Ion Absorption by the Distal Tubules of Onychophoran Nephridia

The distal tubules of the 4th and 5th pairs of nephridia of Peripatus acacioi (Onychophora) absorb ions from the fluid passing through them. Since water follows this movement only to a limited extent, the fluid in the lumen becomes hypoosmotic. Absorption of ions stops when sodium is replaced by choline in the perfusion fluid and is much reduced by treatment with ouabain, particularly when this is applied to the basolateral faces of the cells. These results suggest that ion absorption may be driven by an Na/K exchange pump. Cyclic AMP at 10−2moll−1 increases the rate of salt uptake by about four times; it seems likely from this that the tubules are hormonally controlled.