Paraxanthine increases [Ca 2+]1 in resting mouse skeletal muscle

It was hypothesized that the caffeine derivative paraxanthine (PX) results in an increase in intracellular calcium concentration ([Ca²⁺]i) in resting skeletal muscle. Mouse flexor digitorum brevis were removed and single fibres obtained by collagenase treatment. Single fibres were then loaded with the membrane permeant fluorescent indicator, INDO-1 AM. The fibres were placed in a chamber superfused with a physiological salt solution. The fibre was illuminated at 360 nm and fluorescence was simultaneously collected at 405 and 500 nm. When a stable baseline was recorded, the flow was switched and the fibre was superfused with a physiological salt solution containing 0.5, 1.0, 2.5, or 5.0 mM PX. Exposure of single fibres to PX at 0.5, 1.0, 2.5, and 5.0 mM resulted in [Ca²⁺]i increases of 15%, 15%, 26% and 43% respectively. It is concluded that PX results in subcontracture increases in [Ca²⁺]i in resting mammalian skeletal muscle. (Graph Presented).