E. M. Volkov

Mechanisms of hyperpolarizing effect of GABA on resting potential of the Lumbricus terrestris muscular wall somatic cells.

GABA, baclofen, isoguvacine increase, and cis-4-aminocrotonic acid does not modify resting membrane potential of muscle cells. Bicuculline, phaclofen, N-ethylmaleimide, chlorpromazine, verapamil, and removal of Ca2+ from bathing solution abolished the effect of baclofen, while U73122 and D609 were ineffective in this respect. The authors conclude that the Lumbricus terrestris muscle cells contain GABAergic structures similar to a- and b-receptors. Activation of GABA receptors induced Cl- inward current and Ca2+ entry with subsequent activation of calmodulin-like proteins, which causes membrane hyperpolarization by increasing the effect of “pumping potential” on resting membrane potential.

Effect of cholinergic agonists on resting membrane potential of earthworm body wall muscle cells.

Carbachol in a concentration of 5×10-8 mol/liter does not hyperpolarize, and in a concentration of 5×10-6 mol/liter depolarizes the membrane of somatic muscle cell in earthworm. d-Tubocurarine, α-bungarotoxin, atropine, and hexamethonium added to the incubation medium did not abolish the carbachol-induced decrease in resting membrane potential. Each of these drugs alone had no effect on resting membrane potential in muscle cells. Presumably, the acetylcholine-sensitive receptor-channel membrane complex in earthworm muscle cell differs from acetylcholine receptor in skeletal muscle fibers and peripheral neurons of vertebrates.

Cholinergic and GABAergic innervation regulate activity of electrogenic ionic pump in earthworm somatic muscle cells

Carbacholine reduced, while baclofen and norepinephrine increased resting membrane potential in earthworm somatic muscle cells. In the presence of carbacholine, neither norepinephrine, nor baclofen hyperpolarized the membrane. Ouabain decreased resting potential and abolished the effects of carbacholine, norepinephrine, and baclofen on membrane potential. It was hypothesized that carbacholine directly inhibited the ouabain-sensitive component of Na+/K+ pump and abolished the activating effect of norepinephrine and baclofen.

Effects of cholinergic receptor agonists and antagonists on miniature stimulatory postsynaptic ionic currents in somatic muscle cells of lumbricus terrestris.

Miniature stimulating postsynaptic currents of Lumbricus terrestris somatic muscle cells were recorded. Atropine, d-tubocurarin, α-bungarotoxin, carbacholine, and proserin did not modify the amplitude and temporal parameters of miniature stimulatory postsynaptic currents, while carbacholine and nicotine depolarized the muscle membrane. Presumably, Lumbricus terrestris muscle cells contain acetylcholine-sensitive channel-receptor complexes not belonging to classical nicotinic or muscarinic acetylcholine receptors.

The effect of modelling of hypogravity on postsynaptic acetylcholine receptors and activity of acetylcholinesterase in neuromuscular synapses of fast and slow muscles of rat

Rats were subjected to antiorthostatic hindlimb unloading for 35 days. Using immunofluorescent techniques, we found increased intensity of fluorescence and decreased area of staining of acetylcholine receptors and the increased intensity and area of staining of acetylcholinesterase in neuromuscular synapses of “fast” and “slow” muscles. Changes in the ratio of the number of acetylcholine receptors on the postsynaptic membrane and acetylcholinesterase and the alterations in their spatial location relative to each other in neuromuscular synapses of “fast” and “slow” muscles were also observed. These modifications are in accordance with the electrophysiological data on the decreased amplitudes of miniature end plate currents in both muscles. They were accompanied by a decrease in the volume of muscle fibers. Antiorthostatic unloading, a model of hypogravity, resulted in increased functional activity of acetylcholinesterase associated with decreased area of the postsynaptic membrane occupied by acetylcholine receptors, which led to a decrease in the amplitude of postsynaptic excitatory potentials and thus, to the decreased reliability threshold of excitation transmission from a nerve to a muscle.

The Role of Chloride Ions in the Maintenance of Resting Membrane Potential in Rat Fast and Slow Muscles during Hypogravity Modeling

Antiorthostatic hindlimb suspension reduces resting membrane potential of rat fast and slow muscles within 7 days. Changes in Na+/K+ pump activity and shifts of the equilibrium potential for chloride ions are the main mechanism of the changes in the resting potential of muscle fibers. The latter is presumably associated with increased intracellular ion current due to activation of the second active Na+, K+, 2Cl– symport. Reduction of the membrane potential is related to muscle denervation. However, membrane depolarization of muscle fi bers during antiorthostatic suspension cannot be explained solely by changes in the mechanisms of neurotrophic control from motor neurons.

Study of the vesicular cycle in nerve structures in somatic muscle of earthworm (Lumbricus terrestris)

In the muscle wall of the earthworm Lumbricus terrestris, with the aid of fluorescent endocytotic dyes FM1-43, FM2-10, and FM4-64, there are revealed fluorescent spots 1–2 μm in diameter that represent clusters of “synaptic boutons.” Application takes place onto ganglia of the abdominal nerve chain of the Dil membrane probe capable of translocation by axoplasmic transport; the subsequent (next day) staining of nerve structures with the endocytotic marker FM4-64 showed the complete superposition of fluorescence of these dyes fluorescing in different specter areas. The fluorescent marker DiBAC4(3) revealed an enhancement of fluorescence of nerve elements with increase of K+ concentration in the extracellular medium. Use of FM2-10 showed that, the higher the K+ content in solution and, accordingly, the nerve cell depolarization, the faster the release of the marker and, on the contrary, the slower the process in the absence of K+ in the medium. In the Ca2+-free solution and in the presence of the Ca2+ chelator BAPTA or BAPTA-AM, there uptake and release of FM2-10 are blocked, but only after preliminary 40-min incubation in such solution. In clusters of synaptic boutons, exo- and endocytosis processes take place that are also preserved under conditions of rest. This vesicular cycle depends on the membrane potential of nerve structures and on the content of K+ and Ca2+ in the medium, the calcium sensor working most likely by the “all or nothing” principle.

Ionic selectivity of chloride ion symport in mechanisms controlling resting potential and osmotic homeostasis in earthworm somatic muscle cells

Replacement of Cl− for Br− in bathing solution did not reduce resting potential and had no effect on modulation of transmembrane potential in hyper-and hypoosmotic solutions. Under these conditions, baclofen, an agonist of GABAergic B-type receptors, failed to activate Na+/K+-pump in earthworm somatic muscle cells. It was hypothesized that the contribution of Cl− symport to osmotic homeostasis is not highly selective in respect to replacement of Cl− to Br− ions, whereas in case of activation of electrogenic ion pumps, this replacement is equivalent to removal of Cl− ions from the bathing solution.

Effect of GABAergic and adrenergic agents on activity of Na+/K+ pump and Cl−-cotransport in somatic muscle cells of earthworm Lumbricus Terrestris

GABA, baclofen, epinephrine, and norepinephrine hyperpolarized the membrane of earthworm somatic cells. This effect was prevented by furosemide, removal of Cl− from the medium, or activation of Na+/K+ pump by 3-fold increase external potassium concentration. It was hypothesized that GABA, baclofen, epinephrine, and norepinephrine stimulate Na+/K+ transport via specific receptor inputs, but their effect on resting potential can be realized only under conditions of working Cl− symport.

Two Populations of Miniature Excitatory Synaptic Ionic Currents in Somatic Muscle Cells of Lumbricus Terrestris Earthworm Body Wall

Three types of miniature excitatory synaptic currents were recorded in the same synaptic region of earthworm muscle cells: monoexponential (τ=1.2 msec) and biexponential (τ1=1.2 and τ2=8.0 msec). It was hypothesized that earthworm muscle cells contain at least two populations of acetylcholine-sensitive ionic channels, which do not belong to classical nicotinic and muscarinic acetylcholine receptors.