Intracellular microelectrodes were used to record from individual cells in midguts isolated from Spodoptera littoralis caterpillars. Recorded potentials, referenced to the basal (haemolymph) surface, showed a bimodal distribution, with maxima in the ranges 0 to −10 and −30 to −40 mV. In experiments where the fluorescent dye Lucifer Yellow CH was ionophoresed from the recording microelectrode, fluorescence was associated with single cells only for membrane potentials more negative than −25 mV. Examination of tissue sections showed these cells to be of both columnar and goblet types, in an approximate 2:1 ratio. This conclusion conflicts with that of a previous study on other caterpillar species, in which it was concluded that the goblet cells had basal membrane potentials of only a few mV. Attempts to discriminate between the two cell types by resistance measurements were unsuccessful. The resistance values obtained were substantially higher than those in the previous study, although they are consistent with those predicted from the overall tissue resistance. The major electrical effect of potassium ion transport inhibition by 1 m-KCN was on the apical membrane, supporting the view that the potassium pump is located there. The major initial effect of potassium ion removal was on the basal membrane, which is as expected if this membrane is permeable primarily to potassium. Our inability to discriminate between goblet and columnar cells by any electrical criterion suggests that both cell types may be able to transport potassium.

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