Diluting power of thick limbs of Henle. III. Modulation of in vitro diluting power

This paper considers the quantitative interplay of various factors in modulating diluting power of in vitro medullary and cortical thick ascending limbs of Henle (MTAL and CTAL, respectively) segments from mouse and rabbit. Experimentally, the measured diluting power of the in vitro rabbit CTAL is greater than that of the rabbit MTAL, although the inherent rate of net Cl^- absorption at high perfusion rates is considerably greater in the rabbit MTAL than in the rabbit CTAL. Similar results apply when comparing the rabbit CTAL to the mouse MTAL exposed to antidiuretic hormone (ADH). Our calculations show that, in the rabbit CTAL, the measured static head luminal salt concentration can be accounted for quantitatively by the measured rate of net salt absorption at a high perfusion rate together with the passive permeability coefficients for Na⁺ and Cl^-. Moreover, with perfusion rates of 10% of single nephron glomerular filtration rate, the transport properties of the CTAl predict that, at the end of the CTAL, the static head luminal Cl^- concentration occurs if the initial perfusate contains either 50 or 150 mM Cl^-. Thus one can argue that, in vivo the CTAL may be the cardinal determinant of the TAL contribution to diluting power and to external salt balance. The relatively blunted diluting power of in vitro MTAL segments can be accounted for quantitatively by assuming that luminal dilution, and the attendant osmotic gradient from lumen to cell, suppresses the inherent rate of transcellular Cl^- transport. Our calculations also show that prostaglandin E₂ and peritubular osmolality increases blunt tubular diluting power. Thus in vivo, the MTAL segment may be the cardinal determinant of TAL contribution to concentrating power and to intrarenal salt balance.