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THE entry of calcium ions into cells through voltage-activated Ca²⁺ channels in the plasma membrane triggers many important cellular processes. The activity of these channels is regulated by several hormones and neuretransmitters, as well as intracellular messengers such as Ca²⁺ itself (for examples, see refs 1–9). In cardiac muscle, myoplasmic Ca²⁺ has been proposed to potentiate Ca²⁺ influx^1,7–9, although a direct effect of Ca²⁺ on these channels has not yet been demonstrated. Photosensitive 'caged-Ca²⁺ molecules such as nitr-5, however, provide powerful tools for investigating possible regulatory roles of Ca²⁺ on the functioning of Ca²⁺ channels^10,11. Because its affinity for Ca²⁺ is reduced by irradiation, nitr-5 can be loaded into cells and induced to release Ca²⁺ with a flash of light¹⁰. By using this technique we found that the elevation of intracellular Ca²⁺ concentration directly augmented Ca² …