Ca²⁺Transients Induced by Thyrotropin-Releasing Hormone Rapidly Lose Their Ability to Cause Release of Prolactin

To investigate the relationship of changes in cytosolic free calcium concentrations ([Ca2+]c) caused by TRH to changes in PRL secretion, we simultaneously monitored PRL release and [Ca2+]c, using the fluorescent Ca2+ indicator indo-1, in freshly isolated perifused cells from rat anterior pituitary glands. We found that a 30-sec pulse of 100 nm TRH triggered a transient spike of [Ca2+]c, but prolonged PRL release for up to 30 min; continuous administration of TRH caused a sustained elevation in [Ca2+]c, but the same pattern and amount of PRL release as that caused by the pulse of TRH. PRL secretion was refractory to further pulses of TRH given at 10-min intervals for 40 min, but did respond to a second pulse of TRH given 40 min after the first pulse with no intervening pulses. Pulses of TRH given every 10 min still triggered spikes of [Ca2+]c of the same magnitude as the first pulse, indicating that the cause of the refractory state must occur at a postreceptor step that is after the mobilization of [Ca2+]c. A 30-sec pulse of a high concentration of KCI caused a transient spike of [Ca2+]c and transient, not prolonged, release. Additional pulses of KCI cause progressively less PRL release, although the magnitude of the spikes in [Ca2+]c did not change. We conclude that a pulse of TRH must induce a change other than a [Ca2+]c spike to cause prolonged stimulation of PRL secretion, and that administration of continuous TRH or of frequent pulses maintains the cells in a refractory state in which they have no additional response to spikes of [Ca2+]c of the same magnitude. Spikes in [Ca2+]c alone will not cause prolonged secretion, but may contribute to developing the refractory state.