The Influence of Cations on Spontaneous and Chemically Induced Efflux of Noradrenaline from Perfused Rat Hearts

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Abstract
Isolated rat hearts perfused with Krebs bicarbonate solution and labelled with 3H-noradrenaline (3H-NA) were used to study the influence of various cations on the efflux of noradrenaline from sympathetic nerve endings. Evidence is presented that EDTA causes release by influencing Mg more than Ca ions, possibly those associated in a Mg–ATPase complex at the storage granule. Calcium is required for release by excess potassium but inhibits release caused by BaCl2. In contrast to sustained increases in the rate of 3H-NA efflux caused by EDTA and noradrenaline, potassium and barium ions initiate short-duration increases which subside while the initiating agent is still present. It is postulated that potassium and barium may cause 3H-NA release from a limited pool of granules adjacent to the membrane or may make available a limited supply of intracellular free Ca ions to initiate release at the storage granules.

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