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VOL. 71 (1), 127-151, 2005 LA MAQUINARIA MOLECULAR DE LA EXOCITOSIS...

Chromaffin cells from the adrenal medulla are neural crest derivatives that
synthesize and store catecholamines inside large dense-core granules, the chromaf-
fin granules. Catecholamines are release into the blood stream by a typically exo-
cytotic mechanism, with a well-characterised intracellular Ca2+ dependence. Simi-
larly to other neuroendocrine cells, adrenomedullary chromaffin cells express most
of the proteins that have been involved in synaptic exocytosis. Chromaffin cells are
a simple cellular model widely used in neurosecretion research due to the ease of
culture from normal and also from genetically modified animals, and their amena-
bility to the employment of biophysical techniques for the study of exocytosis with
millisecond resolution. These techniques include the patch-clamp, which allows
the measurement of cell capacitance changes as an assay of exocytosis, and flash
photolysis of caged-Ca2+ compounds (DM-nitrophen, nitrophenil-EGTA), which is
able to instantaneously increase (< 1 ms) cytosolic Ca2+ in the cell in a spatially
homogeneous manner. The chromaffin cell exocytotic response to photorelease of
Ca2+ (~ 20 µ M) comprises two kinetically different components: a phasic compo-
nent, occurring during the first second after intracellular Ca2+ elevation, and a
tonic or sustained component that follows the phasic one over tens of seconds.
Phasic release would result from the exocytosis of mature vesicles whereas the
sustained component would reflect the maturation process (priming) and the sub-
sequent exocytosis of vesicles as long as Ca2+ remains elevated. By using specific
reagents (clostridial toxins, antibodies, peptides, etc.) against the proteins of the
secretory machinery it is possible to interfere selectively with each of these two
components, thereby relating vesicle pools in distinct functional states with certain
synaptic proteins. Recent evidence involving SNARES, Munc18-1 and synaptotag-
min I in specific steps of the exocytotic process from chromaffin cells is summa-
rised at the end of this review.

INTRODUCCIÓN

La comunicación intercelular es la base de la integración funcio-
nal en los organismos superiores. Este proceso emplea mensajeros
extracelulares —hormonas y neurotransmisores— que son liberados
por las células en respuesta a múltiples estímulos. Un ejemplo para-
digmático de comunicación intercelular es el que acontece en el
tejido nervioso. La comunicación entre sus células parenquimatosas,
las neuronas, tiene lugar en regiones especializadas denominadas
sinapsis, en las que la terminación nerviosa de una neurona entra en
contacto con la membrana de otra. En las terminaciones nerviosas
el neurotransmisor se almacena en cantidades discretas, denomina-
das cuantos, en el interior de vesículas membranosas desde las que
accede a la hendidura sináptica mediante el mecanismo de la exoci-

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