Paraxanthine, a caffeine metabolite, dose dependently increases [Ca2+](i) in skeletal muscle

It was hypothesized that the caffeine derivative paraxanthine results in su bcontracture increases in intracellular calcium concentration ([Ca2+](i)) i n resting skeletal muscle. Single fibers obtained from mouse flexor digitor um brevis were loaded with a fluorescent Ca2+ indicator, indo l-acetoxymeth yl ester. After a stable baseline was recorded, the fiber was superfused wi th physiological salt solution (Tyrode) containing 0.5, 1.0, 2.5, or 5 mM p araxanthine, resulting in [Ca2+](i) increases of 6.4 +/- 2.5, 9.7 +/- 3.6, 26.8 +/- 11.7, and 39.6 +/- 9.6 nM, respectively. The increases in [Ca2+](i ) were transient and were also observed with exposure to 5 mM theophylline and theobromine. Six fibers were exposed to 5 mM paraxanthine followed by 5 mM paraxanthine in the presence of 10 mM procaine (sarcoplasmic reticulum Ca2+ release channel blocker). There was no increase from baseline [Ca2+](i ) when fibers were superfused with paraxanthine and procaine, suggesting th at the sarcoplasmic reticulum is the primary Ca2+ source in the paraxanthin e-induced response. In separate experiments, intact flexor digitorum brevis (n. = 13) loaded with indo l-acetoxymethyl ester had a significant increas e in [Ca2+](i) with exposure to 0.01 mM paraxanthine. It is concluded that physiological and low pharmacological concentrations of paraxanthine result in transient, subcontracture increases in [Ca2+](i) in resting skeletal mu scle, the magnitude of which is related to paraxanthine concentration.