Single Cell Monitoring of Cytosolic Calcium Reveals Subtypes of Rat Lactotrophs with Distinct Responses to Dopamine and Thyrotropin-Releasing Hormone*

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Abstract
The cytosolic free calcium concentration, [Ca2+]i, was monitored in single rat lactotrophs in primary culture with the fluorescent probe Fura 2. It was found that lactotrophs are very heterogeneous in their [Ca2+]i response to TRH and dopamine, the major physiological regulators of PRL secretion. While in most lactotrophs TRH raises [Ca2+]i, the kinetics of this rise and the magnitude of its first and second phases vary considerably. For dopamine two clearly divergent response types can be observed. In part of the lactotrophs dopamine causes a lowering of [Ca2+]i from elevated levels, whereas in about 40% of the lactotrophs dopamine leads to a transient rise of [Ca2+]i. The present study reveals subclasses of lactotrophs with distinct [Ca2+]i response characteristics. It is suggested that such response type heterogeneity is a means of optimizing the secretory response to the complex regulatory influences on the pituitary.

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