Mechanisms for the emergence of catecholamine‐sensitive adenylate cyclase and β‐adrenergic receptors in cultured hepatocytes

Abstract

Adult male rat hepatocytes, which normally respond poorly to β‐adrenergic agents, acquire such responsiveness during primary monolayer culture. We here show that the rise in catecholamine‐sensitive adenylate cyclase activity in hepatocytes in vitro is closely paralleled by an increase in the ability to bind the β‐adrenoceptor ligand [125I]cyanopindolol. The emergence of β‐adrenergic responsiveness did not require cell attachment or serum. Addition of dexamethasone, insulin, thyroxine or dihydortestosterone to the cultures, singly or in combination, did not prevent the augmented β‐adrenergic responsiveness. The increase in catecholamine‐sensitive adenylate cyclase activity and [125I]cyanopindolol binding could be blocked by cycloheximide or actinomycin D. Exposure of the cultures to isoproterenol at 3‐hourly intervals led to a dose‐dependent suppression of the rise in isoproterenol‐responsive adenylate cyclase and prevented the increase in β‐adrenoceptor binding.

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

“…We [24] and Nakamura et al [23] found that the increase in isoproterenol-sensitive adenylate cyclase activity as measured at 24 h required protein synthesis. To investigate the possibility that the rise might contain an early protein-synthesis-independent component (see also [30]) we examined the period from 0 h (plating) to 3 h in cycloheximide-treated vs control cells.…”

“…In the period between about 5 h and 24 h of culturing, the increases in P-adrenoceptor binding and isoproterenol-sensitive adenylate cyclase showed close parallelism (Fig. 1, inset and also [24]). When the cells were kept in culture for more than 24 h, the receptor binding in most experiments continued to rise while the P-responsive These results indicate that the emergence of P-adrenoceptors is a very early event during primary culture of hepatocytes and that there is a relatively close relationship between this rise in receptor number and the increase in catecholaminesensitive adenylate cyclase activity.…”

“…1. The binding of '251CYP started to rise within 1 -2 h, and thereafter increased progressively up to at least 40 h. Scatchard analysis at 2 h, 3 h and 24 h showed that the enhanced binding was a result of an increased number of receptors, with no change in affinity (data not shown, see also [24]). In the first 1 -2 h after plating, i.e.…”

“…The rise in catecholamine-sensitive adenylate cyclase activity occurring in hepatocyte monolayer cultures was previously found to be associated with an increased number of P-adrenoceptors [23,24,261, but in hepatocytes incubated for 4 h in suspension the glycogenolytic response to isoproterenol was reported to increase without any concomitant elevation of the number of P-receptors [29]. This raises the possibility of an early facilitation of the transduction of the P-adrenergic signal, preceding the receptor increase.…”

“…The mechanisms underlying these response modulations are not fully clarified. An increased /?-adrenoceptor density is found in association with enhanced P-adrenergic responsiveness in various conditions in vivo [9, 10, 14, [16][17][18][19][20][21] as well as in cultured hepatocytes [23,24,261. On the other hand, some studies suggest that altered receptor coupling or other post-receptor mechanisms are important in regulating hepatocyte /?-adrenergic responses both in vivo and in vitro [18, 19,27 -301.…”