Rapid stimulation of human renal ENaC by cAMP in Xenopus laevis oocytes

Among the compensatory mechanisms restoring circulating blood volume after severe haemorrhage, increased vasopressin secretion enhances water permeability of distal nephron segments and stimulates Na+ reabsorption in cortical collecting tubules via epithelial sodium channels (ENaC). The ability of vasopressin to upregulate ENaC via a cAMP-dependent mechanism in the medium to long term is well established. This study addressed the acute regulatory effect of cAMP on human ENaC (hENaC) and thus the potential role of vasopressin in the initial compensatory responses to haemorrhagic shock. The effects of raising intracellular cAMP (using 5 mmol/L isobutylmethylxanthine (IBMX) and 50 ìmol/L forskolin) on wild-type and Liddle-mutated hENaC activity expressed in Xenopus oocytes and hENaC localisation in oocyte membranes were evaluated by dual-electrode voltage clamping and immunohistochemistry, respectively. After 30 min, IBMX + forskolin had stimulated amiloride-sensitive Na+ current by 52 % and increased the membrane density of Na+ channels in oocytes expressing wild-type hENaC. These responses were prevented by 5 ìmol/L brefeldin A, which blocks antegrade vesicular transport. By contrast, IBMX + forskolin had no effects in oocytes expressing Liddle-mutated hENaC. cAMP stimulated rapid, exocytotic recruitment of wild-type hENaC into Xenopus oocyte membranes, but had no effect on constitutively over-expressed Liddle-mutated hENaC. Extrapolating these findings to the early cAMP-mediated effect of vasopressin on cortical collecting tubule cells, they suggest that vasopressin rapidly mobilises ENaC to the apical membrane of cortical collecting tubule cells, but does not enhance ENaC activity once inserted into the membrane. We speculate that this stimulatory effect on Na+ reabsorption (and hence water absorption) may contribute to the early restoration of extracellular fluid volume following severe haemorrhage (AU)

Avances en la fisiopatología del edema en el síndrome nefrótico / New insights into the pathophysiology of oedema in nephrotic syndrome

El edema es una manifestación clínica frecuente del síndrome nefrótico (SN); sin embargo, el mecanismo fisiopatológico responsable de la retención de sodio ha sido un tema de intenso debate durante décadas. Muchas observaciones clínicas y experimentales no apoyan a la hipótesis clásica o del underfill en la formación del edema nefrótico. En numerosos pacientes, el edema propio del SN se produce por un defecto renal intrínseco en la excreción de sodio y es independiente de factores sistémicos (p. ej., hipoalbuminemia, disminución del volumen arterial efectivo o hiperaldosteronismo secundario). El punto de la nefrona donde se produce la retención de sodio en el SN es el túbulo colector cortical. La activación del canal de sodio epitelial a ese nivel es responsable de la retención de sodio en la patología que nos ocupa. Una barrera glomerular defectuosa propia del SN permitiría el paso de enzimas proteolíticas o sus precursores que a su vez activarían el canal de sodio epitelial causando de esa manera su retención y consiguiente edema (AU)

Oedema is a common clinical manifestation of nephrotic syndrome. However, the pathophysiological mechanism of sodium retention in nephrotic syndrome has been intensely debated for decades. Several clinical and experimental observations argue against the classic or "underfill" hypothesis of oedema formation in nephrotic syndrome. In many patients, oedema formation in nephrotic syndrome is due to the kidney being intrinsically unable to excrete salt and is unrelated to systemic factors (i.e. hypoalbuminaemia, decreased "effective" arterial blood volume, and secondary hyperaldosteronism). The cortical collecting duct is the nephron site of sodium retention in nephrotic syndrome. Activation of the epithelial sodium channel in the cortical collecting duct is responsible for sodium retention in nephrotic syndrome. In nephrotic syndrome, a defective glomerular filtration barrier allows the passage of proteolytic enzymes or their precursors, which have the ability to activate the epithelial sodium channel, thereby causing the the subsequent sodium retention and oedema (AU)