We have recently demonstrated a novel role for "short" actin filaments, a distinct species of polymerized actin different from either monomeric (G-actin) or long actin filaments (F-actin), in the a...

Activation of epithelial Na+ channels by protein kinase A requires actin filaments.

To study the mechanisms by which antidiuretic hormone and prostaglandins regulate Na transport at the apical membranes of the cells of anuran tissues, studies were done with fluctuation analysis. Epithelia of frog skin (Rana pipiens) were treated with vasopressin alone, or treated with vasopressin after inhibition of Na transport by indomethacin. The tissues were bathed symmetrically with a Cl-HCO3 Ringer solution and short-circuited continuously. In this experimental circumstance, the amiloride-induced current noise power density spectra were of the Lorentzian type with little or no l/f noise, provided that "scraped" skins were used for study. Despite large changes of Na transport, especially in epithelia treated with indomethacin and vasopressin, the single-channel Na current remained essentially unchanged, whereas the density of amiloride-inhibitable, electrically conductive Na channels was increased by vasopressin and decreased by indomethacin.

/pdf/hormonal-control-of-apical-membrane-na-transport-in-15aiqey9zx.pdf

Hormonal control of apical membrane Na transport in epithelia. Studies with fluctuation analysis.

Incubating toad bladder with 10 mU/ml vasopressin increases the amiloride-blockable Na+ flux in membrane vesicles derived from the epithelial cells by about twofold. This stimulation is further enhanced by 3-isobutyl-1-methylxanthine and can be mimicked by 8-bromoadenosine 3', 5'-cyclic monophosphate. Thus the natriferic action of cAMP involves a sustained change of the apical membrane preserved by the isolated vesicles. The possibility that transport is modulated by direct phosphorylation/dephosphorylation of the Na+ channel was tested. Trapping purified cAMP-dependent protein kinase, cAMP, and ATP in apical vesicles failed to alter Na+ transport even though the enzyme proved active and could phosphorylate intravesicular proteins. Trapping several phosphatases partially purified from toad bladder in vesicles was ineffective as well. These data suggest that the cAMP-induced increase in Na+ conductance involves processes other than phosphorylation of the channel protein or direct channel-cAMP interaction.

Characterization of cAMP-induced activation of epithelial sodium channels

The effects of ethanol on urinary excretion of arginine vasopressin (AVP), sodium, and potassium were investigated in two rat strains specially selected for their different alcohol preferences. The alcohol preferring (AA) strain excreted more AVP and the water preferring (ANA) strain more urine and sodium during six hours after ethanol intubation (2.4 g/kg b.w.; 20% v/v). The data is insufficient to establish a causal relationship between differences in water and electrolyte metabolism and voluntary ethanol consumption.

AL Finn

Papers

Control of vasopressin stimulation of sodium transport in the toad bladder.