Learned control of heart rate during dynamic exercise in nonhuman primates

The purpose of this study was to describe an animal model to test the relationships among the cardiovascular, pulmonary, and somatomotor command systems during exercise. Using operant conditioning, three chronically chaired monkeys (Macaca mulatta) were trained to exercise (to lift weights repeatedly), to attenuate their heart rate responses, and finally, both conditions were combined so that the animals were required to exercise and attenuate their heart rates. Heart rate, systolic and diastolic blood pressure, rate-pressure product, O₂ and CO₂ concentration in expired air, and number of weight lifts were recorded and compared between the two conditions, i.e., exercise only and combined exercise and heart rate slowing. In all animals heart rate increases in response to exercise were significantly less (P < 0.05) during combined conditions than during exercise only: the mean heart rate increase was 41 beats/min less during combined sessions than during exercise only sessions for monkey 1, 13.5 beats/min less for monkey 2, and 9 beats/min less for moneky 3. Rate-pressure product showed a consisten difference across animals paralleling the heart rate differences. This acquired response did not involve other cardiovascular and pulmonary parameters, which did not change systematically across animals. However, the pattern of cardiovascular reactivity in relation to O₂ consumption (linear regression of heart rate and systolic or diastolic blood pressure on change in O₂ consumption over many experiments) was attenuated during combined sessions relative to exercise only experiments. The relative attenuation of heart rate during combined sessions also remained significant when both experimental conditions were equated on the basis of work done. Therefore, this animal model shows a dissociation of cardiovascular, somatomotor, and pulmonary effects of central command.