MECHANISMS OF ACTION OF SOMATOSTATIN ON HUMAN TSH-SECRETING ADENOMA CELLS

The mechanisms of somatostatin (SRIH) action on thyroid-stimulating ho rmone (TSH) secretion were examined using human TSH-secreting adenoma cells. SRIH (10(-7) M) inhibited TSH secretion through a pertussis tox in-sensitive G protein. SRIH also inhibited forskolin- and 8-bromo-ade nosine 3',5'-cyclic monophosphate (8-BrcAMP)-induced TSH secretion. Th e mechanisms of this inhibition were investigated by measuring intrace llular Ca2+ concentration ([Ca2+](i)) and by electrophysiological expe riments. Application of 10(-7) M SRIH reduced the [Ca2+](i), whereas f orskolin and 8-BrcAMP increased the [Ca2+](i). Simultaneous applicatio n of SRIH abolished the forskolin- and the 8-BrcAMP-induced [Ca2+](i) increase, indicating that the SRIH-induced decrease in [Ca2+](i) was i ndependent of the reduction in intracellular cAMP. Under current clamp using the whole cell clamp, 10(-7) M SRIH hyperpolarized the membrane and arrested Ca2+-dependent action potentials, which accounted for th e SRIH-induced decrease in [Ca2+](i). Voltage clamp experiments reveal ed that this membrane hyperpolarization resulted from the activation o f an inward-rectifying K+ current through a pertussis toxin-sensitive G protein. Intracellular injection of cAMP (100 mu M) through the patc h pipette did not abolish the SRIH-induced K+ current, indicating that the activation of SRIH-induced K+ channels was independent of intrace llular cAMP. From these data, we concluded that SRIH-induced membrane hyperpolarization was responsible for the [Ca2+](i) decrease, which in turn inhibited TSH secretion. Application of thyrotropin-releasing ho rmone (TRH; 10(-7) M) caused an increase in the [Ca2+](i), composed of an initial transient increase followed by a sustained increase. SRIH inhibited the sustained increase in [Ca2+](i). SRIH also inhibited the TRH-induced decrease in the membrane conductance. These phenomena app ear to explain the inhibition of TRH-induced TSH secretion by SRIH.