Calcium and phosphate transport in isolated segments of rabbit Henle's loop.

Abstract

A B S T R A C T Calcium and phosphate transport was examined in rabbit thin descending, thin ascending, and thick ascending limbs of Henle by in vitro perfusion of isolated tubular segments. Permeability coefficients for these segments with 45Ca and 32P04 were determined for both lumen-to-bath and bathto-lumen directions. Both the thin descending and thin ascending limbs were found to be relatively impermeable to both 45Ca and 32PO4. In neither segment were we able to show evidence for net transport of calcium or phosphate. In contrast, the thick ascending limb of Henle showed a decrease in calcium lumen-to-bath concentration from 0.97±0.02 to 0.88 +0.02 when perfused at 4.8 nl min-'. 45Ca lumen-tobath and bath-to-lumen fluxes were 19.96+1.05 and 9.89±0.02 peq min-l cm-1, respectively, and the potential difference was +3.8±0.3 mV (lumen positive).The observed calcium flux ratio was significantly higher than that predicted by Ussing's equation. When ouabain was added to the bath the potential difference fell to + 1.1±0.3 mV, whereas the calcium efflux was only slightly diminished (29.5+5.3-23.7+5.1 peq cm-l min-1). Ouabain had no effect on the influx of Ca across the thick ascending limb of Henle. There was no net transport of phosphate across the thick ascending limb. Phosphate permeability was exceedingly low bidirectionally across the thick ascending limb. Our findings indicate: (a) all segments of Henle's loop are relatively impermeable to calcium and phosphate; (b) net transport of phosphate seems to be absent in Henle's loop; (c) net calcium reabsorption, which cannot be explained by Preliminary reports of a portion of this work have been presented at the VI International Congress of Nephrology.1975.

Nephrolithiasis: site of the initial solid phase

“…The basement membrane of the thin limb appears an unlikely site for initial crystallization in patients with idiopathic hypercalciuria. It is not the site of either vectorial calcium or phosphorus transport (8) and, since even the transtubular permeabilities of these ions are very low (8), it is difficult to link supersaturation within the thin limbs (9) to the surrounding interstitium. However, anatomically, the thin limbs are in very close proximity to the vasa recta and the collecting ducts, and all are situated in a highly concentrated, hypertonic environment.…”

Calcium transport in the pars recta and thin descending limb of Henle of the rabbit, perfused in vitro.

“…Calciuim absorption in the loop of Henle had been attributed to the thick ascending limb, and recent isolated tubule microperfuisioin studies have confirmed the presence of significant calcium transport in this segment (2)(3)(4).…”

Ethnic Differences in Renal Responses to Furosemide

“…11 The luminal transepithelial potential difference in TAL that promotes paracellular uptake of Ca and Mg is coupled to the activity level of NKCC2. [12][13][14] (3) Urinary excretion of K is lower in blacks than in whites. 15 Greater Na uptake anywhere in proximal nephron regions, including TAL, would result in suppression of aldosterone levels, as well delivery of less Na downstream, both causing a reduction in distal nephron function, which is coupled to K secretion.…”