Adaptation to antiorthistasis with modified extracellular fluid volume

Gravity (orthostasis) and body fluid balance (blood volume) are important determinants of cardiovascular regulation; central vascular baroreceptors play a major role. Dietary sodium supply influences extracellular fluid and plasma volume, central venous pressure, and various hormone levels. A recently discovered hormone the task of which in cardiovascular and volume homeostasis needs to be clarified is adrenomedullin. Aim of the study is to investigate hormonal responsiveness (adrenomedullin, aldosterone, renin, atrial natriuretic peptide, vasopressin, norepinephrine) to graded unloading of vascular receptors by means of lower body suction (LBNP). Extracellular fluid is influenced by oral sodium intake (20, 100 and 300 mM/d) for 12 days each: 4 days ambulatory, and 8 days adaptation to antiorthostatic positioning (-6 head down tilt bed rest). Body mass, body electric impedance, plasma osmolality, and 24-hr sodium excretion will be recorded every day. It is hypothesized that stimulus responses within measured variables strongly depend on extracellular fluid volume and adaptation to antiorthostasis. In particular, responses of adrenomedullin to altered sodium and gravity loading, and LBNP are yet unknown. Information derived from this study can be used for further physiological and clinical research and application.