ROLE OF ARGININE-VASOPRESSIN IN THE REGULATION OF EXTRACELLULAR FLUIDVOLUME

The Henry-Gauer hypothesis postulates that changes in left atrial pres sure induce changes in the release of arginine vasopressin (AVP), whic h subsequently modulates the renal output of fluid. Results of the pas t decades indicate that this hypothesis is too simplistic in explainin g the complexity of extracellular fluid volume (ECFV) regulation in hu mans. Factors controlling renal sodium excretion are the primary modul ators of ECFV. AVP is probably important in the related adjustments of renal water excretion whereby changes in plasma sodium concentration induce changes in plasma osmolality and, subsequently, in release of A VP. Evidence has accrued that changes in arterial variables, e.g., art erial pulse pressure, induce changes in the release of AVP during acut e changes in central blood volume. Thus, arterial baroreflex regulatio n of AVP release might constitute one of several pathways of ECFV regu lation. Recent results from the D2-Spacelab mission on ECFV regulation are surprising. Following an isotonic saline infusion, renal sodium a nd fluid output were lower than expected from results of simulation ex periments, and venous plasma NE and renin higher. Since plasma AVP was low, high levels of this variable cannot constitute an explanation fo r the attenuated renal output of fluid during flight. Thus, the curren tly used models (in particular head-down bad rest) for simulating micr ogravity should be critically reevaluated. In addition, the relationsh ip between central cardiovascular variables, endocrine mediators, and renal function during microgravity should be a focus of future researc h.