Hemorrhage-induced vasopressin release in the paraventricular nucleus measured by in vivo microdialysis.

Experiments were carried out, using the technique of in vivo microdialysis in conscious rats, to determine whether hemorrhage, a potent stimulus for the release of vasopressin from the posterior pituitary into the circulation, would also result in a local release of vasopressin from the paraventricular nucleus (PVN), and whether this release is affected by gender. Male and non-estrous female rats were prepared with a microdialysis probe adjacent to the PVN and femoral arterial and venous catheters the day before the experiment. On the day of the experiment, rats was bled either 20% or 30% of blood volume. The concentration of vasopressin in the dialysate increased significantly in the males following both hemorrhages and in the females following the 30% hemorrhage. There were no statistically significant differences in the post-hemorrhage dialysate vasopressin concentration with respect to either gender or magnitude of the hemorrhage. The plasma vasopressin concentration increased markedly in response to the hemorrhage and this response was greater in females following the 30% hemorrhage. There were no gender differences in the reduction in arterial pressure following either hemorrhage. It is concluded that physiological stimuli for the release of vasopressin into the circulation also result in intrahypothalamic release of this hormone.