Electrophysiology of K+ Transport by Midgut Epithelium of Lepidopteran Insect Larvae: II. the Transapical Electrochemical Gradients

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Abstract
The apical surface of the midgut of Manduca sexta larvae is composed of the apical membranes of columnar cells, in the form of microvilli, and the apical goblet of goblet cells. Considerable evidence has suggested that the apical electrogenic pump that is responsible for transepithelial K+ transport is located on the apical membrane of goblet cells. In the present study the transapical potentials and K+ chemical activity [(K+)] gradients of columnar and goblet cells of posterior midgut were examined in the short-circuited gut. In some experiments the recording site was localized by ionophoresis of NiCl2 followed immediately by fixation in rubeanic acid. The (K+) of goblet cavities was substantially higher than that of the free solution on the gut luminal side (mean value of 94mmoll−1 in standard bathing solution). The goblet cavity was electrically positive to the gut lumen (mean value of 40 mV in standard bathing solution). When the rate of pumping of K+ into the goblet cavity was decreased by hypoxia or decreased bathing solution [K+], the electrical potential gradient between cytoplasm and goblet cavity decreased while intracellular (K+) and goblet cavity (K+) were relatively stable. These studies provide evidence that a negatively charged goblet matrix is present in goblet cavities. Furthermore, they suggest that it is the voltage-sensitivity of the apical pump to the electrical component of the transapical electrochemical gradient, and not a concentrationdependence of the pump, that exercises the major role in determining the relationship between extracellular (K+) and net K+ transport by the isolated gut.