Histamine causes Ca 2+ entry via both a storemoperated and a store-independent pathway in bovine adrenal chromaffin cells

Abstract

The characteristics and properties of the increase in cytosolic [Ca 2+] that occurs in bovine adrenal medullary chromaffin cells on exposure to histamine have been investigated. Specifically, these experiments were conducted to determine how much external Ca 2+ enters the cell through a (capacitative) Ca 2+ entry pathway activated as a consequence of intracellular Ca 2+ store mobilization, relative to that which enters independently of store depletion via other channels activated by histamine. In Fura-2 loaded cells continued exposure to histamine (10 μM) caused a rapid but transient increase in cytosolic [Ca 2+] followed by a lower plateau that was sustained as long as external Ca 2+ was present. In the absence of external Ca 2+ only the initial brief transient was observed. In cells previously treated with thapsigargin (100 nM) in Ca 2+-free medium to deplete the internal Ca 2+ stores, histamine caused no increase in cytosolic [Ca 2+] when external Ca 2+ was absent. Re-introduction of external Ca 2+ to thapsigargin-treated store-depleted cells caused a sustained increase in cytosolic [Ca 2+] that was further increased ( P < 0.0002) upon exposure to histamine. The histamine-evoked increase was prevented by the H 1-receptor antagonist, mepyramine (2 μM). A comparison was made between store-dependent Ca 2+ entry consequent upon store mobilization with histamine in Ca 2+-free medium and plateau phase Ca 2+ entry resulting from stimulation with histamine in Ca 2+-containing medium. The latter was found to be approximately 3 times greater in magnitude than the former (P ⪡ 0.0001) at the same concentration of histamine (10 μM). It is concluded that histamine causes Ca 2+ entry not only via a capacitative entry pathway secondary to internal store mobilization, but also causes substantial Ca 2+ entry through other pathways.