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 L-7 and S-1 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 L-7 and S-1 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.