Cation transport in the ampulla of the semicircular canal and in the endolymphatic sac

Summary We examined the effects of anoxia and ethacrynic acid on the endolymphatic potential and cation activity in the superior ampulla of the guinea pig, using double-barrelled ion-exchanger microelectrodes. In normal guinea pigs the ampullar endolymphatic potential was +3.9±1.2 mV (n=32), the Cl^– activity 130±4.6 mM (n=9), and the Na⁺ activity 18.4±4.4 mM (n=20). After anoxia, the ampullar DC potential decreased rapidly and reversed its polarity within 5 min. It then decreased gradually for 60 min and increased afterwards to approximately zero. K⁺ activity decreased gradually after a latency of 10 min, whereas Na⁺ activity increased. During the gradual decrease of a negative ampullar endolymphatic potential, an increase in Na⁺ activity was observed. Thirty minutes after the intravenous injection of ethacrynic acid (100 mg/kg), the potential began to decrease, changed to a negative polarity, and approached a maximum negative level 100 min after the injection. The decrease in K⁺ activity corresponded to the reduction of potential whereas Na⁺ activity remained unchanged. The DC potential of the endolymphatic sac in normal guinea pigs was + 14.7±5.1 mV (n=17). The Na⁺ concentration was 103.3±14.7 mM (n=14) and the K⁺ concentration was 11.6 ±0.8 mM (n=4). After anoxia, the DC potential decreased rapidly and approached 0 mV within 8 min. No negative potential could be observed. The Na⁺ concentration began to increase 2 min after anoxia and reached the extracellular Na⁺ concentration about 30 min later. No significant effect of intravenous administration of ethacrynic acid (100 mg/kg) on DC potential and Na + concentration could be observed. The results suggest the presence of a different ion transport system in the endolymphatic sac from that of the cochlea and the ampullae of the semicircular canals.