| Abstract: | Dopaminergic mechanisms may be involved in the regulation of aldosterone secretion in humans and in the rat. Whether these effects are indirect or are exerted directly at the adrenal level has not yet been resolved. We now report the identification of dopaminergic binding sites in the bovine adrenal zone glomerulosa using [3H]spiperone, a butyrophenone with high affinity for D2 dopamine receptors. Specific [3H]spiperone binding (defined as binding displaceable by 10 microns (+)-butaclamol) reached equilibrium within 20 minutes at 22 degrees C, was reversible, and was heat labile (60 degrees C). Binding was of high affinity and saturable with a Kd of 1.8 +/- 0.2 nM and maximal specific binding of 38 +/- 8 fmol/mg (means +/- SEM; n = 18). [3H]Spiperone binding was unaffected by coincubation with angiotensin II, adrenocorticotropic hormone, or KCl. Binding characteristics, including a dissociation constant at the nanomolar range, greater potency of the D2-agonist LY 171555 relative to the D1-agonist SKF 38393 in inhibiting [3H]spiperone binding, and lack of stimulation of cyclic adenosine 3',5'-monophosphate by dopamine (10(-4) M), were consistent with a predominantly D2-receptor. In vitro studies with collagenase-dispersed adrenal zona glomerulosa cells showed that dopamine (10(-4) M) attenuated angiotensin II-stimulated aldosterone secretion. These observations are consistent with a direct inhibitory effect of dopamine on aldosterone secretion in the adrenal zona glomerulosa. |
