Effects of head down tilt on hemodynamics, fluid volumes, and plasma Na-K pump inhibitor in rats.

BACKGROUND

Hindquarter suspension in rats has been used as a model of simulated weightlessness (SW) for ground based study of the effects of microgravity on the cardiovascular system (CVS).

METHODS

Using this rat model of SW we tested the hypothesis that CVS deconditioning following spaceflight results, in part, from a decrease in the circulating concentration of sodium-potassium pump inhibitor (SPI). Control rats similarly prepared were not suspended.

RESULTS

During the first hour of suspension, central venous pressure (CVP), blood pressure (BP), heart rate (HR), cardiac output (CO), plasma volume (PV), extracellular fluid volume (ECFV), urine output (UV), atrial natriuretic peptide (ANP), and the plasma level of SPI increased. Plasma renin activity (PRA) and myocardial Na+, K(+)-ATPase activity (NKA) decreased. By the end of 4 h of SW, the changes in CVP, BP, HR, ECFV, and UV persisted, but PV, plasma ANP and SPI, and myocardial NKA activity returned to control levels. By the end of 1 d of SW, ECFV and plasma SPI levels had decreased but the myocardial NKA had not increased. At day 4, CVP and BP were the same as in control sham treated rats. Plasma SPI levels were decreased at day 4 but the myocardial NKA was not different, whereas renal NKA was increased. At day 7, myocardial NKA and renal NKA were increased and vascular smooth muscle cell (VSMC) membrane potentials were hyperpolarized.

CONCLUSIONS

These data indicate that prolonged SW causes a decrease in plasma SPI level which, by hyperpolarizing VSMC, may play a role in the CVS deconditioning seen in astronauts following spaceflight.