Myocardial adenine nucleotides, hexose phosphates and inorganic phosphate, and the regulation of phosphofructokinase activity during fluoroacetate poisoning in the rat

Abstract

Myocardial levels of adenine nucleotides, inorganic phosphate (Pi) and hexose phosphates were measured as a function of the time after fluoroacetate (FAc) administration (6 mg/kg i.p.) to the rat. ATP content was progressively depleted, reaching at 6 hr 60 per cent and at 18 hr 45 per cent of the control values. AMP and ADP levels increased during the initial 2 hr and later declined. Intracellular Pi accumulated in great amounts, with a maximum at 4 hr, while serum Pi increased continuously throughout the experimental period. Citrate levels reached a maximum at 6 hr and remained nearly constant thereafter. These results suggest that the energy reserves of the tissue are progressively exhausted during the intoxication, and that the reactivation of the citric acid cycle by the accumulated citrate, postulated by previous authors, does not occur under the present conditions. Fructose diphosphate (FDP) levels were unaltered during the intoxication, while fructose-6-phosphate (F6P) and glucose-6-phosphate only showed transient initial increases. The FDP/F6P ratio, which is indicative of intracellular phosphofructokinase (PFK) activity, was not significantly altered, in spite of the striking changes produced in the levels of PFK effectors. This suggests that the PFK activation usually associated with a depletion of high-energy phosphates and Pi accumulation is blocked in the poisoned tissue. In vitro experiments were performed in which PFK activity was evaluated in the presence of concentrations of substrates and metabolites simulating those found in vivo. It was observed that in the physiological pH range (6·9–7·1) PFK is strikingly activated in the assays corresponding to intoxicated hearts, regardless of the high citrate levels. This suggests that the activation associated with the fall of ATP is quantitatively more important than the citrate inhibition. However, at lower pH values this activation is not produced, irrespective of the levels of ATP, AMP, Pi or citrate. It is suggested that intracellular acidification, possibly associated with the accumulation of citric acid, might be the main factor responsible for the blockade of PFK activation observed in the intoxicated hearts in vivo.