Effect of mucosal halides on Ca(2+)-blockable currents through the skin of Rana ridibunda.

The present study deals with the interaction of mucosal anions with apical Ca(2+)-blockable cation channels of the skin of Rana ridibunda. The intracellular potential was depolarized by exposing the basolateral membranes to K2SO4 Ringer solution. The apical bathing medium consisted of nominal Ca(2+)-free K+ or Na+ solutions with SO4(2-), Cl-, Br-, or I- as the major anion. The effects of mucosal anion substitutions were studied by analyzing 1) the fluctuation in K+ current across the apical membrane driven by imposed transepithelial clamping potentials and 2) alterations of the transepithelial current (It) and conductance (Gt) as well as the Lorentzian parameters in response to anion substitution in the mucosal bathing solution. It and current noise spectra were recorded at different transepithelial potentials (Vt). A Lorentzian component was present in the power density spectrum when Vt was clamped at mucosa-positive voltages. Such noise components were never observed with mucosa-negative potentials. These findings suggest a rectifying behavior of the transepithelial cation currents. The Lorentzian noise component and the inward-oriented cation currents were depressed by the addition of micromolar concentrations of Ca2+ to the apical solutions as well as by replacing mucosal K+ or Na+ by N-methyl-D-glucamine. The Ca(2+)-blockable current and Lorentzian noise plateau (So) were gradually increased by raising Vt. Both parameters, as well as the corner frequency (fc), depended strongly on the major anion species in the apical solution; replacing mucosal SO4(2-) by one of the halides tested reduced fc and elevated So, It, and Gt considerably.

Effects of TRH, bovine TSH, and pituitary extracts on thyroidal T4 release in Ambystoma mexicanum.

Circulating levels of T4 are measured by radioimmunoassay after intravenous injection of TRH, bovine (b) TSH, and pituitary extracts in the Mexican axolotl (Ambystoma mexicanum). Very low control levels of T4 are found (53 +/- 3 pg/ml (n = 27), but they are increased sevenfold following injection of 1/2 pars distalis extract or 1/2-1/10 IU b-TSH. Increased levels following these injections are found in plasma up to 48 hr after the injection. An in vitro assay also indicates that a 1/2 pars distalis of the axolotl is able to release T4 from the thyroid of R. ridibunda somewhat less effectively than a 1/50 pars distalis of the same size of R. ridibunda itself. TRH (50 and 100 micrograms) is unable to stimulate the release of T4 when injected intravenously into the axolotl. It is concluded that both the hypophysis and the thyroid gland of A. mexicanum may release optimal amounts of hormones necessary for metamorphosis following proper stimulation, but that TRH cannot function as a releasing hormone in this respect.