We have examined the dynamics of thyrotropin-releasing hormone (TRH)-stimulated secretion of prolactin (PRL) and thyrotropin-stimulating hormone (TSH) using enzymatically dispersed rat adenohypophyseal cells suspended in a perifusion chamber with a volume of 0.2 ml to minimize mixing and dilution. One-min exposure to 3–300 nMTRH, the effective dose range, elicited immediate pulses of PRL and TSH secretion with dose-dependent amplitudes. At all TRH concentrations, following a brief burst of secretion lasting < 1 min, release of both hormones declined precipitously. Increasing the duration of stimulation up to 30 min with half-maximal TRH concentrations did not alter the dynamics of the initial response and was ineffective in maintaining the initial amplitude of secretion. This phenomenon could not be attributed to exhaustion of readily releasable intracellular PRL and TSH, since an increment in TRH concentration elicited a second pulse of hormone secretion with temporal response characteristics identical to the first. The amplitude of the second pulse was dependent on both the initial concentration of TRH and the magnitude of the increment in TRH concentration. With a stepwise increase in TRH concentration during continuous perifusion, the sum of PRL or TSH secreted from all bursts of secretory activity approximated that achieved with a single exposure to the highest concentration of TRH employed. The high-amplitude secretory response to a given concentration of TRH was restored after an 8-min perifusion with medium alone. The data indicate that: (1) multiple pools of immediately secretable hormone, differing with respect to their TRH stimulation thresholds, exist for both PRL and TSH: each pool or ‘secretory unit’ has an individual threshold of response to TRH and is refractory to further stimulation by TRH after discharging its readily releasable hormone; (2) secretion of both PRL and TSH rapidly becomes refractory to continuous TRH stimulation, but is maintained by pulsatile TRH stimulation, and (3) the dynamics of PRL and TSH secretion by dispersed perifused rat adenohypophyseal cells are indistinguishable.

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