Effect of hemorrhagic shock on release of norepinephrine by tyramine

Abstract

This study was undertaken to determine if the sympathetic discharge which accompanies hemorrhagic shock modifies the response to tyramine. In eight dogs the dose of tyramine equipressor to 1.0 µg/kg min of norepinephrine was 85 ± 8 µg/kg min in the control state, but had risen to 301 ± 55 µg/kg min after 4 hr of hypotension. In four dogs in which the norepinephrine pool had been labeled with tritiated norepinephrine, injection of tyramine into femoral arterial blood released large quantities of norepinephrine into the femoral venous blood during the control period. After 4 hr of hypotension, the output of radioactive norepinephrine by tyramine was greatly reduced. These findings support the view that the diminished responsiveness to tyramine after prolonged hypotension is due to decreased release of norepinephrine. Since there was no detectable decrease in the norepinephrine concentration of atrial appendage in eight dogs after 4 hr of shock, the diminished release of norepinephrine by tyramine in shock may have been due to the depletion of the small portion of the norepinephrine store releasable by tyramine.