The NAD concentration as well as the14C-incorporation in NAD and the disappearance of14C-NAD were studied in spontaneously beating atria of guinea pigs at high and low concentrations of the precursors nicotinamide or nicotinic acid. Atria were incubated in Krebs-Henseleit solution containing 15 mM glucose and the appropriate precursors at 30°C. The control NAD concentration (33 nMol/100 mg w.w.) remained unchanged during a 24-h-incubation time. -20 mM14C-nicotinamide increased the total NAD about three-fold (90 nMol/100 mg w.w.) after an incubation period of 24 h, with positive effects on the performance. The incorporation rate in14C-NAD was calculated to be 43.7 nMol/100 mg w.w. -24 h. The ADPR moiety for the NAD synthesis stemmed from an endogenous pool. Between 5 and 20 mM nicotinamide the increase in the NAD concentration followed an apparent Michaelis-Menten kinetics with a Km of 6.1 mM nicotinamide and a Vmax of 70.92 nMol NAD/100 mg w.w. -24 h. This can be explained as a new synthesis of NAD by a high concentration of nicotinamide and also by a decreased degradation of NAD, due to inhibition of the glycohydrolase by the high concentration of nicotinamide. The ratio of incorporation and disappearance of14C-NAD during the 8th and 16th h incubation period was 2:1. After pre-incubation with 20 mM nicotinamide for an 8-h period the NAD concentration decreased to normal values after incubation for 8 h in a nicotinamide free medium. -20 mM14C-nicotinic acid did not change the total NAD level and no significant incorporation in14C-NAD could be detected, whereas negative effects on the performance occurred. -10 μM14C-nicotinamide showed a slight increase in the total NAD concentration (39.7 nMol/100 mg w.w.) and in the14C-incorporation (4.8 nMol/100 mg w.w.) within 24 h. -10 μM14C-nicotinic acid seemed here to be the better precursor in this concentration. The NAD concentration increased to 49.8 nMol/100 mg w. w. after a 16 h incubation period and the incorporation in14C-NAD was 12.1 nMol/100 mg w.w. after an incubation time of 24 h. As consequence of the observed different influences of each precursor on NAD turnover and NAD concentration the pathways of NAD synthesis and degradation must be studied. The importance of an increased NAD level for the energy metabolism of the cardiac muscle under aerobic and anaerobic conditions is discussed. © 1979 Dr. Dietrich Steinkopff Verlag.
