Heart rate and oxygen uptake response to angiotensin in the squirrel monkey at 10 degrees C

Unanesthetized squirrel monkeys exposed to an ambient temperature of 10 degrees C showed elevations in total body oxygen consumption (VO2), systemic arterial blood pressure (BP), and heart rate (hr) above values recorded at 28 degrees C. Further elevation of BP in the cold by intravenous infusion of phenylephrine (5-50 microgram/kg-min) was accompanied by reduction in both VO2 and HR, and the changes in VO2 were proportional to those in HR. When BP was raised by intravenous infusion of angiotensin (0.05-1.0 microgram/kg-min), large elevations in BP were again accompanied by reductions in HR and VO2. However, for equivalent elevations in BP, the depressions in both HR and VO2 were much smaller with angiotensin than they were with phenylephrine. Previous studies in this laboratory have demonstrated that in response to experimental elevation of BP, reflexes originating at the sinoaortic baroreceptors depress not only HR but also VO2. The present results suggest that angiotensin modulates baro-reflexive responses to elevation in BP. The reductions in HR and VO2 that ordinarily occur in response to baroreceptor stimulation may be modified by an action of angiotensin on the central nervous system.