Decreased in vitro LH releasing activity of oligomeric LH-RH derivatives

Primary rat pituitary cell culture system was used to investigate LH releasing activity of dimeric and oligomeric LH-RH agonists. In addition, peptides containing both agonistic as well as inhibitory segments were tested in order to find out which sequence overrules the other part of the molecule. In addition, intracellular cAMP levels were determined. All peptides but D-Phe2, D-Trp3, D-Lys6-LH-RH which was noted to be an inhibitor were found to increase intracellular cAMP contents. Only 72% LH releasing activity of LH-RH was registered for the analog which was composed of an agonistic and inhibitory chain. Dimeric and tetrameric derivatives of D-Lys6-LH-RH exhibited significantly lower LH releasing activity than the parent compound. But, no differences between the dimer and tetramer with agonistic chains was noted. The data show that cAMP may be involved in certain steps of LH-RH induced LH release, but does not play a role in overall LH release. In addition, dimerization and tetramerization of LH-RH agonist chains by short bridges is not sufficient for binding to more than one receptor molecule. Such derivatives could, however, reveal the length of the connecting bridge which is necessary for inducing receptor aggregation.