Maruyama S, Kemuriyama T, Manabe T, Takahata T, Shoji I, Nishida Y. Severe hypotension during the decreasing phase of G_z stress in anesthetized rats wearing an anti-G suit. Aviat Space Environ Med 2011; 82:1030–6.Introduction: Physiological responses to +G_z stress have been reported in several studies. However, no reports exist on differences in arterial pressure responses between increasing and decreasing G phases. We hypothesized that +G_z stress and/or an anti-G support might disturb the circulation system and cause potential brain hypoperfusion, even if the anti-G support protects against G-induced loss of consciousness. Methods: Dependency of +G_z magnitude, hemodynamic changes, renal sympathetic nerve activity (RSNA), and aortic blood flow (AoBF) were estimated in anesthetized rats to analyze the effects of +G_z stress and/or an anti-G support on arterial pressure at a level of the brain (APLB). The rats were exposed to +G_z using a centrifuge for small animals while wearing an anti-G suit. Results: APLB remained at the control level while the anti-G suit was inflated. However, a decrease in APLB was observed twice during increasing and decreasing G phases using the anti-G suit. Hypotension in the decreasing G phase at +5 G_z was significantly deeper than that in the increasing G phase (47.5 ± 7.7 vs. 29.6 ± 3.0 mmHg). RSNA responses to G_z loads were greater in the decreasing G than in the increasing G phase (129.7 ± 8.6 vs. 147.3 ± 10.4%). Both AoBF and calculated vascular resistance were suppressed more significantly in the decreasing G than in the increasing G phase (38.3 ± 4.4 vs. 34.4 ± 3.4 ml · min⁻¹, 1.44 ± 0.22 vs. 1.09 ± 0.14 mmHg · min⁻¹ · ml⁻¹). Discussion: We conclude that transient excessive decreasing G hypotension may occur during the decreasing G phase, which may be due to anti-G suit functioning.