The relationship between active transport and the exchange diffusion effect

Ion regulatory patterns of mosquito larvae collected from breeding sites in the Amazon rain forest

We examined the ion composition of mosquito breeding sites located in the Amazon rain forest and the ion regulatory patterns of larvae from these habitats. We found larvae of Toxorhynchites haemorroidalis, Limatus durhamii, Culex (Carrollia) bonnei, and Culex (Culex) sp. residing in fallen palm bracts, leaves, and tree holes that were filled with water. These breeding sites had micromolar levels of Na(+) (1.6-99 micromol L(-1)), but K(+) and Cl(-) concentrations were higher and varied over a large range (231-17,615 micromol L(-1) K(+); 355-2,700 micromol L(-1) Cl(-)). Despite the variability in environmental ion levels and ratios, all four species maintain high hemolymph NaCl levels (80-120 mmol L(-1) Na(+); 60-80 mmol L(-1) Cl(-)). However, the species differed in the means by which they maintain hemolymph ion balance, as indicated by the range of unidirectional Na(+) and Cl(-) uptake rates. Toxorhynchites haemorroidalis had extremely low rates of Na(+) uptake and undetectable Cl(-) uptake, whereas L. durhamii had high rates of uptake for both ions. This variability in rates of uptake may reflect species differences in rates of diffusive ion loss (i.e., permeability). We observed the same curious pattern of Na(+) inhibition and Cl(-) stimulation by low-pH exposure in all four species of mosquitoes, as has been documented in other mosquitoes and aquatic insects. Kinetic analyses of Na(+) and Cl(-) uptake in C. bonnei larvae revealed an unusual pattern of Na(+) uptake that increases linearly (nonsaturable) to extremely high rates, while Cl(-) uptake is a low-affinity, low-capacity system. This pattern contrasts with L. durhamii and Culex (Culex) sp. larvae, which had large increases in both Na(+) and Cl(-) uptake when external NaCl levels were increased. Our results suggest that although these rain forest mosquito larvae are residing in habitats with similar low Na(+), high Cl(-) composition and maintain similar hemolymph NaCl levels, the underlying mechanisms of ion regulation differ among the species.

Characterization of the transport of potassium ions in the cyanobacterium Anabaena variabilis Kütz

Interrelationships between potassium-ion transport and transplasmalemma electrical-potential difference (delta psi m) have been investigated in Anabaena variabilis (ATCC 29413) by measuring K+ translocation and membrane potential in parallel. At pH 7.0, 5 mmol . dm-3 external K+, there was a thirtyfold accumulation of K+. The K+ equilibrium potential was lower (more negative) than the measured membrane potential by up to 20 mV, (delta psi K+ = -90 mV; delta psi m = -70 mV to -75 mV, respectively). Dark pretreatment and low temperature (4 degrees C) reduced internal K+ and depolarized delta psi m. External pH affected K+ translocation and membrane potential; delta psi m was hyperpolarized at high external pH; transplasmalemma K+ fluxes and internal K+ concentration were also increased at high pH. The effects of pH upon delta psi m, coupled with the finding that the membrane potential was relatively insensitive to external K+, suggest that delta psi m is unlikely to be due primarily to a diffusion potential of K+, but that the membrane potential is maintained by an electrogenic proton-extrusion mechanism. There was no close (obligate) link between K+ transport and changes in delta psi m. Carbonylcyanide m-chlorophenylhydrazone decreased K+ fluxes, internal K+ and delta psi m when added in amounts up to 100 mumol . dm-3. However, delta psi K+ was always more negative than delta psi m. Valinomycin up to concentrations of 50 mumol . dm-3 increased transplasmalemma K+ fluxes by up to 300%, while changes in delta psi m were negligible. Internal K+ was unaffected by valinomycin. N,N'-Dicyclohexylcarbodiimide at concentrations up to 100 mumol . dm-3, reduced K+ flux rates and caused a hyperpolarization of delta psi m. These observations suggest that delta psi m is primarily due to electron transport reactions at the plasmalemma and that K+ transport is energy-dependent. In the presence of dicyclohexylcarbodiimide, internal K+ redistributed in accordance with the membrane potential, suggesting that passive uniport in response to delta psi m (i.e. secondary active transport) is not usually important but may operate when primary active mechanisms are blocked.