Quantitative analysis of resting membrane electrogenesis in insect (diptera) skeletal muscle. I. Intracellular K+, Na+ and Cl- activities, measured using liquid ion-exchanger and neutral ion-carrier microelectrodes.

Electrophysiological properties of skeletal body-wall muscles of prepupal Calliphora erythrocephala were investigated using double-barrelled intracellular ion-sensitive microelectrodes. The most realistic estimate of the intracellular K+ activity, obtained using K+-sensitive microelectrodes based on a neutral carrier, was 115 mmoll-1. The K+ equilibrium potential was consistently more negative than the prevailing resting potential, the average difference being -15 mV. The intracellular Na+ activity and the Na+ equilibrium potential were 7 mmoll-1 and +46 mV on average, respectively. The mean value of the intracellular Cl- activity was 40 mmoll-1, and this was apparently higher than that required for passive distribution of Cl-. However, when reversibly exposed to a Ringer containing no Cl-, cells could rapidly exchange most of their intracellular Cl-, although the resting membrane potentials were only transiently affected. It is concluded that an anionic interferent exists inside muscles, that this artefactually elevates the measured intracellular Cl- activities, and that Cl- makes no contribution to resting membrane electrogenesis.