Abstract
The effects of porcine growth hormone (pGH) or ovine prolactin (oPRL) alone and in combination with triiodothyronine (T~3~) on renal PRL receptors were determined in both pre‐ and post‐metamorphic tiger salamanders (Ambystoma tigrinum). The protein hormones were given at a dose of 1.0 μg/gm body weight/day and the T~3~ was given at 10.0 ng/gm body weight/day. The duration of treatment was 7 days. Effects on growth, and plasma thyroid hormone levels were also determined. Ovine PRL increased growth in both larvae and adults and reversed metamorphic changes. Administration of T~3~ increased the plasma T~3~ concentration, as measured by radioimmunoassay, and when given alone caused weight loss at both stages. The GH decreased plasma T~4~ and increased plasma T~3~ concentrations, indicating that it caused an increase in T~4~ deiodination. In adults the renal PRL receptor affinity of 2.9 ± 0.7 × 10^10^ L/mol and capacity of 160 ± 22 fmol/mg protein were higher than the corresponding values of 1.8 ± 0.3 × 10^10^ L/mol and 29.2 ± 3.8 fmol/mg in larvae. In adults only, there is an additional low‐affinity, high capacity PRL binding site. The oPRL treatment decreased the binding capacity to 33.2 ± 1.2 and 5.9 ± 4.9 fmol/mg in adults and larvae, respectively. By contrast, pGH increased the capacities to 249 ± 18 and 62.1 ± 6.8 fmol/mg in adults and larvae, respectively. Treatment with T~3~ alone doubled the oPRL binding capacity to 58.3 ± 4.7 fmol/mg in larvae, but there was no effect in adults. In both developmental stages the effects of oPRL and pGH on the receptors were not changed by the simultaneous T~3~ treatment. Porcine GH treatment both with and without T~3~ reduced the PRL binding affinity, but the effect was significant only in larvae. The effect of oPRL on binding affinities could not be determined, because the binding capacity was so low. In adults the low‐affinity site was detectable in all treatment groups, but none of the treatments caused the appearance of the low‐affinity binding site in larval kidneys, even when they increased the binding capacity to adult levels (T~3~, pGH, or both together). It therefore seems likely that some other factors are involved in the development of the adult binding pattern. The increased oPRL binding capacity caused by pGH in larvae and adults may be secondary to its effect on serum T~3~ levels. However, the reduction in K~a~ values in the larvae may be caused by GH itself, possibly due to an increase in the number of the low‐affinity sites. © 1993 Wiley‐Liss, Inc.