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Department of Physiology, St George's Hospital Medical School, London, UK.

1. Voltage-gated Ca2+ currents (ICa) and Ca(2+)-activated Cl- currents (ICl(Ca)) were recorded from cultured rat dorsal root ganglion (DRG) neurones using the whole-cell configuration of the patch clamp technique. Intracellular photorelease of Ca2+ by flash photolysis of DM-nitrophen elicited transient inward currents only in those cells which possessed Ca(2+)-activated Cl- tail currents following ICa. The reversal potential of the flash responses was hyperpolarized when extracellular Cl- was replaced by SCN-. The flash responses and the Ca(2+)-activated Cl- tail currents were inhibited by the Cl- channel blockers niflumic acid (10-100 microM) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) (10 microM). 2. After activation by ICa, the Ca(2+)-activated Cl- current could be reactivated during its decay by photorelease of caged Ca2+. Experiments carried out on neurones held at 0 mV demonstrated that ICl(Ca) could be chronically activated due to residual Ca2+ influx. These data directly demonstrated that the decay of ICl(Ca) is not due to inactivation but rather to deactivation as a result of removal of the Ca2+ load from the cell cytoplasm. 3. Photorelease of caged inositol 1,4,5-trisphosphate (IP3) failed to activate any Ca(2+)-dependent current responses in cultured DRG neurones, although application of caffeine elicited transient inward currents, and responses to photoreleased IP3 could be obtained from freshly dissociated smooth muscle cells. 4. Photorelease of Ca2+ provides a useful method for investigating the properties of ICl(Ca) independently from other physiological parameters. In addition, we have directly demonstrated that ICl(Ca) in DRG neurones does not inactivate, and so may continue to modulate membrane excitability as long as the intracellular Ca2+ concentration ([Ca2+]i) close to the cell membrane is elevated.(ABSTRACT TRUNCATED AT 250 WORDS)

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