Cytoplasmic pH and Goblet Cavity pH in the Posterior Midgut of the Tobacco Hornworm Manduca Sexta

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Abstract
In the larval lepidopteran midgut, the major energy-requiring step of transepithelial K+ secretion occurs across the goblet cell apical membrane. Studies of vesicles of goblet cell apical membrane suggest that K+ secretion across this membrane is a secondary active process in which electroneutral K+/H+ antiport is driven by primary electrogenic H+ secretion. Transbasal K+ movement is thermodynamically passive under standard conditions, but the presence of an active process is revealed in hypoxic solutions or at low extracellular K+ concentration. We measured the pH of cytoplasm and goblet cell cavities, together with the corresponding transmembrane voltages, using double-barreled pH and voltagesensing microelectrodes. For short-circuited midguts in standard bathing solution (pH 8.0) the weighted mean of cytoplasmic pH was 7.14±0.06 (mean±S.E.M.), an average of 0.34 units more acid than expected for electrochemical equilibrium with the hemolymphal solution. The mean pH of goblet cavities was 7.23±0.11, 1.62 units more acid than expected for equilibrium with the luminal solution. The pH gradient across the goblet cell apical membrane is thus of the wrong polarity to drive K+ secretion by electroneutral K+/H+ antiport; however, if the exchange ratio were two or more H+ per K+, the cavity-positive electrical potential could drive H+ back to the goblet cell cytoplasm coupled to K+ secretion from cytoplasm to the goblet cavity. Insensitivity of the goblet cavity pH to the change in the transvalve voltage caused by open-circuiting suggests either that the goblet cavity pH is well regulated or that the valve connecting the cavity to the gut lumen poses a significant barrier to protons. Transbasal potential and cytoplasmic pH were insensitive to a decrease of hemolymphal pH to 6.8, suggesting that the basal membrane is relatively nonconductive to H+. Intracellular pH was unaffected by a decrease of hemolymphal K+ to 5 mmol l−1, suggesting that K+/H+ exchange is not important for transbasal K+ uptake.