Characteristics of Ca<SUP>2&#8290;</SUP> Stores in Rabbit Cerebral Artery Myocytes

Characteristics of Ca<SUP>2&#8290;</SUP> Stores in Rabbit Cerebral Artery Myocytes

<P> In a myocyte freshly isolated from rabbit cerebral artery, the characteristics of Ca<SUP>2&#8290;</SUP> release by histamine or caffeine were studied by microspectrofluorimetry using a Ca<SUP>2&#8290;</SUP>-binding fluorescent dye, fura-2. Histamine (5 &#12557;M) or caffeine (10 mM) induced a phasic rise of cytoplasmic free Ca<SUP>2&#8290;</SUP> concentration ([Ca<SUP>2&#8290;</SUP>]<SUB>c</SUB>) which could occur repetitively with extracellular Ca<SUP>2&#8290;</SUP> but only once or twice in Ca<SUP>2&#8290;</SUP>-free bathing solution. Also, the treatment with inhibitor of sarcoplasmic reticulum Ca<SUP>2&#8290;</SUP>-ATPase suppressed the rise of [Ca<SUP>2&#8290;</SUP>]<SUB>c</SUB> by histamine or caffeine. In Ca<SUP>2&#8290;</SUP>-free bathing solution, short application of caffeine in advance markedly attenuated the effect of histamine, and <I>vice versa</I>. In normal Ca<SUP>2&#8290;</SUP>-containing solution with ryanodine (2 &#12557;M), the caffeine-induced rise of [Ca<SUP>2&#8290;</SUP>]<SUB>c</SUB> occurred only once and in this condition, the response to histamine was also suppressed. On the other hand, in the presence of ryanodine, histamine could induce repetitive rise of [Ca<SUP>2&#8290;</SUP>]<SUB>c</SUB> while the amplitude of peak rise became stepwisely decreased and eventually disappeared. These results suggest that two different Ca<SUP>2&#8290;</SUP>-release mechanisms (caffeine-sensitive and histamine-sensitive) are present in rabbit cerebral artery myocyte and the corresponding pools overlap each other functionally. Increase of [Ca<SUP>2&#8290;</SUP>]<SUB>c</SUB> by histamine seems to partially activate ryanodine receptors present in caffeine-sensitive pool.