In vivo Ca2+ dynamics during cooling after eccentric contractions in rat skeletal muscle

The effect of cooling on in vivo intracellular calcium ion concentration ([Ca2+]i) after eccentric contractions (ECs) remains to be determined. We tested the hypothesis that cryotherapy following ECs promotes an increased [Ca2+]i and induces greater muscle damage in two muscles with substantial IIb and IIx fiber populations. The thin spinotrapezius (SPINO) muscles of Wistar rats were used for in vivo [Ca2+]i imaging and tibialis anterior (TA) muscles provided greater fidelity and repeatability of contractile function measurements. SPINO [Ca2+]i was estimated using fura 2-AM and the magnitude, location and temporal profile of [Ca2+]i determined as the temperature near the muscle surface post-ECs was decreased from 30oC (control) to 20oC or 10oC. Subsequently, in the TA the effect of post-ECs cooling to 10oC on muscle contractile performance was determined at 1 and 2 days after ECs. TA muscle samples were examined by hematoxylin and eosin staining to assess damage. In SPINO reducing the muscle temperature from 30oC to 10oC post-ECs resulted in a 3.7-fold increase in the spread of high [Ca2+]i sites generated by ECs (P<0.05). These high [Ca2+]i sites demonstrated partial reversibility when rewarmed to 30oC. Dantrolene, a ryanodine receptor Ca2+ release inhibitor, reduced the presence of high [Ca2+] sites at 10oC. In the TA cooling exacerbated ECs-induced muscle strength deficits post-ECs via enhanced muscle fiber damage (P<0.05). By demonstrating that cooling post-ECs potentiates [Ca2+]i derangements, this in vivo approach supports a putative mechanistic basis for how post-exercise cryotherapy might augment muscle fiber damage and decrease subsequent exercise performance.