Figure 20-16 Action of aldosterone. Aldosterone enters the distal tubular cytoplasm, attaches to a receptor, and then migrates to the nucleus, where it induces the formation of messenger RNA (mRNA). The mRNA in turn induces the synthesis of a protein that enhances the permeability of the apical (luminal) membrane to sodium and potassium. Reabsorption of sodium stimulates the basolateral membrane Na-K-ATPase pump, the intracellular concentration of potassium rises, and it follows its concentration gradient out into the lumen. The net effect of aldosterone's action is sodium reabsorption and potassium loss. CO, cotransporter (= symporter); P, sodium-potassium-ATPase pump; R, receptor. (Redrawn from Wingard LB, Brody TM, Larner J, Schwartz A: Diuretics: Drugs that increase excretion of water and electrolytes. In Wingard LB, Brody TM, Larner J, Schwartz A [eds]: Human Pharmacology: Molecular-to-Clinical. London, Wolfe, 1991, p 249.)