Reversed effect of caffeine on non-contractile and contractile Ca²⁺ mobilization operated by acetylcholine receptor in mouse diaphragm muscle

Caffeine effects on contractile and acetylcholine receptor-related non-contractile Ca²⁺ mobilization were investigated in phrenic nerve-diaphragm muscles of mice with neostigmine. Caffeine enhanced at 0.25-5 mM, and decreased at 7-20 mM the total amount of contractile Ca²⁺-aequorin luminescence (Ca²⁺ transients), but only decreased at 2-10 mM non-contractile Ca²⁺ transients. Pretreatment with formamide (2 M for 30 min) abolished contractile Ca²⁺ transients, but did not affect non-contractile ones. These results suggest that non-contractile Ca²⁺ mobilization is not due to direct Ca²⁺ release from sarcoplasmic reticulum, but due to direct modulation by nicotinic acetylcholine receptor.