Renin-angiotensin-aldosterone system in hyper- and hypothyroid rats during sodium depletion

A study was made of the responses of the renin-angiotensin-aldosterone system (RAAS) produced in hyper- and hypothyroid rats maintained in a state of sodium depletion, as part of a wider study to effect a comparison with results previously obtained for the same parameters in rats maintained on a normal sodium diet and during experimental alterations of thyroid function. In hyperthyroid rats maintained on sodium deficient and normal sodium diet, and in euthyroid animals on a sodium deficient regimen, elevations of both plasma renin concentration (PRC) and serum aldosterone concentration (AC) were observed, while the plasma renin substrate (PRS) was decreased. In hypothyroid rats with normal sodium intake, PRC and AC were observed to be decreased relative to similar sodium-deficient animals. Relationships were observed between PRC and AC, and between AC and urinary sodium excretion. These results suggest that: i) Sodium depletion is a more effective stimulus than the thyroid hormones on the RAAS. ii) Aldosterone changes in sodium depletion are mediated by the RAS. iii) Sodium depletion appeared to induce a decrease in the hepatic synthesis of angiotensinogen.