Treatment of isolated hepatocytes with the a1-adrenergic agonist norepinephrine induced a dose-dependent increase in free cytosolic Ca2+, as judged by fluorescence increases, in cells loaded with the Ca2+ indicator (2- [(2-bis-[carboxymethyl]amino-5-methylphenoxy)methyl]-6-methoxy-8-bis [carboxymethyl]aminoquinoline (quin-2). Pretreatment with either glucagon or dibutyryl cAMP increased the rate and magnitude of the quin-2 fluorescence response in hepatocytes treated with submaximal doses of norepinephrine and increased the cell sensitivity such that a physiological concentration of norepinephrine (7.5 nM) was able to provoke a quin-2 fluorescence response. Similar enhancement of norepinephrine-induced phosphorylase activation and pyridine nucleotide reduction in isolated hepatocytes and Ca2+ efflux from the perfused liver was also observed in the presence of glucagon. These potentiated responses correlated with a cAMP-dependent increase (mediated by glucagon, dibutyryl cAMP, or forskolin) in the binding of [3HJnorepinephrine or [3Hlepinephrine to sites present on isolated hepatocytes bearing the characteristics of a,-adrenergic receptors. The data suggest that a cAMPdependent mechanism is involved in the regulation of al-agonist binding to liver cells and, thereby, in the control of hepatic carbohydrate metabolism in response to catecholamines.The regulation of carbohydrate metabolism in rat liver can be controlled by two distinct mechanisms, involving, respectively, an increase in cell cAMP (e.g., glucagon, 82-adrenergic agonists) or an increase in cytosolic free Ca2+ (e.g., a1-adrenergic agonists, vasopressin) (1). However, recent evidence has shown that these two separate systems can interact with each other in a manner that leads to modulation of their respective responses (2, 3). It has also been shown that the hepatic a1-adrenergic receptor becomes simultaneously coupled to both Ca2+ mobilization and cAMP generation in mature male rats (3, 4), although the significance of such dual coupling has not, to date, been determined. Paradoxically, interactions between the cAMP-dependent and Ca2+-dependent systems in liver have revealed a mutual antagonism that appears inconsistent with their roles in promoting hepatic glucose release (2, 3, 5). However, such interactions have generally been observed at relatively high (supraphysiological) agonist concentrations.In the present study in rat liver, we have used lower, more physiological, hormone doses to examine whether increases in cAMP can exert any control over Ca2+ mobilization induced by a1-adrenergic agonists. The data presented show that modest increases in hepatic cAMP can markedly enhance Ca2+ mobilization, phosphorylase activation, and pyridine nucleotide reduction in response to low doses of a1-adrenergic agonists. This correlates with a specific enhancement of the binding of agonists to a1-adrenergic receptors on isolated hepatocytes in the presence of agents that raise cAMP levels. The present report therefore represents the documentation of a s...