Rat cardiovascular responses to whole body head-down and non-head-down tilt suspension:

JUDY DOM-BROWSKI. Rat cardiovascular responses to whole body suspen-sion: head-down and non-head-down tilt. J. Appl. Physiol. 73(4):1504-1509, 1992. --The rat whole body suspension techniquemimics responses seen during exposure to microgravity andwas evaluated as a model for cardiovascular responses with twoseries of experiments. In one series, changes were monitored inchronically catheterized rats during 7 days of head-down tilt(HDT) or non-head-down tilt (N-HDT) and after several hoursof recovery. Elevations of mean arterial (MAP), systolic, anddiastolic pressures of _20% (P < 0.05) in HDT rats began asearly as day I and were maintained for the duration of suspen-sion. Pulse pressures were relatively unaffected, but heart rateswere elevated _ 10%. During postsuspension (2-7 h), most car-diovascular parameters returned to presuspension levels. N-HDT rats exhibited elevations chiefly on days 3 and 7. In thesecond series, blood pressure was monitored in 1- and 3-dayHDT and N-HDT rats to evaluate responses to rapid head-uptilt. MAP, systolic and diastolic pressures, and HR were ele-vated (P < 0.05) in HDT and N-HDT rats during head-up tiltafter 1 day of suspension, while pulse pressures remained un-changed. HDT rats exhibited elevated pretilt MAP and failedto respond to rapid head-up tilt with further increase of MAPon day 3, indicating some degree of deconditioning. The wholebody suspended rat may be useful as a model to better under-stand responses of rats exposed to microgravity.mean arterial blood pressure; systolic blood pressure; diastolicblood pressure; pulse pressure; heart rate; weightlessness; mi-crogravityEXPOSURE of humans to microgravity is known to resultin cardiovascular changes. These include a transient ini-tial increase in cardiac volume and cardiac output thatrecovers within several days (1). In Shuttle astronauts,cardiovascular deconditioning has been noted as ele-vated basal and passive tilt-induced heart rates, in-creased basal mean arterial pressure (MAP), and a tilt-induced fall in MAP (2). On exposure to microgravity,there is a redistribution of body fluids and an increase incentral blood volume and pressure (1). The head-down-tilted (HDT) rat has been utilized as an animal model tosimulate the effects of microgravity on the cardiovascu-lar system and fluid redistribution (5, 7, 12, 15, 17). In therat, HDT is known to induce cephalad fluid shifts (5)with concurrent diuresis, natriuresis, and kaliuresis (3). Preliminary studies of HDT rats (9, 10) indicated ele-vated MAP, systolic pressure (SP), and diastolic pres-sure (DP). Popovic (13) found that HDT rats initiallyhad elevated right atrial pressures that returned to base-1504line after 2 days of suspension. Shellock et al. (15) re-ported a direct relationship between the increase in cen-tral venous pressure and the degree of tilt (20 and 45 °) inHDT rats during a 24-h period. Previous observations ofblood pressure changes in the peripheral vascular systemare inconsistent, with results suggesting no significantchange in MAP (16), a transitory elevation {17), or per-sistent elevation in MAP during the first 3 days of sus-pension (9). An examination of the literature indicatesthat pressure measurements in suspended rats have beenmade with various techniques, with different methods ofsuspension (including a |ack of consistency in the angleof HDT), and over different periods of HDT {acute 24-hperiods and chronic periods of several days). It is likelythat variations in published results reflect these differentexperimental protocols.The first 2 clays of suspension appear to be a period ofstress and adjustment. Thus we considered it relevant toassess blood pressure and heart rate (HR) responsesduring the first 3 days of suspension. Our objectives inthis present study were 1) to ascertain HR and arterialpressure responses in rats during a 7-day period of HDTor non-head-down tilt (N-HDT) and after removal fromwhole body suspension (WBS) and 2) to determinewhether the rat exhibits indications oforthostatic intoler-ance after rapid reversal of the HDT posture during thefirst 3 days of suspension. An overall goal of these exper-iments was to examine further the versatility of the WBSsystem specifically as a ground-based model for cardio-vascular effects of microgravity.MATERIALS AND METHODSMale Sprague-Dawley rats in both series of experi-ments were suspended in a WBS harness (8) in either anHDT (20 °) or an N-HDT position. N-HDT rats are ani-mals suspended in harnesses like those of HDT rats ex-cept in a horizontal orientation. The animal room was ona 12:12-h light-dark cycle with ambient temperatures of23-24°C. Both HDT and N-HDT rats lost 2-7% of theirbody weights during the first 3 days of suspension, but byday 7 they had regained or surpassed their startingweights. Presuspension body weights in HDT (208 _+5.4g) and N-HDT rats (210 + 6.6 g) were comparable to7-day values (215 + 5.7 and 211 + 7.9 in HDT and N-HDT rats, respectively). Pentobarbital sodium (6 mg/100 g) was used to anesthetize rats for cannulation 1 wkbefore suspension. In each animal a polyethylene cath-eter (PE-10) was surgically placed in the right carotid0161-7567/92 $2.00 Copyright © 1992 the American Physiological Society