ENHANCEMENT BY CALCITONIN-GENE-RELATED PEPTIDE OF NONCONTRACTILE CA2-INDUCED NICOTINIC RECEPTOR DESENSITIZATION AT THE MOUSE NEUROMUSCULAR-JUNCTION()

1 Nicotinic acetylcholine receptor (AChR)-operated non-contractile Ca2 + mobilization (unaccompanied by muscle contraction) depressed contrac tile Ca2+ mobilization (accompanied by muscle contraction) in mouse di aphragm muscles. In the process of nicotinic AChR desensitization, the enhancing role of calcitonin gene-related peptide (CGRP) on the non-c ontractile Ca2+-induced depression of contractile Ca2+ mobilization wa s investigated by measurement of Ca2+-aequorin luminescence in the pre sence of neostigmine (0.1 mu M). 2 When the phrenic nerve was stimulat ed with paired pulses at intervals of 150, 300, 600, 1000 and 2000 ms, contractile Ca2+ transients were elicited during the generation of no n-contractile Ca2+ mobilization. The amplitude of the contractile Ca2 transients elicited by the second pulse (S-2) was depressed at the sh orter pulse intervals, but not at the longer pulse intervals. 3 The ex tent of depression of S-2 was enhanced when the duration of non-contra ctile Ca2+ mobilization was prolonged by CGRP (10 nM). However, CGRP f ailed to enhance the depression of S-2 when noncontractile Ca2+ mobili zation was not observed at the low external Ca2+ concentration (1.3 mM ). 4 The enhancing effect by CGRP on the depression of S-2 was counter acted by staurosporine (3 nM), a protein kinase-C inhibitor, despite p rolongation of the duration of non-contractile Ca2+ mobilization. 5 Wh en H-89 (1 mu M), a protein kinase-A inhibitor, completely blocked non -contractile Ca2+ mobilization, the depression of S? was diminished. T he prolongation of the duration of non-contractile Ca2+ mobilization b y AA373 (300 mu M), a protein kinase-A activator, enhanced the depress ion of S-2. The enhancing effect was observed neither with CGRP nor wi th AA373, in the presence of H-89 (0.1 mu M). 6 These findings suggest that the CGRP mobilizes non-contractile Ca2+ through activation of pr otein kinase-A, which in turn may activate protein kinase-C, then enha nce the desensitization of postsynaptic nicotinic AChRs at the neuromu scular junction.