Dual action of thapsigargin on calcium mobilization in sensory neurons: inhibition of Ca2+ uptake by caffeine-sensitive pools and blockade of plasmalemmal Ca2+ channels.
The action of thapsigargin on intracellular calcium homeostasis and voltage-activated calcium currents was studied on freshly isolated adult mouse dorsal root ganglia neurons. The cytoplasmic Ca2+ concentration ([Ca2+]i) was measured using indo-1-based microfluorimetry; transmembrane Ca2+ currents were recorded under voltage-clamp in the whole-cell configuration of the patch-clamp technique. Extracellular applications of thapsigargin at concentrations of 20-2000 nM did not cause substantial changes of basal [Ca2+]i level in the majority of neurons studied. However, 5-10 min incubation of neurons with 20 nM thapsigargin completely and almost irreversibly inhibited caffeine-mediated Ca2+ release from intracellular pools. This inhibition was associated with deceleration of the recovery of depolarization-induced [Ca2+]i transients, presumably due to the inhibition of Ca2+ uptake by intracellular calcium stores. At concentrations between 200 and 2000 nM, thapsigargin markedly depressed the amplitudes of depolarization-triggered [Ca2+]i transients due to the inhibition of transmembrane Ca2+ entry through voltage-activated Ca2+ channels. We found that thapsigargin discriminates between low- and high-voltage-activated Ca2+ channels: 2000 nM of thapsigargin decreased the amplitudes of high-voltage-activated currents by 60%, while the amplitudes of low-voltage-activated Ca2+ currents were reduced by only 25%. Thus, thapsigargin exerts a dual action on [Ca2+]i handling mechanisms in mouse sensory neurons: at low concentrations (< 50 nM) it inhibits Ca2+ accumulation by endoplasmic reticulum pools, whereas at higher concentrations (200-2000 nM) thapsigargin blocks high-voltage-activated Ca2+ currents, reducing Ca2+ entry into the cell.
Shmigol
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A
Department of General Physiology of the Nervous System, Bogomoletz Institute of Physiology, Kiev, Ukraine.
Kostyuk
P
P
Verkhratsky
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A
D000818:Animals; D002110:Caffeine / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D005727:Ganglia, Spinal / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D009475:Neurons, Afferent / Q000187:drug effects / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D013729:Terpenes / Q000494:pharmacology*; D019284:Thapsigargin
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D013729:Terpenes; D002110:Caffeine; D019284:Thapsigargin; D000252:Calcium-Transporting ATPases; D002118:Calcium