CEREBRAL CORTICAL BLOOD-FLOW IN RABBITS DURING PARABOLIC FLIGHTS (HYPERGRAVITY AND MICROGRAVITY)

We studied the effect of gravity on cerebral cortical blood flow (CBF) , mean arterial blood pressure (<(BP)over bar>(a)) and heart rate in s ix rabbits exposed to parabolic flights. The CBF was obtained using a laser-Doppler probe fixed on to a cranial window. Before weightlessnes s, the animals were exposed to chest-to-back directed acceleration (1. 8-2.0 g). The CBF values were expressed as a percentage of CBFo (mean CBF during 60 s before the 1st parabola). Propranolol (1 mg . kg(-1) I V) was given after the 11th parabola and pentobarbital (12-15 mg . kg( -1) IV) after the 16th parabola. Before the administration of the drug s, CBF increased (P < 0.01) during hypergravity [i.e. maximal CBF 151 (SD 64)% CBFo. Simultaneously <(BP)over bar>(a) increased [maximal <(B P)over bar>(a), 119 (SD 11) mmHg (P < 0.01)]. At the onset of weightle ssness, CBF and <(BP)over bar>(a) reached maximal values [194 (SD 96)% CBFo (P < 0.01) and 127 (SD 19) mmHg, (P < 0.01) respectively]. The m icrogravity-induced increase in CBF was transient since CBF returned t o its baseline value after 8 (SD 2) s of microgravity. After propranol ol administration, CBF was not statistically different during hypergra vity but an elevation of CBF was still observed in weightlessness. The increases in CBF and BP, also persisted during weightlessness after p entobarbital administration. These data would indicate that CBF of non anesthetized rabbits increases during the first seconds of weightlessn ess and demonstrate the involvement of rapid active regulatory mechani sms since CBF returned to control values within 8 (SD 2) s. We conclud ed that this elevation in blood flow was not related to stress because it persisted after the administration of propranolol and pentobarbita l.