Pardaxin induces exocytosis in bovine adrenal medullary chromaffin cells independent of calcium.
Pardaxin, an excitatory neurotoxin, is a new tool for studying the machinery of neurotransmitter secretion. At noncytotoxic concentrations (< 1 x 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 (> 2 x 10(-5) M), pardaxin was increasingly cytotoxic, as inferred from trypan blue uptake, release of lactate dehydrogenase and 51Cr (ED50 = 100 microM). 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 (ED50 = 1 microM). 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.
Lazarovici
P
P
Department of Pharmacology and Experimental Therapeutics, School of Pharmacy, Faculty of Medicine, Hebrew University of Jerusalem, Israel.
Lelkes
P I
PI
D000255:Adenosine Triphosphate / Q000378:metabolism; D000313:Adrenal Medulla / Q000187:drug effects* / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D002118:Calcium / Q000502:physiology*; D002395:Catecholamines / Q000378:metabolism; D002417:Cattle; D002837:Chromaffin Granules / Q000378:metabolism; D004299:Dopamine beta-Hydroxylase / Q000378:metabolism; D005089:Exocytosis / Q000187:drug effects*; D005397:Fish Venoms / Q000494:pharmacology*; D008969:Molecular Sequence Data; D009498:Neurotoxins / Q000494:pharmacology*
D002395:Catecholamines; D005397:Fish Venoms; D009498:Neurotoxins; C017603:pardaxin; D000255:Adenosine Triphosphate; D004299:Dopamine beta-Hydroxylase; D002118:Calcium