Attenuated Intracellular Calcium Response to Local Heat Stress in Mouse Model of Muscle Denervation

Background: Repetitive heat stress exceeding 40°C induces skeletal muscle hypertrophy. This heat stress-induced hypertrophy is associated with an increase in intracellular calcium ion concentration ([Ca 2+ ]i), a second messenger regulating protein synthesis. However, the heat-dependent intracellular Ca 2+ dynamics and hypertrophic responses in denervation-induced muscle atrophy remain unclear. Purpose: This study tested the hypothesis that, in a denervation-induced muscle atrophy model, the attenuation of the Ca 2+ handling protein-sensing mechanism in response to heat stress results in: 1) suppression of the [Ca 2+ ]i elevation, and 2) consequent suppression of the skeletal muscle hypertrophic response to chronic heat stress. Methods: The predominantly fast-twitch plantaris muscles of (8 – 13 weeks, 20.9 – 27.0 g) male C57BL/6J mice were investigated. Sham-operated (Sham) and denervated (DEN; right sciatic nerve transection) limbs were established. Seven days post-surgery, mice were assigned to either a control group (CONT) or a heat stress group (HEAT). To investigate intracellular Ca 2+ dynamics in response to heat stress, [Ca 2+ ]i in both Sham and DEN-treated muscles was evaluated using in vivo Ca 2+ imaging with the fluorescent indicator Fura-2 AM. The HEAT group was subjected to local heating at 40°C (30 minutes/day, 14 sessions total), while the CONT group was kept at room temperature for the same duration. Twenty-four hours after the final session, the plantaris muscles were excised. Muscle wet weight and the expression levels of [Ca 2+ ]i regulatory proteins, Transient receptor potential vanilloid 1 (TRPV1), Sarcoplasmic reticulum Ca 2+ -ATPase (SERCA), Ryanodine receptor1 (RyR1), and Mitochondrial calcium uniporter (MCU), were quantified. Results: Muscle weight decreased in both the CONT and HEAT groups following denervation (CONT: 36 %, HEAT: 39 %). Following chronic heat stress, plantaris wet weight in the Sham limbs increased by 9.0 ± 1.0% in the HEAT group compared to the CONT group (P = 0.0374) whereas this response was abolished in the DEN plantaris muscles (P = 0.7474). The [Ca 2+ ]i response to heat stress was attenuated in the DEN plantaris muscles compared to the Sham limbs. Specifically, Sham limbs showed a significant increase in [Ca 2+ ]i across the range of 38°C to 43°C relative to the 35°C condition (P < 0.05), whereas the DEN limbs exhibited a significant increase in [Ca 2+ ]i only at 43°C (P < 0.05). Analysis of Ca 2+ handling proteins revealed that denervation per se downregulated thermosensitive TRPV1 (43%, P = 0.0011), while RyR1 remained unchanged. Furthermore, SERCA levels were reduced (37%, P = 0.0028), whereas MCU expression was increased (14%, P = 0.0302). Conclusion: This investigation demonstrates that skeletal muscle hypertrophic adaptation to heat stress is suppressed by denervation likely subsequent to downregulation of TRPV1 expression which inhibits heat-dependent intracellular Ca 2+ accumulation even in the face of decreased SERCA1 expression. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.