Membrane potentials and chloride activities in epithelial cells of Aplysia intestine

Microelectrodes were used to measure the potential difference across the mucosal membrane (psi m) of cells lining the mucosa of isolated Aplysia californica intestine. In glucose-free sodium chloride seawater medium psi m was -68.2 +/- 2.2 mV, the cell interior negative relative to the mucosal bathing medium. Simultaneously, the transmural potential difference (psi m leads to s) was recorded and its value averaged -0.4 +/- 0.1 mV (serosa negative). The addition of D-glucose to the mucosal compartment depolarized psi m to -62.1 +/0 2.5 mV and hyperpolarized psi m leads to s to -1.5 +/- 9.1 mV. Intracellular chloride activity (aCli) was also determined in Aplysia intestinal cells using single-barreled ion-exchange microelectrodes. In the absence of D-glucose the mean aCli was 10.1 +/- 0.5 mM, whereas after mucosal glucose addition the paired mean aCli increased significantly to 14.2 +/- 0.6 mM. Both aCli values were less than those predicted by the electrochemical equilibrium for chloride across the mucosal membrane. An active chloride transport mechanism located in the lateral-serosal membrane of the surface epithelial cells is proposed to explain these observations.