Cardiorespiratory and phrenic nerve responses to graded muscle stretch in anesthetized cats.

This study examined the cardiovascular, ventilatory, and phrenic nerve responses to graded activation of mechanically sensitive muscle afferents. Using eight alpha-chloralose anesthetized cats, the left and right triceps surae muscles were stretched individually and simultaneously at progressive increments (0.5, 1.0, 1.5, 1.75 cm). Muscle stretch elicited sustained increases in mean arterial blood pressure (MAP) and heart rate (HR). These changes were related to the degree of stretch, as stretching one muscle 0.5 cm increased MAP 15 +/- 2 mmHg and HR 7 +/- 2 beats/min, while stretching both legs (1.75 cm) increases these variables 40 +/- 11 mmHg and 11 +/- 3 beats/min. By contrast, muscle stretch initially decreased ventilation and phrenic nerve activity. After the initial fall, ventilation, but not tidal phrenic activity, increased above baseline. These results show that a divergence exists between the initial cardiovascular and ventilatory responses to activation of mechanically sensitive muscle afferents. Further, the hyperpnea elicited by muscle stretch in spontaneously breathing anesthetized cats appears to be the result of excitation of non-diaphragmatic muscles of ventilation.