Anterior pituitary vasodilation after hemorrhage in the dog

A miniature thermoelectric probe was used to record continuously tissue blood flow in the anterior pituitary gland of anesthetized dogs. The output of the flow probe is linear in the range of tissue blood flow encountered in the pituitary. Probe placement required minimal tissue trauma, and zero blood flow was recorded at the end of each experiment. Little or no change in flow followed hemorrhage of 10 ml/kg body weight. However, hemorrhages of 20 or 30 ml/kg were followed by a biphasic deviation from control level, with an initial transient decrease in flow, followed by recovery to control at about 19 min and a rise above control to 90% of maximum at about 32 min. Hypothalamohypophysial vascular resistance was found to decrease significantly following hemorrhages of 20 or 30 ml/kg, and this was shown to be independent of resistance changes in the femoral vascular bed. It is concluded that anterior pituitary blood flow is well maintained following moderate hemorrhage as a result of intrinsic vasodilation in the hypothalamo-hypophysial vascular bed.