Ca2+ influx in resting rat sensory neurones that regulates and is regulated by ryanodine-sensitive Ca2+ stores.
1. Store-operated, voltage-independent Ca2+ channels are activated by depletion of intracellular Ca2+ stores and mediate Ca2+ influx into non-excitable cells at resting membrane potential. We used microfluorimetry, patch-clamp and Mn2+-quench techniques to explore the possibility that a similar mechanism exists in rat dorsal root ganglion (DRG) neurones in primary culture. 2. Following caffeine-induced depletion, ryanodine-sensitive Ca2+ stores refilled with Ca2+ at resting membrane potential. The refilling process required extracellular Ca2+, was blocked by 2 mM Ni2+, and was facilitated by membrane hyperpolarization from -55 to -80 mV, indicating a key role for Ca2+ influx. This influx of Ca2+ was not affected by the voltage-operated Ca2+ channel (VOCC) antagonists nicardipine (10 microM), nimodipine (10 microM) or omega-grammotoxin SIA (1 microM). 3. When ryanodine-sensitive Ca2+ stores were depleted in Ca2+-free media, a return to 2 mM external Ca2+ resulted in a pronounced [Ca2+]i overshoot, indicating an increased permeability to Ca2+. Depletion of Ca2+ stores also produced a 2-fold increase in the rate of Mn2+ influx. The [Ca2+]i overshoot and Mn2+ entry were both inhibited by Ni2+, but not by VOCC antagonists. 4. Caffeine induced periodic Ca2+ release from, and reuptake into, ryanodine-sensitive stores. The [Ca2+]i oscillations were arrested by removal of extracellular Ca2+ or by addition of Ni2+, but they were not affected by VOCC antagonists. Hyperpolarization increased the frequency of this rhythmic activity. 5. These data suggest the presence of a Ca2+ entry pathway in mammalian sensory neurones that is distinct from VOCCs and is regulated by ryanodine-sensitive Ca2+ stores. This pathway participates in refilling intracellular Ca2+ stores and maintaining [Ca2+]i oscillations and thus controls the balance between intra- and extracellular Ca2+ reservoirs in resting DRG neurones.
Usachev
Y M
YM
Department of Pharmacology, University of Minnesota Medical School, 3-249 Millard Hall, 435 Delaware Street SE, Minneapolis, MN 55455, USA.
Thayer
S A
SA
D000200:Action Potentials / Q000187:drug effects / Q000502:physiology; D000818:Animals; D000831:Animals, Newborn; D001692:Biological Transport; D002110:Caffeine / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002478:Cells, Cultured; D005727:Ganglia, Spinal / Q000166:cytology / Q000502:physiology*; D008564:Membrane Potentials; D009475:Neurons, Afferent / Q000166:cytology / Q000502:physiology*; D009529:Nicardipine / Q000494:pharmacology; D009553:Nimodipine / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D010456:Peptides, Cyclic / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012433:Ryanodine / Q000494:pharmacology*; D019837:Ryanodine Receptor Calcium Release Channel / Q000502:physiology*
D002121:Calcium Channel Blockers; D010456:Peptides, Cyclic; D019837:Ryanodine Receptor Calcium Release Channel; C082807:omega-grammotoxin SIA; D012433:Ryanodine; D002110:Caffeine; D009553:Nimodipine; D009529:Nicardipine; D002118:Calcium