Intracellular cross‐talk between receptors coupled to phospholipase C via pertussis toxin‐sensitive and insensitive G‐proteins in DDT1MF‐2 cells

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Abstract
1 The effect on intracellular free calcium concentration ([Ca2+]i) of simultaneous activation of receptors coupled to phospholipase C via pertussis toxin (PTX)-sensitive and -insensitive G-proteins has been investigated in the hamster vas deferens smooth muscle cell line, DDT1MF-2. 2 In fura-2-loaded DDT1MF-2 cells, activation of adenosine A1-receptors (which are linked to PTX-sensitive G-proteins) with a maximal concentration of N6-cyclopentyladenosine (CPA; 300 nm) increased [Ca2+]i from 121 ± 5 nm to 254 ± 20 nm (n = 8). These experiments were performed in the presence of extracellular Ca2+. Stimulation of histamine H1-receptors (which are linked to PTX-insensitive G-proteins) with a low concentration of histamine (1 μm) increased [Ca2+]i from 128 ± 8 nm to 150 ± 13 nm (n = 8). When combined, CPA (300 nm) and histamine (1 μm) synergistically raised [Ca2+]i from 134 ± 6 nm to 607 ± 61 nm (n = 8). 3 Removal of extracellular Ca2+ (experiments performed in Ca2+-free buffer containing 0.1 mm EGTA) had no effect on the synergistic interaction between CPA (300 nm) and histamine (1 μm). 4 The addition of maximal concentrations of CPA (300 nm) and histamine (100 μm) resulted in a rise in [Ca2+]i which was additive when compared to the Ca2+ responses obtained with the two agonists alone. Low (30 nm) and subthreshold (3 nm) concentrations of CPA did not alter the Ca2+ response elicited by maximal concentrations of histamine (100 μm). 5 Subthreshold concentrations of CPA (3 nm) and low concentrations of histamine (1 μm) elicited synergistic rises in [Ca2+]i. 6 Synergistic Ca2+ responses were not observed between histamine H1- and ATP-receptors when cells were simultaneously stimulated with either 1 μm or 10 μm of each agonist. 7 These data suggest that adenosine A1-receptors linked to PTX-sensitive G-protein(s) and histamine H1-receptors linked to PTX-insensitive G-proteins interact synergistically to raise [Ca2+]i. In contrast, activation of ATP-receptors which are linked to PTX-insensitive G-protein(s) do not interact synergically with histamine H1-receptors.