Membrane Transport and Osmoregulation (2A) Practice Test

A freshwater fish was exposed to water containing an inhibitor of gill Na$^+$/Cl$^-$ uptake transporters. Plasma osmolality and urine flow were measured after 4 hours.

Table: Condition vs plasma osmolality (mOsm/kg) and urine flow (mL/kg/hr)

• Control: 300; 8
• • uptake inhibitor: 270; 10

Principle tested: active ion uptake in freshwater supports osmotic balance against passive ion loss and water gain.

Which outcome is most consistent with the osmoregulatory process described?

Researchers investigated epithelial glucose absorption to assess the principle that secondary active transport can be limited by the Na$^+$ gradient established by the Na$^+$/K$^+$-ATPase. Intestinal epithelial monolayers were studied with a fixed luminal glucose concentration. The basolateral Na$^+$/K$^+$-ATPase was inhibited with ouabain, and apical glucose uptake rate was measured over 5 minutes. In a separate condition, luminal Na$^+$ was reduced while maintaining osmolality with an impermeant substitute.

Which response would be expected under the given conditions?