The resting membrane potential of Drosophila melanogaster larval muscle depends strongly on external calcium concentration

The resting membrane potential (RMP) of most cells is not greatly influenced by the transmembrane calcium gradient because at rest, the membrane has very low permeability to calcium. We have observed, however, that the resting membrane potential of muscle cells in the larval bodywall of Drosophila melanogaster varies widely as the external calcium concentration is modified. The RMP depolarized as much as 21.8 mV/mM calcium at low concentrations, and on average, about 10 mV/mM across a range typical of neurophysiological investigations. The extent to which muscle RMP varies has important implications for the measurement of synaptic potentials as well. Two parameters of excitatory junctional potential (EJP) voltage were compared across a range of RMPs. EJP amplitude (ΔV) and peak voltage (maxima) change as a function of RMP; on average, a 10 mV change in RMP elicits a 4-5 mV change in EJP amplitude and peak voltage. The influence of the calcium gradient on resting and synaptic membrane potentials led us to investigate the endogenous ion concentrations of larval hemolymph. In addition to the major monovalent ions and calcium, we report the first voltammetric analysis of magnesium concentration in larval fruit fly hemolymph.