We have developed a thin-slice preparation of whole rat carotid body that allows us to perform patch-clamp recording of membrane ionic currents and to monitor catecholamine secretion by amperometry in single glomus cells under direct visual control. In normoxic conditions (PO 2 Ϸ 140 mmHg; 1 mmHg ‫؍‬ 133 Pa), most glomus cells did not have measurable secretory activity, but exposure to hypoxia (P O 2 Ϸ 20 mmHg) elicited the appearance of a large number of spike-like exocytotic events. This neurosecretory response to hypoxia was fully reversible and required extracellular Ca 2؉ influx. The average charge of single quantal events was 46 ؎ 25 fC (n ‫؍‬ 218), which yields an estimate of Ϸ140,000 catecholamine molecules per vesicle. Addition of tetraethylammonium (TEA; 2-5 mM) to the extracellular solution induced in most (>95%) cells tested (n ‫؍‬ 32) a secretory response similar to that elicited by low PO 2 . Cells nonresponsive to hypoxia but activated by exposure to high external K ؉ were also stimulated by TEA. A secretory response similar to the responses to hypoxia and TEA was also observed after treatment of the cells with iberiotoxin to block selectively Ca 2؉ -and voltage-activated maxi-K ؉ channels. Our data further show that membrane ion channels are critically involved in sensory transduction in the carotid body. We also show that in intact glomus cells inhibition of voltage-dependent K ؉ channels can contribute to initiation of the secretory response to low P O 2 .carotid-body slice ͉ O2 sensing ͉ glomus cell secretion

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