Non-electrolyte permeability through black lipid membranes with different surface charge

The permeability coefficients of some non-electrolytes has been measured across black lipid membranes of different composition. Large discrepancies between permeability coefficients and oil-water partition coefficients has been observed. The discrepancy level seems to be related to the degree of organization of the membrane.

Facilitated transport of urea across the gall-bladder luminal membrane

Counterflow experiments demonstrate the existence of urea counter-transport on the epithelium luminal surface. This phenomenon disappears when 10(-4) M phloretin is added to the perfusion fluid. Moreover counterflow experiments made using thiourea as elicitor, demonstrate that the phenomenon is specific for the urea.

Colchicine inhibition of ADH effect on frog skin permeability

ADH and AMPc enhance both thiourea unidirectional fluxes in frog skin. This effect is completely abolished by colchicine pretreatment. The ADH increase of thiourea discharge with or without colchicine led us to suppose that colchicine does not directly affect ADH action on outer membrane permeability, but exerts its effects on a site which is limiting for the ADH action on transepithelial permeability.

Noradrenaline induced secretion of nonelectrolytes through frog skin

Addition of noradrenaline (4 x 10(-5) M) to the inner bathing fluid in the skin of the frog Rana esculenta results in increased unidirectional fluxes of urea, thiourea, N-methyl-thiourea, N-N'-dimethylthiourea and mannitol. Fluxes towards the external medium (phi o) undergo a much greater increase than those moving the opposite direction (phi i). The effect of noradrenaline on phi o is higher for urea and thiourea than mannitol, while its effect on phi o thiourea derivatives is related to lipid solubility. This phenomenon does not occur for phi i of the same molecules. FCCP (10(-6) M) pretreatment strongly inhibits the noradrenaline effect on phi o. In skin pretreated with colchicine (2 x 10(-5) M) both urea fluxes are increased to the same extent by noradrenaline. Noradrenaline is concluded to exert two separate effects: (1) a change in permeability in both directions; (2) a secretion of nonelectrolytes towards the external fluid. Such secretion is most probably associated with the hormone-induced secretion of fluid and electrolytes, perhaps mediated by an exocytotic mechanism.

Facilitated transport of urea across the toad gallbladder

The toad gallbladder epithelium is much more selective than that of the rabbit especially as to the permeability of two molecules like urea and thiourea. These observations can probably be attributed to different permeation mechanisms of the 2 molecules. Neither active transport nor solvent drag can explain these phenomena. 10(-4) M phloretin strongly inhibits urea movement, but does not alter either thiourea fluxes or isotonic net water transport: these results suggest that a specific mechanism is involved in urea movement. The urea transport shows saturation kinetic which is consistent with the presence of a facilitated mechanism.

Permeability pathways for non-electrolytes through Bufo bufo gall-bladder

Amphotericin B treatment increases the thiourea, D-xylose and mannitol fluxes and lowers those of urea, N-methyl-urea, acetamide, formamide, and N-N'-dimethyl-thiourea. The degree of flux inhibition is related to the cellular permeability of these compounds. Most probably Amphotericin B increases the permeability of all those molecules across the luminal plasma membrane, but simultaneously elicits a cellular swelling, which reduces the diffusion across the lateral plasma membranes. This effect masks the polyene effect especially for molecules showing a mainly cellular permeation pathway such as amides and lipid soluble molecules.