Peripheral skin cooling during Hypergravity: Neuroendocrine, leucocytic and hematological reactions

Abstract Introduction: Optimal neuroendocrine responses, particularly noradrenergic responses, are critical during hypergravity (+ Gz) exposure. Peripheral skin cooling (PSC) has the potential to enhance noradrenergic function, which could improve + Gz resiliency and may also affect leukocyte and hematologic factors. We predicted that PSC would augment cumulative + Gz stress index (CGSI) neuroendocrine, leukocyte and hematologic activity compared to controls, and that CGSI would shift to a noradrenergic dependency in PSC compared to controls. Methods 18 men in a cross-over design underwent a graded + Gz profile. PSC was applied using Arctic Sun cooling pads around the thighs with 8°C water infusion. Neuroendocrine parameters and a complete blood profile with differential were obtained before and after + Gz. CGSI was calculated as + Gz * seconds. Pre / post comparisons were made within the groups with paired t-tests and between groups with independent t-tests. Pearson's correlation analysis was performed between CGSI and neuroendocrine/leukocyte/hematological parameters. Results CSGI did not differ between groups. In both groups, all neuroendocrine, leukocyte and hematological factors changed significantly from pre to post + Gz, except for serum osmolality, which increased only in PSC (p = 0.03). In PSC, CGSI correlated with noradrenaline (p < 0.01, r = 0.71), metanephrine (p = 0.02, r = -0.54), neutrophiles (p = 0.03, r = 0.51), and thrombocytes (p = 0.04, r = 0.48), while in controls, CGSI correlated with metanephrine (p = 0.02, r = -0.54). Pre + Gz metanephrine correlated with CGSI in both groups. Discussion PSC did not confer a higher CGSI nor induce a greater neuroendocrine, leukocytic or hematologic response compared to controls. However, the increase in serum osmolality may indicate increased vasopressin activity. The strong correlations between CGSI and norepinephrine in PSC suggest that despite equal average serum levels, the responsiveness of norepinephrine is enhanced. This may serve as an optimal cardio-protective countermeasure for manned space missions and possible ICU patients. The correlations between neutrophiles and thrombocytes in PSC suggest a leukocytic and hematologic involvement in CSGI. In addition, this is the first study to demonstrate that baseline serum metanephrine may serve as a potential neuroendocrine parameter to assess + Gz resilience.