The resting potential of the muscle membrane of moths

1. An analysis of the resting potential ( E m ) of the muscle membrane of the hawk moth Sphinx ligustri suggests that it is compounded of two components, an ionic electrode, dominated by E K (contributing some −41 mV) and an active component (contributing some −19 mV). 2. Total E m in vivo is approximately −60 mV. 3. The active component is sensitive to lowered temperature and to omission of Cl − from the saline. 4. E m of Sphinx muscle has very similar properties to those described for cockroach. Comparisons are also made with mammalian smooth muscle. 5. A new examination of the ionic content of haemolymph and myoplasm gave much higher values for [K +] 1 and [Na +] 0 and lower values for [K +] 0 and [Na +] 0 than hitherto reported. Calculated E K is therefore higher and it is suggested that it is consistent with an ionic electrode of approximately −40 mV. Comparable results were obtained with ionic analyses of the haemolymph of other species of moth. 6. The formula is given for a saline for Sphinx muscle which is as satisfactory as the insect's own haemolymph in maintaining E m .