UBC Theses and Dissertations
Phospholipids as adjuncts for chromaffin granule release Nayar, Rajiv
The chromaffin granules of adrenal medulla cells are membrane bound entities which act as storage vesicles for catecholamines, ATP, and protein. Release of contents to the extracellular medium appears to occur via calcium-stimulated exocytosis which involves fusion with the plasma membrane. The molecular mechanism of exocytosis was approached from two points of view. First, it was shown by ³¹P-NMR techniques that the endogenous phospholipids assume a liquid-crystalline configuration at physiological temperature both in the intact membrane and in model systems composed of the extracted lipid. This is consistent with a structural role of phospholipids in vivo, serving to maintain membrane integrity. Addition of calcium to these systems resulted in little change in the biological membrane spectra and in the appearance of a relatively small component (<10 %) , possibly arising from phospholipid in the hexagonal H₁₁ phase in the model systems composed of the isolated total lipids. Second, incubation of intact granules in the presence of upto 10 mM calcium did not cause significant release of contents. Similarily, incubation in the presence of exogenous lipid vesicles alone did not induce release. However, incubation with phospholipid systems, which have the ability to undergo structural transitions in the presence of calcium, followed by the introduction of calcium caused immediate and total release of granule contents. This behaviour is attributed to disruption of membrane integrity arising from calcium induced fusion of the phospholipid vesicles with the granules. In contrast, incubation with phospholipid systems which do not undergo structural transitions in the presence of calcium is quite ineffective. On the basis of this information, a mechanism of calcium-induced exocytotic release of catecholamines in vivo is proposed.
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