Calcium-Induced Modulation of the Tubulin Pool in Parathyroid Glands
Participation of cytoplasmic microtubules has been implicated in various secretory processes such as release of hormone from secretory cells and secretion or movement of granular products (see ref. 1, 2 for review). The great bulk of these works is based on the inhibitory effects of colchicine (3) and vinca-alkaloids (4). The mechanism of action has been attributed to the ability of these agents to bind to tubulin, the protein subunit of the cellular microtubules, and as a consequence, to prevent microtubule assembly. With these techniques it has been shown that colchicine and vinca-alkaloids may affect Parathyroid Hormone (PTH) secretion in in vitro culture of Parathyroid Glands (PTG) (5, 6). It has been suggested that these effects may indicate a role of microtubules in the sequence of events leading to release of PTH. Nevertheless although ultrastructural studies of PTG have demonstrated the presence of microtubules (6, 7, 8, 9), anatomical evidence to support a mechanism by which microtubules might affect intracellular processing of PTH are rare. On the other hand in in vivo studies the precise site of action of colchicine is difficult to ascertain because it has been shown that this product may produce hypocalcemia (10) and may inhibit bone resorption (11). Moreover recent studies have shown that the renal handling of phosphate and its regulation by PTH may depend on cytoplasmic microtubules, which have been described in proximal renal tubule cells (12). To further assess the presence of a microtubular system (MTS) in PTG and its possible functional significance, we initiated the following studies which extend previous work in our laboratory (13) in order to demonstrate tubulin in PTG on a biochemical basis. After demonstration of a specific Colchicine Binding Protein (Co1BP) in PTG, the tubulin pool size level in PTG slices incubated in vitro in various extracellular calcium concentrations was assayed.
KeywordsParathyroid Hormone Parathyroid Gland Extracellular Calcium Concentration Precise Site Cytoplasmic Microtubule
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