Muscle receptors close to the myotendinous junction play a role in eliciting exercise pressor reflex during contraction

Abstract

Although a muscle mechanosensitive reflex contributes to regulation of the cardiovascular responses during exercise, the precise location of muscle mechanoreceptors responding to contraction has not been identified yet. We have recently reported that mechanosensitive receptors located at or close to the myotendinous junction play a role in eliciting the cardiovascular responses to passive stretch of skeletal muscle. The mechanoreceptors located at or near the myotendinous junction are hypothesized to respond to static contraction as well. To test this hypothesis, we had two interventions for the reflex cardiovascular responses to static contraction of the triceps surae muscle with the same tension development in decerebrate or pentobarbital-anesthetized rats; cutting the Achilles tendon and local injection of lidocaine into the myotendinous junction. The cardiovascular responses were evoked by static contraction regardless of the achillotomy, suggesting that mechanoreceptors terminating in the more distal part of the cut Achilles tendon did not contribute to the reflex cardiovascular responses. Lidocaine (volume, 0.04-0.1 ml) injected into the myotendinous junction blunted the reflex cardiovascular responses, indicating that muscle afferent fibers terminating at or passing through the myotendinous junction contribute to the exercise pressor reflex. The achillotomy did not affect the cardiovascular responses to passive stretch with the same tension as static contraction, but the localized injection of lidocaine similarly blunted the responses to passive stretch as contraction. We conclude that the mechanosensitive receptors eliciting the reflex cardiovascular responses may at least partly locate close to the myotendinous junction, to monitor tension development during muscular activity.