Direct in vivo monitoring of sarcoplasmic reticulum Ca²⁺ and cytosolic cAMP dynamics in mouse skeletal muscle

Skeletal muscle contraction depends on the release of Ca²⁺ from the sarcoplasmic reticulum (SR), but the dynamics of the SR free Ca²⁺ concentration ([Ca²⁺]_SR), its modulation by physiological stimuli such as catecholamines, and the concomitant changes in cAMP handling have never been directly determined. We used two-photon microscopy imaging of GFP-based probes expressed in mouse skeletal muscles to monitor, for the first time in a live animal, the dynamics of [Ca²⁺]_SR and cAMP. Our data, which were obtained in highly physiological conditions, suggest that free [Ca²⁺]_SR decreases by ∼50 μM during single twitches elicited through nerve stimulation. We also demonstrate that cAMP levels rise upon β-adrenergic stimulation, leading to an increased efficacy of the Ca²⁺ release/reuptake cycle during motor nerve stimulation.