Cardiac Synthesis and Degradation of Pyridine Nucleotides and the Level of Energy-Rich Phosphates Influenced by Various Precursors

Abstract

This chapter is concerned with the question of whether under normoxic and anoxic conditions the myocardial concentrations of pyridine nucleotides (NAD, NADP), adenine nucleotides (AN), and creatine phosphate (CP) can be influenced by addition of various precursors of these compounds to the perfusion solution in order to improve anoxic survival of cardiac cells. After i.p. injection of 10 mmol nicotinamide/kg guinea pig, a NAD level increased by 40–58% can be observed for a period of 12–24 hr, but there is no change in the AN and CP concentrations. In isolated atria of guinea pigs under normoxic conditions, the atrial concentration of NAD increases threefold over a 24-hr period if 10–20 mM nicotinamide is added to the Krebs-Henseleit solution with 15 mM glucose. Analysis of metabolites after incubation with 20 mM [¹⁴C]nicotinamide showed that this increase resulted from new synthesis of NAD by activation of the nicotinamidase that deamidates nicotinamide to nicotinic acid via the Preiss-Handler pathway. Simultaneously, the high concentration of nicotinamide inhibits the degradation of NAD by the glycohydrolase. However, a high concentration of nicotinamide also impairs the new synthesis of AN from adenine and ribose in the salvage pathway. The anoxic degradation of NAD could be protected, compared with controls, by this high nicotinamide concentration. After aerobic incubation with 10 μM [¹⁴C]nicotinamide, no deamidation to nicotinic acid could be observed, and a small but significant incorporation into NAD by the Dietrich pathway could be measured. This pathway is obviously located in the mitochondria. No increased NAD concentration could be observed, and in anoxia there was no protective effect. With 10 μM [¹⁴C]nicotinic acid, a twofold higher synthesis of [¹⁴C]-NAD could be observed than is seen with 10 μM [¹⁴C]nicotinamide. Here, the Preis s-Handler pathway was operating for NAD synthesis. The glycohydrolase was not inhibited as observed with 20 mM nicotinamide. The total NAD level increased by 42%. The degradation of [¹⁴C]-NAD to [¹⁴C]-nicotinamide (N_am) could be observed in atria as well after its penetration in the nutritive solution. [¹⁴C]Nicotinamide was also used for new synthesis of mitochondrial NAD via the Dietrich pathway. Therefore, we assume that the physiological precursor for cardiac NAD synthesis is nicotinic acid. Higher concentrations of nicotinic acid (50 μM) together with adenine and ribose significantly enhanced the total NAD level under aerobic conditions by 77%, ATP by 37%, and AN by 24% during a 24-hr period. Adenine (100 μM) and ribose (500 μM) increased the ATP and AN level in the same range under these conditions. Low concentrations of nicotinamide and nicotinic acid have little effect on the change of anoxic NAD concentration which is shifted to NADH or degraded. The anoxic loss of AN can be protected effectively only if the PRPP pool and the ATP pool have not been decreased to a great degree in the myocardial cell. Standstill before anoxic conditions are started or strongly reduced cardiac work (30 beats/min) in the anoxia test reported can save ATP by 70% through unrestricted anaerobic glycolysis during a 2-hr anoxic period, as compared with the aerobic values. In contrast, in spontaneously beating atria with a high frequency of 200 beats/min when anoxia was started, after 2 hr of anoxia the ATP level decreased strongly to 23%. The addition of adenine (100 μM) and ribose (500 μM) during anoxia produced only a weak but significant protective effect, whereas with higher anoxic ATP values in the anoxia test, the ATP and AN concentrations could be protected with concentrations of ribose (15 mM) and adenine (0.1 mM) comparable to the aerobic values. No protection of CP could be obtained by addition of CP(5–10 mM) or precursors during the 2-hr anoxic period. Therefore, strongly reduced contractile work or cardiac arrest before anoxic conditions are started, as most energy protective effect, may be beneficial for anoxic myocardial survival combined with addition of AN and NAD precursors to myocardial perfusion solutions during cardioplegia.