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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

Ca²⁺ pathway involved in the refilling of store sites in rat adrenal medullary cells

¹Department of Cell and Systems Physiology and ²Department of Anesthesiology, School of Medicine, University of Occupational and Environmental Health, Kitakyushu, Japan

Submitted 26 August 2008 ; accepted in final form 21 January 2009

It has been suggested that store-operated Ca²⁺ entry (SOC) facilitates catecholamine secretion and synthesis in bovine adrenal medullary (AM) cells. However, there has been no experimental result clearly showing that cation channel activity is enhanced by store Ca²⁺ depletion. Thus the present experiments were undertaken to address the issue of whether rat AM cells have SOC channels. Inhibition of the sarco(endo)plasmic reticulum Ca²⁺ (SERCA) pump resulted in a sustained increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) in rat AM cells. This increase was completely suppressed by 2 mM Ni²⁺ but not by 100 µM D600. A bath application of Ni²⁺, but not D600, produced an outward current at –60 mV in rat AM cells, whereas exposure to a SERCA pump inhibitor did not affect either the whole cell current level or the Ni²⁺-induced outward current. The refilling of intracellular store sites was suppressed by the addition of Ni²⁺ to the perfusate. RT-PCR revealed that transcripts for transient receptor potential channels 1 (TRPC1) and 5 (TRPC5) were present in rat adrenal medullas. Immunocytochemistry showed that TRPC1 channels, which have been implicated in SOC in certain types of cells, were mainly localized in the endoplasmic reticulum (ER) and not in the plasma membrane, and that STIM1, a Ca²⁺ sensor in the ER, was not expressed in rat AM cells. On the basis of these results, we conclude that rat AM cells lack the SOC mechanism.

calcium store sites; transient receptor potential channel; endoplasmic reticulum