A calcium–sensing receptor (CaR) has functionally been described in the cortical thick ascending limb of Henle's loop (CTAL) of rat and mouse. This G protein–coupled receptor activates phospholipase C and increases the intracellular Ca²⁺ concentration. We observed that in the mouse CTAL cAMP formation, induced by 10^–8 mol/l AVP, was inhibited by more than 90% when the extracellular Ca²⁺ concentration ([Ca²⁺]_e) was increased from 0.5 to 3 mmol/l. Measurements of transepithelial potential difference (PD_te) in rat and mouse CTAL and medullary thick ascending limb (mTAL) segments and of transepithelial ion net fluxes in the mouse CTAL (isotonic perfusion conditions: 150 mmol/l NaCl in the lumen and bath) showed that an increase in the [Ca²⁺]_e had no effect on basal and arginine vasopressin (AVP, 10^–10 mol/l)–stimulated transepithelial PD_te, NaCl and Mg²⁺ transport. However, Ca²⁺ reabsorption was strongly inhibited by increased [Ca²⁺]_e. Addition of AVP reversed this inhibitory effect of increased [Ca²⁺]_e. Under hypotonic perfusion conditions (lumen 50 mmol/l NaCl; bath 150 mmol/l NaCl), a high [Ca²⁺]_e induced a 50% decrease in Mg²⁺ reabsorption which was restored by AVP. Under these conditions, the effects on Ca²⁺ transport described above were still observed. In conclusion, activation of the CaR in the mouse TAL has no effect on basal and AVP–stimulated transepithelial NaCl reabsorption despite its large inhibitory effect on cAMP synthesis. The CaR, however, could play a role in the regulation of transepithelial Ca²⁺ and Mg²⁺ reabsorption.

Previous studies from our laboratory have shown that Ca2+ and Mg2+ absorption in the mouse cortical thick ascending limb of Henle's loop (cTAL) is a passive, paracellular process driven by the transepithelial voltage. The passive permeability of the epithelium is enhanced by peptide hormones. The present study investigated whether divalent cation absorption in the cTAL is influenced by cell maturation and/or gender. For this purpose, mouse cTAL segments were microdissected from kidneys of female and male animals aged 4 and 8 weeks. The microdissected tubules were perfused in vitro at a luminal flow rate of 1.5 to 2.5 nl/min. Transepithelial Na+, Cl-, Ca2+ and Mg2+ net fluxes (JX, pmol.min-1.mm-1) were measured using electron microprobe analysis, and the transepithelial potential difference (PDte) was measured continuously. No differences were found in the PDte, JNa and JCl of the various animal groups but the transepithelial Ca2+ and Mg2+ transport capacity of the cTAL was higher in adults (8 weeks) than in young animals (4 weeks). Furthermore, irrespective of age, transepithelial Ca2+ net absorption was greater in male than in female animals. In contrast, the NaCl transport was maximal at 4 weeks in both genders. We conclude therefore that transepithelial divalent cation absorption in the mouse cTAL is an inductive process influenced by cell maturation and gender. The molecular basis of these inductions remains to be elucidated.