A radioisotopic method for measuring the formation of adenosine 3',5'-cyclic monophosphate in incubated slices of brain

Adenosine as inhibitor of myocardial effects of catecholamines

Infusion of adenosine into the coronary arteries of isolated guinea pig hearts produced a dose-dependent inhibition of dP/dtmax caused by bolus injections of isoproterenol (4 X 10(-11) moles). Threshold concentration of adenosine was 10(-7) M and maximal inhibition (90%) occurred at 10(-5) M. Coronary dilation induced by papaverine did not influence the contractile response to catecholamines. In addition to its influence on cardiac performance, adenosine (10(-5) M) effectively inhibited the isoproterenol (10(-7)M) induced initial rise in myocardial levels of cyclic 3'5'-AMP, glucose-1-phosphate and glucose-6-phosphate. Adenosine also antagonized the effect of isoproterenol on adenylate cyclase activity in a crude membrane preparation from guinea pig ventricles; it was without effect on the activity of the membrane phosphodiesterase. Theophylline inhibited the actions of adenosine both on adenylate cyclase activity and on contractile force development. Upon infusion of isoproterenol (3 X 10(-7)M) into the coronary arteries of the isolated heart (perfusion at constant pressure), the adenosine concentration in the effluent perfusate increased within 45 s from 10(-8) M to about 10(-6) M. It thus appears conceivable that in ventricular myocardium endogenously formed adenosine may serve 2 functions: dilation of the coronary arteries and limitation of the inotropic and metabolic effects of catecholamines.

Effects of phenytoin on the release of 14C-adenine derivatives

The release of 14C-containing compounds from rat cortical slices prelabeled with 14C-adenine consisted largely of adenosine (6-7%), inosine (13-18%), and hypoxanthine (70-74%), with small amounts of nucleotides including cyclic AMP and adenine. This efflux was increased by both ouabain (0.1 mM) and veratridine (0.05 mM), the increment in released radioactivity consisting almost entirely of these three compounds. However, relatively more inosine than adenosine output was evoked by ouabain while the reverse was true with veratridine. Phenytoin partially reversed the effect of both depolarizing agents. After prelabeling, the efflux from astrocytoma cell cultures contained predominantly inosine (74%) and hypoxanthine (23%) with little adenosine. Ouabain increased the release of 14C-adenine derivatives, and this increase was diminished by phenytoin. Preliminary studies with neuroblastoma cell cultures have shown considerable variability in the composition of the effluent, with hypoxanthine the prevalent compound and almost no adenosine. Ouabain enhanced the efflux from these cells, and this effect was apparently reversed by phenytoin.

Cyclic AMP accumulation in cerebral cortical slices: effect of carbamazepine, phenobarbital, and phenytoin

Recent investigations have suggested that abnormal increases in brain cyclic 3',5'-adenosine monophosphate (cAMP) may play a role in epileptogenesis. Therefore, the effect of three commonly used antiepileptic drugs on cAMP accumulation in rat cortex slices was investigated. Ouabain, a depolarizing agent which produces seizures when applied to rat cortex, produced a five- to sevenfold increase in cAMP accumulation, and both carbamazepine and and phenytoin inhibited this increase. Ouabain stimulation may be mediated by the release of endogenous adenosine, and carbamazepine antagonized adenosine stimulation of cAMP accumulation whereas phenytoin did not. Carbamazepine had no effect on adenosine efflux. The augmentation of cAMP accumulation by norepinephrine was inhibited by carbamazepine and phenobarbital but slightly increased by phenytoin. If increases in brain cAMP are involved in epileptogenesis, the antagonism of cAMP accumulation by antiepileptic drugs may play a role in their anticonvulsant action.

Preliminary study of the use of dansyl chloride to determine cyclic-3',5'-AMP in tissues

A highly sensitive method for the determination of cyclic-3',5'-AMP has been developed which involves the reaction of the substance with dansyl chloride and the subsequent separation of the dansyl-cyclic-3',5'-AMP derivative by thin-layer chromatography. Experiments with standard solutions of 3H-cyclic-3',5'-AMP have shown that there is a direct relationship between the amount of dansyl-3H-cyclic-3',5'-AMP recovered and that dansylated. The procedure is exceedingly sensitive, allowing milligram quantities of material to be analysed for its endogeneous cyclic-3',5'-AMP content. With the use of 14C-adenine as substrate, this method permits the separation of 14C-cyclic-3',5'-AMP formed from the substrate and other 14C-containing compounds, thus allowing the turn-over of cyclic-3'-5'-AMP to be studied. The usefullness of the method is demonstrated by analysing the turn-over and endogenous content of cyclic-3'-5'-AMP in rat nervous tissue.

Ontogeny of beta-adrenergic receptors in rat cerebral cortex

The ontogeny of beta-adrenergic receptors in rat cerebral cortex has been studied using [125I]iodohydroxybenzylpindolol as a ligand in an in vitro binding assay. The concentration of beta-adrenergic receptors was very low during the first week after birth. Between days 7 and 14 there was a rapid increase in the density of receptors. Adult levels were reached by the end of the second week. The affinities of 1-isoproterenol and iodohydroxybenzylpindolol for beta-adrenergic receptors did not vary with the age of the animal. Fluoride stimulated adenylate cyclase activity in the cerebral cortex was 40% of the adult level at birth and gradually increased to maximal levels over the next two weeks. On the other hand, catecholamine stimulated cyclic-3',5'-adenosine monophosphate accumulation was barely detectable during the first week after birth, but it increased rapidly to adult levels between days 7 and 14. The results suggest that it is the development of beta-adrenergic receptors that permits the expression of catecholamine sensitive adenylate cyclase activity. Norepinephrine stores in the cerebral cortex developed slowly reaching adult levels approximately two months after birth. There is therefore little correlation between the ontogeny of presynaptic adrenergic nerve terminals and the postsynaptic development of beta-adrenergic receptors.

Compartmentation of cardiac adenine nucleotides and formation of adenosine

After prelabeling the adenine nucleotides (ATP, ADP, AMP) of isolated perfused guinea pig hearts with either 14C-adenine or 14C-adenosine for 35 min, labeled adenosine, inosine, hypoxanthine and cyclic 3'5'-AMP (cAMP) were continuously released into the cardiac perfusate. Determination of the specific activities (SA) of the adenine nucleotides, cAMP, and their breakdown products (adenosine, inosine, hypoxanthine) in tissue and perfusate revealed: Under steady state conditions the SA of adenosine and cAMP in the perfusate were of the same order of magnitude and proved to be many times higher than the SA of the respective precursor adenine nucleotides. This difference was observed regardless whether adenine or adenosine was used as prelabeling substances. The SA of inosine and hypoxanthine in the perfusate were constantly lower than the SA of adenosine. Cardiac ischemia of 6 min, which resulted in a markedly increased formation of adenosine, led to a pronounced decrease in the SA of adenosine released from the heart. Our findings provide evidence that at least two different adenine nucleotide compartments of the heart severe as precursors for the formation of adenosine and cAMP, one characterized by a high, the other by a lower SA. Under normoxic conditions adenosine and cAMP released into the cardiac perfusate are derived mainly from a nucleotide fraction of high SA, which appears to be rather small. During ischemia a second compartment of much lower SA in addition contributes to the formation of adenosine.

beta-Adrenergic receptor involvement in 6-hydroxydopamine-induced supersensitivity in rat cerebral cortex

The intraventricular administration of 6-hydroxydopamine, a procedure which destroys noradrenergic nerve terminals in the central nervous system, caused an increase in the density of beta-adrenergic receptors in rat cerebral cortex, without affecting their affinity for isoproterenol. The results suggest that changes in the density of adrenergic receptors are involved in 6-hydroxydopamine-induced supersensitivity at central noradrenergic synapses.

Regulation of ovarian steroidogenesis. The disparity between 125I-labelled choriogonadotropin binding cyclic adenosine 3',5'-monophosphate formation and progesterone synthesis in the rat ovary

The binding of 125I-labelled human choriogonadotropin, formation of cyclic adenosine 3',5'-monophosphate (cyclic AMP), and synthesis of progesterone were examined in ovarian cells from immature rats. Collagenase dispersed ovarian cells were found to respond specifically to lutropin-like activity. The equilibrium dissociation constant (Kd) for the binding of 125I-for the binding of 125I-labelled choriogonadotropin was 1.7-10(-10) M. Progesterone synthesis was increased at least 40-fold and cyclic AMP formation 10-fold in response to maximum hormonal stimulation. The concentration of choriogonadotropin which stimulated progesterone synthesis maximally in Eagle's minimum essential medium--0.1% gelatin (2 ng/ml), resulted in minimal (less than 30% of maximum) increases in cyclic AMP accumulation and hormone binding. Similarly, binding of choriogonadotropin was not saturated at a hormone concentration (50 ng/ml) that stimulated maximal cyclic AMP formation. These results are consistent with the existence of receptor reserve in the ovarian cell. A marked shift in the dose vs. response relationship for progesterone synthesis occurred when fetal calf serum was used to supplement Eagle's minimum essential medium, however. Under these experimental conditions, progesterone synthesis reached a maximum at a hormone concentration of the same order of magnitude as did cyclic AMP formation. It is concluded that the degree of spare receptor effect observed may depend not only on an absolute amount of excess receptor, but also on the readiness of the system to respond in a given fashion.

Altered brain cyclic AMP-responses in rats reared in enriched or impoverished environments

The accumulation of radioactive cyclic AMP elicited by various neurohormones has been examined in adenine-labeled telencephalon slices from rats raised in enriched or impoverished environments. Basal levels of cyclic AMP and responses of the brain slice cyclic AMP-generating systems to norepinephrine, isoproterenol and adenosine did not differ between the two group of rats, while responses to prostaglandin E1 were significantly greater with the impoverished group and responses to histamine appeared to be greater with the enriched group.

The effects of dbcAMP on adreanl chromaffin cells in organotypic culture

Chromaffin cells in organotypic cultures of adrenal glands from 19 day rat embryos developed a marked increase in the volume of their rough endoplasmic reticulum (RER) and enlargement of nucleoli in response to 1 mM dibutyryl cyclic AMP (dbcAMP) treatment for 17 days in vitro. 0.1 mM dbcAMP had a similar but less dramatic effect. When dbcAMP was removed from the medium after 11 days in vitro, cells became more like control explants treated with no additives, suggesting the effects of dbcAMP were reversible. ACTH which selectively stimulates adrenal cortical cells had no effect on RER morphology of chromaffin cells. The 'neuronal-like' configuration of dbcAMP treated chromaffin cells is discussed.

Biochemical studies of taste sensation: II. Labelling of cyclic AMP of bovine taste papillae in response to sweet and bitter stimuli

Labeling of cyclic AMP of tase papillae and its responsiveness to tast stimuli has been measured using whole papillae from bovine tongue prelabeled with [ 8-(14) C] adenine. Labeling was measured in circumvallate and fungiform papillae, both of which contain taste buds, and in filiform papillae and small blocks of tongue epithelium, which are devoid of taste buds. No differences were observed in the levels of activity. The labeling of cyclic AMP of circumvallate papillae showed only small increases (12-22%) in the presence of the taste stimulus sucrose (sweet), and the stimulatory effects were not statistically significant. The increase due to sucrose was not potentiated by theophylline. No stimulation by sucrose was observed with epithelium controls. Lactose, which is a poor taste stimulus, did not stimulate labeling of cyclic AMP in taste papillae. Theophylline, caffeine, and quinine (bitter) stimulated labeling of cyclic AMP by up to 2-fold, as did L-Epinephrine. Evidence for a specific role of cyclic AMP as a second messenger in taste sensation was not obtained. It is suggested that cyclic AMP might provide a mechanistic basis for studying some of the effects of sweet and bitter compounds in mixtures.

Analogs of cyclic adenosine monophosphate: correlation of inhibition of Purkinje Neurons with Protein Kinase Activation

Cyclic adenosine monophosphate (cyclic AMP) and 11 derivatives were applied to rat cerebellar Purkinje cells by iontophoresis. Cyclic AMP inhibited 63 percent of the cells, while the 8-parachlorophenylthio- and 8-benzylthio- analogs of cyclin AMP inhibited the spontaneous firing of 92 and 89 percent of cells, respectively. The ability of the 11 analogs to inhibit neuronal firing correlated ( r= + .78) with their reported potency in activating cyclic AMP-dependent protein kinase. These results extend previous studies, pointing to the mediation by cyclic AMP of the noradrenergic inhibition of Purkinje neurons, and provide new physiological evidence that protein phosphorylation is a major step in the action of cyclic AMP.

The stimulatory effect of the boiled supernatant on cyclic AMP formation in synaptosomes from rat cerebral cortex

The effect of the boiled supernatant on synaptosomal adenyl cyclase activity of rat cerebral cortex was investigated. The boiled supernatant markedly increased the accumulation of cyclic AMP in synaptosomes when added to adenyl cyclase system by a mechanism presumably unrelated to inhibition of cyclic AMP phosphod iesterase or adenosine triphosphatase. Magnesium ion was required for synaptosomal adenyl cyclase activity and its stimulation by the boiled supernatant. The result discerned by doubl reciprocal plots showed an increase in Vmax value of adenyl cyclase by addition of the boiled supernatant without significantly altering the affinity for substrate. The enzyme activity was not stimulated by dopamine, histamine and serotonin in either the absence and presence of the boiled supernatant.

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.