Characterization of hypothalamic low-voltage-activated Ca channels based on their functional expression in Xenopus oocytes.
Ca-channel currents expressed in Xenopus oocytes by means of messenger RNA extracted from rat thalamohypothalamic complex were studied using the double microelectrode technique. Currents were recorded in Cl(-)-free extracellular solutions with 40 mM Ba2+ as a charge carrier. In response to depolarizations from a very negative holding potential (Vh = -120 mV), inward Ba2+ current activated at around -80 mV, peaked at -30 to -20 mV and reversed at +50 mV indicating that it may be transferred through the low voltage-activated calcium channels. The time-dependent inactivation of the current during prolonged depolarization to -20 mV was quite slow and followed a single exponential decay with a time-constant of 1550 ms and a maintained component constituting 30% of the maximal amplitude. The current could not be completely inactivated at any holding potential. As expected for low voltage-activated current, steady-state inactivation curve shifted towards negative potentials. It could be described by the Boltzmann equation with half inactivation potential -78 mV, slope factor 15 mV and maintained level 0.3. Expressed Ba2+ current could be blocked by flunarizine with Kd = 0.42 microM, nifedipine, Kd = 10 microM, and amiloride at 500 microM concentration. Among inorganic Ca-channel blockers the most potent was La3+ (Kd = 0.48 microM) while Cd2+ and Ni2+ were not very discriminative and almost 1000-fold less effective than La3+ (Kd = 0.52 mM and Kd = 0.62 mM, respectively). Our data show that messenger RNA purified from thalamohypothalamic complex induces expression in the oocytes of almost exclusively low voltage-activated calcium channels with voltage-dependent and pharmacological properties very similar to those observed for T-type calcium current in native hypothalamic neurons, though kinetic properties of the expressed and natural currents are somewhat different.
Dzhura
I O
IO
A. A. Bogomoletz Institute of Physiology, National Academy of Sciences of Ukraine, Kiev.
Naidenov
V G
VG
Lyubanova
O P
OP
Kostyuk
P G
PG
Shuba
Y M
YM
D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000235:genetics / Q000502:physiology*; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D005260:Female; D007031:Hypothalamus / Q000378:metabolism*; D007267:Injections; D007700:Kinetics; D009865:Oocytes / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics; D051381:Rats; D014982:Xenopus laevis / Q000378:metabolism
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D012333:RNA, Messenger