Properties of Angiotensin II Receptors of Domestic Turkey (Meleagris gallopavo) Adrenal Steroidogenic Cells

In the present study, the properties of angiotensin II (AII) receptors of intact domestic turkey adrenal steroidogenic cells were characterized. AII (but not ACTH) induced an immediate and sustained increase in intracellular Ca²⁺. In addition, dithiothreitol inhibition of maximal AII-induced aldosterone production was closely correlated with its inhibition of binding suggesting that these receptors are type 1-like and operate through a non-"spare" receptor mode. Equilibrium-binding analysis revealed a single class of binding sites at a concentration of 63,500 sites/cell and having an apparent dissociation constant (K_d) of 1.21 nM. However, the K_d derived from kinetic analyses, 0.27 nM, was lower. Both empirically determined and model-based calculated distributions of bound hormone indicated that at equilibrium, about 30% of hormone-receptor complexes were internalized whereas 70% remained on the surface. This distribution contrasts sharply with that reported for mammalian (rat) adrenocortical cells. In keeping with recent cloning studies, these avian AII receptors of intact adrenal steroidogenic cells discriminated angiotensins and mammalian peptidic and nonpeptidic antagonists differently from mammalian adrenocortical and duck adrenal receptor preparations. Importantly, turkey adrenal steroidogenic cell AII receptors poorly discriminated the nonpeptide antagonists, losartan (DuP 753) (type-1 specific) and PD123177 (type-2 specific). Thus, AII receptors of freshly isolated, intact turkey adrenal steroidogenic cells are pharmacologically distinct from mammalian adrenocortical type-1 receptors.