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F Feng, WA Pettinger, PW Abel and WB Jeffries
Midwest Hypertension Research Institute, Omaha, Nebraska.
alpha 1-Adrenoceptors are expressed in high density in the rat kidney. Recent studies have shown that alpha 1-adrenoceptors are heterogeneous and can be subtyped based on their affinity for the antagonists WB 4101 (alpha 1A greater than alpha 1B) and chloroethylclonidine (alpha 1B- selective). We therefore investigated the distribution of alpha 1- adrenoceptor subtypes using [3H]prazosin binding in membranes prepared from cortex, the outer stripe of the outer medulla (OSOM), the inner stripe of the outer medulla (ISOM) and inner medulla dissected from rat kidney. [3H]Prazosin binding was detectable in each region with the following maximum binding site values for [3H]prazosin (femtomoles per milligram of protein): cortex, 186 +/- 20; OSOM, 76 +/- 14; ISOM, 34 +/- 2; and inner medulla, 8 +/- 2. Pretreatment of membranes with chloroethylclonidine (10 microM for 10 min) reduced maximum binding sites to 41 +/- 4% (75 +/- 9 fmol/mg) of control in cortex, 41 +/- 9% of control in OSOM (29 +/- 8 fmol/mg) and 15 +/- 3% of control in ISOM (5 +/- 1 fmol/mg). In competition studies, WB 4101 labeled both high and low affinity sites in cortex (respective pKi values = 10.01 +/- 0.3 and 8.23 +/- 0.1) and OSOM (pKi values = 9.6 + 0.3 and 8.3 +/- 0.5), but only low affinity sites in ISOM (pKi = 8.41 +/- 0.1). The relative prevalence of high:low affinity sites revealed by WB 4101 was 53:47 for cortex, 52:48 for OSOM and virtually all low affinity for ISOM. Prior treatment with chloroethylclonidine greatly reduced the low affinity component of the WB 4101 competition curve in cortex and OSOM. We conclude that: 1) the density of alpha 1-adrenoceptors is highest in the cortex and decreases from cortex to papilla and 2) the alpha 1A and alpha 1B subtypes are approximately equally distributed in the cortex and OSOM, but the alpha 1B subtype predominates in ISOM.
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