Histamine receptors and cyclic AMP

The identification and characterization of histamine receptors in the organ systems of various species has been made possible in recent years by the introduction of relatively selective agonists and antagonists of H1 and H2 receptors. H2 receptors have now been clearly demonstrated in gastric mucosa, heart, rat uterus, brain, and adipose tissue. Less well-defined H2 receptor systems have also been described in the vasculature, bronchioles, and other smooth muscles as well as in the thyroid gland and lymphocytes. In tissues where it has been examined a close correlation between H2 receptors and the adenylate cyclase--cyclic AMP system has been found. With the exception of the central nervous system stimulation of H1 receptors does not seem to be involved with cyclic AMP. In the case of the brain the H1 receptor stimulation of adenylate cyclase can be differentiated from H2 receptor stimulation of the enzyme by the use of blocking agents and by the fact that the H1 receptor response is enhanced in the presence of adenosine. Studies of the involvement of histamine with the adenylate cyclase--cyclic AMP system have been concentrated on such tissues as gastric mucosa, heart, rat uterus, brain, and adipose tissue. The present review will concentrate on the literature concerning those tissues.     

Interdomain interaction of cyclic AMP receptor protein in the absence of cyclic AMP

Interdomain interaction of apo-cyclic AMP receptor protein (apo-CRP) was qualified using its isolated domains. The cAMP-binding domain was prepared by a limited proteolysis, while the DNA-binding domain was constructed as a recombinant protein. Three different regions making interdomain contacts in apo-CRP were identified by a sequence-specific comparison of the HSQC spectra. The results indicated that apo-CRP possesses characteristic modules of interdomain interaction that are properly organized to suppress activity and to sense and transfer the cAMP binding signals. Particularly, the inertness of the DNA-binding motif in apo-CRP was attributable to the participation of F-helices in the interdomain contacts.     

The measurement of cyclic GMP and cyclic AMP phosphodiesterases

Methods are described for measuring phosphodiesterases for cGMP and cAMP in the range of activity yielding 10⁻¹² to 10⁻⁸ mol of product. The 5′-GMP formed is measured by conversion to GDP with guanylate kinase. Amounts of GDP greater than 10⁻¹⁰ mol are measured directly with an enzyme system which results in stoichiometric oxidation of NADH. This is either determined by the decrease in fluorescence or the excess NADH is destroyed with acid and the NAD⁺ measured by its fluorescence in strong NaOH. With smaller amounts of GDP, sensitivity is amplified 1000-fold with the succinic thiokinase-pyruvate kinase cycle. In the case of cAMP diesterase, larger amounts of 5′-AMP are measured in the same way as 5′-GMP, except that adenylate kinase is substituted for guanylate kinase. With smaller amounts, the 5′-AMP is converted to ATP, and sensitivity is amplified with the adenylate kinase-pyruvate kinase cycle. As little as 20 ng dry weight of average brain is sufficient for accurate assay of the diesterase activity toward either cAMP or cGMP. When there is danger of significant destruction of AMP or GMP by tissue 5′-nucleotidase, this is prevented by adding GMP to the cAMP reagent, AMP to the cGMP reagent, or 5′-UMP to either reagent.     

Induction of cyclic AMP phosphodiesterase in Blastocladiella emersonii and its relation to cyclic AMP metabolism.

Extracts of vegetative cells of Blastocladiella emersonii contain 5% or less of the cyclic AMP phosphodiesterase activity in zoospore extracts. This difference in activity could be accounted for entirely by an increase in the differential rate of phosphodiesterase synthesis during sporulation, beginning after a lag period of about 60 min and extending for at least an additional 90 min into the 4-h sporulation process. To examine the relation between enzyme synthesis and cyclic nucleotide metabolicm, we determined the substrate specificity of phosphodiesterase synthesized during sporulation and partially purified from zoospores. Zoospore extracts contain two components, separable by gel filtration chromatography, with cyclic AMP phosphodiesterase activity. The larger component accounts for 20% of the total activity and the smaller component for 80%. Both components show essentially an absolute substrate specificity for cyclic AMP among several cyclic purine and cyclic pyrimidine nucleotides tested. Nevertheless, we found no change in the total cyclic AMP content of sporulating cells before, during, or after enzyme activity increased. We speculate that some other component of cyclic AMP metabolism or function limits the rate of cyclic AMP hydrolysis in sporulating cells.     

Endogenous cyclic AMP in thyroid cell culture. Effect of thyroid-stimulating hormone and dibutyryl cyclic AMP

We have studied the variations of endogenous cyclic AMP levels in thyroid cells cultured over a period of 7 days in several conditions: in the presence of thyroid-stimulating hormone or dibutyryl cyclin AMP which both promote the aggregation of isolated cell into follicles, and in their absence when cells develop as a typical monolayer. In follicle-forming cells, the cyclic AMP level was found to rise during the first day of culture, then to fall rapidly. In monolayer-forming cells, the cyclic AMP content slightly increases attaining the same level as found in other cells at the fourth day, which remains stable till the seventh day. We have investigated the response of these cells to the acute effect of thyroid-stimulating hormone: only cells cultured in the presence of dibutyryl cyclic AMP retain the capability of increasing their cycli AMP concentration whereas monolayer-forming cells do not preserve this quality of thyroid cells.     

Rapid assay for cyclic AMP and cyclic GMP phosphodiesterases.

A rapid highly sensitive assay for cyclic AMP phosphodiesterase has been devised. After a 5-min incubation, cyclic AMP is readily resolved from 5′-AMP, adenosine, and inosine by ion-exchange thin-layer chromatography on 1.3 × 6.5-cm strips of PEI-cellulose for 7 to 8 min. This procedure combines the accuracy of the standard paper chromatography assay (1) with the speed of ion-exchange resin techniques (2), while surmounting some of the major drawbacks of the other two methods (3). Since chromatography on PEI-cellulose efficiently resolves cyclic GMP, 5′-GMP, and guanosine, this methodology has also been adapted to the measurement of cyclic GMP hydrolysis.     

Glucose synthesis by isolated guinea-pig hepatocytes. Effect of cyclic AMP and dibutyryl cyclic AMP.

In isolated guinea-pig hepatocytes, dibutyryl cyclic AMP stimulated gluconeogenesis from 2 mM galactose by 25 and 40% respectively. In the presence of 0.5 mM theophylline, cyclic AMP (0.1 mM) increased glucose synthesis from lactate and galactose by 26 and 34% respectively.     

Iontophoresis of cyclic AMP.

The design, calibration, and operation of a source of controlled amounts of cyclic AMP (c-AMP) are described. Typically, 1.5 s pulses containing 10(10)-10(-12) molecules of c-AMP can be delivered to a region about 10 mum in diameter on an agar plate. The resulting concentration profiles are given as functions of distance and time. The diffusion coefficient of c-AMP in agar was measured to be 0.97 times 10(-5) cm2-s-1 at 21 degrees C.     

Extracellular metabolism of cyclic AMP

Intravenously administered cyclic [8-³H]AMP to rats was quickly eliminated from the circulation. After 2 min 93% of the administered radioactivity disappeared from the plasma, and most of it was recovered in the kidney, liver and muscles. The label in the tissues was recovered mainly in the form of nucleotides, ATP, ADP, AMP and IMP.In vitro contact of cyclic AMP with perfused liver, isolated liver cells and adipose tissue resulted in a rapid breakdown of the nucleotide, presumably on the outer surface of the cells. The degradation products have been identified mainly as adenosine and inosine.Incubation of adipose tissue and isolated liver cells with [³H]AMP also resulted in the breakdown of the nucleotide in the medium. The rate of AMP degradation by these tissues was faster than that for cyclic AMP degradation.The data suggest that cyclic AMP is readily metabolized on the outer surface of cells to products which may be converted within the cells to nucleotides. These findings seem of importance for the quantitative assessments of cellular cyclic AMP outflow during hormonal stimulation.     

Inhibition by cyclic AMP and dibutyryl cyclic AMP of transport of organic acids in kidney cortex.

1. Cyclic adenosine 3',5'-monophosphate and N-6-2'-O-dibutyryl cyclic adenosine 3',5'-monophosphate decrease the initial entry rate and the steady-state uptake of p-aminohippurate and uric acid by rabbit kidney cortex slices. 2. N-6-2'-O-Dibutyryl adenosine 3'-5'-monophosphate inhibits the tubular transport of p-aminohippurate competitively. 3. Isoproterenol, known to increase cyclic nucleotide concentration of the cortical tubules by activation of adenyl cyclase, decreases p-aminohippurate transport. Antidiuretic hormone which is known to stimulate only medullary adenyl cyclase has no effect on p-amino-hippurate uptake by cortical slices. 4. Theophylline, which inhibits cyclic nucleotide phosphodiesterase and, therefore, enhances the cellular accumulation of endogenous cyclic nucleotide, depresses p-aminohippurate transport.     

Enzymatic radioiodination of succinyl cyclic AMP tyrosine methyl ester by lactoperoxidase and radioimmunoassay for cyclic AMP.

A highly specific and simple radioimmunoassay for cyclic AMP with a sensitivity of 0.04 picomoles/tube has been developed according to the method of Steiner et al., using 125I-succinyl cyclic AMP tyrosine methyl ester as a tracer. The tracer with higher immunoactivities could be simply and constantly prepared by an enzymatic iodination procedure utilizing lactoperoxidase, radioactive iodide and hydrogen peroxide generated by glucose-glucose oxidase system, rather than by chloramine-T procedure.     

Endogenous cyclic AMP in thyroid cell culture. Effect of thyroid-stimulating hormone and dibutyryl cyclic AMP.

We have studied the variations of endogenous cyclic AMP levels in thyroid cells cultured over a period of 7 days in several conditions: in the presence of thyroid-stimulating hormone or dibutyryl cyclic AMP which both promote the aggregation of isolated cells into follicles, and in their absence when cells develop as a typical monolayer. In follicle-forming cells, the cyclic AMP level was found to rise during the first day of culture, then to fall rapidly. In monolayer-forming cells, the cyclic AMP content slightly increases attaining the same level as found in other cells at the fourth day, which remains stable till the seventh day. We have investigated the response of these cells cultured in the presence of dibutyryl cyclic AMP retain the capability of increasing their cyclic AMP concentration whereas monolayer-forming cells do not preserve this quality of thyroid cells.     

The dependence of Escherichia coli asparaginase II formation on cyclic AMP and cyclic AMP receptor protein.

The amount of asparaginase II in an Escherichia coli wild-type strain (cya+, crp+) markedly increased upon a shift from aerobic to anaerobic growth. However, no such increase occurred in a mutant (cya) lacking cyclic AMP synthesis unless supplemented with exogenous cyclic AMP. Since a mutant (crp) deficient in cyclic AMP receptor protein also did not support the anaerobic formation of this enzyme, it is concluded that the formation of E. coli asparaginase II depends on both cyclic AMP and cyclic AMP receptor protein.     

Rabbit atrial histamine receptors and cyclic AMP

The present study confirms the presence of H2 receptors in rabbit right and left atrium by measuring changes in cyclic AMP. Histamine produced significant increases in cyclic AMP in both atria although the increases were less in left atria. Since H2 receptors are always associated with the adenylate cyclase--cyclic AMP system, the data provide further support for the suggestion that H2 receptors exit in rabbit right and left atria.