Abstract

A preparation is described in which infusion at an exponentially decreasing rate into the thoracic aorta of the rabbit permits the sudden elevation of tracer concentration in arterial plasma perfusing the kidneys to a high and constant level. Under these conditions inulin in plasma and urine equilibrates at a rapid rate. By contrast, isotopic equilibration of potassium proceeds at a much slower rate, believed to reflect the passage of K through a large and initially unlabeled secretory pool. Stimulation of K excretion by chlorothiazide consists of the addition to urine of K with low specific activity. The conclusion is drawn that chlorothiazide accelerates distal secretion of slowly equilibrating K. Chlorothiazide does not appear to inhibit reabsorption of the high specific activity filtered K, which would approach isotopic equilibration in urine at the same rapid rate as inulin.