Early Distal Tubule Transport

Overview
  • The early distal tubule of the nephron is responsible for resorption of approximately 5% of the filtered load of sodium and chloride. Like the ascending henle, the early distal tubule is relatively impermeable to water and thus the tubular fluid becomes even further diluted as it travels through this segment. Additionally, the distal tubule is also responsible for 8% of the filtered calcium load and is the key segment of regulated calcium resorption.

Molecular mechanisms of early distal tubule transport
Early Distal Tubule Transport is responsible for resorption of roughly 5% of the filtered sodium load which occurs via a luminal Na-Cl symporter, powered by a basolateral Na-K ATPase. This segment of the tubule is impermeable to water and thus the tubular fluid becomes further diluted. In passing, the Na-Cl symporter is a target of thiazide diuretics. The early distal tubule is also responsible for resorbing 8% of the filtered calcium load which occurs via a luminal Ca++ channel, powered by a basolateral Calcium ATPase. Importantly, the distal tubule is major segment of regulated calcium resorption, and its calcium resorptive capacity is modulated by PTH as described in Regulation of Calcium Excretion.

Transport Mechanisms
  • Sodium
    • The resorptive capacity of the early distal tubule is largely powered by a basolateral NaK ATPase which generates low Na+ concentrations within the tubular epithelial cells]. This basolateral primary active transport of sodium is then used to actuate luminal resorption of sodium and chloride via a luminal NaCl Symporter, which is the target of thiazide diuretics. Basolateral transport of chloride into the interstitial fluid occurs via a chloride channel.
  • Calcium
    • As discussed more fully in regulation of calcium excretion, the early distal tubule resorbs 8% of the filtered calcium load via a luminal calcium channel. The electrochemical gradient for calcium resorption is generated by a basolateral calcium ATPase. The resorptive capacity of distal tubular calcium resorption is regulated by Parathyroid Hormone, and this segment represents the major locus of regulated calcium resorption.