In vivo heat production dynamics during a contraction-relaxation cycle in rat single skeletal muscle fibers

Skeletal muscle generates heat via contraction-dependent (shivering) and independent (nonshivering) mechanisms. While this thermogenic capacity of skeletal muscle undoubtedly contributes to the body temperature homeostasis of animals and impacts various cellular functions, the intracellular temperature and its dynamics in skeletal muscle in vivo remain elusive. We aimed to determine the intracellular temperature and its changes within skeletal muscle in vivo during contraction and following relaxation. In addition, we tested the hypothesis that sarcoplasmic reticulum Ca²⁺ ATPase (SERCA) generates heat and increases the myocyte temperature during a transitory Ca²⁺-induced contraction-relaxation cycle. The intact spinotrapezius muscle of anesthetized adult male Wistar rats (n = 18) was exteriorized and loaded with the fluorescent probe Cellular Thermoprobe for Fluorescence Ratio (49.3 μM) by microinjection over 1 s. The fluorescence ratio (i.e., 580 nm/515 nm) was measured in vivo during 1) temperature increases induced by means of an external heater, and 2) Ca²⁺ injection (3.9 nL, 2.0 mM). The fluorescence ratio increased as a linear function of muscle surface temperature from 25 °C to 40 °C (r² = 0.97, P < 0.01). Ca²⁺ injection (3.9 nL, 2.0 mM) significantly increased myocyte intracellular temperature: An effect that was suppressed by SERCA inhibition with cyclopiazonic acid (CPA, Ca²⁺: 38.3 ± 1.4 °C vs Ca²⁺+CPA: 28.3 ± 2.8 °C, P < 0.01 at 1 min following injection). While muscle shortening occurred immediately after the Ca²⁺ injection, the increased muscle temperature was maintained during the relaxation phase. In this investigation, we demonstrated a novel model for measuring the intracellular temperature of skeletal muscle in vivo and further that heat generation occurs concomitant principally with SERCA functioning and muscle relaxation.