Effect of Prolonged Low Dose Infusion of Angiotensin II and Aldosterone on Rat Smooth Muscle and Adrenal Angiotensin II Receptors*

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Abstract
The present studies were designed to determine the effects of continuous alterations in angiotensin II (Ang II) blood levels on Ang II receptors of rat uterine myometrium and adrenal glomerulosa. These tissues were used in a radioreceptor assay for Ang II. Low dose infusion of Ang II for 7 days caused significant suppression of PRA, whereas plasma aldosterone was not increased above control levels. The Kd of uterine myometrium decreased to 29% of the control value, while the Kd of adrenal glomerulosa increased by 115%. No significant change in the number of receptors of uterine myometrium was observed in spite of the increased number of receptors in the adrenal glomerulosa. The observation that angiotensin infusion was associated with a decrease in uterine Kd and an increase in adrenal receptors suggests that receptor occupancy did not influence the affinity or concentration of sites under steady state conditions. Low levels of PRA and Ang II were achieved by continuous infusion of aldosterone with osmotic minipumps at a rate that increased the plasma aldosterone concentration 10-fold. The number of myometrial receptors increased 2.7-fold, whereas the number of adrenal receptors decreased 42–58%. Animals given supplemental KCl in their drinking water during continuous aldosterone infusion had a smaller decrease in the number of adrenal receptors than those which received no supplementation. In addition, the Kd of adrenal glomerulosa decreased 22% with aldosterone infusion and was not influenced by serum potassium. The present findings of affinity changes as well as changes in receptor number differ from previously reported effects of dietary sodium manipulation in spite of similar blood levels of Ang II. Alterations of dietary sodium content in rats have been associated with changes in receptor number only. These results suggest that one or more factors besides the endogenous Ang II concentration influences Ang receptor kinetics at equilibrium.