Glucocorticoid Binding to Adenohypophysis Receptors and its Physiological Role

Abstract

The binding characteristics of glucocorticoids to cytoplasmic and nuclear fractions of the adenohypophysis were determined and an attempt was made to correlate steroid uptake with physiological action. Binding was of high affinity and exhibited a limited number of acceptor sites for dexamethasone (Dx). It was shown that the steroid-receptor complex dissociated rapidly at 37 °C, but was stable at low temperatures. An inverse relationship was detected between the plasma corticosterone (B) titer and the capacity of receptors to combine ³H-Dx. Shortly after stress or injection of physiological doses of B, 50 to 70% of the acceptor sites were saturated. Progressive desaturation of the occupied receptors appeared to depend upon time and the plasma B concentration. Some evidence was also provided supporting a possible correlation between the extent of Dx inhibition of in vitro ACTH secretion and the degree of binding sites’ occupancy. These results suggest that glucocorticoid binding to pituitary receptors may be correlated with the physiological action of the steroids on ACTH release.

Relationship between ACTH Secretion and Corticoid Binding to Specific Receptors in Perifused Adenohypophyses

“…This agrees well with previous in vitro studies reporting increased ability of synthetic glucocorticoids to initiate induction of tyrosine aminotransferase in hepatoma tissue culture cells [Samuels and Tomkins, 1970] or depress synthesis of nuclei acids and proteins in thymocytes [Makman et al, 1966]. Although correlation does not necessarily mean causality, it is striking to note that DEX exhibited not only a greater inhibitory effect than CORT but also greater incorporation rate into pituitary nuclei [De Kloet et al, 1975;McEwen et al, 1976;Koch et al, 1975],…”

“…The pituitaries were then washed twice with ice-cold TMKS (250 m M ) and binding to both cytoplasmic receptor and nuclei measured as previously described [Koclt et al, 1974]. These studies were performed using (3H) DEX, instead of (3H) CORT, in order to get rid of steroid binding to pi tuitary transcortin-like molecules [DeKloet et al, 1975;Koch et a!., 1975], Protein and deoxyribonucleic acid (DNA) contents were determined by the methods of Lowry et al [1951] and Gilíes and Myers [1965], respec tively.…”

“…pmol/ml). dbcAMPi: perifusion of 50 ng/ml CORT synthetic one [DeKloet et al, 1975;Koch et al, 1975], If one assumes correlation between nuclear uptake and physiological response, it seemed of interest to test the ability of both steroids to initiate blockade of ACTH release. It is evident from data in figure 4 that DEX, when compared to CORT, exerted a signifi cantly greater inhibitory effect on the CRFinduced secretion of ACTH.…”

“…[1974], and Mulder and Smelik [1977], on the basis of experiments conducted with whole pituitaries as well as isolated pituitary cells, likewise showed a direct inhibitory effect of corticoids on ACTH release. Moreover, it was found that inhibition was dose-dependent and seemed to correlate well with the degree of specific glucocorticoid receptor occupancy [Koch et al, 1975],…”

<i>In vitro</i> Nuclear ³H-Hydrocortisone Binding in the Hypothalamus of the Pig

“…To our knowledge, the kinetics of binding to the chromatin component of brain cell nuclei has not been previously reported. However, the value of Kd observed in these ex periments is comparable to that reported by H iggins et al [1973] for binding of 3H-dexamethasone to nuclei of HTC cells (2 * 10~1om) and to that of Koch et al [1975] for the binding of 3H-dexamethasone to pituitary cell nuclei in vitro (0.714 x 10-1°m).…”