Pharmacological characterization of unique prazosin-binding sites in human kidney.
In human kidney, we found unique prazosin-binding sites that were insensitive to phentolamine and were thus unlikely to be alpha(1)-adrenoceptors. As the binding of [(3)H]prazosin to phentolamine-insensitive sites was prevented by 100 microM guanabenz, the insensitive sites were evaluated by subtracting [(3)H]prazosin binding in the presence of 100 microM guanabenz from that in the presence of 10 microM phentolamine. [(3)H]Prazosin bound to the phentolamine-insensitive sites monophasically with a high affinity (pK(d); 9.1+/-0.08, n=8), and the B(max) value (814+/-204 fmol mg(-1) protein, n=8) was more than ten times that of the phentolamine-sensitive alpha(1)-adrenoceptor (pK(d)=9.9+/-0.13, B(max)=66+/-23 fmol mg(-1) protein, n=7). The phentolamine-insensitive sites in human kidney were highly sensitive to other quinazoline derivatives such as terazosin and doxazosin. However, other alpha(1)-adrenoceptor antagonists (tamsulosin, WB4101 and corynanthine) did not inhibit the binding at a range of concentrations that generally exhibit alpha(1)-adrenoceptor antagonism, and noradrenaline, rauwolscine and propranolol were without effect on the [(3)H]prazosin binding. On the other hand, ligands for the renal Na(+)-transporter (amiloride and triamterene) and for imidazoline recognition sites (guanabenz, guanfacine and agmatine) displaced the binding of [(3)H]prazosin to phentolamine-insensitive sites at micromolar concentrations. Photoaffinity labeling with [(125)I]iodoarylazidoprazosin showed phentolamine-insensitive labeling at around 100 kDa, a molecular size larger than that of human alpha(1a)- and alpha(1b)-adrenoceptors expressed in 293 cells (50-60 and 70-80 kDa, respectively) on electrophoresis. In contrast, there was no detectable phentolamine-insensitive binding site but were phentolamine-sensitive alpha(1)-adrenoceptors in human liver (pK(d)=10.0+/-0.06, B(max)=44+/-6 fmol mg(-1) protein, n=3). Phentolamine-insensitive prazosin binding sites were also detected in rabbit kidney (approximately 50% of specific binding sites) but were minor in rat kidney (less than 20%). In conclusion, there are unique prazosin-binding sites in human kidney, the pharmacological profiles of which were distinct from those of known adrenoceptors.
Hiraoka
Yasuko
Y
Department of Pharmacology, School of Medicine, Fukui Medical University, 910-1193 Matsuoka, Fukui, Japan.
Taniguchi
Takanobu
T
Tanaka
Takashi
T
Okada
Kenichiro
K
Kanamaru
Hiroshi
H
Muramatsu
Ikunobu
I
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000317:Adrenergic alpha-Antagonists / Q000378:metabolism / Q000494:pharmacology*; D000818:Animals; D001665:Binding Sites; D006801:Humans; D066298:In Vitro Techniques; D007668:Kidney / Q000187:drug effects* / Q000378:metabolism; D008099:Liver / Q000187:drug effects / Q000378:metabolism; D008297:Male; D019920:Photoaffinity Labels; D011224:Prazosin / Q000378:metabolism / Q000494:pharmacology*; D011817:Rabbits; D051381:Rats; D017208:Rats, Wistar; D018340:Receptors, Adrenergic, alpha-1 / Q000187:drug effects / Q000378:metabolism; D018341:Receptors, Adrenergic, alpha-2 / Q000187:drug effects / Q000378:metabolism; D013045:Species Specificity; D014162:Transfection
D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D019920:Photoaffinity Labels; D018340:Receptors, Adrenergic, alpha-1; D018341:Receptors, Adrenergic, alpha-2; D011224:Prazosin