The Effects of Thyroid Hormone Deprivationin Vivoandin Vitroon Growth Hormone (GH) Responses to Human Pancreatic (Tumor) GH-Releasing Factor (1-40) by Dispersed Rat Anterior Pituitary Cells*

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Abstract
Anterior pituitary cells from euthyroid and hypothyroid male rats were cultured as monolayers for 3 days with or without 5 nM T3 [thyroxine] and stimulated with either human pancreatic GH [growth hormone]-releasing factor 1-40 (hpGRF), TRH or the Ca2+ channel ionophore A23187 [calcimycin]. Basal GH secretion was reduced in the hypothyroid cultures (P < 0.001) and basal TSH secretion increased (P < 0.001). Culture with T3 increased GH secretion and intracellular GH content in euthyroid and hypothyroid cultures but suppressed TSH secretion with no effect on intracellular TSH content in either euthyroid or hypothyroid cultures. hpGRF released more GH from euthyroid [3.52 .+-. 0.2 (SE) .mu.g/6 h .cntdot. 105 cells] than hypothyroid cultures of (0.17 .+-. 0.01 .mu.g/6 h .cntdot. 105 cells, P < 0.001) without a change in ED50 (.apprx. 0.02 nM). The reduction in hpGRF-induced GH release remained significant when corrected for the reduced intracellular GH content in the hypothyroid cultures. hpGRF-induced GH release also declined relative to A23187-induced GH release in hypothyroid cultures. Culture with 5 nM T3 doubled maximum hpGRF-induced GH release in euthyroid cultures and increased maximum release 10-fold in hypothyroid cultures without altering the ED50 of hpGRF action. T3 suppressed TRH-induced TSH release in euthyroid cultures but was without effect on TRH-induced TSH release in the hypothyroid cultures. T3 did not effect the ED50 of TRH action (2-5 nM). Hypothyroid rat anterior pituitary cells in culture have a reduced maximal GH response to hpGRF, but the same ED50. hpGRF activity can be partially restored by physiological concentrations of T3 in vitro.

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