Inhibition of bicarbonate absorption by peptide hormones and cyclic adenosine monophosphate in rat medullary thick ascending limb

In vitro microperfusion experiments were performed to examine the effects of peptide hormones on bicarbonate and ammonium transport by the medullary thick ascending limb (MTAL) of the rat. Arginine vasopressin (AVP; 2.8 × 10^-10 M in the bath) reduced bicarbonate absorption by 50% (from 7.8 to 3.7 pmol/min per mm). AVP caused a similar reduction in bicarbonate absorption in tubules perfused with 10^-4 M furosemide to inhibit net NaCl absorption. Glucagon (2 × 10^-9 M in the bath) also reduced bicarbonate absorption (from 11.7 to 7.6 pmol/min per mm). The inhibition of bicarbonate absorption could be reproduced with either exogenous 8-bromo-cAMP or forskolin. With 8-bromo-cAMP (10^-3 M) in the bath, addition of vasopressin to the bath did not significantly affect bicarbonate absorption. PTH significantly inhibited bicarbonate absorption, but the extent of inhibition was less than that observed with either AVP or glucagon. Vasopressin had no effect on net ammonium absorption in MTAL perfused and bathed with 4 mM NH₄Cl. These findings indicate that: (a) vasopressin, glucagon, and PTH directly inhibit bicarbonate absorption in the MTAL of the rat; (b) this inhibition occurs independent of effects on net NaCl absorption and appears to be mediated in part by cAMP; and (c) HCO^-₃ and NH⁺₄ absorption can be regulated independently in the MTAL.