Simultaneous comparison of the microcirculation in the cheek pouch and mesocaecum of the hamster in response to cold
Brooks, Arthur Maurice
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The physiological behavior of the microcirculation in a skinlike surface area (cheek pouch of hamster) is compared with that in a visceral area (mesocaecum) of the same species (golden hamster). A microelectrode test has been used to measure and compare the threshold sensitivity of vascular smooth muscle on arterioles and venules, the susceptibility of small blood vessels to platelet thromboembolism in response to graded electrical stimulation of the endothelial wall, and the tendency of the vessel to rupture resulting in mtcrohemorrhage as the voltage and duration are increased. At room temperature, no significant differences Here found between the reactivity in the cheek pouch and that in the mesocaecum. The effect of low temperature (15°C and 5°C) on the behavior of the microcirculation in the cheek pouch has been investigated during local cooling of a portion of the pouch, during cooling of the entire pouch and during cooling of the entire animal. At decreased temperatures, the blood vessels were significantly less fragile as shown by increased resistance to rhexis following electrical stimulation of the vessel wall with a microelectrode. The reactivity of arteriolar smooth muscle decreased as shown by an elevation in the threshold to direct electrical stimulation; but no significant change occurred in the sensitivity to venular smooth muscle. The threshold for thrombus formation was not clear-cut at lowered temperatures, but there appeared to be a tendency toward increased thresholds. Vasorhexis was decidedly increased with decrease in temperature. Hematocrits and hemoglobin levels were elevated from normal in cold adapted hamsters. Prothrombin times were unexpectedly decreased rather than increased as reported in the literature. Hamsters cold stressed for approximately two weeks at 7°C were also tested with respect to reactions of the microcirculation to electrical stimuli, and their responses were found to be similar to those with local and total body cooling. Blood velocities and volume flows in individual vessels of the microcirculation were measured at room temperature (2.5°C) and also at 15°C and 5°C. Velocities and volume flows were also measured in hamsters not subjected to cold stress but only under the effects of anesthesia. Blood velocities and blood volume flows decreased with decreasing temperature and no correlation was found between the size of vessels and the rate of volume flow. Cold acclimated hamsters had blood velocities and volume flow rates higher than those found in hamsters at room temperature. Sodium pentobarbital has an apparent slowing effect on blood velocity which approaches normal values as the hamster awakens from the anesthesia. The true effectiveness of the protection possible with the "dehydrating" agents ethylene glycol and glycerol, were not evident in these studies because of the toxic levels necessary for such protection. Nevertheless, glycerol was more efficacious than ethylene glycol in ameliorating the responses of the microcirculation to the sequelae of cold injury. The value of Eastern moccasin venom as an indicator of the integrity of the vessel walls by petechial formation is questioned. Lyophilized and crystallized venom at different concentrations was tested on the cheek pouch, and extremely variable results occurred. Data obtained at 15°C with the venom revealed inconsistent results and varying petechial counts and therefore the variability in petechial counts could not be ascribed to cold per se. Cinephotomicrographic recordings of the effects of cold and the "protection" of dehydrating agents have been made on 16mm Kodachrome film. In addition, split-field cinephotomicrography was used to make simultaneous records of the microcirculation of the cheek pouch and mesocaecum on the same picture frame.
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