Vascular Endothelium Is Severely Perturbed and Undergoes Apoptosis in Experimental Heatstroke in Primates.

Introduction: Ultrastructural evidence of endothelial cell (EC) injury has been associated with diffuse microvascular thrombosis in human heatstroke (HS). In vitro studies have also shown that heat stress accelerates apoptotic cell death. Using a recently described baboon model of heatstroke, we sought to examine pathological changes in the vascular endothelium and whether apoptosis is a mechanism of endothelial cell death.

Hypothesis: Major structural vascular endothelium alterations occur in HS and apoptosis is a mechanism of endothelial cell death in HS.

Methods: Anesthetized baboons (Papio hamadyras) were heat-stressed in a neonatal incubator maintained at 44 1.5 °C, until rectal temperature attained 42.5°C (moderate heatstroke; n =4) or systolic blood pressure fell to < 90 mm Hg (severe heatstroke n =4). Animals were resuscitated with normal saline and allowed to cool at room temperature. Four sham-heated animals served as control group. Spleen, liver, heart, kidney, gut, lung and adrenal tissue were obtained either by immediate autopsy in non-survivors or after euthanasia at 72-h for survivors. Vascular endothelium ultrastructure was evaluated by transmission electron microscopy (TEM) of ultra-thin tissue sections. Biological activity of EC was determined by light microscopy (LM) using a polyclonal antibody targeting von Willebrand Factor (vWF). Apoptosis was assessed, also in tissue sections, by deoxyuridine triphosphate nick end-labeling (TUNEL) procedure.

Results: In heatstroke animals, there were marked EC changes in lungs, spleen, jejunum, kidneys and liver, demonstrated by TEM, as increased cytoplasmic membrane convolutions that included formation of villi projecting into the vessel lumina, and increase in the width of the gaps between ECs. Migration of neutrophils, platelets and erythrocytes through these widened gaps was noted. Weibel-Palade bodies were increased both in size and number in EC of jejunum, lungs and kidneys. This increase correlated with increased endothelial expression of immunologically detectable vWF. TEM also showed that there was increased apoptosis manifested by nuclear chromatin condensation and karyorrhexis and formation of cytoplasmic myelin whorls. Increased EC apoptosis was also observed by TUNEL in the jejunum, lungs, liver and spleen. All these changes were greater in animals with severe HS than in animals with moderate HS, whereas sham heated control animals showed no significant changes.

Conclusion: Widespread EC injury with apoptotic cell death is consistent with the hypothesis that the endothelium may play a pathogenic role in heatstroke.