Postsynaptic nicotinic receptor desensitized by non‐contractile Ca²⁺ mobilization via protein kinase‐C activation at the mouse neuromuscular junction

Non‐contractile Ca²⁺ mobilization (unaccompanied by muscle contraction) was initiated by nerve stimulation in the presence of neostigmine (more than 0.03 μm) at the endplate region of mouse diaphragm muscles. In the process of nicotinic receptor desensitization, the depressant effect of non‐contractile Ca²⁺ on contractile Ca²⁺ mobilization was investigated by measurement of Ca²⁺‐aequorin luminescence. When the phrenic nerve was stimulated with paired pulses having intervals of 150, 300, 600, 1000 and 2000 ms, contractile Ca²⁺ transients were elicited during the generation of non‐contractile Ca²⁺ mobilization. The amplitude of the contractile Ca²⁺ transients elicited by the second pulse (S₂) was depressed at the shorter pulse intervals, but recovered to the initial contractile response (S₁) at longer pulse intervals. The extent of depression of S₂ was enhanced by increasing the concentration of neostigmine (0.03 to 0.3 μm). When a low concentration (0.05 μm) of pancuronium, a competitive nicotinic antagonist, completely blocked non‐contractile Ca²⁺ mobilization, the depression of S₂ was diminished. The depression of S₂ was enhanced when the peak amplitude of non‐contractile Ca²⁺ mobilization was raised by increasing the external Ca²⁺ concentration from 1.3 to 5 mm. Staurosporine (10 nm), a protein kinase‐C inhibitor, diminished the depression of S₂ despite large amounts of non‐contractile Ca²⁺ mobilization. The diminishing effect of staurosporine was counteracted by TPA (0.1 μm), a protein kinase‐C activator. These findings suggest that non‐contractile Ca²⁺ mobilization may enhance the desensitization of the postsynaptic nicotinic receptor via activation of protein kinase‐C at the neuromuscular junction. 1995 British Pharmacological Society