Divergent Effects of Calcium Channel Modulators on H-Reflex Excitability in Fatigued Rat Muscle.

Calcium (Ca2+) release from the sarcoplasmic reticulum is central to excitation-contraction coupling and plays a critical role in the development of skeletal muscle fatigue. Altered Ca2+ dynamics may affect not only contractile function but also neuromuscular excitability. This study examined the effects of pharmacological modulation of Ca2+ channels on fatigue development and spinal reflex activity in rats. Using the Hoffmann reflex (H-reflex) as an indicator of motoneuron excitability, we evaluated the effects of Ca2+ channel blockers (Amiloride, Nifedipine) and an activator ((-)-Bay K8644) on the reflex responses of the plantar muscle before and after fatigue induction. The ratio of the maximum H-reflex to maximum M-wave (Hmax/Mmax) was used to assess alterations in spinal excitability. Compared with the control, both Amiloride and Nifedipine markedly reduce the Hmax/Mmax ratio (77% and 60%, respectively), whereas (-)-Bay K8644 elicited a robust 129% increase. These findings demonstrate that pharmacological modulation of Ca2+ channels has distinct and divergent effects on spinal excitability during fatigue. These results highlight the close interaction between intramuscular Ca2+ regulation and reflex pathways and suggest potential strategies for enhancing muscle performance through targeted Ca2+ channel modulation.