J Membr Biol 1999 Jan 01;1671:25-33. doi: 10.1007/s002329900468.

A calcium homeostasis mechanism induced by heterologous expression of total RNA from chicory leaves in Xenopus oocytes.

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Xenopus oocytes were injected with total RNA from chicory leaf tissues and then examined by the voltage-clamp technique. A double-step voltage protocol was used, with an initial hyperpolarization step from the holding potential of -35 to -140 mV followed by a second depolarization step to +60 mV. Two different outward currents were observed, one noninactivating (Ini), and one inactivating (Ii). Only the noninactivating outward current (Ini) could be induced by depolarization from -35 to +60 mV. The mean amplitude of Ini was 2915 +/- 848 nA (n = 11). This current, carried by chloride ions, declined nearly to the baseline in 153 +/- 64 sec (n = 13), and was highly dependent on intracellular calcium. After the rundown of Ini, the same oocyte was depolarized from -140 to +60 mV. This protocol induced an inactivating outward current (Ii) with a mean amplitude of 4461 +/- 1605 nA (n = 13). Ii was also carried by chloride ions and dependent on extracellular calcium. Ii was strongly inhibited by 100 micron extracellular La3+. These two types of chloride currents were also observed after IP3 injection in control oocytes. Ini and Ii were not observed in noninjected oocytes or water-injected oocytes. We suggest that the expression of total chicory leaf tissue RNA in Xenopus oocytes reveals a calcium homeostasis mechanism responsible for calcium mobilization from internal stores and subsequent calcium entry.

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