Ca2+ Ions Regulate Activity of Na+,Cl–(HCO 3 − )-ATPase in the Mucosa of Rabbit Small Intestine

Effect of Ionophores on Activity of Na+,Cl-(HCO 3- )-ATPase

We studied the effects of ionophores on activity of Na⁺,Cl^-(HCO ₃^- )-ATPase. The most significant effect on the activity of this enzyme was produced by protonophore 2,4-dinitrophenol. The effect of this drug largely depended on the cation and anion composition and pH of the incubation medium and its pH. Activity of Na⁺,Cl^-(HCO ₃^- )-ATPase increased at neutral and weakly alkaline pH and decreased at pH below 6.5-6.7. In control animals (without histamine injection) with very weak or absent Na⁺,Cl^-(HCO ₃^- )-ATPase activity, the observed effect of the protonophore on ATPase activity was also virtually absent. The stimulatory effect of other ionophores (monensine, valinomycin, and A₂₃₁₈₇) was significantly weaker and depended on pH of the incubation medium, its cationic and anionic composition, and concentration of these ionophores.

Substrate Specificity of Na+,Cl-(HCO3-)-ATPase

We studied substrate specificity of Na⁺,Cl^-(HCO₃^-)-ATPase. In most cases, replacement of ATP for other phosphate-containing substances resulted in not only pronounced suppression of phosphohydrolase reactions, but also dramatic changes of their responsiveness to the stimulating effect of monovalent ions. The data showed that Na⁺,Cl^-(HCO₃^-)-ATPase is a highly specific enzyme for ATP.