Voltage dependence of calcium transport in the thick ascending limb of Henle's loop

138

A B S T R A C T Isolated cortical thick ascending limbs of Henle's loop were perfused in order to directly evaluate magnesium transport in this segment. Transepithelial potential difference was altered by varying the NaCl concentration in perfusate and bath and adding 50 ,uM furosemide to the perfusate. Perfusion under standard conditions with isotonic solutions resulted in a mean transepithelial potential difference of +8.8±0.7 mV and net magnesium absorption at a rate of 0.32±0.06 pmol/mm per min. Perfusion with a hypotonic solution significantly increased potential difference and the net absorptive rate of magnesium, calcium, and potassium. Conversely, reversal of the polarity of the potential difference with low NaCl bath and luminal furosemide produced net secretion of magnesium, calcium, and potassium. Parathyroid hormone in a bath concentration of 1.0 U/ml increased magnesium absorption from 0.34±0.06 to 0.63±0.06 pmol/mm per min (P < 0.001) and calcium from 0.52±0.08 to 0.97±0.08 pmol/mm per min (P < 0.001). Dibutyryl cyclic AMP produced similar effects on both calcium and magnesium absorption. Increasing bath calcium concentration twofold significantly inhibited net calcium absorption from 0.79±0.27 to 0.16±0.02 pmol/mm per min but magnesium transport was unaffected. Increasing bath magnesium concentration twofold significantly inhibited net magnesium absorption from 0.56±0.14 to -0.09±0.13 pmol/mm per min but had no effect upon net calcium transport. Net absorption of magnesium was significantly increased with increased concentraPortions of this study were presented at the 12th Annual Meeting of the

Section: Resultssupporting

confidence: 90%

Section: Protocolsmentioning

confidence: 99%

See 1 more Smart Citation

Magnesium transport in the cortical thick ascending limb of Henle's loop of the rabbit.

Shareghi¹,

Agus²

1982

138

“…Precautions taken to prevent loss of 45Ca-counts included the following: (a) To prevent binding of calcium to glass, all pipettes in contact with perfused or collected fluid were siliconized with SC-87 (Pierce Chemical Co., Rockford, IL); (b) All oils used were waterequilibrated and bubbled with 100% CO2 to prevent loss of C02, alkalinization of fluid samples, and precipitation of calcium salts; (c) Collected samples were expelled from the constant-volume constriction pipette with a 10-mM EGTA solution to ensure complete removal of 45Ca; and (d) All counting vials contained two drops of 12 N HCO to acidify the scintillation fluid and prevent loss of counts with time (14). In previous studies, the above maneuvers were sufficient to prevent binding of isotopic calcium from static solutions in the pipettes for at least 2 h. Furthermore, recovery of isotopic calcium in the bath closely matches that which disappears from the perfusate (10,11).…”

Section: Methodsmentioning

confidence: 99%

Calcium transport in the rabbit superficial proximal convoluted tubule.

Ng¹,

Rouse²,

Suki³

1984

Abstract. Calcium transport was studied in isolated S2 segments of rabbit superficial proximal convoluted tubules. '"Ca was added to the perfusate for measurement of lumen-to-bath flux (JibCa), to the bath for bath-to-lumen flux (JbIc), and to both perfusate and bath for net flux (Jnetc). In these studies, the perfusate consisted of an equilibrium solution that was designed to minimize water flux or electrochemical potential differences (PD). Under these conditions, JibCa (9.1 ± 1.0 peq/mm min) was not different from JblCa (7.3±1.3 peq/mm min), and Jntc1 was not different from zero, which suggests that calcium transport in the superficial proximal convoluted tubule is due primarily to passive transport. The efflux coefficient was 9.5±1.2 X 10-5 cm/ s, which was not significantly different from the influx coefficient, 7.0±1.3 X 10-5 cm/s. When the PD was made positive or negative with use of different perfusates, net calcium absorption or secretion was demonstrated, respectively, which supports a major role for passive transport. These results indicate that in the superficial proximal convoluted tubule ofthe rabbit, passive driving forces are the major determinants of calcium transport.

“…The basic approach used to determine Ca permeability involved construction of a transepithelial gradient of NaCl (56). To clamp for Vte, asymmetric solutions were used: the tubule was bathed with a solution containing 144 mmol/l Na and 120 mmol/l Cl, whereas the lumen was perfused with a solution containing 53 mmol/l Na and 29 mmol/l Cl.…”

Section: Measurement Of Transepithelial Permeability To Camentioning

confidence: 99%

PTH-independent regulation of blood calcium concentration by the calcium-sensing receptor

Loupy¹,

Ramakrishnan²,

Wootla³

et al. 2012

181

171

Tight regulation of calcium levels is required for many critical biological functions. The Ca 2+ -sensing receptor (CaSR) expressed by parathyroid cells controls blood calcium concentration by regulating parathyroid hormone (PTH) secretion. However, CaSR is also expressed in other organs, such as the kidney, but the importance of extraparathyroid CaSR in calcium metabolism remains unknown. Here, we investigated the role of extraparathyroid CaSR using thyroparathyroidectomized, PTH-supplemented rats. Chronic inhibition of CaSR selectively increased renal tubular calcium absorption and blood calcium concentration independent of PTH secretion change and without altering intestinal calcium absorption. CaSR inhibition increased blood calcium concentration in animals pretreated with a bisphosphonate, indicating that the increase did not result from release of bone calcium. Kidney CaSR was expressed primarily in the thick ascending limb of the loop of Henle (TAL). As measured by in vitro microperfusion of cortical TAL, CaSR inhibitors increased calcium reabsorption and paracellular pathway permeability but did not change NaCl reabsorption. We conclude that CaSR is a direct determinant of blood calcium concentration, independent of PTH, and modulates renal tubular calcium transport in the TAL via the permeability of the paracellular pathway. These findings suggest that CaSR inhibitors may provide a new specific treatment for disorders related to impaired PTH secretion, such as primary hypoparathyroidism.