The Localization of Ion-Selective Pumps and Paths in the Plasma Membranes of Turtle Bladders
Turtle bladders actively transport Na, Cl, and HCO3 to the serosal fluid; and each ionic flux is independent of the others under short-circuiting conditions. This behavior mimics that of a parallel network of ion-selective, electrically-conductive paths and pumps in each membrane, — a picture consistent with recent evidence along three independent lines. (1) The potential response to increases in mucosal Na concentration indicates that the Na conductance of the apical membrane is 70% of the transepithelial conductance and that the Na transfer across this membrane occurs via an electrically-charged carrier operation. (2) The sidedness and selectivity of transport changes induced by certain agents are the following. Acting from the mucosal side only, amiloride blocks passive Na transfer; and catecholamines (or imidazoles or theophylline) accelerate active anion transport. Acting from the serosal side only, ouabain blocks active Na transport; and disulfonic stilbenes or acetazolamide block passive anion transfers. (3) The surface charge density of the apical membrane differs from that of basal-lateral during free-flow electrophoresis (FFE) of a mixed membrane fraction of epithelial cells. Basal-lateral membrane fragments (containing ouabain-sensitive ATPase and a stilbene-binding protein) migrate toward the positive electrode while apical membrane fragments (contain nor-epinephrine-sensitive adenylate cyclase and cAMP-activated protein kinase) migrate toward the negative electrode. (4) Thus, ouabain, nor-epinephrine, and a disulfonic stilbene are shown to be useful membrane probes for the Na pump, the anion pumps, and the passive anion transfer paths, respectively.
KeywordsAdenylate Cyclase Apical Membrane Anion Transport Membrane Fragment Bathing Fluid
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