Interactive roles of CD73 and tissue nonspecific alkaline phosphatase in the renal vascular metabolism of 5'-AMP.

CD73 metabolizes extracellular 5'-AMP to adenosine; yet recent experiments in brain tissue suggest that CD73 is not required for the metabolism of 5'-AMP to adenosine because of tissue nonspecific alkaline phosphatase (TNAP), which like CD73 is a GPI-anchored ecto-enyzme with 5'-nucleotidase activity. Because adenosine importantly regulates renovascular function, we investigated whether both TNAP and CD73 are involved in the renovascular metabolism of 5'-AMP. To test this, we examined in isolated, perfused mouse kidneys the metabolism of 5'-AMP (applied to the lumen of the renal vasculature via intrarenal artery administration) to adenosine by measuring renal venous levels of 5'-AMP, adenosine, and inosine (adenosine metabolite) by mass spectrometry. In one study, we compared 5'-AMP metabolism in naive CD73+/+ (wild-type, n = 16) vs. CD73-/- (knockout, n = 16) kidneys; and in a second study, we compared 5'-AMP metabolism in CD73+/+ (n = 9) vs. CD73-/- (n = 8) kidneys pretreated with levamisole (1 mmol/l; TNAP inhibitor). In naive kidneys, 5'-AMP increased renal venous 5'-AMP, adenosine, and inosine, and these responses were similar in CD73+/+ vs. CD73-/- kidneys. Levamisole per se did not inhibit renovascular 5'-AMP metabolism; however, in the presence of levamisole, 5'-AMP increased renal venous 5'-AMP threefold more in CD73-/- vs. CD73+/+ kidneys and knockout of CD73 inhibited 5'-induced adenosine and inosine by 81 and 86%, respectively. TNAP mRNA, protein, and activity were similar in CD73+/+ vs. CD73-/- kidneys. In conclusion, CD73 and TNAP play interactive roles to metabolize luminally applied 5'-AMP in the renal vasculature such that inhibition of both is required to inhibit the production of adenosine.