Enhancement by calcitonin gene‐related peptide of nicotinic receptor‐operated noncontractile Ca 2+ mobilization at the mouse neuromuscular junction

1 The involvement of calcitonin gene‐related peptide (CGRP) in the mechanism of nicotinic acetylcholine receptor‐operated noncontractile Ca 2+ mobilization (not accompanied by twitch tension) was investigated by measuring Ca 2+ ‐aequorin luminescence at the neuromuscular junction of mouse diaphragm muscle treated with neostigmine. 2 Noncontractile Ca 2+ transients were enhanced by 4‐aminopyridine (100 μ m ), a K + channel blocker, and inhibited by botulinum toxin (1–100 μg, i.p.) and hexamethonium (10–100 μ m ), a neuronal nicotinic receptor antagonist. 3 Noncontractile Ca 2+ transients were diminished by CGRP 8–37 (10–20 μ m ), a CGRP antagonist. CGRP (0.3–10 n m ) prolonged the duration of noncontractile Ca 2+ transients. The effect of CGRP was suppressed by CGRP 8 _ 37 (0.1 μ m ). 4 Noncontractile Ca 2+ transients were inhibited by H‐89 (0.1–1 μ m ), a protein kinase‐A inhibitor. The catalytic subunit of protein kinase‐A and AA373 (300 μ m ), a protein kinase‐A activator, prolonged the duration of noncontractile transients. The prolongations either by CGRP or by AA373 were not observed in the presence of H‐89 (0.1 μ m ). 5 Contractile (accompanied by twitch tension) but not noncontractile Ca 2+ transients were decreased by 12‐O‐tetradecanoyl phorbol 13‐acetate (TPA, 0.3–1 μ m ), a protein kinase‐C activator. Phospholipase A 2 increased only contractile Ca 2+ transients. Calmodulin‐related agents affected neither type of Ca 2+ transients. 6 These results provide the first evidence that nicotinic acetylcholine receptor‐operated noncontractile Ca 2+ mobilization is promoted by nerve‐released CGRP activating protein kinase‐A, and is dependent on the accumulated amounts of acetylcholine at the neuromuscular junction where desensitization might readily develop.