Annual Meeting of the Swiss Physiological Society, October 8, 2004
Asher-Hess Prize
Abstracts selected for oral presentation
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Cardiovascular autonomic regulation in response to water and saline drinking in humans

 

Clive M. Brown, Luc Barberini, Abdul G. Dulloo and Jean-Pierre Montani.

Department of Medicine, Division of Physiology, University of Fribourg.  

Water drinking elicits several acute physiological effects including concurrent increases in sympathetic vasomotor discharge and cardiac vagal tone. The trigger for these responses is unknown but might be related to either gastric distension or to osmotic factors. To test whether osmotic stimulation is involved, we compared the cardiovascular responses to a water drink with those to an isotonic saline drink.

Nine healthy volunteers (aged 20-32 years) drank 7.5 ml/kg body weight of distilled water or 0.9% saline on different days in a randomized order. For 15 min before and 60 min after the drink, we recorded arterial blood pressure (finger plethysmography), heart rate (ECG) and its reciprocal, cardiac interval, and cardiac output (impedance cardiography). Total peripheral resistance was calculated as mean blood pressure/cardiac output. An index of cardiac vagal tone was obtained by using spectral analysis to assess cardiac interval variability in the high frequency range. Baroreflex control of heart rate was quantified from the responses of cardiac interval to spontaneously occurring fluctuations in systolic blood pressure.

Water drinking significantly decreased heart rate (from 62.4±2.0 to 59.6±2.3 beats/min) and increased total peripheral resistance (from 16.15±0.88 to 17.12±1.00 mmHg/ l min -1 ), cardiac interval high frequency power (from 967±140 to 1532±273 ms 2 ) and baroreflex sensitivity (from 24.2±2.1 to 29.0±2.5 ms/mmHg), but did not change blood pressure or cardiac output. In contrast, drinking the saline significantly increased systolic (from 112.5±3.8 to 116.9±4.0 mmHg) and diastolic (74.4±2.8 to 77.8±3.1 mmHg) blood pressure, and cardiac output (from 5.40±0.34 to 5.62±0.38 l/min) but had no effects on total peripheral resistance, heart rate, cardiac interval high frequency power or baroreflex sensitivity.

With water drinking, the decrease in heart rate and increase in cardiac interval variability and baroreflex sensitivity indicate enhanced vagal tone. The increased total peripheral resistance suggests a rise in sympathetic vascular tone. Saline drinking did not seem to activate the autonomic nervous system but did increase blood pressure, probably through the increased cardiac output. These results confirm that water drinking activates both the sympathetic and parasympathetic branches of cardiovascular autonomic regulation. That these effects were absent after drinking isotonic saline suggests that the cardiovascular responses to drinking are influenced by the osmotic properties of the solution.