Abstract
This chapter presents the concept of the involvement of the microtubular-microfilamentous system in the intracellular transport of beta granules and the mechanism by which insulin is released from the beta cell. Glucose stimulates both the release and the synthesis of insulin in beta cells. Insulin release occurs within a few seconds after stimulation with glucose. The sequence of events in the formation of insulin are as follows: (1) proinsulin is formed within the endoplasmic reticulum and is transferred by an energy-requiring process to the Golgi complex, (2) distinct beta granules form within the Golgi complex and are pinched off into the cytoplasm, (3) the C peptide of proinsulin is removed in the Golgi area, and (4) zinc binds with the insulin in the mature beta granules, resulting in the formation of a crystalline matrix. Thus, the storage form of insulin in beta cells is the beta granule that resembles a microcrystal of zinc insulin. The beta granule is surrounded by a smooth membranous sac with a space between the granule and the encompassing membrane. Biochemical studies on microtubular protein isolated from the brain indicate that these structures contain an actin-like substance.
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