Ouabain distinguishes between nicotinic and muscarinic receptor‐mediated catecholamine secretions in perfused adrenal glands of cat

1 The effect of ouabain on catecholamine (adrenaline and noradrenaline) secretion induced by agents acting on cholinoceptors was studied in perfused cat adrenal glands. Acetylcholine (ACh) (5 × 10 −7 to 10 −3 m), pilocarpine (10 −5 to 10 −3 m ) and nicotine (10 −6 to 5 × 10 −5 m ) caused dose‐dependent increases in catecholamine secretion. Both ACh and nicotine released more noradrenaline than adrenaline and the reverse was the case for pilocarpine. 2 Ouabain (10 −5 m ) enhanced catecholamine secretion induced by ACh (10 −5 m), pilocarpine (10 −3 m ) and nicotine (3 × 10 −6 m ) during perfusion with Locke solution. The ratio of adrenaline to noradrenaline was not affected by ouabain. 3 In the absence of extracellular Ca 2+ , ACh and pilocarpine, but not nicotine, still caused a small increase in catecholamine secretions, which were enhanced by treatment with ouabain (10 −5 m ) plus Ca 2+ (2.2 m m ) for 25 min. The effect of ouabain was much more significant on noradrenaline secretion than on adrenaline secretion. The enhanced response was blocked by atropine (10 −6 m ) but not by hexamethonium (5 × 10 −4 m ). 4 Nifedipine (2 × 10 −6 m ) inhibited the responses to pilocarpine and nicotine. The treatment with ouabain (10 −5 m ) reversed only the response to pilocarpine and resulted in a significant increase in the proportion of noradrenaline released. 5 It is suggested that ouabain enhances evoked catecholamine secretions by facilitating Ca 2+ entry through nicotinic receptor‐linked Ca 2+ channels and by increasing the intracellular Ca 2+ pool linked to muscarinic receptors.