Muscle mechanosensitive reflex is suppressed in the conscious condition: effect of anesthesia

To test the hypothesis that a muscle mechanosensitive reflex is suppressed in the conscious condition, we examined the effect of anesthesia on the cardiovascular responses to passive mechanical stretch of the hindlimb triceps surae muscle in six conscious cats. The triceps surae muscle was manually stretched for 30 s by extending the hip and knee joints and subsequently by dorsiflexing the ankle joint; the lateral gastrocnemius muscle was lengthened by 19 +/- 2.6 mm. Heart rate (HR) and mean arterial blood pressure (MAP) did not change significantly during passive stretch of the muscle in the conscious condition. At 10-40 min after intravenously administering pentobarbital sodium (20-25 mg/kg), the identical passive stretch of the triceps surae muscle was able to induce the cardiovascular responses; HR and MAP were increased by 14 +/- 1.3 beats/min and 14 +/- 1.4 mmHg, respectively, and the cardiovascular responses were sustained throughout the passive stretch. In contrast, stretching skin on the triceps surae muscle evoked no significant changes in HR and MAP in the anesthetized condition. When anesthesia became light 40-90 min after injection of pentobarbital and the animals started to show spontaneous body movement, the cardiovascular response to passive muscle stretch tended to be blunted again. It is therefore concluded that passive mechanical stretch of skeletal muscle is capable of evoking the reflex cardiovascular response, which is suppressed in the conscious condition but exaggerated by anesthesia.