Accumulation of intramyocyte TRPV1-mediated calcium during heat stress is inhibited by concomitant muscle contractions

Heat stress promotes intramyocyte calcium concentration ([Ca2+](i)) accumulation via transient receptor potential vanilloid 1 (TRPV1) channels. We tested the hypothesis that muscle contractile activity concomitant with heat stress would accelerate the increase in [Ca2+](i) via TRPV1, further impairing [Ca2+](i) homeostasis. Spinotrapezius muscles of adult Wistar rats were exteriorized in vivo and loaded with the fluorescent Ca2+ probe fura 2-AM. Heat stress (muscle surface temperature 40 degrees C) was used as TRPV1 activator. An isometric contraction (100 Hz, 5-10 V, 30 s) was induced electrically concomitant with heat stress. [Ca2+](i) was determined for 20 min using in vivo fluorescence microscopy, and the phosphorylation response of TRPV1 was determined by Western blotting. Heat stress induced a significant [Ca2+](i) increase of 18.5 +/- 8.1% at 20 min and TRPV1 phosphorylation (+231%), which was inhibited by addition of the TRPV1 inhibitor (capsazepine). However, contrary to expectations, the heat stress and isometric contraction condition almost completely inhibited TRPV1 phosphorylation and the consequent [Ca2+](i) elevation (<2.8% accumulation during heat stress, P > 0.05). In conclusion, this in vivo physiological model demonstrated that isometric muscle contraction(s) can suppress the phosphorylation response of TRPV1 and maintain [Ca2+](i) homeostasis during heat stress. NEW & NOTEWORTHY This investigation is the first document the dynamics of intramyocyte calcium concentration ([Ca2+](i)) increase in the myoplasm of skeletal muscle fibers in response to heat stress where the muscle blood flow is preserved. Heat stress at 40 degrees C drives a myoplasmic [Ca2+](i) accumulation in concert with transient receptor potential vanilloid 1 (TRPV1) phosphorylation. However, muscle contraction caused TRPV1 channel deactivation by dephosphorylation of TRPV1. TRPV1 inactivation via isometric contraction(s) permits maintenance of [Ca2+](i) homeostasis even under high imposed muscle temperature.