Potentiation of angiotensin II action by corticosteroids in vascular tissue

Studies were performed to determine if corticosteroids act directly on the vasculature to potentiate the vasoconstrictor action of angiotensin II and to determine whether corticosteroids upregulate angiotensin II receptors by receptor redistribution or by synthesis of new receptors.

Aortic rings from normal Sprague-Dawley rats were incubated ex vivo with corticosteroids in aerated Krebs-Henseleit buffer to avoid secondary systemic effects prior to stimulated contraction. In cultured vascular smooth muscle cells, these experimental techniques were used: colchicine (blocker of microtubule assembly), chloroquine (inhibitor of endosomal pH gradients), measuring surface-bound 125I-Ang II internalization rate, immunoblotting of angiotensin AT1 receptor protein, and incorporation of [35S]methionine into AT1 receptor protein.

Contractions to 100 nM angiotensin II in rings incubated with 1 microM aldosterone or dexamethasone for 10 min ex vivo were not different from contractions in control rings. However, angiotensin II-stimulated (but not KCl-stimulated) contractions were enhanced by almost 100% if ex vivo incubation with aldosterone (or corticosterone) lasted for 24 h. Endothelium-dependent relaxation was not significantly reduced by aldosterone pre-incubation. Incubation of cultured vascular smooth muscle cells with a number of corticosteroids for > 8 h resulted in concentration-dependent upregulation of angiotensin II receptor binding and was reversible upon removal of the corticosteroid. Aldosterone did not affect the rate of internalization of surface-bound angiotensin II. In addition, concomitant incubation of colchicine or chloroquine with aldosterone did not hamper angiotensin II receptor upregulation. Incubation of cells with various concentrations of aldosterone for 24 h resulted in concentration-dependent increases in total cell angiotensin II receptor protein content and increases in [35S]methionine incorporation into immunoprecipitated AT1 receptor protein.

At least a portion of the enhancement of angiotensin II action by corticosteroids is via direct interaction of corticosteroids with the vasculature. Corticosteroids appear to upregulate angiotensin II receptors by synthesis of new receptor protein rather than by alterations in receptor trafficking.