Cavernous sinus and carotid rete of sheep and sows as a possible place for countercurrent exchange of some neuropeptides and steroid hormones

Abstract

This study was conducted in order to scrutinise the countercurrent exchange of luteinising hormone releasing hormone (LH-RH), β-endorphin and progesterone in the carotid rete and cavernous sinus of sheep and gilts. The ewes ( n=30) and gilts ( n=20) were anaesthetised and had catheters inserted into their jugular veins and carotid common arteries from which heparinised blood was then collected. After exsanguination, the head with the neck was immediately removed and about 30 min later supplied with oxygenated and heated autologous blood mixed with saline (1:1), through one of the carotid arteries at a stable flow rate. Blood samples were collected every minute from the carotid artery contralateral to the arterial blood supply (blood from the carotid rete) and from both jugular veins. Arterial blood pressure was continuously measured and regulated. Radiolabelled hormones 3H-progesterone ( 3H-P 4), 125I-luteinising hormone releasing hormone ( 125I-LH-RH), 125I-β-endorphin ( 125I-En), or 51Cr-labelled red blood cells (control) were infused for 5 min into the cavernous sinuses through catheterised angularis oculi veins. Infusions were carried out on gilts and sheep which were in different phases of the oestrous cycle and on sheep during anoestrus and after oestrogen treatment. As a control, 3H-P 4 was infused into the facial vein instead of into the cavernous sinus. In all experiments with 3H-P 4 and 125I-En infusions, labelled hormones were found in arterial blood collected from the carotid rete. No radioactivity was present in arterial blood in the experiment with seasonally anoestrous sheep. However, 125I-LH-RH was found in the arterial blood of anoestrous oestrogen-treated sheep and of sheep in the early luteal phase. No radioactivity was ascertained in arterial blood after 3H-P 4 infusion into the facial vein (control) or 51Cr-labelled red blood cells into the cavernous sinus (control). These results demonstrate that two neuropeptides (LH-RH and β-endorphin) and a steroid hormone (progesterone) can be transferred from venous to arterial blood at the cavernous sinus and carotid rete in the base of the brain in sheep and pigs. This indicates that there is a powerful exchange system for resupplying hormones to the brain and that this system is dependent on the phase of the oestrous cycle.