An electrophysiological study of the non-junctional membrane of epidermal cells in Tenebrio molitor

The contribution of K and Cl to the membrane potential of the epidermal cells of the recently-ecdysed larva of the mealworm was examined. The ionic basis for the membrane potential is complex. Although increasing the external K level depolarized the cell membrane, the relationship obtained suggests that ions other than K contribute largely to the recorded membrane potential. In particular, exposing the cells to K concentrations below the normal level of 40 mM has only slight effects on membrane potential, irrespective of whether K is lowered by direct substitution with Na or under conditions in which Na and Cl levels are held constant. Increasing the external Cl levels from 4 mM to 154 mM while holding K and Na levels constant resulted in a 10 mV hyperpolarization. The slight hyperpolarizing effects of high external Cl could be mimicked by citrate, but not by acetate, the latter drastically hyperpolarizing the cell membrane at levels of K that normally maintain a reduced membrane potential. External Na has little effect on the membrane potential at normal physiological levels of K, but may depolarize the cell at low K levels. The results suggest that several inorganic ions, and possibly organic acids, participate in generating the membrane potential of the epidermal cell. The passive ionic properties of non-junctional epidermal membrane and muscle membrane appear to the similar in this insect.The electrical resistance on the non-junctional membrane is highly dependent on the external K level, and can be reduced by three orders of magnitude by increasing external K from 1 mM to 120 mM. The resistance of the junctional membrane remains constant over this range of external K concentrations.