Responses of Plasma Proenkephalin Peptide F in Rats Following 14 Days of Spaceflight

Introduction. Proenkephalin peptide F [107-140] is related to the enhancement of immune function, while microgravity has been shown to cause immuno-suppression. We investigated the physiological response of proenkephalin peptide F to microgravity. Methods: There were 12 Fischer 344 female rats, ovariectomized at 10.5 wk of age, used to determine plasma concentrations of peptide F in response to a 14-d flight aboard the Columbia Space Shuttle mission STS-62. There were 36 other such rats that served as ground-based controls to separate the effects of microgravity from those of thermal stress, flight stress, and crowded habitats. Control groups of 12 rats each were kept under the following conditions: 1) 22°C vivarium, 2) 28°C vivarium, and 3) variable (Var) to mimic flight. The flight and control groups were housed in animal enclosure modules 21 d prior to flight and for the duration of the study. The rats were sacrificed within 4-5 h after landing, at which time blood samples were obtained. Results: Body weights were obtained prior to sacrifice; mean values were flight, 199 g; 22°C, 193 g; 28°C, 192 g; and Var, 194 g. The flight group produced a significantly greater (p ≤ 0.05) level of plasma peptide F (0.056 pmol · ml ^-1) compared with the controls (0.016, 0.022, and 0.016 pmol · ml ^-1 for 22°C, 28°C, and Var, respectively). Flight animals demonstrated higher corticosterone concentrations and reduced T and B cell splenocyte counts than controls. Conclusions: These data indicate that the increases in proenkephalin peptide F observed with exposure to microgravity may present an adrenal-medullary response to cope with the decreased immune function and increased stress experienced during spaceflight and landing.