Microelectrode Evidence for the Electrical Isolation of Goblet Cell Cavities in Manduca Sexta Middle Midgut

Abstract

The middle midgut of Manduca sexta is known to transport K⁺ actively from the basal to the apical side, and is thought to be involved in the generation of an extremely high _pH in the midgut lumen An experimental configuration is described which allows visually guided microelectrode impalements to be made of any compartment in the midgut tissue, with simultaneous verification by fluorescent dye ionophoresis in the live tissue. In this way, many impalements can be performed and verified on a single gut The potential differences between impalement sites and the apical compartment were plotted against transepithelial potential difference (TEP) for three sites: goblet cell cytoplasm, columnar cell cytoplasm and goblet cell cavities. In all three cases, the potentials recorded varied significantly and linearly with gross TEP Goblet and columnar cell cytoplasm impalements resembled each other both in size and direction of potential, being always negative relative to the lumen. This implied electrical coupling was confirmed by the observation that the two cell types were linked by gap junctions Goblet cavity impalements were characterized by a positive potential relative to the lumen. The goblet cavities were isolated from the lumen by a high resistance, and no leakage of Lucifer Yellow or 6-carboxyfluorescein markers through the apical valve was detected Using sensible estimates for midgut intracellular _pH and in vivo TEP, it was shown that the potential difference across the goblet cavity apical membrane in vivo would agree closely with the Nernst potential for protons across the membrane, in agreement with a model for generation of high _pH