The influence of KCl on the resting potential of Tenebrio molitor larva and imago muscle.

The study of mechanisms underlying the production of resting potential in insect muscle fibers has revealed some differences as compared with other excitable cells. In some systems the resting membranes of the insect muscle fibers are potassium electrodes according to the Nernst equation (Janiszewski and Skubalanka 1967; Wareham et al. 1974; Ashcroft 1981). On the other hand, many authors have found that membrane potentials of many invertebrate muscle fibers vary with [K] 0 with a considerable divergence from the slope of 58 mV, and that Em is very different from Ek, usually exceeding it. Belton and Grundfest (1962) showed that muscle fibers of the larva of Tenebrio molitor which are normally surrounded by the haemolymph with a high concentration of potassium ions, are insensitive to changes in [K] 0 over a wide range between 0 and 120 mmol/1. It has been shown that in these systems in the resting state, the permeability of the muscle membrane to other ions allows to introduce the concept of a so-called "multiionic electrode" (Usherwood 1969; Janiszewski 1981). Detailed studies have tried to evaluate the contribution of various ions to the resting potential in the Tenebrio larva muscle. The results obtained have suggested that since complete substitution of choline or Tris for 70 mmol/1 Na does not modify the resting potential significantly, the contribution of Na to this potential would be rather small. Also it has been shown that the resting potential seems not to be sensitive to changes in divalent cations, such as Mg , Ca 2 + and Ba . When 70 mmol/1 Cl~ ions were replaced with various anions, the membrane hyperpolarized. The effectiveness of hyperpolarization was of the following order: C r = B r " < N O j « acetate" = propionate (Belton and Grundfest 1962; Grund­ fest and Kusano 1970; Janiszewski and Olszewska 1984; Kusano and Janiszewski .1984). Data obtained on muscle fibers from larval stages have posed the question whether ionic requirements underlying the resting potential may change during