POSTSYNAPTIC NICOTINIC RECEPTOR DESENSITIZED BY NONCONTRACTILE CA2-KINASE-C ACTIVATION AT THE MOUSE NEUROMUSCULAR-JUNCTION( MOBILIZATION VIA PROTEIN)

1 Non-contractile Ca2+ mobilization (unaccompanied by muscle contracti on) was initiated by nerve stimulation in the presence of neostigmine (more than 0.03 mu M) at the endplate region of mouse diaphragm muscle s. In the process of nicotinic receptor desensitization, the depressan t effect of noncontractile Ca2+ on contractile Ca2+ mobilization was i nvestigated by measurement of Ca2+-aequorin luminescence. 2 When the p hrenic nerve was stimulated with paired pulses having intervals of 150 , 300, 600, 1000 and 2000 ms, contractile Ca2+ transients were elicite d during the generation of non-contractile Ca2+ mobilization. The ampl itude of the contractile Ca2+ transients elicited by the second pulse (S-2) was depressed at the shorter pulse intervals, but recovered to t he initial contractile response (S-1) at longer pulse intervals. 3 The extent of depression of S-2 was enhanced by increasing the concentrat ion of neostigmine (0.03 to 0.3 mu M). When a low concentration (0.05 mu M) Of pancuronium, a competitive nicotinic antagonist, completely b locked non-contractile Ca2+ mobilization, the depression of S-2 was di minished. 4 The depression of S-2 was enhanced when the peak amplitude of non-contractile Ca2+ mobilization was raised by increasing the ext ernal Ca2+ concentration from 1.3 to 5 mM. 5 Staurosporine (10 nM), a protein kinase-C inhibitor, diminished the depression of S-2 despite l arge amounts of non-contractile Ca2+ mobilization. The diminishing eff ect of staurosporine was counteracted by TPA (0.1 mu M), a protein kin ase-C activator. 6 These findings suggest that non-contractile Ca2+ mo bilization may enhance the desensitization of the postsynaptic nicotin ic receptor via activation of protein kinase-C at the neuromuscular ju nction.