Postnatal growth in vertebrates is primarily regulated by growth hormone (GH) released by somatotrophs of the anterior pituitary. GH secretion by these cells is itself controlled by hypothalamic and peripheral peptide hormones. In many secretory cells, hormone secretion is tightly coupled to the cell's electrical activity, most often through voltage-sensitive calcium (Ca2+) channels. The membrane currents which underly electrical activity in somatotrophs, and the modulation of these channels by the specific peptide hormones have not been studied. This knowledge will be essential for an understanding of the physiological mechanisms which regulate GH secretion and growth in vertebrates. In the proposed studies, identified somatotrophs will be obtained for study in culture using either discontinuous gradient centrifugation or the reverse hemolytic plaque assay. These cells will be studied with standard intracellular and patch voltage clamp techniques.
The aims of the study will be: 1) to describe the electrical properties of cultured rat pituitary somatotrophs and to identify and characterize the underlying membrane currents: 2) to identify the ionic conductance changes produced in somatotrophs by the three peptide hormones which are known to regulate GH secretion in vivo. These peptides include growth hormone releasing factor (GRF), somastatin, and insulin-like growth factor-I (IGF-I); 3) to determine whether intracellular messengers such as cAMP, diacylglycerol, and GTP-binding proteins regulate channel activity in somatotrophs and, relatedly, if these molecules mediate peptide-induced conductance changes; 4) to find whether Ca2+ entry through voltage-sensitive channels can couple electrical activity to hormone synthesis in somatotrophs and other pituitary cells. The techniques used to identify somatotrophs and explore their ionic currents can be applied to other pituitary cells types and, presumably, to the peptide secreting cells of the hypothalamus, which control hormone release from the anterior pituitary. Thus, a description of the biophysical mechanisms by which these central elements reulate the function of the entire endocrine system seems feasible.
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