Changes in blood pressure, blood flow towards the head and heart rate during 90deg head-up tilting for 30min in anaesthetized male rats

New Findings What is the central question of this study? How does the baroreceptor reflex contribute to systemic blood pressure (BP) control during prolonged 90deg head-up tilting (HUT) in unconscious rats? What is the main finding and its importance? In intact rats, heart rate (HR) increased after the transition to upright by HUT, and BP was maintained in this posture throughout the 30min experimental period. After sinoaortic denervation, which results in lack of discharge of the baroreflex afferents, HR and BP decreased gradually during 30min of 90deg HUT, suggesting that the baroreceptor reflex is an important factor for maintenance of BP during long-term upright posture. Changes in cardiovascular parameters during prolonged 90deg head-up tilting (HUT) in animals have not been elucidated in detail. We clarified changes in systemic blood pressure (BP), blood flow towards the head (BF) and heart rate (HR) and the role of the baroreceptor reflex after a transition from the supine posture to 90deg HUT for 30min in anaesthetized rats (n=13). In control rats with the baroreceptor reflex afferents intact, mean BP and BF after the onset of 90deg HUT decreased significantly by -15.4 +/- 5.9 and -26.2 +/- 11.5% at 2.9 +/- 1.1s (mean +/- SD, n=12), respectively, compared with control values and then immediately increased and steadied at 30.7 +/- 13.1s (plateau; -2.8 +/- 8.5% in BP and -17.5 +/- 17.4% in BF compared with control values; BP was maintained during 90deg HUT). After acute sinoaortic denervation in seven rats, initial reductions in BP and BF after 90deg HUT were observed at 3.9 +/- 1.0s, similar to the reductions in the nerve-intact rats; the percentage changes from control were -19.2 +/- 3.7 and -32.3 +/- 8.4%, respectively. These parameters reached a plateau at 22.4 +/- 5.8s at -8.6 +/- 7.7 and -29.5 +/- 15.0%, respectively, and then BP decreased gradually throughout 90deg HUT. Heart rate increased slightly after 90deg HUT in nerve-intact rats, but after sinoaortic denervation this increase disappeared and HR decreased gradually during 90deg HUT. These results suggest that the baroreceptor reflex contributes to the maintenance of adequate BP during long-term 90deg HUT.