Fluid shifts and endocrine responses during lower body negative pressure and dynamic arm exercise

The use of lower body negative pressure (LBNP) is proposed as a means of reducing the effect of spaceflight on body water loss by stimulation of renin angiotensin aldosterone system (RAAS) activity. Seven subjects were successively submitted to LBNP exposure, arm cranking physical exercise, and to a combination of both procedures (LBNP + arm cranking) in order to check whether this combination enhances RAAS activity. The results showed that exposure to 40 min of LBNP to a level of -40 mm Hg was a more potent stimulus for renin secretion than submaximal and maximal arm cranking. The combination of LBNP with exercise does not further enhance the RAAS activity induced by LBNP alone. These data suggest that the fluid shift toward the lower body induced by LBNP counteracts triggering of renin secretion due to physical exercise.