Permeation by zinc of bovine chromaffin cell calcium channels: relevance to secretion.
Zn2+ increased the rate of spontaneous release of catecholamines from bovine adrenal glands. This effect was Ca2+ independent; in fact, in the absence of extracellular Ca2+, the secretory effects of Zn2+ were enhanced. At low concentrations (3-10 microM), Zn2+ enhanced the secretory responses to 10-s pulses of 100 microM 1,1-dimethyl-4-phenylpiperazinium (DMPP, a nicotinic receptor agonist) or 100 mM K+. In the presence of DMPP, secretion was increased 47% above controls and in high-K+ solutions, secretion increased 54% above control. These low concentrations of Zn2+ did not facilitate the whole-cell Ca2+ (ICa) or Ba2+ (IBa) currents in patch-clamped chromaffin cells. Higher Zn2+ concentrations inhibited the currents (IC50 values, 346 microM for ICa and 91 microM for IBa) and blocked DMPP- and K(+)-evoked secretion (IC50 values, 141 and 250 microM, respectively). Zn2+ permeated the Ca2+ channels of bovine chromaffin cells, although at a much slower rate than other divalent cations. Peak currents at 10 mM Ba2+, Ca2+, Sr2+ and Zn2+ were 991, 734, 330 and 7.4 pA, respectively. Zn2+ entry was also evidenced using the fluorescent Ca2+ probe fura-2. This was possible because Zn2+ causes an increase in fura-2 fluorescence at the isosbestic wave-length for Ca2+, i.e. 360 nm. There was a slow resting entry of Zn2+ which was accelerated by stimulation with DMPP or high-K+ solution. The entry of Zn2+ was concentration dependent, slightly antagonized by 1 mM Ca2+ and completely blocked by 5 mM Ni2+. The entry of Ca2+ evoked by depolarization with high-K+ solution was antagonized by Zn2+.(ABSTRACT TRUNCATED AT 250 WORDS)
Vega
M T
MT
Departamento de Farmacología, Facultad de Medicina, Universidad Autónoma de Madrid, Spain.
Villalobos
C
C
Garrido
B
B
Gandía
L
L
Bulbena
O
O
García-Sancho
J
J
García
A G
AG
Artalejo
A R
AR
D000818:Animals; D001464:Barium / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D015220:Calcium Channels / Q000502:physiology*; D002395:Catecholamines / Q000378:metabolism; D002417:Cattle; D002463:Cell Membrane Permeability; D002838:Chromaffin System / Q000378:metabolism* / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D013324:Strontium / Q000494:pharmacology; D015032:Zinc / Q000378:metabolism*
D015220:Calcium Channels; D002395:Catecholamines; D001464:Barium; D015032:Zinc; D002118:Calcium; D013324:Strontium