A choline-evoked [Ca2+]c signal causes catecholamine release and hyperpolarization of chromaffin cells.
In bovine chromaffin cells fast-superfused with Krebs-HEPES solution containing 1-2 mM Ca2+, 5 s pulses of choline (1-10 mM), elicited catecholamine secretory responses that were only approximately 10% of those evoked by ACh (0.01-0.1 mM). However, in high-Ca2+ solutions (10-20 mM) the size of the choline secretory responses approached those of ACh. The choline responses (10 mM choline in 20 mM Ca2+, 10Cho/20Ca2+) tended to decline upon repetitive pulsing, whereas those of ACh were well maintained. The confocal [Ca2+]c increases evoked by 10Cho/20Ca2+ were similar to those of ACh. Whereas 10Cho/20Ca2+ caused mostly hyperpolarization of chromaffin cells, 0.1ACh/20 Ca2+ caused first depolarization and then hyperpolarization; in regular solutions (2 mM Ca2+), the hyperpolarizing responses did not show up. In Xenopus oocytes injected with mRNA for bovine alpha7 nicotinic receptors (nAChRs), 10Cho/20 Ca2+ fully activated an inward current; in oocytes expressing alpha3beta4, however, the inward current elicited by choline amounted to only 4% of the size of alpha7 current. Our results suggest that choline activates the entry of Ca2+ through alpha7 nAChRs; this leads to a cytosolic concentration of calcium ([Ca2+]c) rise that causes the activation of nearby Ca2+-dependent K+ channels and the hyperpolarization of the chromaffin cell. This response, which could be unmasked provided that cells were stimulated with high-Ca2+ solutions, may be the underlying mechanism through which choline exerts a modulatory effect on the electrical activity of the chromaffin cell and on neurotransmitter release at cholinergic synapses.
Fuentealba
Jorge
J
Instituto Teófilo Hernando y Departamento de Farmacología y Terapéutica, Facultad de Medicina, Universidad Autónoma de Madrid, Madrid, Spain.
Olivares
Román
R
Alés
Eva
E
Tapia
Laura
L
Rojo
Jonathan
J
Arroyo
Gloria
G
Aldea
Marcos
M
Criado
Manuel
M
Gandía
Luis
L
García
Antonio G
AG
D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D001285:Atropine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D020013:Calcium Signaling / Q000187:drug effects*; D002395:Catecholamines / Q000378:metabolism*; D002417:Cattle; D002794:Choline / Q000494:pharmacology*; D019439:Chromaffin Cells / Q000187:drug effects* / Q000378:metabolism*; D004553:Electric Conductivity; D008464:Mecamylamine / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects*; D009865:Oocytes / Q000187:drug effects / Q000378:metabolism; D011188:Potassium / Q000494:pharmacology; D011978:Receptors, Nicotinic / Q000235:genetics / Q000378:metabolism; D014982:Xenopus laevis
D002395:Catecholamines; D011978:Receptors, Nicotinic; D008464:Mecamylamine; D001285:Atropine; D002794:Choline; D000109:Acetylcholine; D011188:Potassium; D002118:Calcium