Altered adrenergic response and specificity of the receptors in rat ascites hepatoma AH130

Abstract

Adenylate cyclase activation through adrenergic receptors in rat ascites hepatoma (AH) 130 cells in response to adrenergic drugs was studied, and receptor binding and displacement were compared with those of normal rat hepatocytes. Epinephrine (Epi) and norepinephrine (NE) activated AH130 adenylate cyclase about half as much as isoproterenol (IPN) but equaled IPN after treatment with the alpha-antagonist phentolamine or islet-activating protein (IAP). The three catecholamines in hepatocytes were similar regardless of phentolamine or IAP. These catecholamines activated adenylate cyclase in order of IPN greater than NE greater than Epi in AH130 cells but IPN greater than Epi greater than NE in hepatocytes. We then used the alpha 1-selective ligand [3H]prazosin, the alpha 2-selective ligand [3H]clonidine, and the beta-ligand [125I]iodocyanopindolol [( 125I]ICYP), and found that AH130 cells had few prazosin-binding sites, about eight times as many clonidine-binding sites with high affinity, and many more ICYP-binding sites than in hepatocytes. The dissociation constant (Ki) of the beta 1-selective drug metoprolol by Hofstee plots for AH130 cells was lower than that for hepatocytes. The inhibition of specific ICYP binding by the beta 2-selective agonist salbutamol for AH130 cells gave only one Ki value which was much higher than both high and low Ki values of the drug for hepatocytes. These findings indicate that the alpha- and beta-adrenergic receptors in hepatocytes are predominantly alpha 1-type and beta 2-type, but that those in AH130 cells are predominantly alpha 2-type and beta 1-type, and the low adrenergic response of AH130 cells is due to the dominant appearance of alpha 2-adrenergic receptors, linked with the inhibitory guanine-nucleotide binding regulatory protein, instead of alpha 1-adrenergic receptors, and beta 1-adrenergic receptors with low affinity for the hormone.