Metabolism of cAMP to Adenosine in the Renal Vasculature 1

Abstract

ABSTRACT We recently demonstrated that cAMP added to the perfusate increased the renal venous recovery of adenosine in the isolated rat kidney, an effect blocked by inhibition of ecto-phosphodiesterase and ecto-5Ј-nucleotidase. Although our previous study established the cAMP-adenosine pathway, i.e., the conversion of cAMP to adenosine, as a viable metabolic pathway within the kidney, that study did not determine whether conversion of arterial cAMP to adenosine recoverable in the venous effluent occurred in the tubules versus nontubular sites. In the current study, we addressed this issue by determining the effects of blocking cAMP transport into the renal tubules with probenecid (0.1, 0.3 and 1 mM) on the increase in renal venous output of adenosine induced by adding cAMP (30 M) to the perfusate of isolated rat kidneys. Addition of cAMP to the perfusate caused a marked increase in renal venous secretion of adenosine, an effect that was augmented, rather than inhibited, by probenecid. To test the hypothesis that the renal vasculature supports a cAMP-adenosine pathway, cultured rat preglomerular vascular smooth muscle cells were incubated with cAMP (30 M) for 1 hr in the presence and absence of 3-isobutyl-1-methylxanthine (a phosphodiesterase inhibitor). Incubation with cAMP increased extracellular adenosine levels 41-fold, and this effect was abolished by 3-isobutyl-1-methylxanthine. In a third experimental series, addition of cAMP (0.3, 1, 3, 10 and 30 M) to the perfusate of isolated rat kidneys and mesenteric vascular beds increased the renal venous, but not mesenteric venous, output of AMP, adenosine and inosine. We conclude that the renal vasculature supports a cAMP-adenosine pathway, that administering cAMP into the renal artery and measuring adenosine in the venous effluent of the perfused rat kidney most likely monitors primarily the renal vascular cAMPadenosine pathway and that the quantitative importance of the cAMP-adenosine pathway is not equivalent in all vascular compartments. Renal adenosine participates importantly in the regulation of renin release, renal hemodynamics, tubuloglomerular feedback, erythropoietin production and tubular transport The existence of a cAMP-adenosine pathway in the kidneys is supported by two lines of evidence. First, infusion of IBMX (a phosphodiesterase inhibitor) into the renal cortical interstitium via a microdialysis probe decreases renal cortical interstitial levels of adenosine and inosine (a metabolite of adenosine) The above-mentioned studies in the isolated perfused rat kidney strongly suggest that in the kidney cAMP is converted to adenosine extracellularly. However, those studies do not determine whether conversion of perfusate cAMP to adenosine recoverable in the venous effluent occurs mostly in the tubules versus nontubular sites such as the renal vasculature. Several studies demonstrate that cAMP is efficiently transported by the probenecid-inhibitable organic anion transport system in the proximal tubul