Aldosterone has effects on most or all cells of the body but, clinically, the most important actions are in the kidney, on cells of the late distal convoluted tubule and medullary collecting duct. In the principal cells aldosterone increases activity of basolateral membrane sodium-potassium ATPase and apical epithelial sodium channels, ENaC, as well as potassium channels, ROMK. These actions increase sodium reabsorption and potassium secretion. Since more sodium is reabsorbed than potassium secreted, it also makes the lumen more electrically negative, causing chloride to follow sodium. Water then follows sodium and chloride by osmosis. In Conn syndrome, these actions cause increased extracellular sodium and fluid volume and reduced extracellular potassium. Aldosterone also acts on intercalated cells to stimulate an apical proton ATPase, causing proton secretion that acidifies urine and alkalizes extracellular fluid.
In summary, hyperaldosteronism causes hypernatremic, hypokalemic, metabolic alkalosis.
Finer notes on aldosterone include the fact that it stimulates sodium-potassium ATPase in muscle cells, increasing intracellular potassium and also increases sodium reabsorption all along the intestine and nephron, possibly due to widespread stimulation of sodium-potassium ATPase. Finally, epithelial cells of sweat gland ducts and distal colon surface respond exactly the same as the principal cells of the nephron. These responses are important in climate adaptation and as a cause of constipation with elevated aldosterone.
The sodium retention leads to plasma volume expansion and elevated blood pressure. The increased blood pressure will lead to increased glomerular filtration rate and cause a decrease in renin release from the granular cells of the juxtaglomerular apparatus in the kidney. If there is a primary hyperaldosteronism, the decreased renin (and subsequent decreased angiotensin II) will not lead to a decrease in aldosterone levels (a very helpful clinical tool in diagnosis of primary hyperaldosteronism).