Overall open-loop gain of rapidly acting arterial pressure control system in rabbits

Norepinephrine was infused intravenously in two groups of normal, awake rats. In one group local cerebral glucose utilization (LCGU) was measured by the deoxyglucose method (Sokoloff et al. 1977b); in the other group local cerebral blood flow (LCBF) was determined by the iodoantipyrine method (Sakurada et al. 1978). The experiments were performed during a stable state in which the heart rate was reduced between 36% (LCGU experiments) and 27% (LCBF experiments). Norepinephrine infusion reduced LCGU in all 39 structures measured between - 18 and - 37% from control values obtained in a group of normal non-infused rats. The decrease in LCGU was significant (P less than 0.05) in 38 of the 39 structures tested. LCBF was increased but not statistically significantly in most of the structures examined. When the LCGU values of the various structures during norepinephrine infusion were correlated with their corresponding LCBF values, a tight correlation (r = 0.94) was found indicating a close coupling between LCGU and LCBF during norepinephrine infusion. When compared to the relationship between LCGU and LCBF in a normal, non-infused control group, the slope of the regression line was increased significantly (P less than 0.01) by the norepinephrine infusion, indicating a resetting of the coupling mechanism. This means that, at a given metabolic rate, a higher blood flow is needed to perfuse a brain structure during norepinephrine infusion than during control conditions.