Tetraethylammonium blocks muscarinically evoked secretion in the sheep parotid gland by a mechanism additional to its blockade of BK channels

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Abstract
Since the secretory cells of the sheep parotid gland contain large numbers of high-conductance, voltage-and Ca2+-activated K+ channels (BK channels), we have used tetraethylammonium (TEA), a commonly employed blocker of BK channels, to investigate their role in secretion by this gland. In patch-clamp studies we found that 10 mmol/l TEA applied extracellularly inhibits the BK channel but not a 30-pS K+ channel also seen in this gland. We then showed by in-vivo perfusion that muscarinically evoked secretion is inhibited almost completely by 10 mmol/l TEA. We next used microspectrofluorimetry with fura-2 to demonstrate that muscarinic agonists cause the intracellular free Ca2+ concentration to increase. Unexpectedly, however, we found that 0.3–10 mmol/l TEA inhibited the increase in intracellular free Ca2+ induced by 5.0 μmol/l bethanechol or by 0.1 μmol/l acetylcholine. Consequently we conclude that the inhibition of muscarinically evoked secretion by the sheep parotid gland by TEA cannot be attributed solely to blockade of the BK channel — rather it must be attributed, at least in part, to blockade of some step in muscarinic signal transduction, for instance, receptor-agonist binding or Ca2+ release into the cytosol.