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

Laboratório de Pesquisa Básica-LIM 12, Hospital das Clínicas da Faculdade de Medicina-Nefrologia, Universidade de São Paulo, São Paulo, Brazil

Submitted 13 June 2008 ; accepted in final form 27 September 2008

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; µm/s) at 37°C and pH 7.4 in normal rat IMCDs (n = 36) perfused with Ringer/HCO₃ was determined. Gl (10^–7 M) in absence of Vasopressin (AVP) enhanced the Pf from 4.38 ± 1.40 to 11.16 ± 1.44 µm/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-His¹-[Glu⁹] 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.

glucagon; aquaporin 2; water transport; glucagon antagonist; vasopressin

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