Norepinephrine-induced calcium mobilization in C6 glioma cells

摘要

ATM: To investigate the mechanism underlying the norepinephrine-induced elevation in intracellular calcium concentration ([Ca~(2+)];) in C6 glioma cells. METHODS: Measurement of [Ca~(2+)]_i was carried out using the dual-wavelength fluorescence method with fura-2 as the indicator. RESULTS: Norepinephrine was found to induce concentration-dependent increases in [ Ca~(2+)]_i through α_1-adrenoreceptors. The [Ca~(2+)]_i elevations were extracellular-calcium independent and not influenced by the treatment of pertussis toxin. Pretreatments with either U73122 or thapsigargin abolished the subsequent cellular calcium responses to norepinephrine. Preincuba-tion with phorbo1 12-myristate 13-acetate ( PMA) significantly reduced the [Ca~(2+)]_i elevations, while protein kinase C inhibitors Ro31-8220 or GF-109203X completely blocked the inhibitory action of PMA. However, drugs either activating or inhibiting the function of protein kinase A had no effect on the [Ca~(2+)]_i elevations. CONCLUSION: Norepinephrine induces calcium mobilization from internal stores by activation of phospholipase C in C6 cells. The [Ca~(2+)]_i elevation is negatively regulated by the activation of protein kinase C. demonstrated recently that neurotransmitters released from neurons can also bind to the specific receptors on astrocytes, which results in elevations of intracellular calcium concentration ([Ca~(2+)]_i) in the cells. The calcium waves trigger the release of certain chemical substances, particularly glutamate, from the astro- cytes, and these substances will serve as astrocyte-to-neuron signaling molecules because they bring about [Ca~(2+)]_i elevations in adjacent neurons and even change the ongoing states of synaptic neurotransmission and neuronal activity. This bidirectional communication between neurons and astrocytes has been demonstrated in many experimental models, and is assumed as a general property of astrocytes. It has also been demonstrated that an elevation in [Ca~(2+)]_i is both necessary and sufficient to produce glutamate release from astrocytes, no matter what the stimulation is in nature. Nevertheless, depending on the extracellular stimulation intensities and the cellular context, the magnitude of the astrocytic calcium response seems to be graded, which may in turn lead to a graded feedback modulation to the synaptic transmission. It is therefore important to figure out the conditions or factors by which the cellular calcium responses are affected. Astrocytes are known to express α_1-adrenoreceptors, the activation of which causes [Ca~(2+)]_i increment. However, the modulation of the response by protein kinases is not well studied. Rat C6 glioma cells show many characteristics of astrocytes and have been widely employed as model cells for astrocyte research. In this study, we investigated the mechanism underlying the norepinephrine (NE)-induced elevation [Ca~(2+)]_i in C6 cells, as well as its modulation by protein kinases.