The effects of caffeine on the resting level and agonist-induced changes in intracellular calcium ([Ca2+]i) have been studied in the vascular smooth muscle cell line A7r5. Caffeine (1-30 mM) lowers the resting [Ca2+]i by reducing the entry of Ca2+ and inhibits completely the mobilization of Ca2+ by arginine vasopressin. Application of forskolin, to elevate cAMP, does not affect the resting level of Ca2+i but does abolish the agonist-induced rise. These data add to the complexity of caffeine-induced changes in [Ca2+]i and point to a possible interaction between cAMP and other second messenger systems mobilizing Ca2+i in this cell type.
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Otun et al. (1991) studied this question.