The resting membrane potential (Vm) of isolated somatic longitudinal muscles of the earthworm Lumbricus terrestris was studied by glass microelectrodes. The inhibition of chloride permeability by low pH did not affect Vm of the muscle fibers in isolated somatic longitudinal muscles of the earthworm Lumbricus terrestris which was -48.7 mV (inside negative) at pH 7.3 and -49.1 at pH 5.6. On the other hand, bathing the muscles in Cl- and Na+-free solutions, or application of the chloride transporter inhibitor furosemide and Na+-K+-ATPase inhibitor ouabain depolarized the Vm by 3-5 mV. The effects of a Cl- -free solution and ouabain were not additive. This demonstrates relatively small contribution of equilibrium potential for Cl- to the resting membrane potential and electrogenic effect of Na+K+-ATPase which is dependent on the supply of Na+i ions by furosemide-sensitive and Cl-e- and Na+e-dependent electroneutral transport (most probably Na+K+Cl- cotransport)., E. M. Volkov, L. F. Nurullin, E. Nikolsky, J. Krůšek, F. Vyskočil., and Obsahuje bibliografii
All secretory anterior pituitary cells fire action potentials spontaneously and exhibit a high resting cation conductance, but the channels involved in the background permeability have not been identified. In cultured lactotrophs and immortalized GH3 cells, replacement of extracellular Na+ with large organic cations, but not blockade of voltage-gated Na+ influx, led to an instantaneous hyperpolarization of cell membranes that was associated with a cessation of spontaneous firing. When cells were clamped at –50 mV, which was close to the resting membrane potential in these cells, replacement of bath Na+ with organic cations resulted in an outward-like current, reflecting an inhibition of the inward holding membrane current and indicating loss of a background-depolarizing conductance. Quantitative RTPCR analysis revealed the high expression of mRNA transcripts for TRPC1 and much lower expression of TRPC6 in both lactotrophs and GH3 cells. Very low expression of TRPC3, TRPC4, and TRPC5 mRNA transcripts were also present in pituitary but not GH3 cells. 2-APB and SKF-96365, relatively selective blockers of TRPC channels, inhibited electrical activity, Ca2+ influx and prolactin release in a concentration-dependent manner. Gd3+, a common Ca2+ channel blocker, and flufenamic acid, an inhibitor of non-selective cation channels, also inhibited electrical activity, Ca2+ influx and prolactin release. These results indicate that nonselective cation channels, presumably belonging to the TRPC family, contribute to the background depolarizing conductance and firing of action potentials with consequent contribution to Ca2+ influx and hormone release in lactotrophs and GH3 cells., M. Kučka ... [et al.]., and Obsahuje seznam literatury
The early postdenervation depolarization of rat diaphragm muscle fibres (8-10 mV) is substantially smaller (3 mV) when muscle strips are bathed with 1 mM L-glutamate (GLÜ) or N-methyl-D-aspartate (NMDA). The effects of GLÜ and NMDA are not seen in the presence of aminophosphonovaleric acid (APV), a blocker of NMDA-subtype of glutamate receptors, 5 mM Mg2+ (which blocks NMDA-controlled ion channels) and L-nitroarginine methylester (NAME), an inhibitor of NO-synthase. This indicates that NMDA-subtype of GL(J receptors might be involved in the regulation of the membrane potential in muscle fibres, most probably through the NO-synthase system.
There are two principal mechanisms of acetylcholine (ACh) release from the resting motor nerve terminal: quantal and non-quantal (NQR); the former being only a small fraction of the total, at least at rest. In the present article we summarize basic research about the NQR that is undoubtedly an important trophic factor during endplate development and in adult neuromuscular contacts. NQR helps to eliminate the polyneural innervation of developing muscle fibers, ensures higher excitability of the adult subsynaptic membrane by surplus polarization and protects the RMP from depolarization by regulating the NO cascade and chloride transport. It shortens the endplate potentials by promoting postsynaptic receptor desensitization when AChE is inhibited during anti-AChE poisoning. In adult synapses, it can also activate the electrogenic Na+/K+-pump, change the degree of synchronization of quanta released by the nerve stimulation and affects the contractility of skeletal muscles., F. Vyskočil, A. I. Malomouzh, E. E. Nikolsky., and Obsahuje seznam literatury
The effect of Na+-K+-ATPase inhibitor ouabain on the resting membrane potential (Vm) was studied by glass microelectrodes in isolated somatic longitudinal muscles of the earthworm Lumbricus terrestris and compared with frog sartorius muscle. In earthworm muscle, Vm was -49 mV (inside negative) in a reference external solution with 4 mmol/l K+. The electrogenic participation of Na+-K+-ATPase was absent in solutions with very low concentrations of 0.01 mmol/l K+, higher in 4 and 8 mmol/l K+ (4-5 mV) and maximal (13 mV) in solutions containing 12 mmol/l K+ where Vm was -46 mV in the absence and -33 mV in the presence of 1x10-4 M ouabain. The electrogenic participation of Na+-K+-ATPase was much smaller in m. sartorius of the frog Rana temporaria bathed in 8 and 12 mmol/l K+. The results indicate that the Na+-K+-ATPase is an important electrogenic factor in earthworm longitudinal muscle fibres and that its contribution to Vm depends directly on the concentration of K+ in the bathing solution., E. M. Volkov, L. F. Nurullin, I. Švandová, E. E. Nikolsky, F. Vyskočil., and Obsahuje bibliografii
Antiorthostatic hindlimb suspension (unloading) decreased the resting membrane potential (RMP) of skeletal muscle fibers in fast extensor digitorum longus (EDL) and slow soleus (SOL) muscle of the rat by about 10 % within 7 days and more. Inactivation of the membrane Na+,K+-pump by ouabain brought about similar depolarization as unloading. The increased sodium permeability of the membrane was excluded as the major cause of this depolarization by experiments in which TRIS was substituted for Na+ in the medium. On the other hand, the decrease in the electrogenic participation of the Na+,K+-pump is apparently one of the causes of RMP decrease during hypogravity, in EDL muscle in particular., O. Tyapkina ... [et al.]., and Obsahuje seznam literatury