The inhibitory effect of hCG on counter current transfer of GnRH and the presence of LH/hCG receptors in the perihypophyseal cavernous sinus--carotid rete vascular complex of ewes.

The existence of the hormone passage from venous blood into arterial blood in the area of the perihypophyseal vascular complex has been demonstrated in some mammals, but its mechanism has not been defined. To study the regulatory mechanism we infused hCG into perihypophyseal cavernous sinus of ovariectomized, conscious ewes to test if the hCG would affect putative LH/hCG receptors and inhibit counter-current transfer of GnRH from the venous cavernous sinus to the arterial carotid rete. The latter study was done on an isolated head model. Ewes were ovariectomized in mid-anestrus and, after 4 to 5 wk were used in the experiments. On the day of experiment ewes were treated intramuscularly with estradiol benzoate or oil vehicle, and 18 to 20 h later were infused either with a multielectrolyte solution or hCG for 2 h via the venae angularis oculi. Immediately thereafter the ewes were anesthetized and exanguinated, and subsequently decapitated. The isolated head was perfused with Dextran in multielectrolyte. The 125I-GnRH was infused into the cavernous sinus via the venae angularis oculi for 5 min; contemporaneous samples were taken from the carotid rete and both jugular veins at 1-min intervals. Transfer of 125I-GnRH from the cavernous sinus to the carotid rete was inhibited by hCG in ewes pretreated with estradiol benzoate but not with oil (P<0.005). We collected tissue samples from the vascular complex of the cavernous sinus and carotid rete of cyclic ewes to determine the presence of LH/hCG receptors. In situ hybridization showed the presence of LH/hCG receptor mRNA transcripts in the walls of both arterial and venous compartments of the cavernous sinus-carotid rete complex, and immunohistochemistry revealed the presence of receptor proteins. These novel findings confirm previously obtained data suggesting that LH is a modulatory factor for the counter-current transfer of neuropeptides from the venous blood of the cavernous sinus to the arterial blood supplying the brain and hypophysis. The LH could modulate 125I-GnRH transfer acting directly on the vascular smooth muscle.