Multiple transduction mechanisms of dopamine, somatostatin and angiotensin II receptors in anterior pituitary cells.

The concept of multifactorial pituitary control is now well established. As in other cell systems, integration of complex messages involves dynamic interactions of receptors and coupling mechanisms. Regulation of adenohypophyseal secretions has been shown to involve cyclic AMP production, the modulation of phosphatidylinositol phosphate breakdown and Ca2+ mobilization. Dopamine, somatostatin and angiotensin II receptors are negatively coupled to adenylate cyclase in anterior pituitary cells. In the case of angiotensin, this effect on adenylate cyclase appears paradoxical since the peptide markedly stimulates prolactin secretion. In fact, angiotensin II also markedly stimulates inositol phosphate production and this effect could account for the stimulated hormone secretion. In addition, dopamine could inhibit inositol phosphate production stimulated by angiotensin II and thyrotropin-releasing hormone. Dopamine and somatostatin also directly modulate voltage-dependent calcium channels, perhaps through a direct coupling with potassium channels. On the other hand, steroids modulate the sensitivity of adenohypophyseal cells to neurohormones by different mechanisms. In the case of somatostatin, it increases the number of specific binding sites, while in the case of dopamine estradiol affects the transduction mechanisms of D2 dopamine receptors. In conclusion, dopamine and somatostatin receptors appear coupled to various transduction mechanisms through pertussis-sensitive G proteins in anterior pituitary cells.