Angiotensin stimulates glucose and fluid absorption by rat proximal straight tubules

I investigated the effect of angiotensin on glucose and fluid absorption by isolated, perfused rat proximal straight tubules. During the control period, tubules absorbed fluid at 0.68 +/- 0.05 nL/mm.min and glucose at 12.5 +/- 1.3 pmol/mm.min. After 10(-10) M angiotensin was added to the bath, tubules absorbed fluid at 0.82 +/- 0.06 nL/mm.min and glucose at 16.0 +/- 2.4 pmol/mm.min--a stimulation of both parameters by 30%. Time controls showed no significant change in the rate of glucose or fluid absorption. This stimulation was due to an increase in the maximum rate of transport. The maximum rate of glucose absorption with 5.5 mM in bath and perfusate increased from 14.5 +/- 1.6 pmol/mm.min to 18.5 +/- 1.4 pmol/mm.min after 10(-10) M angiotensin was added to the bath. In contrast, the Km for glucose did not change, since a solution containing 0.55 mM glucose (approximately Km) supported 63 +/- 6% of the maximum rate during the control period and 60 +/- 5% of the maximum rate after angiotensin was added to the bath. Angiotensin also had no effect on glucose permeability, which was (3.7 +/- 0.4) x 10(-6) cm/s before treatment and (3.8 +/- 0.4) x 10(-6) cm/s after treatment. From these data, it is concluded that: (1) angiotensin stimulates glucose absorption in the rat proximal straight tubule; (2) this stimulation is the result of an increase in the maximum rate of transport rather than a change in Km or permeability; and (3) stimulation of Na/glucose absorption by angiotensin accounts for a significant portion of the increase in fluid absorption caused by angiotensin.