Muscle mechanosensitive receptors close to the myotendinous junction of the Achilles tendon elicit a pressor reflex.

Feedback regulation by activation of mechanosensitive afferents in the exercising muscle causes the cardiovascular and sympathetic nerve responses, which follow tension development and are almost identical between static contraction and passive stretch. The precise location of the mechanoreceptors contributing to the exercise pressor reflex, however, remained unknown. To test the hypothesis that the mechanoreceptors will be located around the myotendinous junction to monitor a change in muscle tension than a change in muscle length, we examined the reflex cardiovascular responses to passive stretch of the triceps surae muscle in anesthetized rats with three interventions; systemic injection of gadolinium, cutting the Achilles tendon, and local injection of lidocaine into the myotendinous junction. Gadolinium (42 micromol/kg iv) blunted the increases in heart rate and mean arterial blood pressure during passive stretch by 36 and 22-26%, respectively, suggesting that the reflex cardiovascular responses were evoked by stimulation of muscle mechanosensitive receptors. The cardiovascular responses to passive stretch were not different between the cut Achilles tendon and the intact tendon in the same rats, suggesting that any mechanoreceptors, terminated in the more distal part of the tendon, did not contribute to the reflex cardiovascular responses. Lidocaine (volume, 0.04-0.1 ml) injected into the myotendinous junction blunted the stretch-induced increases in heart rate and mean arterial blood pressure by 37-49 and 27-34%, respectively. We conclude that the muscle mechanosensitive receptors evoking the reflex cardiovascular responses at least partly locate at or close to the myotendinous junction of the Achilles tendon.