Essential hypernatremia. The experimental model

The importance of intracellular volume in the control of vascular volume

The effect of change in extracellular (interstitial) volume on vascular volume (VV) is mediated through interstitial pressure. Since the effect of swollen or shrunken cells on interstitial pressure should be identical to that of changes in interstitial volume on interstitial pressure, it would be appropriate to consider that intracellular volume (ICV) as well as extracellular volume (ECV) contributes to control of VV. Total body water rather than ECV, therefore, should be considered as the fluid volume which regulates VV. Support for these conclusions has been provided by theoretical analysis of the factors that regulate capillary fluid exchange. In further support of this hypothesis, clinical examples are described, in which renal salt retention was observed despite increases in ECV (ICV was markedly decreased), and in which renal salt loss occured despite decrease in ECV (in the presence of increased cell volume).

A central osmosensitive receptor for renal sodium excretion

1. The effect on renal Na and water excretion of increasing the NaCl concentration of blood supplying the brain was investigated in conscious water-loaded sheep. Intracarotid infusion ot 4 M-NACl at 0-8 ml./min for 60 min was compared with equivalent intrajugular infusion. 2. A more rapid increase in renal Na excretion and urine osmolality occurred with the intracarotid infusions than with intrajugular infusions. 3. Intracarotid infusions of 2 M sucrose or fructose at 1-6 ml./min for greater increase in renal Na excretion, urine osmolality and a decrease in urine flow rate. 4. The results suggest that there are receptors in the brain sensitive to changes in extracellular tonicity which influence renal Na excretion. It is possible that changes in ADH secretion alone mediate the early natriuresis seen with intracarotid hypertonic infusions although an alternative concurrent mechanism cannot be ruled out.