Isoprenaline-stimulated differential adrenergic response of K+ channels in skeletal muscle under hypokalaemic conditions

Synthesis and BK channel-opening activity of 2-amino-1,3-thiazole derivatives

A series of 2-amino-5-arylmethyl- or 5-heteroarylmethyl-1,3-thiazole derivatives were synthesized and evaluated for BK channel-opening activities in cell-based fluorescence assay and electrophysiological recording. The assay results indicated that the activities of the investigated compounds were influenced by the physicochemical properties of the substituent at benzene ring.

Beta-adrenergic modulation of tremor and corticomuscular coherence in humans

Coherence between the bioelectric activity of sensorimotor cortex and contralateral muscles can be observed around 20 Hz. By contrast, physiological tremor has a dominant frequency around 10 Hz. Although tremor has multiple sources, it is partly central in origin, reflecting a component of motoneuron discharge at this frequency. The motoneuron response to ~20 Hz descending input could be altered by non-linear interactions with ~10 Hz motoneuron firing. We investigated this further in eight healthy human subjects by testing the effects of the beta-adrenergic agents propranolol (non-selective β-antagonist) and salbutamol (β(2)-agonist), which are known to alter the size of physiological tremor. Corticomuscular coherence was assessed during an auxotonic precision grip task; tremor was quantified using accelerometry during index finger extension. Experiments with propranolol used a double-blind, placebo-controlled crossover design. A single oral dose of propranolol (40 mg) significantly increased beta band (15.3-32.2 Hz) corticomuscular coherence compared with placebo, but reduced tremor in the 6.2-11.9 Hz range. Salbutamol (2.5 mg) was administered by inhalation. Whilst salbutamol significantly increased tremor amplitude as expected, it did not change corticomuscular coherence. The opposite direction of the effects of propranolol on corticomuscular coherence and tremor, and the fact that salbutamol enhances tremor but does not affect coherence, implies that the magnitude of corticomuscular coherence is little influenced by non-linear interactions with 10 Hz oscillations in motoneurons or the periphery. Instead, we suggest that propranolol and salbutamol may affect both tremor and corticomuscular coherence partly via a central site of action.

A sodium channel knockin mutant (NaV1.4-R669H) mouse model of hypokalemic periodic paralysis

Hypokalemic periodic paralysis (HypoPP) is an ion channelopathy of skeletal muscle characterized by attacks of muscle weakness associated with low serum K+. HypoPP results from a transient failure of muscle fiber excitability. Mutations in the genes encoding a calcium channel (CaV1.1) and a sodium channel (NaV1.4) have been identified in HypoPP families. Mutations of NaV1.4 give rise to a heterogeneous group of muscle disorders, with gain-of-function defects causing myotonia or hyperkalemic periodic paralysis. To address the question of specificity for the allele encoding the NaV1.4-R669H variant as a cause of HypoPP and to produce a model system in which to characterize functional defects of the mutant channel and susceptibility to paralysis, we generated knockin mice carrying the ortholog of the gene encoding the NaV1.4-R669H variant (referred to herein as R669H mice). Homozygous R669H mice had a robust HypoPP phenotype, with transient loss of muscle excitability and weakness in low-K+ challenge, insensitivity to high-K+ challenge, dominant inheritance, and absence of myotonia. Recovery was sensitive to the Na+/K+-ATPase pump inhibitor ouabain. Affected fibers had an anomalous inward current at hyperpolarized potentials, consistent with the proposal that a leaky gating pore in R669H channels triggers attacks, whereas a reduction in the amplitude of action potentials implies additional loss-of-function changes for the mutant NaV1.4 channels.

First order phase transition and hysteresis in a cell's maintenance of the membrane potential--An essential role for the inward potassium rectifiers

Hysteretic behavior is found experimentally in the transmembrane potential at low extracellular potassium in mouse lumbrical muscle cells. Adding isoprenaline to the external medium eliminates the bistable, hysteretic region. The system can be modeled mathematically and understood analytically with and without isoprenaline. Inward rectifying potassium channels appear to be essential for the bistability. Relations are derived to express the dimensions of the bistable area in terms of system parameters. The selective advantage and evolutionary origin of inward rectifying channels and hysteretic behavior is discussed.

Effects of calcitonin gene-related peptide on rat soleus muscle excitability: mechanisms and physiological significance

Intense exercise causes a large loss of K+ from contracting muscles. The ensuing elevation of extracellular K+ ([K+]o) has been suggested to cause fatigue by depressing muscle fiber excitability. In isolated muscles, however, repeated contractions confer some protection against this effect of elevated K+. We hypothesize that this excitation-induced force-recovery is related to the release of the neuropeptide calcitonin gene-related peptide (CGRP), which stimulates the muscular Na+-K+ pumps. Using the specific CGRP antagonist CGRP-(8-37), we evaluated the role of CGRP in the excitation-induced force recovery and examined possible mechanisms. Intact rat soleus muscles were stimulated to evoke short tetani at regular intervals. Increasing extracellular K+ ([K+]o) from 4 to 11 mM decreased force to ∼20% of initial force ( P < 0.001). Addition of exogenous CGRP (10−9 M), release of endogenous CGRP with capsaicin, or repeated electrical stimulation recovered force to 50–70% of initial force ( P < 0.001). In all cases, force recovery could be almost completely suppressed by CGRP-(8-37). At 11 mM [K+]o, CGRP (10−8 M) did not alter resting membrane potential or conductance but significantly improved action potentials ( P < 0.001) and increased the proportion of excitable fibers from 32 to 70% ( P < 0.001). CGRP was shown to induce substantial force recovery with only modest Na+-K+ pump stimulation. We conclude that the excitation-induced force recovery is caused by a recovery of excitability, induced by local release of CGRP. The data suggest that the recovery of excitability partly was induced by Na+-K+ pump stimulation and partly by altering Na+ channel function.

Noradrenaline inhibits substantia gelatinosa neurons in mice trigeminal subnucleus caudalis via α2 and β adrenoceptors

The actions of noradrenaline (NA) in the substantia gelatinosa (SG) are important for their antinociceptive effects. In order to identify the possible mechanisms underlying NA actions in the SG of trigeminal subnucleus caudalis (Vc), the direct membrane effects were examined by gramicidin-perforated patch clamp recording using brain slice preparation from immature mice brainstem. The majority (60/71, 85%) of neurons tested were hyperpolarized by NA application, and these hyperpolarizing effects were mimicked both by the alpha(2) adrenergic agonist, clonidine (18/28, 64%) and the beta adrenergic agonist, isoproterenol (9/24, 38%). NA-induced hyperpolarizing effect was also blocked by the alpha(2) adrenergic antagonist, yohimbine in five out of six neurons tested. However, a minority (5/71, 7%) of neurons tested were depolarized by NA, and these depolarizing effects were mimicked by the alpha(1) adrenergic agonist, phenylephrine (11/26, 42%). NA-induced hyperpolarizing effects were maintained in the presence of tetrodotoxin (TTX), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), d,l-2-amino-5-phosphonopentanoic acid (AP5), picrotoxin and strychnine, a Na(+) channel, ionotropic glutamate receptor, GABA(A) and glycine receptor antagonists, respectively, indicating that the effects of NA are direct on the postsynaptic SG neurons. These results indicate that alpha(2) and beta adrenoceptor mediate inhibition, and alpha(1) adrenoceptor mediates facilitation of orofacial nociceptive processing in mouse trigeminal brainstem SG neurons by postsynaptic actions.

Ionic mechanisms of excitation-induced regulation of Na+-K+-ATPase mRNA expression in isolated rat EDL muscle

This study investigated the effects of electrical stimulation on Na+-K+-ATPase isoform mRNA, with the aim to identify factors modulating Na+-K+-ATPase mRNA in isolated rat extensor digitorum longus (EDL) muscle. Interventions designed to mimic exercise-induced increases in intracellular Na+and Ca2+contents and membrane depolarization were examined. Muscles were mounted on force transducers and stimulated with 60-Hz 10-s pulse trains producing tetanic contractions three times at 10-min intervals. Ouabain (1.0 mM, 120 min), veratridine (0.1 mM, 30 min), and monensin (0.1 mM, 30 min) were used to increase intracellular Na+content. High extracellular K+(13 mM, 60 min) and the Ca2+ionophore A-23187 (0.02 mM, 30 min) were used to induce membrane depolarization and elevated intracellular Ca2+content, respectively. Muscles were analyzed for Na+-K+-ATPase α1–α3and β1–β3mRNA (real-time RT-PCR). Electrical stimulation had no immediate effect on Na+-K+-ATPase mRNA; however at 3 h after stimulation, it increased α1, α2, and α3mRNA by 223, 621, and 892%, respectively ( P = 0.010), without changing β mRNA. Ouabain, veratridine, and monensin increased intracellular Na+content by 769, 724, and 598%, respectively ( P = 0.001) but did not increase mRNA of any isoform. High intracellular K+concentration elevated α1mRNA by 160% ( P = 0.021), whereas A-23187 elevated α3mRNA by 123% ( P = 0.035) but reduced β1mRNA by 76% ( P = 0.001). In conclusion, electrical stimulation induced subunit-specific increases in Na+-K+-ATPase mRNA in isolated rat EDL muscle. Furthermore, Na+-K+-ATPase mRNA appears to be regulated by different stimuli, including cellular changes associated with membrane depolarization and increased intracellular Ca2+content but not increased intracellular Na+content.