Acetylcholine sensitivity of biphasic Ca2+ mobilization induced by nicotinic receptor activation at the mouse skeletal muscle endplate

Effects of nicotine on rat sternohyoid muscle contractile properties

Obstructive sleep apnoea (OSA) is a major clinical disorder characterised by recurring episodes of pharyngeal collapse during sleep. At present, there remains no satisfactory treatment for OSA. Pharmacological therapies as a potential treatment for the disorder are an attractive option and include agents that increase the contractility of the pharyngeal muscles. The aim of the present study was to examine the effects of nicotine on upper airway muscle contractile properties. In vitro isometric contractile properties were determined using strips of rat sternohyoid muscle in physiological salt solution containing nicotine (0-100 microg/ml) at 25 degrees C. Isometric twitch and tetanic tension, contraction time, half-relaxation time and tension-frequency relationship were determined by electrical field stimulation with platinum electrodes. Fatigue was induced by stimulation at 40 Hz with 300 ms trains at a frequency of 0.5 Hz for 5 min. Nicotine at a concentration of 1 microg/ml was associated with a significant increase in sternohyoid muscle specific tension compared to control data. Dose-dependent increases in contractile tension were not observed. Nicotine had effects on tension-frequency relationship and endurance properties of the sternohyoid muscle at some but not all doses. A leftward shift in the tension-frequency relationship was observed at low stimulus frequencies (20-30 Hz) for nicotine at a concentration of 1 and 5 microg/ml and a significant increase in fatigue resistance was observed with nicotine at a concentration of 10 microg/ml. As fatigue of the upper airway muscles has been implicated in obstructive airway conditions, a pharmacological agent that improves muscle endurance may prove useful as a potential treatment for such disorders. Therefore, further studies of the effects of nicotinic agonists on upper airway function are warranted.

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.