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

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 with 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.