Effects of postural changes and removal of vestibular inputs on blood flow to and from the hindlimb of conscious felines

Abstract

Considerable data show that the vestibular system contributes to blood pressure regulation. Prior studies reported that lesions that eliminate inputs from the inner ears attenuate the vasoconstriction that ordinarily occurs in the hindlimbs of conscious cats during head-up rotations. These data led to the hypothesis that labyrinthine-deficient animals would experience considerable lower body blood pooling during head-up postural alterations. The present study tested this hypothesis by comparing blood flow though the femoral artery and vein of conscious cats during 20-60 degrees head-up tilts from the prone position before and after removal of vestibular inputs. In vestibular-intact animals, venous return from the hindlimb dropped considerably at the onset of head-up tilts and, at 5 s after the initiation of 60 degrees rotations, was 66% lower than when the animals were prone. However, after the animals were maintained in the head-up position for another 15 s, venous return was just 33% lower than before the tilt commenced. At the same time point, arterial inflow to the limb had decreased 32% from baseline, such that the decrease in blood flow out of the limb due to the force of gravity was precisely matched by a reduction in blood reaching the limb. After vestibular lesions, the decline in femoral artery blood flow that ordinarily occurs during head-up tilts was attenuated, such that more blood flowed into the leg. Contrary to expectations, in most animals, venous return was facilitated, such that no more blood accumulated in the hindlimb than when labyrinthine signals were present. These data show that peripheral blood pooling is unlikely to account for the fluctuations in blood pressure that can occur during postural changes of animals lacking inputs from the inner ear. Instead, alterations in total peripheral resistance following vestibular dysfunction could affect the regulation of blood pressure.

Fig. 1.

Blood flow responses to head-up tilt. Mean femoral vein blood flow (top trace) was obtained from a flow module (model TS420, Transonic Systems); pulsatile and mean femoral artery blood flow traces are also shown. Bottom trace is a recording of table position provided by a potentiometer.

Fig. 2.

Average changes in femoral artery (top trace) and vein (middle trace) blood flow during 20°, 40°, and 60° head-up tilts before removal of vestibular inputs. Bottom trace: instantaneous blood accumulation at each time period, determined by subtraction of percent difference from baseline in venous blood flow from percent difference from baseline in arterial blood flow. Symbols designate changes in blood flow and blood accumulation elicited by 40° and 60° tilt that were significantly different from those resulting from 20° tilt. For points where no symbols are present, significant differences were not detected. Error bars, SE. P values reflect effects on results of the following parameters: interaction of time from tilt onset and tilt amplitude (time * amplitude), time from tilt onset (time), tilt amplitude (amplitude), and matching of subjects (matching).

Fig. 3.

Effects of removal of vestibular inputs on changes from pretilt levels in femoral artery and vein blood flow and hindlimb blood accumulation at different times during 60° head-up rotation. A–G: responses recorded from each animal. Prelesion, data recorded before vestibular lesion; Postlesion W1, values ascertained during the 1st wk after vestibular neurectomy; Postlesion W2–4, responses recorded subsequently. Symbols designate postlesion changes in blood flow or accumulation during tilts that were significantly different from those determined when vestibular inputs were present; relative significance of differences is clarified in Fig. 4. For points where no symbols are present, significant differences were not found. Error bars, SE.

Fig. 4.

Relative significance of differences in postlesion blood flow and hindlimb blood accumulation from prelesion values in each animal. Symbols indicate whether mean femoral artery and vein blood flow and hindlimb blood accumulation determined for the 1st wk (W1) and subsequent weeks (W2–4) after removal of vestibular inputs differed significantly from prelesion values. Mean data are plotted in Fig. 3.

Fig. 5.

Average effects of removal of vestibular inputs on femoral artery and vein blood flow and hindlimb blood accumulation relative to pretilt baseline values. Symbols designate postlesion changes in blood flow and blood accumulation during tilt that were significantly different from those recorded when vestibular inputs were present. For points where no symbols are present, significant differences were not found. Error bars, SE. P values reflect effects on results of the following parameters: interaction of time from tilt onset and lesion state (time * lesion), time from tilt onset (time), lesion state (lesion), and matching of subjects (matching).