The relationship between beta-adrenoceptor regulation and beta-adrenergic responsiveness in hepatocytes. Studies on acquisition, desensitization and resensitization of isoproterenol-sensitive adenylate cyclase in primary culture

Abstract

The role of β‐adrenoceptor regulation in the mechnisms controlling β‐adrenergic responsiveness in hepatocytes was explored, using primary monolayer cultures. When plated in vitro, these cells gradually acquire a strong catecholamine‐sensitive adenylate cyclase activity and an enhanced ability to bind the β‐adrenoceptor ligand [125I]iodocyanopindolol (125ICYP). Examination of the time course showed that the increase in the number of 125ICYP binding sites was detectable within 1–2 h of culturing and slightly preceded the elevation of isoproterenol‐responsive activity. Then the responsiveness rose steeply and between about 5–24 h it closely followed the increase in β‐receptor binding. Addition of isoproterenol (10 μM) to cells after 20 h culturing caused a rapid homologous desensitization of the adenylate cyclase (50% after about 5 min). This was paralleled by a down‐regulation of β‐adrenoceptors measured both in membrane particles and in total cell lysates. Removal of isoproterenol led to a resensitization of the adenylate cyclase, which was rapid and protein‐synthesis‐independent after a brief (10‐min) desensitization, or slow and cycloheximide‐sensitive after prolonged (4‐h) exposure to the agonist. In both cases an up‐regulation of the 125ICYP binding paralleled the recovery from refractoriness. In contrast, no concurring changes in 125ICYP binding were measured when the β‐adrenoceptor‐linked adenylate cyclase activity was enhanced by pretreatment with pertussin toxin (islet‐activating protein, IAP) or was desensitized by exposure of the cells to glucagon or 8‐bromo‐cAMP; however, these modulations of the adenylate cyclase were nonselective, since the pretreatments with IAP, glucagon or 8‐bromo‐cAMP affected both isoproterenol‐sensitive and glucagon‐sensitive activities. The results suggest that, in hepatocytes, regulation at the β‐adrenoceptor level is a major determinant for both short‐term and long‐term selective changes of the β‐adrenergic responsiveness.

Pertussis toxin abolishes the inhibitory effects of prostaglandins E1, E2,I2 and F2α on hormone‐induced cAMP accumulation in cultured hepatocytes

“…Itoh et al [47] reported that treatment of membranes from hepatocytes with pertussis toxin increased the isoproterenol-sensitive adenylate cyclase activity. This is supported by our data, but, in contrast to the results of Itoh et al [47], we found as in other studies [45,49,501 that the glucagon responsiveness was also augmented, which argues against a selective effect of the toxin on /?-adrenoceptor-mediated functions.…”

“…[42]) was amplified when the cells were pretreated with pertussis toxin, as demonstrated by the augmented effect of epinephrine and isoproterenol (Table 1). These results, and other recent data [45,461, also indicate that hepatocytes cultured in vitro as primary monolayers retain the capacity to react to pertussis toxin, and thus do not support the finding in another study [47] that the response of the liver cells to the toxin is lost during culturing. Pertussis toxin was also strongly effective when added to cultured hepatocytes as much as 5 days after plating (data not shown).…”

“…Pertussis toxin augmented the cAMP response of the hepatocytes to hormones, confirming data on adenylate cyclase in lysates from these cells [45]. This suggests the existence in the hepatocytes of an inhibitory tonus on the adenylate cyclase mediated by Gi.…”

Cardiac Gap Junction Channels Are Upregulated by Metoprolol: An Unexpected Effect of Beta-Blockers

“…maximum left ventricular pressure, heart rate and blood pressure were severely attenuated in the isoprenaline-treated desensitized rats. Moreover, other working groups demonstrated in cultured heart cells or cultured hepatocytes that downregulation of ␤ -adrenoceptors might occur within the range of hours and that ␤ -blocker treatment is able to overcome this downregulation [19,20] . Thus, it is in good accordance with the literature that in our cell culture model ␤ -adrenoceptors are downregulated after isoprenaline treatment and that S-or RS-metoprolol upregulate ␤ 1 -adrenoceptors, thereby restoring the ␤ 1 -adrenoceptor/G s /adenylyl cyclase pathway, whereas the R-enantiomer, due to its lack of activity on ␤ -adrenoceptors, is not able to upregulate ␤ 1 -adrenoceptors.…”

Stimulatory and inhibitory effects of catecholamines on DNA synthesis in primary rat hepatocyte cultures: Role of alpha1‐ and beta‐adrenergic mechanisms

“…Moreover, p-adrenoceptors are linked to adenylyl cyclase activation, and bidirectional effects of cyclic AMP on hepatocyte DNA synthesis have been found (Brgnstad et al, 1983;Thoresen et al, 1990). One reason why it is particularly desirable to obtain more direct information about growth effects mediated through P-adrenoceptors is the fact that in various conditions associated with hepatocyte growth, such as the perinatal period (Christoffersen et al, 1973), regenerating liver (Brgnstad and Christoffersen, 1980;Aggerbeck et al, 1983;HuertaBahena et al, 1983;Sandnes et al, 19861, and upon culturing of hepatocytes (Nakamura et al, 1983;Refsnes et al, 1983Refsnes et al, , 1987Christoffersen et al, 19841, there is a dramatic elevation of the P-adrenergic responsiveness of the hepatic adenylyl cyclase, and a several-fold increase in the number of P-adrenoceptors. The functional consequence of these changes, as related to growth control, is not clear.…”