Biphasic release of thyrotropin in response to thyrotropin-releasing hormon (TRH) from rat anterior pituitary cells in vitro: Possible dependence on protein synthesis.

Abstract

Mechanisms related to the biphasic release of TSH were studied using primary cultured cells of the rat anterior pituitary gland on micro-carrier beads in a superfusion system. Release of TSH in response to continuous exposure to TRH exhibited a biphasic pattern ; the first phase was characterized by a rapid, transient and high-rate release (phase I) and the second phase by a chronic and low-rate release (phase II). The shift of the release from phase I to phase II occurred by treatment with TRH at concentrations from submaximal to maximal. When the Ca2+ concentration in the medium was decreased, the phase I release was partially inhibited, while the phase II release was completely inhibited, suggesting a difference between the mechanisms in phase I and phase II release. The phase I release was not suppressed by cycloheximide.This protein synthesis-independent release of phase I seemed to be linked to the intracellular releasable pool of TSH. The phase II release was suppressed by the presence of a protein synthesis inhibitor.After the phase II release was suppressed by cycloheximide, the magnitude of phase I release in response to re-exposure to TRH markedly decreased.The decreased phase I release in response to TRH was observed with the cells which were previously stimulated by high K+ instead of TRH, suggesting that the decrease in the response of phase I reflects the depletion of a releasable pool of TSH rather than homologous desensitization of thyrotrophs with TRH.These results suggest that the phase I release of TSH depends on a portion of the previously prepared-releasable pool while phase II release depends on previously prepared plus newly prepared pools of TSH. Replenishment of the releasable TSH pool was considered to involve protein synthesis.

Assessment of TRH as a potential MSH release stimulating factor in Xenopus laevis

“…In mammalian studies, TRH stimulation of thyrotropin release [7,31] and prolactin release [2,26] have been described as biphasic, consisting of a very rapid initial phase of hormone secretion, followed by a plateau of elevated secretion. It has been suggested that different intracellular mechanisms are responsible for the two phases of TRH-induced stimulation [2,26,34]. Interestingly, studies with rat thyrotropes [2] and GH3 tumor cells [34] indicate that the second phase of the response to TRH is dependent on protein synthesis.…”

“…It has been suggested that different intracellular mechanisms are responsible for the two phases of TRH-induced stimulation [2,26,34]. Interestingly, studies with rat thyrotropes [2] and GH3 tumor cells [34] indicate that the second phase of the response to TRH is dependent on protein synthesis. The melanotropes of white-background adapted Xenopus are known to have an extremely low level of biosynthetic activity [19,24,48].…”

Calcium Metabolism, Serum Thyroid-Stimulating Hormone, Prolactin, and Growth Hormone in Spontaneously Hypercholesterolemic Rats

“…PRL, GH, and TSH in the serum were assayed by radioimmunoassay using the antisera provided by the NIADDK Rat Pituitary Hormone Distribution Program, and the results were expressed in terms of NIAMDD-rat PRL-RP-2, NIAMDD-rat GH-RP-1, and NIADDK-rat TSH RP-2, respectively, according to the method described earlier (8,9). Thyroxine (T,) and triiodothyronine (T3) in the serum were assayed using kits (Mallinckrodt Inc., St. Louis, MO).…”