S. Regunathan et al., EXPRESSION OF I-2-IMIDAZOLINE SITES IN RAT PROSTATE - EFFECT OF CASTRATION AND AGING, Biochemical pharmacology, 51(4), 1996, pp. 455-459
Clonidine, idazoxan, and related imidazoline adrenergic drugs bind to
non-adrenergic sites in brain and several peripheral tissues. These si
tes, termed imidazoline receptors, appear to exist in two major subcla
sses, I-1 sites labeled by clonidine and I-2 sites labeled by idazoxan
. In this study, we investigated whether rat prostate expresses imidaz
oline receptors and, if so, whether their expression can be regulated
by circulating testosterone. Studies in rat ventral prostate membrane
revealed that [H-3]idazoxan, but not [H-3]P-aminoclonidine, bound to n
on-adrenergic sites. The binding of [H-3]idazoxan was saturable (B-max
: 941 +/- 105 fmol/mg protein) and high affinity (K-D: 16.4 +/- 2.3 nM
). The rank order of the inhibition of binding by imidazoline ligands
was cirazoline > clonidine > UK 14,304 > guanabenz, indicating an I-2
subclass of imidazoline receptors. Bilateral orchiectomy increased the
number of binding sites (B-max) for [H-3]idazoxan without changing th
e affinity (K-D). Testosterone replacement, while completely restoring
the plasma testosterone levels, only partially B-max. In contrast, th
e binding of [H-3]idazoxan to prostate membranes of rats in different
reversed the increase in B age groups (4, 7, and 16 months) revealed a
progressive decrease in the B-max without any change in K-D. We concl
ude that the rat prostate expresses the I-2 subclass of imidazoline re
ceptors and that the expression is regulated by circulating testostero
ne.