The hydrolysis of 5′-AMP by 5′-nucleotidase is the main source of adenosine. In various tissues adenosine is a local mediator adjusting the organ work to the available energy. In the kidney it regulates renal hemodynamics, glomerular filtration rate and renin release via specific receptors of the arteriolar walls. By immunocytochemistry we identified interstitial and tubular sites of 5′-nucleotidase in the rat kidney. In the interstitium the enzyme was detected only in the cortical labyrinth, the compartment that comprises all arteriolar vessels besides other putative targets of adenosine. The 5′-nucleotidase-positive cells of the interstitium were identified as fibroblasts. The fibroblasts are in close contact with the tubules as well as with the vessels. Thus, any 5′-AMP released by the tubules into the interstitial space would be converted to adenosine in the direct vicinity of its assumed targets. Adenosine produced by tubular cells would hardly have access to its known targets, since 5′-nucleotidase is restricted to the luminal cell surface. Pathological events affecting the fibroblasts might influence renal function by modifying the interstitial adenosine
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