We have studied the effect of hypoxia and ouabain on the muscle resting potential (RP) of the cockroach Periplaneta americana L. The experiments were done in insects reared either in normoxic or hypoxic conditions and treated, or not treated, with ouabain. Hypoxic conditions decreased the RP by about 11.6 mV. Ouabain decreased the RP by about 8.3 mV (ouabain 10(-5) mol/l) or 12.3 mV (ouabain 5 x 10(-5) mol/l). Hypoxia and ouabain decreased the RP by about 28.5 mV (ouabain 10(-5) mol/l) or 31.3 mV (ouabain 5 x 10(-5) mol/l).

The catecholamine (CA) depletion degree in rat adrenal medulla, the survival time (ST) and rectal temperature changes induced by combined thermal and immobilization stress were examined with the aim to prove tha alpha- and beta-adrenoreceptor sensitivity decrease implication in 24 hour fasting-induced changes of the above mentioned phenomena. The significant ST increase in strong stressful situations and the adrenal CA turnover augmentation in fed and fasted- propranolol or dihydroergotamine pretreared rats compared to untreated ones provided evidence that alpha- and beta adrenoreceptor blockade causes adrenal CA turnover increase and ST prolongation similar to effects observed in our previous experiments. Thus the implication of fasting induced adrenoreceptor downregulation in adrenal CA turnover augmentation and ST prolongation of fasted rats in strong stressful conditions was suggested and the possible mechanisms of these phenomena have been discussed.

The author discusses the recent findings concerning the influence of selected neurotoxins on the voltage-gated sodium channel. Sodium voltage-gated channels are blocked or modified by four of five classes of neurotoxical agents: guanidinum toxins (tetrodotoxin, saxitoxin), lipid soluble compounds (veratridine, grayanotoxin, batrachotoxin, aconitine, pyrethroids, brevetoxins), polipeptide toxins (alpha-scorpion and sea anemone toxins) and beta-scorpion toxins. The mode of operation of these toxins at the different binding sites within the channel is discussed.

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

Experiments were carried out on longitudinal ventral muscles of the mealworm larva (Tenebrio molitor) using conventional microelectrode methods. Both sodium nitrate and sodium sulphate depolarized the fibres markedly, whereas sodium propionate exerted an hyperpolarizing influence. It is concluded that the differences in the mode of action are due to different permeabilities of the membrane to the applied anions. The results obtained confirm also the concept of a multi-ionic electrode with respect to insect muscle fibres.

The effect of temperature on muscle resting potential was studied in Acheta domesticus, Leptinotarsa decemlineata and Tenebrio molitor. The experiments were performed using the conventional microelectrode method and specific physiological solution for each insect species. The measurements were taken at three temperature levels: +/- 4 degrees C, +/- 20 degrees C and +/- 35 degrees C. Placing the preparations into a bath at 4 degrees C caused in all three species a rapid decrease of the muscle resting potential (+/- by 1/3). Increased temperature (+/- 35 degrees C) led to a transient increase of the resting potential (10 min after placing in the solution), and then a decrease of the potential below the control values. The possible mechanisms of these findings based both on ionic gradients, permeabilities and metabolic activity are discussed.

Experiments were carried out on tergosternal muscle fibres of the house cricket Acheta domesticus. Conventional microelectrode methods were used. All the investigated fibres depolarized with time (0, 60, 120 min) in statistically significant manner. Replacing CL- in the physiological solution by carbonate prevents this depolarization. Citrate anions added to the bath depolarize the fibers. Acetate does not change the resting potential. Increasing osmolarity by adding 100 or 200 mM saccharose exerts no significant influence on the level of the resting potential. Lowering the level of sodium ions leads to a slight but significant depolarization. The present work confirmed the opinion that skeletal muscle fibers of Acheta domesticus are at rest multiionic electrodes.

Experiments were carried out on the skeletal muscle fibres of the cricket Acheta domesticus using conventional microelectrode methods. Both potassium and ammonium ions depolarized the fibres but with a considerable divergance from the slope of 58 mV. Calcium ions hyperpolarized the fibres. High concentrations of magnesium ions or dramatic increase in the osmotic pressure of the bathing medium exerted no influence on the resting potential. It seems likely that skeletal muscle fibres of Acheta domesticus at rest are multionic electrodes, but the role played by various ions needs further studies.