Wednesday, November 18, 2009

Distal Convoluted Tubule

The distal convoluted tubule is the final portion of the nephron which is situated between the loop of Henle and the collecting duct system. The distal convoluted tubule is lined with cuboidal epithelial cells with abundant mitochondria and lateral membrane interdigitation with neighboring cells.

The distal convoluted tubule is mainly responsible for the reabsorption of water and sodium. It regulates pH through the absorption of bicarbonate and secretion of protons (H+) into the filtrate. Sodium absorption by the distal tubule is regulated by the hormone aldosterone. It also takes part in calcium regulation by secretion of excess Ca2+.

The distal tubule reabsorbs ~10% of the filtered Mg2+, but this is 70-80% of that delivered from the loop of Henle. Because there is little Mg2+ reabsorption beyond the distal tubule, this segment plays an important role in determining the final urinary excretion. The distal convoluted segment (DCT) is characterized by a negative luminal voltage and high intercellular resistance so that Mg2+ reabsorption is transcellular and active. This review discusses recent evidence for selective and sensitive control of Mg2+ transport in the DCT and emphasizes the importance of this control in normal and abnormal renal Mg2+ conservation.

Normally, Mg2+ absorption is load dependent in the distal tubule, whether delivery is altered by increasing luminal Mg2+ concentration or increasing the flow rate into the DCT. With the use of microfluorescent studies with an established mouse distal convoluted tubule (MDCT) cell line, it was shown that Mg2+ uptake was concentration and voltage dependent. Peptide hormones such as parathyroid hormone, calcitonin, glucagon, and arginine vasopressin enhance Mg2+ absorption in the distal tubule and stimulate Mg2+ uptake into MDCT cells. Prostaglandin E2 and isoproterenol increase Mg2+ entry into MDCT cells.

Distal Convoluted Tubule