Cushing’s Syndrome can be caused by ACTH secretion from pituitary adenomas or from ectopic tumours and is characterized by glucocorticoid excess, central obesity, altered metabolism of glucose and hypertension. Hypertension is an important predictor of outcome in Cushing’s Syndrome and although the causes are not fully known, renal sodium retention may be a contributory factor (Ferrari, 2003). Cushing’s has been modeled experimentally in rats but this is not ideal as ACTH excess causes a profound weight loss, rather than the central obesity seen in humans. Our previous studies indicate that ACTH infusion in the C57BL6 mouse causes hypertension without the catabolic effects seen in rats (Kenyon et al, 2007). In the present study, we have investigated the effect of ACTH infusion on renal sodium handling. Male C57BL6 mice (~25g) were infused via osmotic minipump (under halothane anaesthesia (2% with oxygen by inhalation)) with ACTH (n=7. Synacthen; 3μg/day) or saline (n=7) for two weeks before being anaesthetized (Inactin; 100mg/kg IP) and prepared surgically for renal function experiments as described (Bailey et al, 2008). Mice were infused IV with a saline solution (0.2ml/h/10g) containing FITC-inulin for the measurement of glomerular filtration rate (GFR). After a control urine collection, amiloride (2mg/kg; IV) was injected and a second urine collection made. At the end of the experiments, mice were killed with an overdose of anaesthetic and the kidneys removed for extraction of RNA. The expression of mRNA encoding subunits of the epithelial sodium channel (ENaC) and the Na-K-2Cl cotransporter (NKCC2) were measured by quantitative RT-PCR. Data are mean± S.E. and statistical comparisions were made using Student’s t test. Mean arterial blood pressure was significantly elevated in mice receiving ACTH infusion (114±3 vs 90±3 mmHg; P<0.01). Sodium excretion was similar in both groups. Since plasma sodium (157±1 vs 148±1 mmol/l; P<0.01) and GFR (0.27±0.03 vs 0.16±0.02 ml/min; P<0.01) were elevated in the ACTH-treated group, fractional sodium excretion was significantly reduced compared to controls (0.76±0.1 vs 1.31±0.2%; P<0.01). The ENaC-inhibitor, amiloride, caused an increase in sodium excretion in both groups of animals but the natriuretic effect was significantly greater in mice receiving ACTH (0.63±0.13 vs 0.17±0.07 µmol/min; P<0.01). mRNA expression was expressed as a ratio to that of 18S RNA: expression of ENaCα was increased by ACTH treatment (0.65±0.09 vs 0.46±0.05; P<0.05), whereas that of NKCC2 was reduced (0.7±0.08 vs 1.24±0.11; p<0.01). Our data show that ACTH treatment causes hypertension in the mouse and this is associated with inappropriately elevated tubular reabsorption of sodium via ENaC. After 14 days of treatment, absolute sodium excretion is normalized, possibly via down-regulation of sodium reabsorption in the loop of Henle.