The effect of orotic acid treatment on the energy and carbohydrate metabolism of the hypertrophying rat heart

• 1.1. Adenine nucleotide concentrations in normal and one day hypertrophied hearts of untreated, orotic acid (OA), uridine, uracil, dihydroorotate and reserpine pretreated rats were measured. OA treatment increased the ADP concentration 5-fold in one day hypertrophied hearts. Neither uracil, uridine, dihydroorotate nor reserpine treatments changed ADP or total adenylate concentrations at one day of hypertrophy.
• 2.2. The adenine nucleotide ratio (ANR) at 0.263 × 10³ M⁻¹ and the energy charge (0.66) were at their lowest values in OA and in reserpine treated one day hypertrophying hearts. The temporal decline in the indices of energy metabolism corresponded with the OA induced maximum stimulation of contractility and maximum rates of protein, RNA and glycogen synthesis.
• 3.3. The phosphorylation state of the adenine nucleotides (PSAN) was both the most sensitive and the best predictive index of the cellular energy status in normal and hypertrophying hearts. The pronounced ability of OA treatment to energize myocyte cytoplasm was shown by the 9- and 6-fold greater values of PSAN over ANR in one and three day hypertrophied hearts. The enhanced PSAN may be the key factor in the mechanism of OA induced enhancement of contractile and synthetic functions of the heart in compensatory hypertrophy.
• 4.4. The development of myocardial hypertrophy in untreated rats resulted in a 36% reduction in the cytoplasmic NAD/NADH ratio. In rats treated with OA this redox couple of the hypertrophying heart was more oxidized and was increased by 30% to restore it to the value range of normal heart.
• 5.5. The regulatory status of the glycolytic pathway in untreated and OA treated hypertrophying hearts was assessed by comparisons of the mass action ratio (MAR) and equilibrium constants for each of the individual glycolytic reactions. There was an OA induced 2.7-fold increase in glycogen, UDP-glucose and total uridine nucleotides in hypertrophied hearts. The concentrations of seven out of ten glycolytic intermediates, including pyruvate and lactate were increased as a consequence of OA treated hypertrophy. Glycolytic flux was not stalled, rather the pathway was “more open” permitting greater throughput of intermediates with individually increased levels of selected metabolites. OA stimulated hypertrophy did not change the canonical control of glycolysis by the activities and individual MAR values of phosphofructokinase and pyruvic kinase.
• 6.6. Elevated levels of Glu 6-P, Fru 6-P and DHAP can force glycolytic intermediate entry into the non-oxidative reaction segment of the pentose pathway (PP), thereby elevating Rib 5-P concentration by reversal of the conventional flux direction of PP. Rib 5-P is rate limiting for PRPP and nucleotide synthesis and increases in its concentration in OA treated hypertrophying hearts can inter alia explain the elevation in adenylate concentrations.
• 7.7. OA does not act directly on the isolated normal or hypertrophying heart neither as an inotropic agent nor as a metabolic substrate. Its myocardial action requires whole-body integration and its principal metabolic fate involves the liver and the activation of the salvage pathway of pyrimidine biosynthesis. Preformed bases and nucleosides, mostly as uridine, enter and enhance the domains of pyrimidine, purine nucleotide and RNA metabolism in the OA stimulated hypertrophying myocardium.