Effect of vasopressin on sodium transport across inner medullary collecting duct

We have used rat inner medullary collecting ducts (IMCD) perfused "in vitro" to study the effect of vasopressin (VP) on the unidirectional Na+ flux (in nmol.cm-2.s-1). We found that, at a high perfusion rate in the basal state, 24Na lumen-to-bath flux (Jl----b) was greater than the bath-to-lumen flux (Jb----l) (4.88 +/- 0.15 vs. 2.57 +/- 0.21), resulting in a significant net flux (Jnet) (P less than 0.001). Addition of 10 microU/ml of lysine vasopressin (LVP) to the bath produced a stable increase in Jl----b to 6.66 +/- 0.35 (P less than 0.001) without significant effect on Jb----l. Measuring directly the net flux absorption at lower perfusion rate (8 nl/min), we observed that LVP (10 microU/ml) produced a reversible stimulation on Jnet from 1.39 +/- 0.14 to 2.79 +/- 0.23 (P less than 0.01). The transtubular potential difference (PD) measured in the middle and final third of IMCD showed a small but significant PD (0.30 +/- 0.02 mV lumen positive) that increased significantly to 0.60 +/- 0.04 mV in the presence of LVP. However, dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP, 10(-4) M) added to the bath fluid did not change the JNa+l----b, nor was 1-desamino-8-D-arginine vasopressin (50 microU/ml), a specific V2-agonist, able to increase the Na+. We also demonstrated that JNa+l----b stimulated by LVP from 4.70 +/- 0.08 to 6.33 +/- 0.26 (P less than 0.01) was completely and reversibly inhibited by V1-antagonist, d(CH2)Tyr(Me)AVP, to 4.79 +/- 0.05. On the other hand, the absence of Ca2+ in the bath or the addition of amiloride to the lumen fluid or ouabain to the bath fluid completely inhibited AVP-stimulated JNa+l----b. Therefore, AVP and LVP increase Na+ absorption in the rat IMCD by increasing the Na+ outflux, probably generated by an increase of luminal membrane Na+ permeability modulated by extracellular Ca2+ and mediated through V1-receptors and independent of cAMP cascade.