Factors influencing the effect of hormones on the accumulation of cyclic AMP in cultured human astrocytoma cells.

The characteristics of the effects of catecholamines, prostaglandins, and adenosine on the adenosine 3',5'-monophosphate (cAMP) content of human astrocytoma cells are described. Catecholamines interact with a typical beta-adrenergic receptor, i.e., the order of potency of catecholamines is isoproterenol larger than or equal to epinephrine greater than norepinephrine greater than dopamine, and propranolol is an inhibitor but phentolamine is not. The prostaglandins interact with a receptor that recognized PGE-1, PGE-2, and PGA-1 but not PGF-2-alpha. The effects of PGE-1 are blocked by 7-oxa-13-prostynoic acid, indomethacin, and meclofenamic acid in a rapid, reversible manner. The cells contain another adenylate cyclase-linked receptor that recognizes adenosine and the adenine nucleotides but not guanosine, deoxyadenosine, or adenine. Theophylline and other methylxanthines are competitive inhibitors of the effect of adenosine. Each class of effector appears to stimulate adenylate cyclase by interacting with a structure-specific receptor. This follows from the observation that the effect of each class of agonists can be blocked selectively by the various inhibitors and is consistant with the observation that co-addition of different agonists results in additive effects on accumulation of cAMP. The magnitude of the effect of any of the classes of agonists can be influenced by a variety of factors, some of which may be related to the peculiarities of growth in culture: (1) The cells secrete cAMP into the medium, and the magnitude of this secretion for a given rise in intracellular cAMP is different for different agonists. (2) The exposure of the cells to catecholamines or prostaglandins leads to a loss of responsiveness to a subsequent challenge by the same agonist. The magnitude of the agonist-induced loss of responsiveness is dependent on the concentration of the agonist and the time of exposure. The process is at least partially agonist specific in that exposure of cells to isoproterenol can lead to greater than 90% loss in catecholamine responsiveness with less than 20% loss in responsiveness to prostaglandins. (3) The responsiveness of the cells also changes as a function of the age of the culture and as a function of cell density. (4) Finally, it can be demonstrated that cells maintained in culture for prolonged periods (months to years) may lose responsiveness to specific agonists while responsiveness to other agonists remains unchanges or actually increases. The advantages and disadvantages of the use of cells in culture for studies of the regulation of cAMP metabolism are discussed.