EFFECTS OF PROLACTIN, GROWTH-HORMONE, AND TRIIODOTHYRONINE ON PROLACTIN RECEPTORS IN LARVAL AND ADULT TIGER SALAMANDERS (AMBYSTOMA-TIGRINUM)

The effects of porcine growth hormone (pGH) or ovine prolactin (oPRL) alone and in combination with triiodothyronine (T3) on renal PRL recep tors were determined in both pre- and post-metamorphic tiger salamande rs (Ambystoma tigrinum). The protein hormones were given at a dose of 1.0 mug/gm body weight/day and the T3 was given at 10.0 ng/gm body wei ght/day. The duration of treatment was 7 days. Effects on growth, and plasma thyroid hormone levels were also determined. Ovine PRL increase d growth in both larvae and adults and reversed metamorphic changes. A dministration of T3 increased the plasma T3 concentration, as measured by radioimmunoassay, and when given alone caused weight loss at both stages. The GH decreased plasma T4 and increased plasma T3 concentrati ons, indicating that it caused an increase in T4 deiodination. In adul ts the renal PRL receptor affinity of 2.9 +/- 0.7 x 10(10) L/mol and c apacity of 160 +/- 22 fmol/mg protein were higher than the correspondi ng values of 1.8 +/- 0.3 x 10(10) L/mol and 29.2 +/- 3.8 fmol/mg in la rvae. In adults only, there is an additional low-affinity, high capaci ty 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, respect ively. 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 l arvae, 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 T3 treatment. Porcine GH treatment both with and without T3 reduced the PRL binding affinity, but the effect was significant on ly in larvae. The effect of oPRL on binding affinities could not be de termined, because the binding capacity was so low. In adults the low-a ffinity site was detectable in all treatment groups, but none of the t reatments caused the appearance of the low-affinity binding site in la rval kidneys, even when they increased the binding capacity to adult l evels (T3, pGH, or both together). It therefore seems likely that some other factors are involved in the development of the adult binding pa ttern. The increased oPRL binding capacity caused by pGH in larvae and adults may be secondary to its effect on serum T3 levels. However, th e reduction in K(a) values in the larvae may be caused by GH itself, p ossibly due to an increase in the number of the low-affinity sites. (C ) 1993 Wiley-Liss, Inc.