Role of catecholamines in regulating ovine median eminence blood flow

Blood flow was measured in the ovine median eminence and neural lobe before and after the intravenous infusion of dopamine (n = 7), the D1 agonist SKF 38393 (n = 4), the D2 agonist bromocriptine (n = 4), and the dopamine antagonist haloperidol (n = 5). It was also measured before and after the intracarotid infusion of dopamine into eight naive sheep and seven sheep pretreated with phenoxybenzamine. Radiolabeled microspheres were used to determine regional cerebral and regional neurohypophysial blood flows (RNHBF) in these 35 adult female sheep anesthetized with pentobarbital sodium. Samples for serum prolactin measurement by radioimmunoassay were obtained before and after drug infusion. Intravenous dopamine infusion did not change median eminence or neural lobe blood flow (RNHBF) but increased renal and choroid plexus blood flow. Intravenous haloperidol caused a significant fall in RNHBF and blood flow in choroid plexus, caudate nucleus, and kidneys. Intracarotid dopamine infusion decreased RNHBF but increased choroid plexus blood flow. RNHBF was significantly greater in the seven sheep pretreated with phenoxybenzamine than in the eight naive sheep. These findings do not support a role for dopamine in the regulation of median eminence blood flow. The last observation does add support to the hypothesis that norepinephrine or epinephrine interaction with alpha-receptors plays a role in the control of ovine median eminence blood flow and hence in the regulation of delivery of humoral messages from the brain to the anterior pituitary gland.