Differential effects of ADH on sodium, chloride, potassium, calcium and magnesium transport in cortical and medullary thick ascending limbs of mouse nephron

Abstract

The effect of antidiuretic hormone (arginine vasopressin, AVP) on transepithelial Na⁺, Cl⁻, K⁺, Ca²⁺ and Mg²⁺ net transports was investigated in medullary (mTAL) and cortical (cTAL) segments of the thick ascending limb (TAL) of mouse nephron, perfused in vitro. Transepithelial net fluxes (J _Na ⁺,J _Cl ⁻,J _K ⁺,J _Ca ²⁺,J _Mg ²⁺) were determined by electron probe analysis of the collected tubular fluid. Transepithelial potential difference (PD_te) and transepithelial resistance (R_te) were measured simultaneously. cTAL segments were bathed and perfused with isoosmolal, HCO ⁻₃ containing Ringer solutions, mTAL segments were bathed and perfused with isoosmolal HCO ⁻₃ free Ringer solutions. In cTAL segments, AVP (10⁻¹⁰ mol·l⁻¹) significantly increasedJ _Mg ²⁺ andJ _Ca ²⁺ from 0.39±0.08 to 0.58±0.10 and from 0.86±0.13 to 1.19±0.15 pmol·min⁻¹ mm⁻¹ respectively. NeitherJ _Na ⁺ norJ _Cl ⁻, (J _Na ⁺: 213±30 versus 221±28 pmol·min⁻¹ mm⁻¹,J _Cl ⁻: 206±30 versus 220±23 pmol·min⁻¹ mm⁻¹) nor PD_te (13.4±1.3 mV versus 14.1±1.9 mV) or R_te (24.6±6.5Ω cm² versus 22.6±6.4Ω cm²) were significantly changed by AVP. No significant effect of AVP on net K⁺ transport was observed. In mTAL segments, Mg²⁺ and Ca²⁺ net transports were close to zero and AVP (10⁻¹⁰ mol·l⁻¹) elicited no effect. However NaCl net reabsorption was significantly stimulated by the hormone,J _Na ⁺ increased from 107±33 to 148±30 andJ _Cl ⁻ from 121±33 to 165±32 pmol·min⁻¹ mm⁻¹. The rise inJ _NaCl was accompanied by an increase in PD_te from 9.0±0.7 to 13.5±0.9 mV and a decrease in R_te from 14.4±2.0 to 11.2±1.7 Ω cm². No K⁺ net transport was detected, either under control conditions or in the presence of AVP.

To test for a possible effect of HCO ⁻₃ on transepithelial ion fluxes, mTAL segments were bathed and perfused with HCO ⁻₃ containing Ringer solutions. With the exception ofJ _Ca ²⁺ which was significantly different from zero (J _Ca ²⁺: 0.26±0.06 pmol·min⁻¹ mm⁻¹), net transepithelial fluxes of Na⁺, Cl⁻, K⁺ and Mg²⁺ were unaffected by HCO ⁻₃ . In the presence of AVP,J _Mg ²⁺ andJ _Ca ²⁺ were unaltered whereasJ _NaCl was stimulated to the same extent as observed in the absence of HCO ⁻₃ . In conclusion our results indicate heterogeneity of response to AVP in cortical and medullary segments of the TAL segment, since AVP stimulates Ca²⁺ and Mg²⁺ reabsorption in the cortical part and Na⁺ and Cl⁻ reabsorption in the medullary part of this nephron segment.