Using the shrinking droplet method and simultaneous perfusion of the peritubular capillaries the isotonic reabsorption of Ringer's solution from the papillary collecting ducts was measured. Under control conditions the volume reabsorption from the papillary collecting ducts was Jv±SE=2.6±0.1 · 10-5 cm3 · cm-2 · s-1. In rats which were on low Na+ diet, Jv increased to 127%, and in adrenalectomized animals it decreased to 34% of the control value. Three hours after application of aldosterone in the adrenalectomized animals Jv was partially restored to 63% of control rats. Amiloride 10-4 M, added to the luminal perfusate, produced a strong inhibition of Jv (to 32% of control). Acetazolamide, 10-4 M, added to both perfusates, reduced Jv very strongly (to 40% of control), while omission of bicarbonate reduced it only to 77% of control. Acetazolamide, added to bicarbonate-free perfusates, did not result in a significant further reduction of Jv. The data indicate that the Na+ reabsorption from the papillary collecting duct is controlled by mineralocorticoids. Furthermore, they suggest the existence of two transport mechanisms in the luminal cell membrane: 1. An amiloride-sensitive entry step and 2. an entry step via a Na+-H+-countertransport mechanism, the latter being less important. © 1979 Springer-Verlag.
