Fig. 4.

Histamine H₃ receptor activation attenuates the increase in intracellular cAMP and phosphorylation of PKA (PKA activity) elicited by BNP in PC12-H₃ cells. Histamine H₄ receptor activation attenuates the increase in intracellular cAMP and phosphorylation of PKA elicited by BNP in PC12 cells. A, intracellular cAMP levels in PC12-H₃ cells treated with forskolin (10 μM; positive control) or BNP (100 nM) in the absence or presence of the selective H₃ receptor agonist methimepip (1 nM), either alone or in combination with the selective H₃ receptor antagonist JNJ5207852 (30 nM). Columns are means ± S.E.M. (n = 4). Significantly different from control: **, P < 0.01 and ***, P < 0.001 by unpaired t test. Significantly different from BNP: ##, P < 0.01 by unpaired t test. Significantly different from BNP + H₃ receptor agonist + H₃ receptor antagonist: †, P < 0.05 by unpaired t test. B, PKA activity in PC12-H₃ cells treated with forskolin (10 μM; positive control) or BNP (100 nM) in the absence or presence of the H₃ receptor agonist methimepip (1 nM), either alone or in combination with the H₃ receptor antagonist JNJ5207852 (30 nM). Top band, representative immunoblot of PC12-H₃ cell lysate probed with antiphosphorylated PKA antibody. Bottom band, same immunoblot probed with anti-β-actin antibody. Bars represent mean quantitative values (± S.E.M.; n = 4). Significantly different from control: **, P < 0.01 by unpaired t test. Significantly different from BNP and BNP + H₃ receptor agonist + H₃ receptor antagonist: ##, P < 0.01 by unpaired t test. C, intracellular cAMP levels in PC12 cells treated with forskolin (10 μM; positive control) or BNP (100 nM) in the absence or presence of the H₃ receptor agonist methimepip (1 nM; negative control) or H₄ receptor agonist 4-methylhistamine (20 μM), either alone or in combination with the H₄ receptor antagonist A943931 (300 nM). Columns are means ± S.E.M. (n = 4). Significantly different from control: **, P < 0.01 and ***, P < 0.0001 by unpaired t test. Significantly different from BNP: ##, P < 0.01 by unpaired t test. Significantly different from BNP + H₄ receptor agonist + H₄ receptor antagonist: †, P < 0.05 by unpaired t test. D, PKA activity in PC12 cells treated with forskolin (10 μM; positive control) or BNP (100 nM) in the absence or presence of the H₃ receptor agonist methimepip (1 nM) or H₄ receptor agonist 4-methylhistamine (20 μM), either alone or in combination with the H₄ receptor antagonist A943931 (300 nM). Top band, representative immunoblot of PC12 cell lysate probed with antiphosphorylated PKA antibody. Bottom band, same immunoblot probed with anti-β-actin antibody. Bars represent mean quantitative values (± S.E.M.; n = 4). Significantly different from control: ***, P < 0.001 and **, P < 0.01 by unpaired t test. Significantly different from BNP and BNP + H₄ receptor agonist + H₄ receptor antagonist: ##, P < 0.01 by unpaired t test.