The hindlimb unweighting (HLU) rat model mimics cardiovascular deconditioning following microgravity or human bed rest, particularly for the development of orthostatic intolerance. We have examined vascular responses to α₁ adrenergic and non-α₁ adrenergic agonists in vitro . We have also explored the reversibility of the contractile abnormalities observed.

Dose-response curves were generated to phenylephrine (PE) and norepinephrine (NE) (10^?? to 10^?? M), U₄₆₆₁₉ (U₄) (10^??0 to 10^?? M) at one-half log order intervals in controls (n = 6), HLU (n = 6), or recovered rats (n = 6). EC₅₀s and maximal responses (E_max) were calculated by nonlinear logistic regression analysis with PRIZM software (Graphpad, Mountain View, CA).

Simulated microgravity results in attenuated contractile responses to both α₁ adrenergic and non-α₁ adrenergic agonists, but the impaired contractile phenomenon reverses with time.

The decreased vascular reactivity after microgravity and prolonged bed rest could cause attenuated baroreflex function and produce orthostatic intolerance, but that problem resolved with time.