Cardiorespiratory reflexes from muscles during dynamic and static exercise in the dog.

Cardiorespiratory reflex responses during the initial phase of dynamic and static contraction of hindlimb muscles were studied in anesthetized dogs. Muscle contractions were elicited by stimulating the femoral and gastrocnemius nerves at 3 and 100 Hz with the intensity of 2.0-2.5 times the motor threshold for a 20-s period. Rhythmic contractions caused a decrease in arterial pressure (Pa) and heart rate (HR) and increased pulmonary ventilation (VE) by increasing frequency (f) without significantly changing VT. Tetanic contractions provoked an increase in Pa and HR and a hyperpnea resulting from a rise in both f and VT. Similar responses were also obtained in anesthetized dogs with carotid sinuses denervated and cervical vagi cut. The abrupt increase in VE at the start of both types of exercise was not associated with immediate significant decreases in end-tidal CO2 values. These two patterns of cardiocirculatory and respiratory responses were closely similar to those reported in anesthetized rabbits in previous studies. Both patterns of responses were reflexes initiated by activation of muscle receptors verified by interrupting the afferents from the contracting muscles. It is concluded that, during dynamic and static work, two distinct muscular reflex mechanisms might exert their drives, related to the muscular metabolic rate, on the circulatory and respiratory function.