We evaluated the effects of osmotic gradients on 22Na+ influx in vesicles prepared from rat outer renal medulla. 22Na+ influx driven in a coflow mode by an inwardly directed 100 mM KCl gradient was measured at 20 and 60 s; 1 mM bumetanide inhibited ~ 30% of 22Na+ influx. The bumetanide-sensitive 22Na+ influx was reduced by ~ 65% when either K+ or Cl- was omitted from the aqueous phases. We found that an osmotic gradient for vesicle shrinkage, that is, 600 mM urea in the extravesicular medium, enhanced the bumetanide-sensitive 22Na+ influx twofold. Conversely, an osmotic gradient for vesicle swelling, that is, with vesicles but not extravesicular media loaded with 600 mM urea, produced a 50% suppression of bumetanide-sensitive 22Na+ influx. Moreover, 600 mM extravesicular urea, an osmotic gradient for vesicle shrinkage, also reduced uptake of the nonspecific marker [14C]mannitol. These effects of osmotic gradients were not due to alterations in ionic driving forces, since bumetanide-sensitive 22Na+ influx driven in a counterflow mode by loading the vesicles with 100 mM NaCl also was activated or suppressed by osmotic gradients for vesicles shrinkage or swelling, respectively. We conclude that osmotic gradients, and/or vesicle volume changes, modulate bumetanide-sensitive Na+:K+:2Cl- activity.
|Original language||English (US)|
|Journal||American Journal of Physiology - Renal Fluid and Electrolyte Physiology|
|Issue number||6 (24/6)|
|State||Published - 1988|
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