The functional vascular integrity of the canine small intestine was examined at 48 and 72 hours postirradiation. The dogs were exposed to 1500 rads midline tissue dose of mixed gamma-neutron radiation. First, the intestinal blood flow values of an in situ jejunal loop were measured in response to sudden systemic blood pressure alterations after exogenous administration of norepinephrine and isoproterenol. Second, the intestinal vasomotion was evaluated after applying a gradual bleeding and reinfusion technique. The regulation of the intestinal blood flow to sudden blood pressure changes, induced by the alpha and beta adrenergic agents, indicates progressive alterations in the neurohumoral vascular regulation during the postirradiation period. In the irradiated animals the so-called autoregulatory escape was not manifested after norepinephrine administration. The intestinal vessels of the irradiated dogs after 72 hours behaved as a rigid tube exhibiting an overshooting and a lack of vasocompensation components. The postirradiation vasoactivity during gradual blood volume alterations did not differ from the responses of the non-irradiated control group. The data indicate that intrinsic intestinal vascular alterations may underlie the development of the so-called gastrointestinal radiation syndrome.

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