Luminal L-alanine stimulates exocytosis at the K⁺-conductive epical membrane of Aplysia enterocytes

In Aplysia intestine, stimulation of Na⁺ absorption with luminal adenine increases apical membrane K⁺ conductance (G(K,a)), which presumably regulates enterocyte volume during stimulated Na⁺ absorption. However, the mechanism responsible for the sustained increase in plasma membrane K⁺ conductance is not known for any nutrient-absorbing epithelium. In the present study, we have begun to test the hypothesis that the alanine-induced increase in G(K,a) in Aplysia enterocytes results from exocytic insertion of K⁺ channels into the apical membrane. We used the fluid-phase marker horseradish peroxidase to assess the effect of alanine on apical membrane exocytosis and conventional microelectrode techniques to assess the effect of alanine on fractional capacitance of the apical membrane (fC(a)). Luminal alanine significantly increased epical membrane exocytosis from 1.04 ± 0.30 to 1.39 ± 0.38 ng·min^-1·cm^-2. To measure fC(a), we modeled the Aplysia enterocyte as a double resistance-capacitance (RC) electric circuit arranged in series. Several criteria were tested to confirm application of the model to the enterocytes, and all satisfied the model. When added to the luminal surface, alanine significantly increased fC(a) from 0.27 ± 0.02 to 0.33 ± 0.04 (n = 10) after 4 min. There are two possible explanations for our findings: 1) the increase in exocytosis, which adds membrane to the apical plasma membrane, prevents plasma membrane fracture, and 2) the increase in exocytosis delivers K⁺ channels to the apical membrane by exocytic insertion. After the alanine-induced depolarization of apical membrane potential (V(a)), there is a strong correlation (r = 0.96) between repolarization of V(a), which reflects the increase in G(K,a), and increase in fC(a). This correlation supports the exocytic insertion hypothesis for activation of G(K,a).