Effects of prolactin on water balance and kidney function in bullfrog tadpoles

In four experiments of similar design, stage III-XII bullfrog tadpoles were treated for 5-13 days with either 10 micrograms/day ovine prolactin (PRL) or 10 microliters/day saline (0.7% NaCl). In all cases, saline-treated tadpoles lost ca. 10% of their body weight, whereas PRL-treated animals either maintained their weight or gained slightly. Compared with controls, PRL-treated tadpoles showed a significantly higher water content and a 10-20% increase in inulin space (= extracellular volume). PRL treatment also significantly decreased plasma [Na+] (but not total extracellular [Na+]), and the rates of renal inulin clearance (= glomerular filtration rate) and urine production. PRL-treated tadpoles showed significant retention of the water taken up in response to a salt load. These data suggest that the PRL-induced weight gain observed in bullfrog tadpoles is accompanied by major changes in renal function and the size, water content, and sodium concentration of the extracellular fluid compartment.