Respiratory and cardiac responses to exercise ? simulating peripheral perfusion in endurance trained and untrained rats

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Abstract
Ventilatory and circulatory drives elicited by exercise-simulating perfusion of the circulatory isolated hindleg were examined in 10 trained (TR) and untrained (UTR) rats. TR were submitted to endurance training on a motordriven treadmill (30·min−1 at a grade of 10%, 5 days a week for 30 min). Exercise was simulated by perfusion with modified tyrode solutions:I.) hypoxic, enriched with lactic acid (15 mmol·l−1), II.) normoxic, enriched with lactic acid. III.) hypoxic without lactic acid. Perfusion was performed in anaesthetized animals through cannulae in the femoral artery and vein; the hindled was connected to the rest of the body only by nerve and bone. 10 min of control perfusion (normoxic tyrode solution) was followed by a 20 min test period and another 10 min control perfusion. Apart from heart rate (HR), respiratory rate (RR) and several outflow parameters were measured ([K+], [Na+], [lactate], pH, PO2, PCO2). During control periods HR was slightly higher in UTR than in TR (375.5±3.9 (SE) vs. 364.1±5.5 beats/min−1,p−1,p−1 and 1.4±0.1 breaths·min−1 respectively, whereas in TR the changes were-2.9±1.5 beats·min−1 and -0.8±0.2 breaths·min−1. A significant difference between UTR and TR can only partly be due to diminished venous [H+] caused by better H+ buffering in TR. Particularly in the tests with lactic acid, lactate was far above threshold level. It can be concluded that the metabolic stimulus and the afferent branch of the cardiorespiratory reflex have been attenuated by endurance training.