Relationships among intravascular volume, total body sodium, arterial pressure, and vasomotor tone.

CHLOROTHIAZIDE administration in normotensive and hypertensive individuals causes saluresis, diuresis, oligemia, increased depressor responses to ganglioplegics, and decreased pressor responses to norepinephrine."" These changes in responses to vasoactive drugs are abolished if plasma volume is expanded by salt-free dextran,^* and we have suggested that they reflect an augmented sympathetic vasomotor tone brought about by oligemia. This suggestion implies that the saluresis evoked by chlorothiazide is important only because it allows the establishment of a negative water balance. It has been considered that sodium plays a primary role in the development and/or maintenance of hypertension. Since studies of the mechanism of action of oral diuretics suggest a secondary role for sodium and a primary one for water, it becomes important to separate the hemodynamic effects of sodium depletion from those of water depletion. Selective sodium depletion can be achieved by intraperitoneal injection of 5 per cent glucose solution. This fluid will gain sodium and other electrolytes, and its removal, in a few hours, effects sodium depletion without concomitant depletion of body water. Transport of sodium into the peritoneal fluid creates extracellular hypotonicity and, as water moves intracellularly, hypovolemia and hypotension result. Measurements were made of arterial pressure, plasma-sodium levels, and intravascular volume and estimates of sympathetic vasomotor tone were obtained' in normal rats first made hyponatremic, hypovolemic, and hypotensive by intraperitoneal injection of 5 per