Downregulation of 5'-nucleotidase in rabbit heart during coronary underperfusion

The hydrolysis of AMP to adenosine during acute coronary underperfusion is temporarily beneficial to myocardial survival yet may cause tissue injury during sustained underperfusion because of depletion of adenine nucleotides. We hypothesized that the enzyme mediating AMP hydrolysis, 5'-nucleotidase (5'-NT), is downregulated during sustained coronary underperfusion to prevent excessive loss of nucleotides. Langendorff-perfused rabbit hearts were subjected to two successive, identical 45-min periods of underperfusion (4-5% of baseline flow) separated by 20 min of reperfusion. Although coronary venous lactate efflux was comparable in the two periods, total coronary purine efflux during the second period of underperfusion was attenuated by 75%. Phosphorus nuclear magnetic resonance data showed that ATP fell 46% in the first period but fell only another 10% in the second period. Phosphocreatine levels fell comparably (75-78%) during both periods of underperfusion. Analysis using a mathematical model describing the kinetics of myocardial energetics revealed that the combined data set was best described by a lower activity of 5'-NT (52% decrease in maximal reaction velocity) during the second period of under-perfusion. Additional time course experiments showed that the decrease in 5'-NT activity was slow in onset, requiring approximately 20 min of underperfusion. The decrease in 5'-NT activity during sustained underperfusion may benefit tissue survival by limiting the depletion of myocardial adenine nucleotides. In conclusion, at the onset of coronary underperfusion, there is a high activity of 5'-NT, but later during sustained under-perfusion, 5'-NT is downregulated, resulting in decreased AMP hydrolysis to adenosine.