The proximal convoluted tubules are cuboidal epithelium with microvilli. Their large surface area reabsorbs glucose, electrolytes, and water. Water is reabsorbed by osmosis, so the tubule fluids are isosmotic with plasma as they enter the loop of Henle. Organic anions (toxins, some drugs) are secreted into the PCT. Examples include benzoate, salicylate, and penicillin.
Active transport of Na by Na-K ATPase on the basolateral membrane (the interstitial fluids side) drives water and glucose reabsorption (secondary active transport). Na ions enter PCT cells through open channels in the apical membrane (on the luminal side). Glucose and amino acids are co-transported into the PCT cells by apical, Na-dependent symporters. Organic nutrients (glucose and amino acids) are >99% reabsorbed. The SGLT (sodium-glucose linked transporter) responsible for co-transport of glucose and sodium saturates if glucose levels are too high, spilling glucose into the urine in hyperglycemia.
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