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

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 depo larization step to +60 mV. Two different outward currents were observed, on e noninactivating (I-ni), and one inactivating (I-i). Only the noninactivat ing outward current (I-ni) could be induced by depolarization from -35 to 60 mV. The mean amplitude of I-ni was 2915 +/- 848 nA (n = 11). This curren t, 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 I-ni, the same oocyte was depolarized from -140 to +60 mV. This protocol induced an inactivating outward current (I-i) with a mean amplitud e of 4461 +/- 1605 nA fn = 13). I-i was also carried by chloride ions and d ependent on extracellular calcium. I-i was strongly inhibited by 100 mu M e xtracellular La3+. These two types of chloride currents were also observed after IP3 injection in control oocytes. I-ni and I-i 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 mob ilization from internal stores and subsequent calcium entry.