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.

The nature of the K⁺ exit across the basolateral membrane of microperfused rabbit cortical thick ascending limbs (cTALs) was investigated using the transepithelial and transmembrane potential difference (PD_te, PD_bl) and conductance measurements. An increase in bath K⁺ concentration from 4 to 10, 25, 50 mmol/l depolarized the basolateral membrane in a concentration-dependent manner, accompanied by a decrease in the fractional resistance of the basolateral membrane (FR_bl). The Cl^– channel blocker, 5-nitro-2-(3-phenylpropyl-amino)-benzoic acid (NPPB), did not prevent these effects. The effect of Ba²⁺ on PD_bl was bimodally distributed: paradoxically, in the tubules in which Ba²⁺ largely depolarized, the effects on PD_bl of the bath K⁺ concentration increases were not inhibited by extracellular Ba²⁺, in tubules in which Ba²⁺ moderately depolarized, Ba²⁺ partially inhibited the K⁺ concentration increase-induced depolarization of the basolateral membrane. However, the parallel decrease in FR_bl was Ba²⁺ insensitive, indicating that the K⁺ channel of the basolateral membrane was not modified by extracellular Ba²⁺. The Ba²⁺-induced depolarizations were prevented by furosemide suggesting that Ba²⁺ acts by inhibiting basolateral KCl extrusion. Finally, the K⁺ concentration increase-induced depolarizations were insensitive to tetraethylammonium, charybdotoxin, apamin and verapamil. In conclusion, the present study provides evidence that, in addition to a Ba²⁺-sensitive KCl cotransport system, the basolateral membrane of rabbit cTAL cells possesses a K⁺ conductance which is insensitive to extracellular Ba²⁺.