Some aspects of the phosphorylation of phenylalanine 4-monooxygenase by a calcium-dependent and calmodulin-dependent protein kinase

A calmodulin-dependent protein kinase purified from liver catalyzed the incorporation of up to 0.7 mol of phosphate per mol subunit of phenylalanine 4-monooxygenase. The phosphorylation was accompanied by a proportional increase in the hydroxylase activity. The reaction was Ca2+-dependent and was inhibited by physiological concentrations of phenylalanine. Phenylalanine 4-monooxygenase was also a substrate for the cGMP-dependent protein kinase, but in this system phenylalanine stimulated the rate of phosphorylation to a similar extent as that observed in the reaction catalyzed by cAMP-dependent protein kinase. The hydroxylase was not a substrate for phosphorylase kinase. The calmodulin-dependent reversal of the kinase reaction in the presence of MgADP, was also inhibited by phenylalanine. Since the kinetics of the reverse reaction was the same using 32P-hydroxylase phosphorylated by calmodulin-dependent and cAMP-dependent kinases, it is likely that both kinases phosphorylate the same site on the enzyme. This conclusion was further supported by peptide mapping of tryptic and peptic digests of 32P-hydroxylase, which revealed one major phosphopeptide with enzyme phosphorylated by either kinase. The Ca2+-dependent and calmodulin-dependent phosphorylation described above may mediate the increased phosphorylation of the hydroxylase [Garrison, J. C., Johnsen, D. E., and Campanile, C. P. (1984) J. Biol. Chem. 259, 3283-3292] and its increased activity [Fisher, M. J., Santana, M. A., and Pogson, C. I. (1984) Biochem. J. 219, 87-90] recently observed in hepatocytes exposed to Ca2+-elevating agents.

Antidepressant tachyphylaxis

We report eleven patients in whom tachyphylaxis to antidepressants, lithium or ECT occurred. These cases are a small sample of patients we have encountered in whom an initial good response to mood regulating drugs was not sustained. We suspect that systematic studies would reveal that tachyphylaxis is a common phenomenon with antidepressant therapies.

Phosphorylation of cytosolic proteins by resting and activated human neutrophils

A study was conducted on the phosphorylation of proteins in the neutrophil cytosol in response to phorbol myristate acetate (PMA) and N-formyl-methionyl-leucyl-phenylalanine (fMLP). Autoradiography of gel electrophoretograms prepared from neutrophils incubated with 32Pi in the presence and absence of the activators showed nine proteins whose state of phosphorylation was affected by neutrophil activation. 32P was gained by eight of these proteins and was lost by the ninth. For all but one of these proteins, the change in the extent of labeling appeared to reach completion by one to two minutes. It was possible to quantitate the changes in 32P content of three of the nine proteins. One of these was the 20-kD protein that lost label when the neutrophils were activated. Quantitation showed that over half the 32P present in this protein in the resting state was gone within 0.2 minutes after activation. The other two were proteins weighing 11 and 69 kD. The phosphorylation characteristics of these two proteins differed, depending on whether activation had been carried out with PMA or fMLP. These differences in protein phosphorylation support other evidence suggesting that PMA and fMLP do not activate neutrophils by identical biochemical pathways. Differences in phosphorylation between resting and activated cells were not affected by dibutyryl cyclic guanosine monophosphate (cGMP), dibutyryl cyclic adenosine monophosphate (cAMP), theophylline, aspirin, hydrocortisone, or colchicine. The differences were abolished, however, by 30 mumol/L trifluoperazine. This finding is consistent with the hypothesis that the calcium/calmodulin system plays a biochemical role in the activation of neutrophils.

[Hypothalamo-hypophyseal-adrenal system in the regulation of immunologic processes]

Glucocorticoids are concluded to be modulators of immune reactions capable of changing different parameters of the immune responses. The conclusion is based on investigation of correlations between endogenous glucocorticoids level and intensity of humoral and cell-mediated immune response in different experimental sets. Changes of cyclic nucleotides level in spleen lymphocytes induced by antigen were shown to depend on the intensity of hormonal shifts. The major significance of the intensity of hormonal changes and the sensitivity of cell populations to hormones for the final hormonal influence, is emphasized.

Regulation of cyclic nucleotides in retinal photoreceptors. An ultracytochemical approach on the role of cyclases

The distribution of cyclases in retinal photoreceptors of dark- and light-adapted brook trout was studied by means of a cytochemical method (lead precipitation). It confirms earlier reports that retinal photoreceptors contain high levels of cyclic nucleotides, and that cAMP predominates in cones and cGMP in rods. There is an apparent difference in the level of the cyclases with the adaptive states. In addition, the catalytic unit of cyclase is interlamellar in cones. In rods, adenylate cyclase is intradiscal, while the location of guanylate cyclase varies with the adaptive state. The variation of cyclase with adaptation indicates that this enzyme has a role in the process of visual transduction.

Effect of papaverine on organic anion transport in rat kidney cortical slices

The present studies were conducted to further investigate a role of cyclic nucleotides for organic anion transport in rat kidney cortical slices by using papaverine as a phosphodiesterase inhibitor. Papaverine was chosen for this study because this drug is an organic cation, a transport system of which is thought to be clearly separable from an organic anion transport system. Papaverine inhibited not only p-aminohippuric acid (PAH) accumulation in the slices but also urate one. Pretreatment of the slices with papaverine was effective for significant inhibition of PAH accumulation during subsequent incubation in the absence of papaverine. The present results suggest that a relationship may exist between cyclic nucleotides and organic anion transport in rat kidney cortical slices, although further work is necessary before the above evaluation can be given.

Mechanism of action of gonadotropin-releasing hormone-stimulated Leydig cell steroidogenesis. I. The stimulatory effect is calcium dependent and not mediated by cyclic nucleotides

The present study was designed to elucidate mechanisms responsible for gonadotropin-releasing hormone (GnRH)-stimulated testosterone formation. Purified Leydig cells from adult Sprague-Dawley rats were incubated with varying concentrations of GnRH agonist (des-Gly10, (D-Ala6) GnRH N-ethylamide), hCG, 8-bromo cAMP or pregnenolone; testosterone, cAMP, cyclic GMP (cGMP) and cAMP-dependent protein kinase activity were measured after various time periods. Basal testosterone levels were 2.54 +/- 0.13 ng/10(5) cells, increasing to 3.18 +/- 0.14, 4.32 +/- 0.08, and 4.63 +/- 0.12 ng within 1 hour after the addition of 10(-9), 10(-8), and 10(-7) M GnRH agonist, respectively. After a 3-hour incubation a 10(-7) M dose of GnRH agonist increased testosterone production four-fold above control. GnRH agonist potentiated hCG-stimulated testosterone formation, but had no significant effects on cGMP levels and cAMP-dependent protein kinase activity. Cyclic AMP levels in the incubation medium increased slightly. GnRH agonist also enhanced 8-bromo-cAMP and pregnenolone-induced testosterone formation. Furthermore, GnRH agonist increased testosterone formation both in the absence and presence of phosphodiesterase inhibitor. These results suggest that the major effect of GnRH agonist is probably beyond the cAMP step. When purified Leydig cells were incubated in a calcium-free medium, the stimulatory effects of GnRH agonist on testosterone formation were completely abolished, but could be restored by the addition of calcium to the incubation medium. GnRH agonist-induced testosterone formation was also blocked by the addition of nifedipine (a calcium channel blocking agent, 0.1 to 10 micrograms/ml). Finally, GnRH antagonist in a concentration of 10 micrograms/ml completely inhibited GnRH agonist-stimulated testosterone formation.

IN CONCLUSION

GnRH agonist stimulated Leydig cell testosterone formation in short-term incubations. The stimulatory effect is calcium dependent and not mediated by cyclic nucleotides.

The role of cyclic nucleotides and calcium in the mediation of the modulatory effects of octopamine on locust skeletal muscle

The role of cyclic AMP and calcium in the mediation of the effects of octopamine has been investigated in the extensor tibiae muscle of the hind leg of the locust. Elevation of cyclic AMP levels in the preparation by means of the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX), by means of the diterpene adenylate cyclase activator, forskolin, and by means of cyclic nucleotide analogues mimics the post-synaptic effects of octopamine application at different frequencies of neuronal stimulation. These conditions also mimic the presynaptic effects of octopamine on spontaneous release of transmitter from the slow motoneurone. The effects of octopamine on the preparation are calcium sensitive, with the maximal sensitivity occurring between 0.5 and 4.0 mM-external calcium. The results are discussed in terms of the role of cyclic AMP and calcium in the mediation of the effects of octopamine.

Brain cyclic nucleotides and the development of convulsion, with reference to the anticonvulsant activity of diazepam

Dibutyryl cyclic GMP (DbcGMP) or dibutyryl cyclic AMP (DbcAMP) given to mice intracerebroventricularly in a dose of 200 or 400 nmol/head produced electroencephalographic and electromyographic changes corresponding to twitches, and clonic and tonic convulsions. Lower doses of DbcGMP or DbcAMP facilitated the pentylenetetrazol-induced clonic and tonic convulsions. Diazepam given intraperitoneally in a dose of 0.5 mg/kg suppressed clonic and tonic convulsions induced by DbcGMP and DbcAMP, but the twitches were not suppressed. These results suggest that these nucleotides lower the threshold for electrical discharges related to convulsion and facilitate the propagation of convulsion. Diazepam suppresses this facilitation.

Na/K/Cl co-transport and its regulation

NaCl and Na/K/Cl co-transport systems in many cell types show a number of similarities, among which are sensitivity to ‘loop’ diuretic inhibition and extremely high anion selectivity. Avian erythrocytes possess a NA+K+2Cl co-transporter that is stimulated by agents raising intracellular cAMP. The system is also inhibited following ATP-depletion of the cells, in agreement with results in other tissues; this nucleotide may exert a regulatory role on the operation of the co-transporter. A Na/K/Cl co-transport system is also present in flounder intestine, where it plays a central role in salt absorption at the luminal border of the tissue. In contrast to the avian erythrocyte this system is inhibited by raising intracellular cyclic nucleotide content, cGMP being more effective than cAMP. From these results it is concluded that second messenger regulation of Na/K/Cl co-transport processes is heterogeneous and may exhibit some tissue specificity.

Role of cyclic nucleotides and energy-rich phosphates during energetic deficiency in frog heart

Mechanical performance, energy-rich phosphates and cyclic nucleotides of frog heart were measured during energetic deficiency and subsequent addition of adrenaline. When oxidative metabolism was inhibited by 3 mM cyanide, tension decrease was accompanied by a decrease in creatine phosphate while ATP and cyclic nucleotides did not vary significantly. After subsequent addition of adrenaline mechanical activity remained less than control value; creatine phosphate (CP) concentration was further decreased while cAMP was increased in the same proportion as when adrenaline alone was added. In the presence of cyanide, the weak inotropic effect of adrenaline is not due to an alteration of cyclic nucleotides but is rather correlated to a further decrease in energy-rich phosphates, mainly in creatine phosphate. These results suggest that creatine phosphate may control contractile activity and may be a limiting factor for inotropic interventions at least during energetic inhibition.

Cyclic nucleotides and calcium: their role in the control of cell communication in the heart

The effects of theophylline and di-butyryl cyclic adenosine 3',5'-monophosphate (db-cAMP) on the electrical coupling of heart cells were investigated in rat trabeculae. Theophylline (4 X 10(-4) M) and db-cAMP (5 X 10(-5) M) increased both the space constant and conduction velocity. The time constant of the membrane was not changed by either drug. Measurements of the time constant of the foot of the action potential and conduction velocity were used to calculate the intracellular longitudinal resistance. Both theophylline and db-cAMP were found to enhance cell-to-cell communication in the heart by decreasing the intracellular longitudinal resistance.

[Participation of histamine and cyclic nucleotides in the regulation of gastric secretion in experimental thyrotoxicosis]

Experiments on 48 Wistar rats evidence that in early times of hyperthyroidism, the action of thyroxine on parietal cells is mediated by histamine and cAMP, while in prolonged hyperthyroidism, its action on parietal cells refractory to endogenous histamine and cAMP is found to be unmediated and direct.

[Effects of colchicine, cyclic nucleotides and biogenic amines on the chemoreceptor apparatus of the tongue]

Colchycine considerably decreased sensitivity of the Rana temporaria tongue's chemoreceptors to adequate stimulation. The effect is reversible and can be abolished by administration of drugs initiating setting of the cells' microtubular apparatus: 3',5'-cAMP, theophylline, adrenaline as well as by activation of the sympathetic system. The restoring effect of these agents is specific since 2',3'-cAMP, 5'-AMP, serotonin, inosine, acetylcholine and 3,'5'-cGMP prove ineffective in restoring the reactions after colchycine administration although they do alter the activity of the receptors in normal conditions. The revealed colchycine-sensitive, 3',5'-cAMP-dependent process seems to be associated with the microtubular apparatus of the receptor cell. Its role in the receptor structures is discussed.