Time course of proximal tubule response to acute arterial hypertension in the rat.

Acute hypertension was previously shown to cause partial inhibition of proximal tubule fluid reabsorption in perfused tubules in the rat. If the inhibition also occurs in unobstructed tubules receiving native glomerular filtrate, hypertension should increase end proximal flow rate despite autoregulation of glomerular filtration rate (GFR). We tested this prediction with a videodensitometric method recently developed for measurement of tubular flow rate that does not interrupt flow to the macula densa. Hypertension was induced by increasing total peripheral resistance in rats receiving several hormones at rates designed to maintain high levels of these agents. End proximal flow rate was increased 18% as early as 1.5-2 min following the induction of hypertension and increased over the course of the next 25-30 min to reach values 50% greater than controls as the hypertension was sustained. Whole-kidney GFR and renal blood flow were fully autoregulated. The results confirm that hypertension increases the fluid load to the loop of Henle, and are consistent with an effect on proximal tubule fluid reabsorption. This increase in fluid load could signal the macula densa and contribute to the efficacy of autoregulation; it could also provide a significant fraction of the increased fluid and salt excretion of pressure natriuresis.