Blood Flow Changes in the Rabbit Cranial and Caudal Venae Cavae during Postural Loads after Propranolol, Bisoprolol and Methyldopa Pretreatment

The β_1,2-adrenergic receptor blockerpropranolol, β₁-blocker bisoprolol, and centralpresynaptic α₂-adrenergic receptor agonistmethyldopa are used in clinical practice to treat the hyperadrenergictype of postural orthostatic tachycardia syndrome. There are noliterature data on the effects of these drugs on vena cava flowsduring postural tests. In acute experiments on anesthetized rabbits,we studied blood flow changes in the cranial vena cava (CrVC) andcaudal vena cava (CdVC) during orthostatic (head-up tilt by 25°)and antiorthostatic (head-down tilt by –25°) tests for 20 s afterpropranolol, bisoprolol and methyldopa intravenous preadministration.Before drug pretreatment, both CrVC and CdVC flows decreased inresponse to orthostasis at the 4th and 20th s. During antiorthostasis,CdVC flow increased at the 4th s and decreased to the baseline valueby the 20th s, while CrVC flow decreased by the 4th s and exceededthe baseline level by the 20th s. After propranolol pretreatment,CdVC flow decreased to a greater extent than in response to bisoprololand methyldopa pretreatment. After methyldopa pretreatment duringorthostasis, by the 20th s, CrVC flow decreased more notably thanCdVC flow, while after propranolol and bisoprolol pretreatment underconditions of orthostasis, both CrVC and CdVC flows decreased aboutequally. During antiorthostasis, 20 s after propranolol pretreatment,CdVC flow increased more considerably compared to CrVC flow. Afterbisoprolol and methyldopa pretreatment, CrVC flow increased in responseto antiorthostasis not only to a greater extent than CdVC flow buteven more pronouncedly compared to its initial increment. Thus,postural tests after the pretreatment with the above drugs revealdifferences in the mechanisms of blood flow redistribution in CrVCand CdVC basins.