POTENTIATION OF K+‐EVOKED CATECHOLAMINE RELEASE IN THE CAT ADRENAL GLAND TREATED WITH OUABAIN

1 A vigorous catecholamine secretory response was evoked by small increments (2–10 mM) of the extracellular concentration of K+ ([K+])0) in cat adrenal glands treated with ouabain (10−4m), and perfused with Krebs‐bicarbonate solution at room temperature. 2 The secretory response depends on [K+]0; increments of [K+]0 as small as 2 mM for 2 min evoked a clear secretory response; at 10–17.7 mM K+, the maximal secretory response was observed. In normal glands, not treated with ouabain, no increase of the rate of catecholamine output was observed by raising [K+]0 up to 17.7 mM for 2 min. 3 The K+ secretory response was time‐dependent, requiring at least 1 min to be initiated; on continued exposure to 10 mM [K+]0, the enhanced response remained for at least 1 h. 4 In low [Na+]0, the K+‐secretory response was unchanged. However, in 0‐Ca2+, high‐Mg2+ solutions, or in the presence of D600, an organic Ca2+ antagonist, it was abolished. 5 The K+‐induced secretory response was not altered in the presence of tetrodotoxin or tetraethylammonium. 6 It is concluded that ouabain potentiated the catecholamine secretory response to raised [K+]D by increasing the amount of Ca2+ available to the secretory machinery through (a) mobilization of an enhanced pool of membrane‐bound Ca2+, (b) activation of membrane Ca2+ inward current; or (c) decrease of intracellular Ca2+ buffering systems. The activation by ouabain of a membrane Na+‐Ca2+ exchange system is not involved in this K+‐secretory response. It is suggested that the plasma membrane ATPase enzyme system, by changing the affinity of its Ca2+ binding sites, might control the availability of this cation to the secretory machinery and, therefore, modulate catecholamine secretion in the adrenal gland.