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 affer ents. Using eight alpha-chloralose anesthetized cats, the left and rig ht triceps surae muscles were stretched individually and simultaneousl y at progressive increments (0.5, 1.0, 1.5, 1.75 cm). Muscle stretch e licited 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) increased these variables 40 +/- 11 mmHg and 11 +/- 3 beats/min. By contrast, muscle s tretch initially decreased ventilation and phrenic nerve activity. Aft er the initial fall, ventilation, but not tidal phrenic activity, incr eased above baseline. These results show that a divergence exists betw een the initial cardiovascular and ventilatory responses to activation of mechanically sensitive muscle afferents. Further, the hyperpnea el icited by muscle stretch in spontaneously breathing anesthetized cats appears to be the result of excitation of non-diaphragmatic muscles of ventilation.