ISSN:
1432-2013
Keywords:
Cyclic GMP
;
Prostaglandins
;
Prostacyclins
;
Thromboxane B2
;
Probenecid
;
Indomethacin
;
Phosphodiesterase inhibitors
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
Abstract Using the stop-flow peritubular capillary microperfusion method the inhibitory potency (apparent K i values) of cyclic nucleotides and prostanoids against contraluminal p-aminohippurate (PAH), dicarboxylate and sulphate transport was evaluated. Conversely the contraluminal transport rate of labelled cAMP, cGMP, prostaglandin E2, and prostaglandin D2 was measured and the inhibition by different substrates was tested. Cyclic AMP and its 8-bromo and dibutyryl analogues inhibited contraluminal PAH transport with an app. K i, PAH of 3.4, 0.63 and 0.52 mmol/l. The respective app. K i,PAH values of cGMP and its analogues are with 0.27, 0.04 and 0.05 mmol/l, considerably lower. None of the cyclic nucleotides tested interacted with contraluminal dicarboxylate, sulphate and N 1-methylnicotinamide transport. ATP, ADP, AMP, adenosine and adenine as well as GTP, GDP, GMP, guanosine and guanine did not inhibit PAH transport while most of the phosphodiesterase inhibitors tested did. Time-dependent contraluminal uptake of [3H]cAMP and [3H]cGMP was measured at different starting concentrations and showed facilitated diffusion kinetics with the following parameters for cAMP: K m=1.5 mmol/l, J max=0.34 pmol s−1 cm−1, r (extracellular/intracellular amount at steady state)=0.91; for cGMP: K m=0.29 mmol/l, J max=0.31 pmol s−1 cm−1, r=0.55. Comparison of app. K i, cGMP with app. K i, PAH of ten substrates gave a linear relation with a ratio of 1.83±0.5. All prostanoids applied inhibited the contraluminal PAH transport; the prostaglandins E1, F1α, A1, B1, E2, F2α, D2, A2 and B2 with an app. K i, PAH between 0.08 and 0.18 mmol/l. The app. K i of the prostacyclins 6,15-diketo-13,14-dihydroxy-F1α (0.22 mmol/l) and Iloprost (0.17 mmol/l) as well as that of leukotrienes B4 (0.2 mmol/l) was in the same range, while the app. K i, PAH of the prostacyclins PGI2 (0.55 mmol/l), 6-keto-PGF1α (0.77 mmol/l), and 2,3-dinor-6-keto-PGF1α (0.57 mmol/l) as well as that of thromboxane Bin2 (0.36 mmol/l) was somewhat higher. None of these prostanoids inhibited contraluminal dicarboxylate transport and only PGB1, E2 and D2 inhibited contraluminal sulphate transport (app. $$K_{i,SO_4^{2--} } $$ 5.4, 11.0, 17.9 mmol/l respectively). Contraluminal influx of labelled PGE2 showed complex transport kinetics with a mixed K m=0.61 mmol/l and J max of 4.26 pmol s−1 cm−1. It was inhibited by probenecid, sulphate and indomethacin. Contraluminal influx of PGD2, however, was only inhibited by probenecid. The data indicate that cyclic nucleotides as well as prostanoids are transported by the contraluminal PAH transporter. For prostaglandin E2 a significant uptake through the sulphate transporter occurs in addition. The hypothesis that prostaglandins as well as 8-bromo and dibutyryl cyclic nucleotides permeate cell membranes by simple diffusion because of their lipid solubility must be considered with reservation.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00550874
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