beta intercalated cell

**Location:** Late distal convoluted tubule (connecting tubule) and cortical collecting duct, interspersed among principal cells.

**Mechanism (mirror image of alpha-intercalated cell):** - Carbonic anhydrase (CA II) inside the cell converts CO2 + H2O to H+ + HCO3-. - HCO3- is secreted into the lumen through the apical Cl-/HCO3- exchanger **pendrin (SLC26A4)**, which brings Cl- into the cell. - H+ is pumped into the blood by the **basolateral H+-ATPase** (and possibly H+/K+-ATPase). - Net effect: HCO3- is added to the urine, so urine alkalinizes and plasma pH falls.

**Physiologic role:** - Activated in **metabolic alkalosis** (for example, volume-replete states with excess HCO3- or alkali load). Alpha-intercalated cells, in contrast, are activated in acidosis. - Pendrin also mediates Cl- reabsorption and indirectly helps Na+ reabsorption (together with ENaC in principal cells and the Na+-driven Cl-/HCO3- exchanger NDCBE). This makes it relevant to volume and blood pressure regulation. - Aldosterone upregulates pendrin and ENaC.

**Contrast with alpha-intercalated cells:** - Alpha: secrete H+ (apical H+-ATPase and H+/K+-ATPase) and reabsorb HCO3- (basolateral Cl-/HCO3- exchanger AE1 or band 3). They are active in acidosis. - Beta: secrete HCO3- (apical pendrin) and reabsorb H+ (basolateral H+-ATPase). They are active in alkalosis.

**Clinical correlations:** - **Pendred syndrome:** AR mutation in SLC26A4 (pendrin). It causes sensorineural deafness and goiter (impaired iodide organification or transport in the thyroid thyroid, with a positive perchlorate discharge test). Renal acid-base problems are generally mild unless the patient is stressed, for example with a bicarbonate load. - **Distal (type 1) RTA:** This is mainly a defect of alpha-intercalated cell H+ secretion (H+-ATPase, AE1 defects, or amphotericin B-induced H+ leak). It is not a primary beta cell disorder. It presents with hyperchloremic non-anion gap metabolic acidosis, urine pH >5.5, hypokalemia, and calcium phosphate stones. - **Carbonic anhydrase inhibitors (acetazolamide):** Act mainly at the proximal tubule, but they also impair CA-dependent H+/HCO3- generation in intercalated cells. - Aldosterone-driven states (hyperaldosteronism) with volume expansion are associated with metabolic alkalosis. Increased distal Na+ delivery or reabsorption through ENaC makes the lumen more negative, which drives H+ secretion by alpha cells and K+ secretion by principal cells.

**High-yield mnemonic:** Alpha = Acid secretion (acidosis). Beta = Base (bicarbonate) secretion (alkalosis).