Cirsimarin and Cirsimaritin, Flavonoids of Microtea debilis (Phytolaccaceae) with Adenosine Antagonistic Properties in Rats: Leads for New Therapeutics in Acute Renal Failure

In traditional medicine Microtea debilis is used against proteinuria. In ligand‐binding studies extracts of Microtea debilis have been shown to inhibit the binding of [3H]1,3‐dipropyl‐8‐cyclopentylxanthine ([3H]DPCPX) to adenosine‐A1 receptors in rat forebrain membranes. Subsequently, cirsimarin, a flavonoid, was isolated as the active component and was shown to function as adenosine antagonist at the adenosine‐A1 receptor in‐vitro. In this study we have investigated the adenosine‐A2 receptor activity of cirsimarin the in‐vivo inhibition of the effects of adenosine by cirsimarin in rats, the absorption of cirsimarin and the inhibition of the binding of [3H]DPCPX to the adenosine‐A1 receptor by urine samples obtained after oral administration of crude extract of Microtea debilis, cirsimarin or cirsimaritin to rats. Cirsimarin inhibited the binding of [3H]5′‐N‐ethylcarboxamidoadenosine ([3H]NECA) to adenosine‐A2 receptors in rat striatum with an inhibition constant, Ki, of 6.5 ± 0.3 μm. The decrease of heart rate and blood pressure induced by adenosine was significantly inhibited by cirsimarin. After oral administration of 8 and 80 mg kg−1 cirsimarin, the compound could not be detected in either plasma or urine, but the presence of cirsimaritin was established. By use of β‐glucuronidase, glucuronides of cirsimaritin were also detected in the urine. The concentrations of cirsimaritin in the plasma were 0.126 ± 0.04, 0.138 ± 0.015, and 0.120 ± 0.022 μm, respectively, 2, 5 and 12 h after administration of 8 mg kg−1 cirsimarin. The concentrations of cirsimaritin in the urine at the same times after administration of the same dose were 205 ± 1.86, 5.05 ± 2.6 and 2.06 ± 0.09 μm, respectively. The inhibition of the binding of [3H]DPCPX to the adenosine‐A1 receptor by urine samples collected 2, 5 and 12 h after oral administration of 8 mg kg−1 cirsimarin or a crude extract of Microtea debilis containing approximately 8 mg kg−1 cirsimarin and 2.8 mg kg−1 cirsimaritin, or 6.8 mg kg−1 cirsimaritin, was not significantly different from that of urine samples collected from untreated rats, in contrast with urine samples collected 1 and 2 days after oral administration of 80 mg kg−1 cirsimarin. Approximately 3% of the cirsimarin was excreted in the urine as cirsimaritin. The results indicate that in the kidney and urinary tract the concentrations of cirsimaritin produced after ingestion of more than 8 mg kg−1 cirsimarin can be high enough to inhibit the interaction of aden