Epac1 null mice have nephrogenic diabetes insipidus with deficient corticopapillary osmotic gradient and weaker collecting duct tight junctions

Abstract

Aim The cAMP‐mediator Epac1 (RapGef3) has high renal expression. Preliminary observations revealed increased diuresis in Epac1−/− mice. We hypothesized that Epac1 could restrict diuresis by promoting transcellular collecting duct (CD) water and urea transport or by stabilizing CD paracellular junctions to reduce osmolyte loss from the renal papillary interstitium. Methods In Epac1−/− and Wt C57BL/6J mice, renal papillae, dissected from snap‐frozen kidneys, were assayed for the content of key osmolytes. Cell junctions were analysed by transmission electron microscopy. Urea transport integrity was evaluated by urea loading with 40% protein diet, endogenous vasopressin production was manipulated by intragastric water loading and moderate dehydration and vasopressin type 2 receptors were stimulated selectively by i.p.‐injected desmopressin (dDAVP). Glomerular filtration rate (GFR) was estimated as [14C]inulin clearance. The glomerular filtration barrier was evaluated by urinary albumin excretion and microvascular leakage by the renal content of time‐spaced intravenously injected 125I‐ and 131I‐labelled albumin. Results Epac1−/− mice had increased diuresis and increased free water clearance under antidiuretic conditions. They had shorter and less dense CD tight junction (TJs) and attenuated corticomedullary osmotic gradient. Epac1−/− mice had no increased protein diet‐induced urea‐dependent osmotic diuresis, and expressed Wt levels of aquaporin‐2 (AQP‐2) and urea transporter A1/3 (UT‐A1/3). Epac1−/− mice had no urinary albumin leakage and unaltered renal microvascular albumin extravasation. Their GFR was moderately increased, unless when treated with furosemide. Conclusion Our results conform to the hypothesis that Epac1‐dependent mechanisms protect against diabetes insipidus by maintaining renal papillary osmolarity and the integrity of CD TJs.

Renal failure in focus

“…2 Dealing with NDI a study showed that Epac1 −/− (exchange protein directly activated by cAMP) mice had an increased diuresis compared to their wild type littermates. 3 However, some parameters were similar, such as aquaporin 2 expression, albumin permeability and sodium excretion. Differences appeared in responsiveness to AVP, urine composition and altered microvascular permeability within the different genotypes.…”

“…This led to the conclusion that Epac1 is essential for maintaining the corticomedullary osmotic gradient. 3 Furthermore, tightly regulated sodium excretion is essential for the electrolyte balance. In the nephrotic syndrome (NS) sodium excretion impedes proteinuria.…”

“…Dealing with NDI a study showed that Epac1 −/− (exchange protein directly activated by cAMP) mice had an increased diuresis compared to their wild type littermates 3 . However, some parameters were similar, such as aquaporin 2 expression, albumin permeability and sodium excretion.…”

“…Differences appeared in responsiveness to AVP, urine composition and altered microvascular permeability within the different genotypes. This led to the conclusion that Epac1 is essential for maintaining the corticomedullary osmotic gradient 3 …”

The Art of Planning—How many samples are enough?

“…In most scientific disciplines the standard for the statistical power and the level of significance are 0.8 and 0.05 respectively 15,16 . There are differences between disciplines, and it is common for some scientists to use alpha levels ( P ‐value <alpha) of 0.01 or below or to differentiate between different levels of certainty within one publication 17‐21 . A more detailed guide to finding the correct levels for alpha‐ and beta‐errors and important caveats is available 22 …”