Gamma melanocyte stimulating hormone (γMSH) possesses cardiac pressor effects and promotes natriuresis by binding to melanocortin 3 receptor (MC3R) in the inner medulla (1). In this study we examined the in vivo and in vitro effects of a stable analogue NDP-γMSH (2) on mean arterial pressure (MAP), heart rate, fractional sodium excretion (FeNa), urinary flow rate, (UV) renal sympathetic nerve activity (RSNA) and expression of epithelial Na+ channel (ENaC), aquaporin-2 (AQP2) and MC3R. Male Wistar rats were fed 0.3% (NNa) or 3% (HNa) sodium for 6 weeks from 3 weeks of age. They were anaesthetised with 1 ml (I.P.) of a chloralose-urethrane (16.5 and 250 mg ml-1, respectively) and maintained with supplemental doses of 0.05 ml every 30 min. Indwelling cannulae were inserted for measurement of blood pressure, i.v. fluid infusion (0.15M saline) and collection of urine. Both kidneys were exposed, the left was cannulated for direct infusion of 100nM NDP-γMSH (300fmol/min) into cortico-medullary border and the right for renal nerve isolation and measurement of RSNA with a bipolar stainless steel electrode. GFR was quantified using FITC-Inulin clearance. Kidneys were removed post infusion and homogenates were blotted for ENaCα, AQP2 and MC3R. Cultured inner medulla collecting duct cells, isolated from male Wistar kidneys (1) were treated with 100nM NDP-γMSH for 3 hours in the presence/ absence of Cytochalsin D & Brefeldin A. Total cell lysates and plasma membrane fractions prepared by differential centrifugation were blotted for ENaCα, AQP2 and MC3R. Infusion of NDP-γMSH increased FeNa more in HNa (165±14% P<0.01) than NNa (10±4%); raised UV more in HNa, 45±12.7µl/min/kg (P<0.03) than NNa, 6.23.1± 2.9µl/min/kg; increased RSNA more in HNa, 46±10% (P<0.001) than NNa, 0.4±0.1%; raised GFR only in HNa, 3.20±0.17ml/min/kg (P<0.05), NNa, 1.86±0.15 ml/min/kg. MAP (NNa, 96±3mmHg; HNa 96±1mmHg) and heart rate (NNa, 369±16bpm; HNa, 415±15bpm) did not change during infusion. Results, presented as means+/-SEM, were compared using paired t-tests and post hoc bonferroni correction. Infusion of NDP-γMSH significantly decreased ENACα expression and increased AQP2 and MC3R expression in the infused kidney (n=4). In vitro treatment of collecting duct cells with NDP-γMSH resulted in a significant decrease in expression of ENaCα and significant increases in expression of AQP2 and MC3R and was accompanied by a decrease in the plasma membrane abundance of ENaCα which was prevented by Cytochalsin D & Brefeldin A (n=3). Data suggests that NDP-γMSH possesses same natriuretic/diuretic, but not cardiac pressor, effects as native γMSH. Natriuretic effects may be achieved by redistribution from apical membranes and a down regulation in expression of ENaC in collecting duct cells mediated by MC3R.