Water and total CO₂ reabsorption along the rat proximal convoluted tubule

Abstract

Micropuncture experiments were performed on rat kidney to evaluate the profile of water and total CO₂ reabsorption along the proximal tubule. Three to eight samples were collected along the same nephron and the puncture-to-glomerulus distances were measured for each site. In Munich rats with accessible glomeruli, the water reabsorption rate was found to be constant all along the first five millimeters of proximal tubule. In Sprague Dawley rats with no accessible glomerulus, the same observation was made for these five millimeters, and the water reabsorption rate per mm along this segment was found to be a function of the glomerular filtration rate. For the two last millimeters of tubule accessible at the kidney surface, the water reabsorption rate was found to decrease in 5 out of the 21 tubules studied and ranged from 0.15–3.5 nl·min⁻¹·mm⁻¹.

In Sprague Dawley rats the fall in the luminal total CO₂ concentration (CO₂)_t along the tubular length was nearly constant (21 mmole·l⁻¹) between Bowman's capsule and the end proximal tubule, irrespective of the plasma (CO₂)_t value. The distance needed to reach half-maximum total CO₂ reabsorption varied from 1.1–1.9 mm from one tubule to another, as a function of the total CO₂ filtered load. These data suggest that the tubular length involved in “avid” bicarbonate reabsorption increases as a function of the filtered load and that in the first millimeters of tubule, bicarbonate reabsorption depends on a rapidly saturable mechanism. However, no close relationship was found between total CO₂ movement or the calculated transepithelial chloride gradient on the one hand and water reabsorption along the convoluted proximal tubule on the other.