In high potassium serum levels, kidney compensation occurs by-

Ans. is 'c' i.e., Increased secretion in DCT Potassium transport* Potassium is one of the few substances that are both reabsorbed as well as secreted by the renal tubules.* The reabsorption of about two-third of the filtered K+ occurs in the proximal tubules and about 15-20% of reabsorption occur in the thick ascending limb of loop of Henle. This absorption is independent of the K+ status of the body. After this, about 10% of filtered K+ is reached the DCT and CD, here K+ reabsorption as well as secretion takes place. Here also, reabsorption of K+ occurs independent of K+ status of the body. It is the secretory process that is regulated by aldosterone in response to the K+ and Na+ status of body.* Potassium is secreted by principal cells. Na+K+ ATPase transports Na+ out of the cell and transports K+ inside the cells across the basolateral membrane. This results in decreased intracellular Na+ with increased K+. As a result Na+ is reabsorbed along concentration gradient from tubular lumen. This increases negativity on the luminal surface of epithelial cells, i.e., increased transepithelial potential difference (TEPD). Both of these factors (increased intracellular K+ and negative TEPD) stimulate secretion of K+ into the tubular lumen to decrease negativity of luminal surface (K+ has positive charge).* Intercalated (I) cells reabsorb K+ in exchange with H+ by apical H+K+ ATPase (hydrogen potassium ATPase), i.e., K+ is reabsorbed and H+ is secreted. Under normal conditions this is not important. This transport becomes prominent in circumstances associated with severe potassium depletion when there is cessation of potassium secretion and net reabsorption of potassium.* The normal day-to-day variation of potassium excretion, however, is regulated mainly by secretion in the distal and collecting tubules rather than by changes in glomerular filtration or tubular reabsorption. Potassium is sometimes reabsorbed in these tubular segments (e.g., during potassium depletion), and at other times it is secreted in large amounts depending on the needs of the body. With high potassium intake, the required extra excretion of potassium is achieved almost entirely by increased secretion of potassium in the distal and collecting tubules.