Chronic repetitive cooling and caffeine-induced intracellular Ca2+ elevation differentially impact adaptations in slow- and fast-twitch rat skeletal muscles

Intracellular Ca2+ concentration ([Ca2+](i)) is considered important in the regulation of skeletal muscle mass. This study tested the hypothesis that chronic repeated cooling and/or caffeine ingestion would acutely increase [Ca2+](i) and hypertrophy muscles potentially in a fiber-type-dependent manner. Control rats and those fed caffeine were subjected to repeated bidiurnal treatments of percutaneous icing, under anesthesia, to reduce the muscle temperature below similar to 5 degrees C. The predominantly fast-twitch tibialis anterior (TA) and slow-twitch soleus (SOL) muscles were evaluated after 28 days of intervention. The [Ca2+] i elevating response to icing was enhanced by caffeine loading only in the SOL muscle, with the response present across a significantly higher temperature range than in the TA muscle under caffeine-loading conditions. In both the TA and SOL muscles, myofiber cross-sectional area (CSA) was decreased by chronic caffeine treatment (mean reductions of 10.5% and 20.4%, respectively). However, in the TA, but not the SOL, CSA was restored by icing ( +15.4 +/- 4.3% vs. noniced, P < 0.01). In the SOL, but not TA, icing + caffeine increased myofiber number (20.5 +/- 6.7%, P < 0.05) and satellite cell density (2.5 +/- 0.3-fold) in cross sections. These contrasting muscle responses to cooling and caffeine may reflect fiber-type-specific [Ca2+](i) responses and/or differential responses to elevated [Ca2+](i).