Nanoscale Dynamics of Growth Hormone Secretion in Pigs
The anterior pituitary gland contains cells that produce and secrete growth hormone (GH) into the circulating blood that causes muscle accretion, lipolysis, and lean growth in the pig. GH-secretagogues, such as L- 692,585, stimulate GH release by action at the pituitary in a dose dependent manner in vivo, and by isolated porcine GH cells in culture. We used atomic force microscopy (AFM) to identify cytoplasmic structures at the plasma membrane of GH-secreting cells of the pituitary and implicate their involvement in hormone release. New cellular structures at the plasma membrane called “pits” and “depressions” were identified where membranebound secretory vesicles dock and fuse to release vesicular contents. Pits containing 100- to 200- nanometer in diameter depressions or “fusion pores” were identified in unstimulated (resting) GH-secreting cells. After stimulation of secretion with L-692,585, the size of depressions enlarged and gold-tagged GH antibody were found to bind to the pit structures in the stimulated GH cells. This study documents, for the first time, the presence of these structures and their involvement in hormone secretion in a neuroendocrine cell.