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1Department of Anesthesiology and 2Institute of Physiology, University of Lübeck, D-23538 Lübeck, Germany; 3Institute of Clinical Biochemistry, Rikshospitalet, University of Oslo, NO-0316 Oslo, Norway; and 4Immundiagnostik AG, D-64625 Bensheim, Germany
Submitted 10 November 2003 ; accepted in final form 25 February 2004
The polypeptide relaxin (RLX) has been suggested to play a role in cardiorenal integration and to be related to the natriuretic peptide system. We hence examined the effects of variations in thoracic blood volume and intravenous volume loading on plasma and urinary RLX levels and associated changes in natriuretic peptide levels in healthy men. Two groups of eight subjects were randomly tilted into a 15° feet-down or a 15° head-down position. Ten volunteers were crossover subjected to an infusion of 15 ml/kg of 0.9% NaCl (over 60 min) or control during an observation period of 10 h. Blood and urine were sampled at timed intervals. RLX, NH2-terminal prohormones of atrial natriuretic peptide (NT-pro-ANP), and NH2-terminal prohormones of brain natriuretic peptide (NT-pro-BNP) were determined by enzyme, radio-, and electrochemoluminescence immunoassays, respectively. NT-pro-ANP levels (in percentage of baseline levels) were higher (P < 0.05) during the head-down (124 ± 13%) than during the feet-down position (82 ± 6%). NT-pro-BNP and RLX were not affected by tilting. Volume loading induced a short-lasting increase in plasma NT-pro-ANP, a delayed increase in plasma NT-pro-BNP, had no effect on plasma RLX, and induced a parallel increase in urine flow, renal excretion of sodium, RLX, and NT-pro-BNP. It is concluded that variations in thoracic blood volume in healthy men are not associated with variations in plasma RLX. Increased urinary RLX and NT-pro-BNP excretion during volume loading suggest renal production and a possible role of kidney-derived RLX and brain natriuretic peptide in sodium homeostasis in men.
intrathoracic blood volume; fluid homeostasis; posture; cardiorenal integration; natriuretic peptides
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