Malgaroli A, Vallar L, Elahi F R, Pozzan T, Spada A, Meldolesi J
Department of Pharmacology, University of Milano, Italy.
J Biol Chem. 1987 Oct 15;262(29):13920-7.
Single rat lactotroph cells were studied after loading with the cytosolic free Ca2+ concentration ([Ca2+]i) indicator fura-2 either 1 or 3 days after cell dispersion. Under unstimulated conditions, two groups of lactotrophs were observed, the first (predominant at day 1) with large [Ca2+]i fluctuations (peaks up to 300 nM) probably due to spontaneous action potentials and the second (predominant at 3 days) with stable [Ca2+]i (values variable between 65 and 200 nM). The effect of dopamine on the resting [Ca2+]i was different in the two groups. Even at high dopamine concentrations, no change occurred in the second group; whereas in the first, disappearance of fluctuations and marked decrease of [Ca2+]i were observed. These effects of dopamine appear to be due to hyperpolarization that was demonstrated by the use of a specific fluorescent indicator, bis(oxonol). Two types of triggered [Ca2+]i transients were studied in detail: those due to redistribution of Ca2+ from the intracellular stores (induced by thyrotropin-releasing hormone) and those due to Ca2+ influx through voltage-gated Ca2+ channels (induced by high [K+]). Dopamine (1 microM) markedly inhibited both these transients by the action of D2 receptors (blocked by 1-sulpiride and domperidone). All effects of dopamine were prevented by treatment of the cells with pertussis toxin, indicating the involvement of one (or more) GTP-binding protein(s). Another consequence of D2 receptor activation is the inhibition of adenylate cyclase. Treatments (cholera toxin, forskolin), known to raise cAMP levels, were found to dissociate the effects of dopamine on [Ca2+]i inasmuch as they markedly relieved the inhibition of the redistributive transients by thyrotropin-releasing hormone but left hyperpolarization and inhibition of K+ transients unaffected. The spectrum of intracellular signals elicited by the activation of D2 receptors is therefore complex and includes at least two mechanisms that involve [Ca2+]i, one related and the other independent of the decrease of cAMP levels.
在细胞分散后的第1天或第3天，用胞质游离Ca2 +浓度（[Ca2 +] i）指示剂fura - 2加载后，对单个大鼠催乳细胞进行了研究。在未刺激条件下，观察到两组催乳细胞，第一组（第1天占优势）具有较大的[Ca2 +] i波动（峰值高达300 nM），可能是由于自发动作电位引起的；第二组（第3天占优势）具有稳定的[Ca2 +] i（值在65至200 nM之间变化）。多巴胺对静息[Ca2 +] i的影响在两组中有所不同。即使在高多巴胺浓度下，第二组也没有变化；而在第一组中，观察到波动消失和[Ca2 +] i显著降低。多巴胺的这些作用似乎是由于超极化引起的，这通过使用特定的荧光指示剂双（氧杂萘）得以证明。详细研究了两种类型的触发[Ca2 +] i瞬变：一种是由于Ca2 +从细胞内储存库重新分布（由促甲状腺激素释放激素诱导）引起的，另一种是由于Ca2 +通过电压门控Ca2 +通道内流（由高[K +]诱导）引起的。多巴胺（1 microM）通过D2受体的作用（被1 - 舒必利和多潘立酮阻断）显著抑制了这两种瞬变。多巴胺的所有作用都被百日咳毒素处理细胞所阻止，表明涉及一种（或多种）GTP结合蛋白。D2受体激活的另一个后果是抑制腺苷酸环化酶。已知能提高cAMP水平的处理（霍乱毒素、福斯可林）被发现可使多巴胺对[Ca2 +] i的影响解离，因为它们显著减轻了促甲状腺激素释放激素对重新分布瞬变的抑制，但对超极化和K +瞬变的抑制没有影响。因此，D2受体激活引发的细胞内信号谱是复杂的，包括至少两种涉及[Ca2 +] i的机制，一种与cAMP水平降低有关，另一种与之无关。
