Demonstration of a heterogenous population of binding sites for thyroliberin in prolactin-producing tumour cells and their possible functional significance.

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The binding of [3H]thyroliberin was studied in intact GH3 cells at 37°C and correlated to accumulation of adenosine 3′,5′-monophosphate (cyclic AMP) and prolactin release. Experiments which were performed at steady state, showed binding of [3H]thyroliberin which was specific and saturable, but the results could not be adequately described on the basis of one homogeneous population of receptors. When two classes of binding sites were assumed, average Kd values of 0.9 nM and 23 nM were found in experiments performed at steady state. The maximum binding capacity was calculated to be 780 fmol [3H]thyroliberin/mg cell protein. Of the calculated 82500 thyroliberin receptors per GH3 cell, about 1/3 represented the higher affinity binding site. The half-times for dissociation of [3H]thyroliberin were 160 min and 5 min for the higher and lower affinity site, respectively. Association and dissociation rate constants measured at 37 °C for the high and low affinity receptor sites gave the following average values, k+1/k−1: 1.1 × 105 M−1 s−1/0.7 × 10−4 s−1 and 0.7 × 105 M−1 s−1/ 12 × 10−4 s−1, respectively. The phenomenon of negative cooperativity was apparently not present in the GH3 cells since the rates of dissociation for [3H]thyroliberin were similar in the presence or the absence of a 100-fold excess thyroliberin at high and low receptor occupancy. There was no evidence for internalization of the [3H]thyroliberin-receptor complex during these experiments (duration ⋝ 4 h) since the binding was easily reversible and the rates of dissociation for the slowly dissociable binding site were similar in experiments using different times of association. At brief incubation periods (10–120 s) the binding of the tripeptide occurred simultaneously with the increase in cvclic AMP accumulation with a lag period of less than 10 s, and maximal concentrations of cyclic AMP occurred at about 10% receptor occupancy. The dose-response curve of thyroliberin for prolactin rciease showed half maximal stimulation at 0.4 nM while half-maximal binding occurred at 10 nM during equilibrium conditions. We conclude that the GH3 cells contain a heterogeneous population of saturable thyroliberin-binding sites, and that the higher affinity receptor may be associated with prolactin release and cyclic AMP formation.