Water and salt permeability of gastric vesicles

Summary

Volume-dependent changes in light scatter have been shown to be a linear function of the osmotic gradient imposed upon gastric vesicles purified from hog gastric mucosa. Observation of the light scattered 90° to incident, using the Durrum stop flow system D-110, indicates that the vesicles exposed to hypertonic medium undergo rapid shrinkage due to water loss from the vesicle interior. The rate constant for this water movement is 1.1±0.09 sec⁻¹ (n=10) and is linearly dependent on temperature between 16 and 36°C. The activation energy of 13.93±0.60 kcal mole⁻¹ (n=3), calculated from an Arrhenius plot, is inconsistent with water movement facilitated by a large-pore aqueous channel. A slower reswell phase, dependent on solute entry into the intravesicular space, follows the water-dependent shrink phase. KCl entry, studied because of the intravesicular requirement for active K⁺/H⁺ transport, exhibits two entry stages. The faster, described by a single exponential imposed upon a constantly sloping background, has a rate constant of 7.75±0.48×10⁻³ sec⁻¹ (n=15). The slower phase, which typically accounts for 90% of the reswell process, demonstrates a rate constant of 1.94±0.23×10⁻⁴ sec⁻¹ (n=15). In the presence of valinomycin or nigericin, two fast rate constants and one slow rate constant of swelling are observed. The rate constant of the faster reswell phase is increased from 7.75±0.48×10⁻³ sec⁻¹ (n=15) to 15.74±3.7×10⁻³ sec⁻¹ (n=5) and 17.23±3.4×10⁻³ (n=3) by the addition of nigericin (1 μg ml⁻¹) and valinomycin (4.5 μm), respectively. The second part of the faster reswell phase is approximately that seen in the control population. Transport-dependent volume changes of significant magnitude can be demonstrated following the addition of ATP to vesicles equilibrated with 150mm KCl. The volume change is a function of HCl leak rate and is abolished by ionophores which eliminate the transport-dependent pH gradient. So ^−-₄ substitution, which eliminates the overshoot phenomena observed in KCl medium, also eliminates the shrinkage resulting from ATP addition.