Hypothalamic influences on cardiovascular response of beagles to dynamic exercise.

We studied nine young adult beagles at rest and during four levels of dynamic exercise before and after electrolytic lesions were made in the hypothalamus in the region of the fields of Forel. The beagles were habituated to run freely on a motor-driven treadmill and were instrumented chronically to allow repeated measurement of cardiovascular variables. Variables measured or calculated included O2 consumption, cardiac output, arteriovenous O2 difference, systemic arterial pressure, systemic vascular resistance, heart rate, and mixed venous blood lactate concentration. In addition, regional blood flow was measured with radioactive microspheres in two beagles at rest and during peak exercise (6.4 km/h, 20% grade). After prelesion treadmill studies, the beagles were anesthetized and the tips of stainless steel electrodes were placed bilaterally in the hypothalamus in locations that when stimulated (100-300 microA) evoked increases in cardiovascular and muscle motor function. Lesioning (5 mA; 15 s) abolished the responses evoked by stimulation. By contrast, the cardiovascular variables measured in the beagles at rest and during dynamic exercise were similar pre- and postlesion. Therefore, loss of hypothalamic sites that produce increases in cardiovascular and muscle motor function when stimulated electrically does not appear to comprise the cardiovascular response of awake beagles to dynamic exercise.