Running Head: Cardiovascular responses and osmolality

Water drinking activates the autonomic nervous system and induces acute hemodynamic changes. The actual stimulus for these effects is undetermined but might be related to either gastric distension or to osmotic factors. In the present study we tested whether the cardiovascular responses to water drinking are related to its relative hypo-osmolality. We therefore compared the cardiovascular effects of a water drink (7.5 ml/kg body weight) with an identical volume of a physiological (0.9%) saline solution in 9 healthy subjects (6 male, 3 female, aged 26±2 years), while continuously monitoring beat-to-beat blood pressure (finger plethysmography), cardiac intervals (electrocardiography) and cardiac output (thoracic impedance). Total peripheral resistance was calculated as mean blood pressure/cardiac output. Cardiac interval variability (high frequency power) was assessed by spectral analysis as an index of cardiac vagal tone. Baroreceptor sensitivity was evaluated using the sequence technique. Drinking water, but not saline, decreased heart rate (P=0.01) and increased total peripheral resistance (P<0.01), high-frequency cardiac interval variability (P=0.03) and baroreceptor sensitivity (P=0.01). Neither water nor saline substantially increased blood pressure. These responses suggest that water drinkin