ATP concentrations and muscle tension increase linearly with muscle contraction

Previous studies have suggested that activation of ATP-sensitive P2X receptors in skeletal muscle play a role in mediating the exercise pressor reflex (Li J and Sinoway LI. Am J Physiol Heart Circ Physiol 283: H2636-H2643, 2002). To determine the role ATP plays in this reflex, it is necessary to examine whether muscle interstitial ATP (ATP_i) concentrations rise with muscle contraction. Accordingly, in this study, muscle contraction was evoked by electrical stimulation of the L₇ and S₁ ventral roots of the spinal cord in 12 decerebrate cats. Muscle ATP_i was collected from microdialysis probes inserted in the muscle. ATP concentrations were determined by the HPLC method. Electrical stimulation of the ventral roots at 3 and 5 Hz increased mean arterial pressure by 13 ± 2 and 16 ± 3 mmHg (P < 0.05), respectively, and it increased ATP concentration in contracting muscle by 150% (P < 0.05) and 200% (P < 0.05), respectively. ATP measured in the opposite control limb did not rise with ventral root stimulation. Section of the L₇ and S₁ dorsal roots did not affect the ATP_i seen with 5-Hz ventral root stimulation. Finally, ventral roots stimulation sufficient to drive motor nerve fibers did not increase ATP in previously paralyzed cats. Thus ATP_i is not largely released from sympathetic or motor nerves and does not require an intact afferent reflex pathway. We conclude that ATP_i is due to the release of ATP from contracting skeletal muscle cells.