Independent regulation of activation and inactivation phases in non-contractile Ca2+ transients by nicotinic receptor at the mouse neuromuscular junction

Methyllycaconitine-sensitive neuronal nicotinic receptor-operated slow Ca2+ signal by local application or perfusion of ACh at the mouse neuromuscular junction

Local application of acetylcholine (ACh; 0.3 mM, 20 microl) elicited bi-phasic elevation of intracellular Ca2+ concentrations (contractile fast and non-contractile slow Ca2- signal measured as aequorin luminescence) in diaphragm muscle preparation. A neuronal nicotinic antagonist methyllycaconitine (MLA; 0.01-1 microM), which did not affect the fast Ca2+ transients and twitch tension, concentration-dependently depressed only the slow Ca2+ component. Ca2+ channel blockers, Cd2+ (200 microM), nitrendipine (1 microM), verapamil (1 microM) and diltiazem (1 microM), or a Na+ channel blocker tetrodotoxin (TTX; 0.1 microM) failed to prevent the generation of slow Ca2+ response. Perfusion of ACh (1 microM) to isolated single skeletal (flexor digitorum brevis) muscle cells pretreated with TTX (0.1 microM) also elicited a slow Ca2+ signal measured as confocal imaging with a fluorescent dye, fluo-3, at the endplate region. MLA (1 microM) antagonized against the ACh perfusion-elicited slow Ca2+ signal. Perfusion of choline (1 mM), a neuronal nicotinic agonist, also elicited the MLA-sensitive slow Ca2+ signal. These results strongly suggest that the ACh-induced slow Ca2+ signal reflects Ca2+ entry through a postsynaptic MLA-sensitive neuronal nicotinic ACh receptor subtype at the neuromuscular junction.

Calcium-dependent desensitizing function of the postsynaptic neuronal-type nicotinic acetylcholine receptors at the neuromuscular junction

Several subunits that commonly have been regarded as neuronal-type nicotinic acetylcholine receptor (nAChR) subtypes, have been found in the postjunctional endplate membrane of adult skeletal muscle fibres. The postsynaptic function of these neuronal-type nAChR subtypes at the neuromuscular junction has been investigated by using aequorin luminescence and fluorescence confocal imaging. A biphasic elevation of intracellular Ca2+ is elicited by prolonged nicotinic action at the mouse muscle endplates. The fast and slow Ca2+ components are operated by a postsynaptic muscle- and colocalized neuronal-type nAChR, respectively. Neuromuscular functions may be regulated by a dual nAChR system to maintain the normal postsynaptic excitability. Certain neuronal-type nAChR may be endowed with the same functional role in the central nervous system also.

Diabetic state-induced rapid inactivation of noncontractile Ca2+ mobilization operated by nicotinic acetylcholine receptor in mouse diaphragm muscle

1. Diabetic modifications of nicotinic receptor-operated noncontractile Ca2- mobilization observed in the presence of anticholinesterase were investigated by measuring Ca(2+)-aequorin luminescence in diaphragm muscles of mice with diabetes induced by injections of streptozotocin (150 mg kg-1, bolus i.v.) and alloxan (85 mg kg-1, bolus i.v.). 2. The diabetic state accelerated the decline of noncontractile Ca2+ transients without affecting their peak amplitude. Insulin treatment reversed this alteration. 3. The increase in contractile Ca2+ transients by cholinesterase inhibition was attenuated 0.6 fold and became resistant to changes in [Ca2+]o in the diabetic state. 4. Changes in extracellular pH from 7.6 to 5.6 depressed the peak amplitude of noncontractile Ca2+ transients without affecting their duration, and enhanced the peak amplitude of contractile Ca2+ transients. 5. These results suggest that the inactivation process of noncontractile Ca2+ mobilization is promoted in diabetic muscles, presumably by desensitization of the nicotinic acetylcholine receptor.

Complementary effects of paeoniflorin and glycyrrhizin on intracellular Ca2+ mobilization in the nerve-stimulated skeletal muscle of mice

Effects of paeoniflorin (PF) and glycyrrhizin (GLR), contained in paeony and licorice roots, respectively, on contractile and non-contractile Ca2+ mobilization were examined by measuring the Ca(2+)-aequorin luminescence (Ca2+ transients) of the nerve-stimulated skeletal muscle of mice in the presence of neostigmine (0.3 microM). PF (0.1-1 mM) prolonged the duration of non-contractile Ca2+ transients, which may induce the desensitization of nicotinic acetylcholine receptor, but did not affect contractile Ca2+ transients. GLR (0.3-1 mM) depressed contractile Ca2+ transients without affecting non-contractile transients. These results suggest that PF and GLR may have complementary effects on intracellular Ca2+ mobilization to block the neuromuscular transmission.

Monoclonal antibody to beta 2 subunit of neuronal nicotinic receptor depresses the postjunctional non-contractile Ca2+ mobilization in the mouse diaphragm muscle

The involvement of subtypes of nicotinic acetylcholine receptor (nAChR) in the postjunctional non-contractile Ca2+ mobilization was investigated in mouse diaphragm muscles treated with an anticholinesterase, using monoclonal antibodies (mAbs) to nAChR subunits. mAb 210 (specific for alpha 1 subunit of muscle nAChR) depressed contractile Ca2+ transients without affecting non-contractile Ca2+ transients. mAb 270 (specific for beta 2 subunit of neuronal nAChR) depressed only non-contractile Ca2+ transients. mAb 210 did not completely block the ACh-activated channel currents in flexor digitorum brevis muscle cells. The present findings indicate that the anti-beta 2 mAb 270-related subtype of nAChR may postsynaptically operate the non-contractile Ca2+ mobilization at the neuromuscular junction, suggesting the involvement of a subtype different from the usual muscle-type nAChR.