RODENT ADRENAL ADAPTATION TO NINE DAY HYPOKINETIC MODEL EXPOSURE1140

Abstract

The 30° head down tilt (HDT) tail suspension model in rodents has been used as a hypokinetic model to study general physiologic deconditioning. To what extent HDT induces adrenal hypoactivation as part of the deconditioning syndrome has not been elucidated. To determine if exposure to HDT for 9 days results in an adrenal hypoactivation the following hypothesis was tested: adrenal corticosterone content and catecholamine synthesizing enzyme activity, tyrosine hydroxylase (TH) and phenylethanolamine-N-methyltransferase (PNMT), will decline in rodents exposed to 9 day HDT. Male Sprague-Dawley rats (95 days of age) were randomly assigned into the following groups: 1) cage control(CAGE-CN, n=20), 2) 9-day head down tilt (HDT, n=20). Tyrosine hydroxylase activity was determined by radiometric assay, PNMT activity was measured spectrophotometrically, and corticosterone content was determined via radioimmunoassay. All assay results were normalized to adrenal protein content. The enzyme PNMT activity (unitsmg-1 protein), and corticosterone content (pgug-1 protein) in adrenal tissue presented respectively were significantly lower in the HDT group(*0.052 ± 0.005, *264.9± 16) compared to the CAGE-CN group (0.073 ± 0.007, 483.7± 34). Tyrosine hydroxylase activity (CPM mg-1 protein) demonstrated no significant difference between SUS (2129 ± 342) and CAGE-CN (2161 ± 356) group. These findings suggest that 9 day exposure to simulated weightlessness results in adrenal hypoactivation which is consistent with general physiologic deconditioning.