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EXERCISE AND RESPIRATORY PHYSIOLOGY

Effects of calcitonin gene-related peptide on rat soleus muscle excitability: mechanisms and physiological significance

Institute of Physiology and Biophysics, University of Aarhus, Århus, Denmark

Submitted 13 December 2007 ; accepted in final form 20 July 2008

Intense exercise causes a large loss of K⁺ from contracting muscles. The ensuing elevation of extracellular K⁺ ([K⁺]_o) has been suggested to cause fatigue by depressing muscle fiber excitability. In isolated muscles, however, repeated contractions confer some protection against this effect of elevated K⁺. We hypothesize that this excitation-induced force-recovery is related to the release of the neuropeptide calcitonin gene-related peptide (CGRP), which stimulates the muscular Na⁺-K⁺ pumps. Using the specific CGRP antagonist CGRP-(8-37), we evaluated the role of CGRP in the excitation-induced force recovery and examined possible mechanisms. Intact rat soleus muscles were stimulated to evoke short tetani at regular intervals. Increasing extracellular K⁺ ([K⁺]_o) from 4 to 11 mM decreased force to 20% of initial force (P < 0.001). Addition of exogenous CGRP (10^–9 M), release of endogenous CGRP with capsaicin, or repeated electrical stimulation recovered force to 50–70% of initial force (P < 0.001). In all cases, force recovery could be almost completely suppressed by CGRP-(8-37). At 11 mM [K⁺]_o, CGRP (10^–8 M) did not alter resting membrane potential or conductance but significantly improved action potentials (P < 0.001) and increased the proportion of excitable fibers from 32 to 70% (P < 0.001). CGRP was shown to induce substantial force recovery with only modest Na⁺-K⁺ pump stimulation. We conclude that the excitation-induced force recovery is caused by a recovery of excitability, induced by local release of CGRP. The data suggest that the recovery of excitability partly was induced by Na⁺-K⁺ pump stimulation and partly by altering Na⁺ channel function.

Na⁺-K⁺ pump; Na⁺ channels; Cl^– channels; extracellular K⁺