Stimulation of adenosine receptor enhances α1 ‐adrenergic receptor‐mediated activation of phospholipase C and Ca2+ mobilization in a pertussis toxin‐sensitive manner in FRTL‐5 thyroid cells

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Abstract
Norepinephrine (NE) stimulated FRTL‐5 thyroid cells via an α1‐adrenergic receptor, resulting in cytosolic Ca2+ ([Ca2+]i) mobilization and activation of phospholipase C. Adenosine and its receptor agonist, phenylisopropyladenosine (PIA), although not exerting a direct effect, markedly enhanced the NE‐induced changes. Basal NE action was not totally abolished whereas the permissive action of adenosine and PIA was completely abolished by pretreatment of the cells with islet‐activating protein (IAP), pertussis toxin. The decrease in cAMP level induced by adenosine or PIA is not the cause of their permissive effect, since this effect was not reversed by the addition of cAMP‐increasing agents. We conclude that an IAP substrate GTP‐binding protein(s) plays a novel role in forming a stimulatory coupling between an adenosine receptor and an α1‐adrenergic receptor‐coupled phospholipase C system.

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