Adenosine-induced cyclic AMP increase in pig lymphocytes is not related to adenylate cyclase stimulation

Mechanisms whereby extracellular adenosine 3',5'-monophosphate inhibits phosphate transport in cultured opossum kidney cells and in rat kidney. Physiological implication

The mechanism of phosphaturia induced by cAMP infusion and the physiological role of extracellular cAMP in modulation of renal phosphate transport were examined. In cultured opossum kidney cells, extracellular cAMP (10-1,000 microM) inhibited Na-dependent phosphate uptake in a time- and concentration-dependent manner. The effect of cAMP was reproduced by ATP, AMP, and adenosine, and was blunted by phosphodiesterase inhibitors or by dipyridamole which inhibits adenosine uptake. [3H]cAMP was degraded extracellularly into AMP and adenosine, and radioactivity accumulated in the cells as labeled adenosine and, subsequently, as adenine nucleotides including cAMP. Radioactivity accumulation was decreased by dipyridamole and by inhibitors of phosphodiesterases and ecto-5'-nucleotidase, assessing the existence of stepwise hydrolysis of extracellular cAMP and intracellular processing of taken up adenosine. In vivo, dipyridamole abolished the phosphaturia induced by exogenous cAMP infusion in acutely parathyroidectomized (APTX) rats, decreased phosphate excretion in intact rats, and blunted phosphaturia induced by PTH infusion in APTX rats. These results indicate that luminal degradation of cAMP into adenosine, followed by cellular uptake of the nucleoside by tubular cells, is a key event which accounts for the phosphaturic effect of exogenous cAMP and for the part of the phosphaturic effect of PTH which is mediated by cAMP added to the tubular lumen under the influence of the hormone.

Evidence against functional adenosine receptors on murine lymphocytes

The effects of adenosine, 2-chloroadenosine (2Cl Ado) and N6-phenylisopropyladenosine (PIA) were examined on peripheral blood and splenic lymphocytes from mice. Lectin-stimulated DNA synthesis was antagonized by adenosine and 2Cl Ado at high concentrations. Lower concentrations of all three nucleosides produced an enhancement of lectin-stimulated thymidine uptake in splenic lymphocytes. Peripheral blood lymphocytes were found to exhibit only inhibitions of mitogenic stimulation suggesting a difference in response to nucleoside exposure between spleen and peripherally circulating cells. The synthesis of antibody to sheep red blood cells was inhibited in a non-cytotoxic manner by 2Cl Ado and PIA while adenosine was without effects. The receptor antagonist 8-phenyltheophylline was found to block nucleoside increases in thymidine uptake and low concentrations of 2Cl Ado with regard to antibody production. The effects of high concentrations of 2Cl Ado or PIA on humoral responses were not antagonized by receptor blockade. The data suggest that functional alterations of lymphocyte responses to nucleoside exposure are not a consequence of surface receptors for adenosine nucleosides in murine cells.

Theophylline-like properties of xanthine analogs

Theophylline and other methylxanthines display a large number of biological effects, some of which are clinically important. The effects of these compounds are commonly ascribed to an inhibition of cyclic AMP breakdown. However, it becomes actually evident that another mechanism, namely adenosine receptor antagonism, could be responsible for certain methylxanthine effects. It could be of interest to find new compounds displaying only one of these mechanisms, either phosphodiesterase inhibition or adenosine receptor antagonism. We have studied several synthetic imidazol[1,2a]pyrazines, some of which display theophylline-like pharmacological properties at lower doses than theophylline. We showed that some of these compounds inhibited mitogen-induced [3H]-thymidine uptake by human lymphocytes, which is consistent with increases in cyclic AMP levels: the most efficient compounds were those which were better phosphodiesterase inhibitors than theophylline and poorer adenosine receptor antagonists.

N6-phenylisopropyladenosine stimulates in normal and inhibits in adrenalectomized rats the low KM cyclic AMP phosphodiesterase in the brain

In crude rat brain membranes, apparent adenylate cyclase activity tested in the absence of phosphodiesterase inhibitors was dose-dependently inhibited by the adenosine 'R-site' agonist N6-phenylisopropyladenosine (N6-PIA). In membranes from adrenalectomized rats, however, N6-PIA induced, under the same conditions, an activation. However, in the presence of the phosphodiesterase inhibitor Ro-20-1724, brain adenylate cyclase responsiveness to N6-PIA resulted in a dose-dependent inhibition in both sham-operated and adrenalectomized rats. Thus, the low KM cyclic AMP phosphodiesterase activity present in these brain membranes was investigated. Although this activity (tested in the presence of GTP) was unaltered by adrenalectomy, the dose-response curve of this enzyme to low concentrations of N6-PIA showed an activation in sham-operated and an inhibition in adrenalectomized rats, two effects which were suppressed by sodium (80 mM). These results showing that N6-PIA modulates both adenylate cyclase and cyclic AMP phosphodiesterase in the brain, provide an additional argument for a potential role of adenosine in the regulation of cyclic AMP metabolism in normal as well as in pathological brain.

Inhibition by xanthine derivatives of adenosine receptor-stimulated cyclic adenosine 3',5'-monophosphate accumulation in rat and guinea-pig thymocytes

The effect of stable adenosine analogues, including adenosine 5'-N-ethylcarboxamide (NECA) and N6-L-phenylisopropyl-adenosine (L-PIA), were studied on cyclic adenosine 3', 5'-monophosphate (cyclic AMP) accumulation in rat and guinea-pig thymocytes. NECA was approximately 10 times more potent than L-PIA, in thymocytes from both species. D-PIA was more potent in guinea-pig than in rat thymocytes. The effect of a number of adenosine analogues followed the order: NECA greater than 2-chloro-adenosine greater than L-PIA greater than N6-cyclohexyl-adenosine (CHA), an order of potency characteristic for adenosine receptors of the A2-subtype. Thymocytes may be used as a model system to study the pharmacology of such receptors. Several xanthines were studied as antagonists of the NECA (1 microM)-induced cyclic AMP accumulation. The order of potency was: 1,3-diethyl-8-phenylxanthine greater than 8-phenyl-theophylline greater than IBMX = 8-p-sulphophenyltheophylline = verrophylline greater than theophylline greater than caffeine greater than enprofylline greater than theobromine greater than pentoxiphylline. The pA2 value for 8-phenyltheophylline was 0.35 microM, and the antagonism was shown to be competitive. The order of potency of the xanthine is virtually identical to that found earlier in several other systems in which the receptors are of the A1-subtype. None of the xanthine derivatives tested thus seem to discriminate between A1 and A2-receptor-mediated adenosine actions.

The effect of ribose and purine modified adenosine analogues on the secretion of histamine from rat mast cells induced by ionophore A23187

The purine nucleosides adenosine and 2',5'-dideoxyadenosine (2',5'ddAdo) enhance and inhibit respectively the anti-IgE-induced secretion of histamine and transient rise in cellular levels of cyclic AMP in rat mast cells. These findings have provided evidence for a role for cyclic AMP in the activation of mast cell secretion. It has been generally accepted that the nucleosides mediate their effects on mast cells by altering adenylate cyclase activity. We have investigated the effect of various purine and ribose modified analogues of adenosine on secretion of histamine from rat mast cells induced by ionophore A23187 for which there is no associated elevation in cyclic AMP and no evidence for the activation of adenylate cyclase in its mechanism of action. Adenosine and N6, phenylisopropyladenosine (0.01-1000 microM) (activators of adenylate cyclase in many tissues) enhanced the secretion of histamine induced by ionophore A23187 and anti-IgE. Two inhibitors of adenylate cyclase had differential effects on secretion. 2',5'ddAdo (100-1000 microM) inhibited both A23187-and anti-IgE-mediated secretion; whilst 9-beta-D-arabinofuranosyladenine had no effect on secretion. These results suggest that the ability of these nucleosides to modulate histamine secretion is unrelated to their effects on adenylate cyclase.

Alamethicin or detergent permeabilization of the cell membrane as a tool for adenylate cyclase determination

Treatment of mouse lymphocytes with very low concentrations of alamethicin or Lubrol PX induces spontaneous permeabilization of the plasma membrane to ATP and allows determination of adenylate cyclase activity in whole cells. The permeabilized cells retain responsiveness to hormones (isoproterenol, adenosine analogs) and to fluoride. The main advantage of this new method is that it does not require any homogenization step, and thus adenylate cyclase activities can be accurately and reproducibly measured with very low amounts of cells. It should be especially useful for the study of purified lymphocyte subpopulations.

Unmasking of membrane enzyme activities and the problem of subcellular localization of adenylate cyclase in pig lymph node lymphocytes

A large-scale purification of plasma membranes from pig lymph node lymphocytes is described. Centrifugation on a discontinuous sucrose density gradient was performed in a zonal rotor. Adenylate cyclase activity of untreated fractions displayed a profile different from that of plasma membrane enzymatic markers and was maximal at higher density. However, when latent adenylate cyclase was unmasked by Lubrol PX treatment, its maximum was shifted to lower density and was no longer significantly different from that of plasma membrane markers. These results are discussed in terms of cell surface topography.

Lymphocyte membrane adenosine receptors coupled to adenylate cyclase: properties and occurrence in various lymphocyte subclasses

Lymphocytes possess membrane adenosine receptors through which this nucleoside or analogs activate adenylate cyclase. Several adenosine analogs - 5'-N-ethylcarboxamide adenosine (NECA), N6-(L-2-phenyl-isopropyl) adenosine (PIA), 2-chloro adenosine (Cl-Ado) - were tested on the adenylate cyclase of mouse thymocyte homogenates; the order of potencies: NECA greater than Cl-Ado greater than PIA is consistent with the order which has been assumed to be characteristic of Ra type adenosine receptors. The stimulation of thymocyte adenylate cyclase by NECA was biphasic while the responses to Cl-Ado and PIA were not, which suggests the existence of Ra sub-types of adenosine receptors. Moreover, these adenosine receptors were found to be present in several lymphocyte subclasses: mature and immature mouse thymocytes, B and T mouse splenocytes, human peripheral blood lymphocytes, and also in lymphoblastoid cell lines. No crucial differences were observed between the adenylate cyclase stimulations of the various population by NECA.