Aquaporin 2 expression increased by glucagon in normal rat inner medullary collecting ducts.

It is well known that Glucagon (Gl) is released after a high protein diet and participates in water excretion by the kidney, principally after a protein meal. To study this effect in in vitro perfused inner medullary collecting ducts (IMCD), the osmotic water permeability (Pf; mum/s) at 37 degrees C and pH 7.4 in normal rat IMCDs (n = 36) perfused with Ringer/HCO(3) was determined. Gl (10(-7) M) in absence of Vasopressin (AVP) enhanced the Pf from 4.38 +/- 1.40 to 11.16 +/- 1.44 microm/s (P < 0.01). Adding 10(-8), 10(-7), and 10(-6) M Gl, the Pf responded in a dose-dependent manner. The protein kinase A inhibitor H8 blocked the Gl effect. The specific Gl inhibitor, des-His(1)-[Glu(9)] glucagon (10(-7) M), blocked the Gl-stimulated Pf but not the AVP-stimulated Pf. There occurred a partial additional effect between Gl and AVP. The cAMP level was enhanced from the control 1.24 +/- 0.39 to 59.70 +/- 15.18 fm/mg prot after Gl 10(-7) M in an IMCD cell suspension. The immunoblotting studies indicated an increase in AQP2 protein abundance of 27% (cont 100.0 +/- 3.9 vs. Gl 127.53; P = 0.0035) in membrane fractions extracted from IMCD tubule suspension, incubated with 10(-6) M Gl. Our data showed that 1) Gl increased water absorption in a dose-dependent manner; 2) the anti-Gl blocked the action of Gl but not the action of AVP; 3) Gl stimulated the cAMP generation; 4) Gl increased the AQP2 water channel protein expression, leading us to conclude that Gl controls water absorption by utilizing a Gl receptor, rather than a AVP receptor, increasing the AQP2 protein expression.

Angiotensin-(1-7) stimulates water transport in rat inner medullary collecting duct: evidence for involvement of vasopressin V2 receptors.

The peptide angiotensin-(1-7) [Ang-(1-7)] is known to enhance water transport in rat inner medullary collecting duct (IMCD). The aim of this study was to determine the mechanism of the Ang-(1-7) effect on osmotic water permeability (Pf). Pf was measured in the normal rat IMCD perfused in vitro in presence of agonists [Ang-(1-7), arginine vasopressin (AVP) and Ang-(3-8)], and antagonists of the angiotensin and the vasopressin cascade. Ang-(1-7), but not Ang-(3-8), increased Pf significantly. The effect of Ang-(1-7) on Pf was abolished by its selective antagonist, A-779, added before or after Ang-(1-7). Prostaglandin E2 and the protein kinase A inhibitor H8 also blocked the Ang-(1-7) effect. Blockade of vasopressin V1 receptors by antagonists did not change the Ang-(1-7) effect, but pre-treatment with a V2 antagonist abolished the effect of Ang-(1-7) on Pf. Similarly, pre-treatment with A-779 inhibited AVP's effect on Pf. Forskolin-stimulated Pf was blocked both by A-779 and by the V2 antagonist. Finally, Ang-(1-7) increased cAMP levels in fresh IMCD cell suspensions whilst the forskolin-stimulated cAMP synthesis was decreased by A-779 and the V2 antagonist. These data provide evidence that Ang-(1-7) interacts via its receptor with the AVP V2 system through a mechanism involving adenylate-cyclase activation.