Effect of prolactin on the active Na transport across Rana catesbeiana skin during metamorphosis.

The effect of long-term application of prolactin (PRL) on active Na transport across the abdominal skin of Rana catesbeiana tadpoles during metamorphosis was investigated. At Taylor-Kollros stage XX, the potential difference (PD) and short-circuit current (SCC) across the skin were absent and resistance to an active Na current (RNa) was infinite. At stage XXI, the PD and SCC clearly appeared and increased thereafter. The RNa after stage XXI remained at a relatively constant value of about 5-10 k omega.cm2, whereas the electromotive force of the active Na current (ENa) greatly increased in stages XXI-XXII. It thus appears that Na channels are first formed at stage XXI, and that the Na pump rapidly develops during stages XXI-XXII. Long-term application of PRL was started at both stage XXI (when PD, SCC, and ENa are still low) and stage XXV (when PD, SCC, and ENa are high). Prolactin (20 micrograms/g body wt) was injected every other day for 2 weeks. Since the PD, SCC, and ENa remained low after the injection was started at stage XXI, early treatment with PRL apparently inhibits the differentiation of the Na pump (i.e., ENa). By contrast, treatment with PRL starting at stage XXV decreased the PD and SCC and increased the RNa, but was without effect of the ENa of this group. It appears that PRL has no effect on the Na pump already differentiated, although it inhibits the maintenance of Na channels formed in earlier stages.