion. This may indicate allosteric activation of native AMP deaminase in situ, perhaps because of collapse of [ATP]/([ADP][P_i]). Similarly, rates of adenosine plus inosine release and of total purines, also including urate, exhibited non-linear sigmoidal rather than linear or rectangular hyperbolic dependences on free cytoplasmic AMP concentration (not total AMP content). Since inclusion of IMP as a co-variable of free AMP appreciably improved the sigmoidal fits, IMP appeared to be a significant precursor of released inosine in guinea pig heart.