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Charge properties and ion selectivity of the rectal intima of the desert locust Lewis, Simon Andrew


The rectal intima of the desert locust was found to possess fixed negative charges, rather than fixed neutral sites. It was suggested that the molecular species responsible for the negative sites might be acidic amino acids. The selective permeability of the intima as estimated from diffusion potentials, for divalent cations was 3a⁺² > Ca⁺² > Sr⁺² > Mg⁺² > Mn⁺², for monovalent cations was NH₄⁺ > Rb⁺ > Cs⁺ > K⁺ > Na⁺ > Li⁺ > TEA⁺ and for monovalent anions was HCO₃⁻ > CN⁻ > F⁻ > NO₃⁻ > CL⁻ > CH₃COO⁻ > Br⁻ > H₂PO₄⁻ > I⁻. Cation affinity for the fixed charged site was found to be in the order of Ca⁺² > Mg⁺⁺ » K⁺ > Na⁺. Similarity of effects of pH and ion concentration on streaming and diffusion potentials indicated that ion movement and water flow might take place through the same route. The intima was found to act as an osmotic compartment such that at high external osmotic pressures, the rate of water flow was reduced due to a shrinkage of the effective pore size in the intima, however the relative permeability of ions did not seem effected by membrane dehydration. Unstirred layers at the membrane-solution interfaces were found to have a minimal effect on diffusion potentials, however half of the value for streaming potentials was found to be due to a diffusion potential caused by an ion concentration difference in opposing unstirred layers. Calcium -45 flux across the intima at pH 5.5 (i.e. possessing fixed charge) was found to be 81 times greater, at a concentration of 10 mM/l CaCl₂, than calcium flux at the same concentration across the uncharged membrane (pH 2.2). The same effect was not significant for rubidium. Conversely, the removal of fixed charge enhanced anion flux. Calcium permeation rate was found to be a function of its dissociation rate from the fixed charge and did not correlate in a simple manner with the membrane binding capacity for calcium. A trans effect on calcium flux was also found in the intima and is believed to be a function of the dissociation rate of calcium from the fixed negative site. It was concluded that electro-osmosis was not the mode of water movement across the rectum, however physiological advantage of electro-osmosis was discussed. Flux experiments possible indicate that the intima might be the rate limiting step for K⁺ reabsorption in a hydrated animal.

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