Pardaxin induces exocytosis in bovine adrenal medullary chromaffin cells independent of calcium
Lazarovici, P.; Lelkes, P.I.
Journal of Pharmacology and Experimental Therapeutics 263(3): 1317-1326
1992
ISSN/ISBN: 0022-3565 PMID: 1469638 Document Number: 395920
Pardaxin, an excitatory neurotoxin, is a new tool for studying the machinery of neurotransmitter secretion. At noncytotoxic concentrations ( lt 1 times 10-5 M), pardaxin stimulated exocytosis, as assessed by the concomitant release of catecholamines, ATP and dopamine-beta-hydroxylase from bovine adrenal medullary chromaffin cells in the presence or absence of extracellular calcium. At higher concentrations ( gt 2 times 10-5 M), pardaxin was increasingly cytotoxic, as inferred from trypan blue uptake, release of lactate dehydrogenase and 51Cr (ED-50 = 100 mu-M). The role of intracellular calcium ((Ca++)-i) homeostasis in pardaxin action was investigated by using the fluorescent calcium indicator Fura-2. In the presence of extracellular calcium, addition of noncytotoxic concentrations of pardaxin yielded a steady, concentration-dependent rise in (Ca++)-i (ED-50 = 1 mu-M). Depolarization of chromaffin cells by high K+ reduced pardaxin binding and abolished the pardaxin-evoked rise in (Ca++)-i. In the absence of extracellular calcium, pardaxin failed to elicit an elevation of (Ca++)-i. These data suggest that, in the presence of extracellular calcium, pardaxin might cause elevations in (Ca++)-i and neurotransmitter release, concomitant with inducing transmembranal Ca++ influx. However, the complex concentration dependence of (Ca++)-i and the fact that pardaxin stimulated secretion without a rise of (Ca++)-i suggest that the toxin, in addition to being a pore-forming molecule, might directly affect exocytosis in a Ca++-independent way. In proposing a pharmacological working model, we hypothesize that pardaxin might present a molecular structure which mimics an essential step in the endogenous docking mechanism between secretory granules and the plasma membrane.