Electrophysiology of K+ transport by midgut epithelium of lepidopteran insect larvae. III. Goblet valve patency

The midgut epithelium of lepidopteran insect larvae contains characteristic goblet cells possessing an apical cavity surrounded by goblet cell apical membrane (GCAM) and guarded from the gut lumen by a valve-like structure. The currently accepted model of active K+ secretion by the midgut of lepidopteran insect larvae locates the major active step at the GCAM, implying that actively transported K+ reaches the gut lumen by passing through the valve. The major question for these studies was whether K+ could pass through the valve by diffusion in free solution. Using tetramethylammonium (TMA+) as a surrogate for K+, and exploiting the high sensitivity of standard K+-selective ion-exchange resin to quaternary amines, we used K+-selective intracellular electrodes to measure the rate of access of apically applied TMA+ to the goblet cavity. TMA+ failed to gain access to more than half of the cavities. For those cavities to which it did gain access, the influx and efflux rates and predicted equilibrium concentrations of TMA+ were too low to be consistent with diffusive exit of transported K+ along the same path. Upon superfusion with cytochalasin E, a disrupter of actin-based cytoskeleton, the TMA+ influx rates immediately increased for those cavities previously accessible to TMA+, but not for those previously inaccessible. Increases in TMA+ influx suggestive of spontaneous valve opening were not observed. The results are consistent with an indirect route of access of TMA+ to goblet cavities. We conclude that goblet valves are closed in vitro. Assuming that the goblet cavity is part of the transport route, actively transported K+ must exit the cavity to the gut lumen by a mechanism that does not involve diffusion in free solution.