Intracellular K⁺ activities and cell membrane potentials in a K⁺-transporting epithelium, the midgut of tobacco hornoworm (Manduca sexta)

Transbasal electrical potential (V_b) and intraepithelial potassium chemical activity ((K⁺)_i) were measured in isolated midgut epithelium of tobacco hornworm (Manduca sexta) using double-barrelled glass microelectrodes. Values of V_b ranging from +8 to -48 mV (relative to blood side) were recorded. For all sites, (K⁺)_i is within a few millivolts of electrochemical equilibrium with the blood side bathing solution. Sites more negative than -20 mV show relatively high sensitivity of V_b to changes in blood side K⁺ concentration: 43% of these sites can be marked successfully with iontophoresed Lucifer yellow CH dye and shown to represent epithelial cells of all three types present in the midgut. In about half of successful marks, "dye-coupling" of several adjacent cells is seen. Low potential sites - those with V_b less negative than -20 mV -typically do not show high sensitivity of Vb to changes of external K⁺, but rather (K⁺)_i rapidly approaches the K⁺ activity of blood side bathing solution. These sites can seldom be marked with Lucifer yellow (4% success). The mean (K⁺)_i of the high potential sites is 95±29 (sd)mm under standard conditions, a value which is in accord with published values for the whole tissue.