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 i n some mammals, but its mechanism has not been defined. To study the regula tory mechanism we infused hCG into perihypophyseal cavernous sinus of ovari ectomized, conscious ewes to test if the hCG would affect putative LH/hCG r eceptors and inhibit counter-current transfer of GnRH from the venous caver nous sinus to the arterial carotid rete. The latter study was done on an is olated 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 trea ted 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 v ia the venae angularis oculi. Immediately thereafter the ewes were anesthet ized and exanguinated, and subsequently decapitated. The isolated head was perfused with Dextran in multielectrolyte. The 125I-GnRH was infused into t he 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 i ntervals. Transfer of 125I-GnRH from the cavernous sinus to the carotid ret e was inhibited by hCG in ewes pretreated with estradiol benzoate but not w ith 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 prese nce of LH/hCG receptors. In situ hybridization showed the presence of LH/hC G receptor mRNA transcripts in the walls of both arterial and venous compar tments of the cavrnous sinus-carotid rete complex, and immunohistochemistry revealed the presence of receptor proteins. These novel findings confirm p reviously obtained data suggesting that LH is a modulatory factor for the c ounter-current transfer of neuropeptides from the venous blood of the caver nous sinus to the arterial blood supplying the brain and hypophysis. The LH could modulate 125I-GnRH transfer acting directly on the vascular smooth m uscle. (C) 1999 by Elsevier Science Inc.