Expression of low voltage-activated calcium channels inXenopus oocytes

Calcium channels were expressed inXenopus oocytes by means of messenger RNA extracted from the rat thalamo-hypothalamic complex, mRNA(h). Inward barium currents,I _Ba, were recorded in Cl^–-free extracellular solution with 40 mM Ba²⁺ as a charge carrier, using two-microelectrode technique. Depolarizations from a very negative holding potential (V _h=–120 mV) began to activateI _Ba at about –80 mV; this current peaked at –30 to –20 mV and reversed at +50 mV, indicating that I _Ba may be transferred through the low voltage-activated (LVA) calcium channels. The time-dependent inactivation of the current during a prolonged depolarization to –20 mV was quite slow, followed a single exponential decay with a time constant of 1550 msec, and contained a residual component constituting 30% of the maximum amplitude. The current could not be completely inactivated at any holding potential. As expected for LVA current, a steady-state inactivation curve was shifted towards negative potentials. It could be described by the Boltzmann's equation with the half-inactivation potential of –78 mV, slope factor of 15 mV, and residual level of 0.3. ExpressedI _Ba could be blocked by flunarizine (K _d=0.42 µM), nifedipine (K _d=10 µM), and amiloride at a 500 µM concentration. Among the inorganic Ca²⁺ channel blockers, the most potent was La³⁺ (K _d=0.48 µM), while Cd²⁺ and Ni²⁺ were not very selective and almost thousand-fold less effective (K _d=0.52 mM andK _d=0.62 mM, respectively) than La³⁺. Our data show that mRNA(h) induces expression in the oocytes of almost exclusively LVA Ca²⁺ channels with voltage-dependent and pharmacological properties very similar to those observed for T-type Ca²⁺ current in native hypothalamic neurons, though kinetic properties of the expressed and natural currents are somewhat different.Neirofiziologiya/Neurophysiology, Vol. 27, No. 3, pp. 183–189, May–June, 1995.