PMA-sensitive protein kinase C is not necessary in TRH-stimulated prolactin release from female rat primary pituitary cells

In GH3 cells and other clonal rat pituitary tumor cells, TRH has been shown to mediate its effects on prolactin release via a rise of cytosolic Ca2+ and activation of protein kinase C. In this study, we examined the role of protein kinase C in TRH-stimulated prolactin release from female rat primary pituitary cell culture. Both TRH and PMA stimulated prolactin release in a dose-dependent manner. When present together at maximal concentrations, TRH and PMA produced an effect which was slightly less than additive. Pretreatment of rat pituitary cells with 10(-6) M PMA for 24 hrs completely down-regulated protein kinase C, since such PMA-pretreated cells did not release prolactin in response to a second dose of PMA. Interestingly, protein kinase C down-regulation had no effect on TRH-induced prolactin release from rat pituitary cells. In contrast, PMA-pretreated GH3 cells did not respond to a subsequent stimulation by either PMA or TRH. Pretreatment of rat pituitary cells with TRH (10(-7) M, 24 hrs) inhibited the subsequent response to TRH, but not PMA. Forskolin, an adenylate cyclase activator, stimulated prolactin release by itself and in a synergistic manner when incubated together with TRH or PMA. The synergistic effects of forskolin on prolactin release was greater in the presence of PMA than TRH. Down-regulation of protein kinase C by PMA pretreatment abolished the synergistic effect produced by PMA and forskolin but had no effect on those generated by TRH and forskolin. sn-1,2-Dioctanylglycerol (DOG) pretreatment attenuated the subsequent response to DOG and PMA but not TRH. The effect of TRH, but not PMA, on prolactin release required the presence of extracellular Ca2+. In conclusion, the mechanism by which TRH causes prolactin release from rat primary pituitary cells is different from that of GH3 cells; the former is a protein kinase C-independent process whereas the latter is at least partially dependent upon the activation of protein kinase C.

Evidence for a role of protein kinase-C in His-D-Trp-Ala-Trp-D-Phe-Lys-NH2-induced growth hormone release from rat primary pituitary cells

We have recently reported that His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 (GHRP-6) synergizes with GH-releasing factor (GRF) to increase GH release and cAMP accumulation in rat pituitary cells in vitro. This study was undertaken to further investigate the mechanism of action of GHRP-6 on GH release, particularly the involvement of protein kinase-C. Forskolin (10(-5) M), A23187 (10(-6) M), and phorbol 12-myristate 13-acetate (PMA; 10(-7) M) all stimulated GH release. However, only PMA can mimic the synergistic effects of GHRP-6 on GRF-stimulated GH release and intracellular cAMP accumulation. 4 alpha-Phorbol 12,13-didecanoate, an inactive phorbol ester, was unable to stimulate GH release or potentiate the effect of GRF. Extracellularly added phospholipase-C not only stimulated GH release in a dose-dependent manner, but also potentiated GRF-induced GH release. Phloretin, a protein kinase-C inhibitor, in a concentration range of 10-250 microM had very little or no effect on basal and GRF-stimulated GH release, but markedly inhibited the stimulatory effects induced by either PMA or GHRP-6. Incubation of rat pituitary cells with 10(-6) M PMA for 24 h completely down-regulated protein kinase-C, since such PMA-pretreated cells did not release GH in response to a second dose of PMA. The protein kinase-C-depleted cells had an attenuated GHRP-6 response, but they responded normally to GRF. Moreover, the synergistic effects of GHRP-6 and GRF on GH release and cAMP accumulation were also greatly inhibited by protein kinase-C down-regulation. These data suggest that the effects of GHRP-6 on GH release, either alone or together with GRF, are at least partially mediated via the activation of protein kinase-C.

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Insulin-like effects of ATP on adipocyte pyruvate dehydrogenase and phosphorylase

Extracellular ATP stimulated adipocyte pyruvate dehydrogenase in a time- and dose-dependent manner with an EC50 of 0.1 mM. The maximal effect was observed at 0.5 mM ATP after a 15-min incubation with a lag period of about 5 min. Depletion of intracellular Ca2+ with ethylene glycol bis(beta-aminoethyl ether) N,N'-tetraacetic acid reduced the effect of ATP by 50% and completely abolished the stimulatory effect of vasopressin on adipocyte pyruvate dehydrogenase but had no effect on the stimulation induced by insulin or adenosine. The effects of insulin and ATP on pyruvate dehydrogenase were glucose-dependent whereas the effect of adenosine was glucose-independent. Furthermore, ATP, like insulin, partially blocked the stimulatory effect of isoproterenol on phosphorylase. Adenosine, at a concentration of 1 mM, did not affect either basal or isoproterenol-stimulated phosphorylase activities. It is concluded that ATP activates adipocyte pyruvate dehydrogenase by at least two separate mechanisms: one is Ca2(+)-dependent and the other is Ca2(+)-independent. However, neither is the result of the formation of adenosine from ATP through hydrolysis.

The synergistic effects of His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 on growth hormone (GH)-releasing factor-stimulated GH release and intracellular adenosine 3',5'-monophosphate accumulation in rat primary pituitary cell culture

His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 (GHRP-6) stimulated GH release from rat primary pituitary cells in a time- and dose-dependent manner. Stimulation was observed after a 15-min, but not a 4-h, incubation. The concentrations of GHRP-6 required for half-maximal and maximal stimulation were 7 x 10(-9) and 10(-7) M, respectively. GH release induced by GHRP-6 was not affected by the addition of either naloxone or the GRF antagonist [N-Ac-Tyr1,D-Arg2]GRF-(1-29)-NH2. The latter inhibited GRF-stimulated GH release by shifting the dose-response curve to the right. His-D-Trp-D-Lys-Trp-D-Phe-Lys-NH2, an analog of GHRP-6, inhibited GH release stimulated by GHRP-6 without affecting that induced by GRF. When present together at maximal concentrations, GHRP-6 and GRF produced a synergistic effect on GH release. GHRP-6 had no effect on intracellular cAMP levels, whereas GRF increased intracellular cAMP concentrations by 3-fold. Combined treatment of pituitary cells with GRF and GHRP-6 resulted in a potentiation of the GRF-induced increase in cAMP levels. Basal GH release was reduced by 30% after pretreatment with GHRP-6 (10(-7) M) for 1 h. Pretreatment with GHRP-6 also decreased the subsequent response to GHRP-6, but not GRF. In contrast, pretreatment with GRF for 1 h had no effect on the subsequent action of GHRP-6 or GRF on GH release. The desensitization induced by GHRP-6 was completely reversed within 1 h after removal of the peptide. Results from this study indicate that GHRP-6 and GRF stimulated GH release from somatotrophs via different receptors and through discrete mechanisms.

Timing light and tone signals in pigeons

Pigeons' ability to time light and tone stimuli was examined in four experiments. In Experiment 1, two groups of pigeons were trained to discriminate between 2- and 8-s durations of lights or tones and then were transferred to reversal or nonreversal discriminations in the alternate modality. Pigeons learned the light discrimination faster than the tone discrimination and showed immediate positive intermodal transfer from tone to light but not from light to tone. In Experiments 2-4, the peak procedure was used to study birds' timing of 15- and 30-s fixed-interval light and tone signals. Peak times on empty trials under baseline conditions closely approximated the length of fixed-interval signals. When pigeons were tested with time-outs and intermodal switches introduced midway through an empty trial, they tended to reset the timing mechanism and begin timing again from 0 s. With both estimation and production procedures, pigeons were less accurate when timing the tone stimuli than when timing the light stimuli. A comparison of these data with data from timing experiments with rats suggests several possible differences in timing processes between pigeons and rats.

Some psychophysics of the pigeon's use of landmarks

1. Three pigeons (Columba livia) were trained to find hidden food in a sunken well (3.3 cm in diameter) at a constant place within an (160 cm x 160 cm) experimental box (Fig. 1). After learning the location, the animals were tested occasionally with the well and food absent. Landmarks in the experimental box might be transformed on such tests. 2. Changing the height or width of a nearby landmark had no systematic influence on the position of peak search. Translating a nearby landmark, however, led to a shift in peak search position. All three birds then searched most somewhere between the original goal location, as defined by the unmoved landmarks, and the goal location as defined by the shifted landmark. Within a limited range of landmark shift, the peak shift as a function of landmark shift is linear (Fig. 3). 3. To explain the data (Fig. 7), the pigeon records at the location of the goal the algebraic vectors from a number of landmarks to the goal. These vectors have both a direction and a distance component. When searching for the goal again in the experimental box, it computes independently for each landmark a navigation vector. This is arrived at by vector-adding the algebraic vector from the bird's current position to the landmark in question, supplied by perception, to the corresponding landmark-goal vector in its record. The pigeon moves in the direction and distance specified by a weighted average of the independently calculated navigation vectors. For positive vector weights, vector geometry guarantees that the bird would search somewhere between the original goal and the goal according to the shifted landmark. The extent to which it shifts toward the shifted goal reflects the vector weight given to the shifted landmark.

Purification of complexes of nuclear oncogene p53 with rat and Escherichia coli heat shock proteins: in vitro dissociation of hsc70 and dnaK from murine p53 by ATP

Oligomeric protein complexes containing the nuclear oncogene p53 and the simian virus 40 large tumor antigen (D. I. H. Linzer and A. J. Levine, Cell 17:43-51, 1979), the adenovirus E1B 55-kilodalton (kDa) tumor antigen, and the heat shock protein hsc70 (P. Hinds, C. Finlay, A. Frey, and A. J. Levine, Mol. Cell. Biol. 7:2863-2869, 1987) have all been previously described. To begin isolating, purifying, and testing these complexes for functional activities, we have developed a rapid immunoaffinity column purification. p53-protein complexes are eluted from the immunoaffinity column by using a molar excess of a peptide comprising the epitope recognized by the p53 monoclonal antibody. This mild and specific elution condition allows p53-protein interactions to be maintained. The hsc70-p53 complex from rat cells is heterogeneous in size, with some forms of this complex associated with a 110-kDa protein. The maximum apparent molecular mass of such complexes is 660,000 daltons. Incubation with micromolar levels of ATP dissociates this complex in vitro into p53 and hsc70 110-kDa components. Nonhydrolyzable substrates of ATP fail to promote this dissociation of the complex. Murine p53 synthesized in Escherichia coli has been purified 660-fold on the same antibody affinity column and was found to be associated with an E. coli protein of 70 kDa. Immunoblot analysis with specific antisera demonstrated that this E. coli protein was the heat shock protein dnaK, which has extensive sequence homology with the rat hsc70 protein. Incubation of the immunopurified p53-dnaK complex with ATP resulted in the dissociation of the p53-dnaK complex as it did with the p53-hsc70 complex. This remarkable conservation of p53-heat shock protein interactions and the specificity of dissociation reactions suggest a functionally important role for heat shock proteins in their interactions with oncogene proteins.

"Stable" effects of insulin and isoproterenol on adipocyte pyruvate dehydrogenase

Insulin, at a concentration of 1 mU/ml, stimulated glycogen synthase and pyruvate dehydrogenase about threefold in isolated rat adipocytes. Upon the removal of insulin, glycogen synthase activity remained in the activated state for 10 min and thereafter rapidly returned to basal level. On the other hand, insulin-stimulated pyruvate dehydrogenase activity remained elevated for at least 30 min. Isoproterenol (10(-8) M) stimulated phosphorylase and inhibited pyruvate dehydrogenase through the activation of beta-adrenergic receptors. Addition of the beta-antagonist, propranolol (10(-5) M), after isoproterenol reversed the action of isoproterenol on phosphorylase but not its action on pyruvate dehydrogenase. Dibutyryl cyclic AMP, when added to intact adipocytes, produced an effect on pyruvate dehydrogenase similar to that induced by isoproterenol. Our results indicate that both insulin and the beta-agonist have a unique action on pyruvate dehydrogenase which is different from their effects on other enzymes such as glycogen synthase and phosphorylase.

A putative mediator of insulin action which inhibits adenylate cyclase and adenosine 3',5'-monophosphate-dependent protein kinase: partial purification from rat liver: site and kinetic mechanism of action

A novel putative mediator of insulin action which acts to inhibit adenylate cyclase and cAMP-dependent protein kinase has been purified from livers of insulin-treated streptozotocin-diabetic rats. It was increased by short term (5-min) insulin injections in vivo and purified several thousand-fold by Sephadex and HPLC. Its mol wt was somewhat larger (2500) than previous mediators identified, and it was more hydrophobic in character. Its mechanism of action or adenylate cyclase was determined and found to be chiefly directed against the catalytic subunit. Its action on the cAMP-dependent protein kinase was found to be competitive with regard to protein substrate, but noncompetitive with regard to ATP and cAMP. Its relationship to other putative insulin mediators and the mechanism of insulin action is discussed.

The total artificial heart as a bridge to transplantation. A report of two cases

In 1985, at the University of Arizona, Tucson, two attempts were made to "bridge" patients from impending death to heart transplantation, using orthotopically positioned total artificial hearts. The first attempt, using an unapproved device on an emergency basis, failed after transplantation because of severe pulmonary edema and Pseudomonas pneumonia and the apparent transmission of a Pseudomonas infection from donor to recipient. The second experience, using a Jarvik-7 device, led to stable support for nine days with one major complication, a reversible neurologic deficit with no associated computed tomographic scan abnormality. This patient survived cardiac transplantation and, after being successfully treated for complications, has made a full recovery and returned to full-time work.

Interim cardiac replacement with a mechanical heart: staged cardiac transplantation

Lack of donor heart availability complicates the management of terminally ill patients who are candidates for cardiac replacement. The total artificial heart has been used as a bridge to transplantation in three patients with terminal cardiomyopathy. Acute allograft rejection and the lack of another donor heart prompted us to use the mechanical heart as a bridge to re-transplantation in a 33-year-old man. The cardiac prosthesis functioned well for 11 hours, when a second transplantation was performed, but the patient died of right heart failure 48 hours after the second transplantation. Critical factors in such cases include (1) a prompt decision to proceed with cardiac replacement; (2) avoidance of long periods of cardiopulmonary bypass; (3) prosthetic device availability; and (4) surgical team preparedness, with technical expertise in transplantation, allograft explantation, and total artificial heart implantation/explantation, with re-transplantation.

Interim heart replacement with a mechanical device: an adjunct to management of allograft rejection

Acute heart rejection; unresponsive to immunosuppressive therapy, results in cardiogenic shock and death. In the absence of another donor heart, a total artificial heart can be used as a suitable bridging device to re-transplantation. A thirty-three year-old man rejected his allograft forty-eight hours after transplantation. A total artificial heart was used for eleven hours until another donor heart became available and was transplanted. During the bridging period, the hemodynamic performance of the mechanical prosthesis was satisfactory. The patient died forty-eight hours after re-transplantation of donor right heart failure due to pulmonary edema. This edema was felt to be related to the long periods of cardiopulmonary bypass. This unique experience illustrates the need for a prompt decision to proceed with cardiac replacement, avoiding long periods of cardiopulmonary bypass, the need for a suitable mechanical device availability, technical expertise in device implantation and allograft transplantation.

Unidirectional actions of insulin and Ca2+-dependent hormones on adipocyte pyruvate dehydrogenase

Norepinephrine and epinephrine, in the presence of the beta-adrenergic antagonist propranolol (10(-5) M), stimulated adipocyte pyruvate dehydrogenase at low concentrations but inhibited the enzyme at higher concentrations. The alpha-adrenergic agonist, phenylephrine, rapidly stimulated pyruvate dehydrogenase activity in a dose-dependent manner with maximal stimulation observed at 10(-6) M. The stimulation of pyruvate dehydrogenase by phenylephrine was mediated via alpha 1-receptors. Inhibition of pyruvate dehydrogenase by catecholamines was mediated via beta-adrenergic receptors, since the beta-agonist, isoproterenol, and dibutyryl cAMP produced similar effects. Like insulin, alpha-adrenergic agonists increased the active form of pyruvate dehydrogenase without changing the total enzyme activity and cellular ATP concentration. The effects induced by maximally effective concentrations of insulin and alpha-adrenergic agonists were nonadditive. The ability of phenylephrine and methoxamine to stimulate pyruvate dehydrogenase and phosphorylase and to inhibit glycogen synthase was not affected by the removal of extracellular Ca2+. Similarly, the stimulation of pyruvate dehydrogenase and glycogen synthase by insulin was also observed under the same conditions. However, when intracellular adipocyte Ca2+ was depleted by incubating cells in a Ca2+-free buffer containing 1 mM ethylene glycol bis(beta-amino-ethyl ether)-N,N,N' -tetraacetic acid, the actions of alpha-adrenergic agonists, but not insulin, on pyruvate dehydrogenase were completely abolished. Vasopressin and angiotensin II also stimulated pyruvate dehydrogenase in a dose-dependent manner with enhancement of glucose oxidation and lipogenesis. Our results demonstrate that the Ca2+ -dependent hormones stimulate pyruvate dehydrogenase and lipogenesis in isolated rat adipocytes, and the action is dependent upon intracellular, but not extracellular, Ca2+.

A novel mechanism for the insulin-like effect of vanadate on glycogen synthase in rat adipocytes

Vanadate activated rat adipocyte glycogen synthase similarly to insulin in a dose- and time-dependent manner. No additional effect was observed when insulin and vanadate were added together. Vanadate also partially counteracted the effect of epinephrine to activate rat adipocyte glycogen phosphorylase similarly to insulin. Inhibition of Na+K+ATPase or stimulation of hydrogen peroxide generation were shown not to be the mechanisms of the insulin-like action of vanadate on glycogen synthase. Vanadate stimulated the phosphorylation of the 95,000-dalton subunit of the insulin receptor on tyrosine residues both in intact adipocytes and in a solubilized insulin receptor fraction. Vanadate also stimulated the phosphorylation of the 95,000-dalton subunit of a highly purified insulin receptor from human placenta. Neither the insulin receptor fraction from rat adipocyte nor the highly purified insulin receptor from human placenta contained any detectable phosphotyrosine phosphatase activity. Potassium fluoride had no stimulatory effect on the phosphorylation of the insulin receptor. Vanadate caused a 10-fold decrease in the Km for ATP, for tyrosine kinase, and enhanced the phosphorylation of histone H2B. These results demonstrate that vanadate enhances the phosphorylation of the insulin receptor by stimulating the kinase reaction in a similar but not identical manner to insulin.

Cell membrane signals in the mechanism of insulin action. Claude P. Brown memorial lecture

Present evidence points to the rapid formation of one or several mediators by proteolysis initiated by insulin and, possibly, other hormones. Mediators act intracellularly at a number of subcellular sites, including cytoplasm, mitochondria, endoplasmic reticulum, cell membrane, and nucleus (figure 9). These mediators control enzymes that are controlled by covalent phosphorylation. As a result, the mediators impart an overall integrated control of metabolism. Evidence strongly suggests that the mediators are peptides. They appear to be formed by limited proteolysis from cell membrane proteins or glycoproteins and act as transmembrane signals following the binding of insulin to its receptor and the activation of the insulin-receptor complex.

'Insulin-like' effects of lithium ion on isolated rat adipocytes. II. Specific activation of glycogen synthase

Lithium ion, like insulin, activated adipocyte glycogen synthase with or without glucose in the medium. However, the effect of lithium ion was much greater than that of insulin under both conditions. The lithium-activated glycogen synthase was stable to both Sephadex chromatography and ethanol precipitation of the enzyme, indicating that the effect of lithium ion on glycogen synthase was through covalent modification of the enzyme. Glycogen synthase was significantly activated by lithium ion under conditions where concentrations of cellular ATP were unaffected. The effect of lithium ion on glycogen synthase was rapid and observed at concentrations as low as 1 to 3 mM, reaching a maximum at the concentration of 40 mM. It was thus the most sensitive of all the effects studied (see previous paper). Insulin further stimulated glycogen synthase at low concentrations but not at maximal concentration of lithium ion. Lithium-activated glycogen synthase was inhibited by both epinephrine and dibutyryl cyclic AMP, but was not affected by the removal of extracellular Ca++. Interestingly, lithium ion had no detectable effect on basal pyruvate dehydrogenase as well as on epinephrine-stimulated phosphorylase. The failure of lithium ion to thus mimic insulin actions on pyruvate dehydrogenase and on phosphorylase suggests that the action of lithium ion on glycogen synthase is quite specific and may be mediated by stimulating a phosphatase or by inhibiting a protein kinase acting specifically on glycogen synthase.

'Insulin-like' effects of lithium ion on isolated rat adipocytes. I. Stimulation of glycogenesis beyond glucose transport

Both insulin and lithium ion stimulated cytochalasin B-sensitive glucose transport in isolated rat adipocytes. As a result of enhanced glucose transport, the incorporation of [14C]glucose into CO2, glycogen and lipid were increased by both agents. However, the action of these two agents was distinguished. Cytochalasin B decreased insulin-stimulated glucose oxidation, glycogenesis and lipogenesis. In contrast, lithium-stimulated glycogenesis was decreased but lithium-stimulated glucose oxidation and lipogenesis were completely blocked. These results indicate that lithium ion in rat adipocytes has a specific insulin-like effect on glycogenesis without affecting glucose oxidation and lipogenesis.

A proteolytic mechanism for the action of insulin via oligopeptide mediator formation

Evidence is presented that the chemical mediator of insulin action is a peptide(s) and most likely glycopeptide(s). The mediator is formed proteolytically because 1) protease inhibitors inhibit insulin action and 2) trypsin mimicks insulin action via mediator formation. Trypsin mediator does not faithfully reproduce the action of insulin mediator, which indicates that the sites of proteolytic cleavage by insulin and trypsin differ. A coordinated multivalent proteolytic mechanism by which insulin acts to trigger an external membrane-bound protease to cleave mediator from a membrane glycoprotein precursor is presented.

Measurement of membrane potentials (psi) of erythrocytes and white adipocytes by the accumulation of triphenylmethylphosphonium cation

The accumulation of the lipophilic cation, triphenylmethylphosphonium, has been employed to determine the resting membrane potential in human erythrocytes, turkey erythrocytes, and rat white adipocytes. The triphenylmethylphosphonium cation equilibrates rapidly in human erythrocytes in the presence of low concentrations of the hydrophobic anion, tetraphenylborate. Tetraphenylborate does not accelerate the uptake of triphenylmethylphosphonium ion by adipocytes. The cell associated vs. extracellular distribution of the triphenylmethylphosphonium ion is proportional to changes in membrane potential. The distribution of this ion reflects the membrane potential determining concentration of the ion with dominant permeability in a "Nernst" fashion. The resting membrane potentials for the human erythrocyte, turkey erythrocyte, and rat white adipocyte were found to be -8.4 +/- 1.3, -16.8 +/- 1.1, and -58.3 +/- 5.0 mV, respectively, values which compare favorably with values obtained by other methods. In addition, changes in membrane potential can be assessed by following triphenylmethylphosphonium uptake without determining the intracellular water space. The method has been successfully applied to a study of hormonally induced changes in membrane potential of rat white adipocytes.

Studies on the insulin mediator. II. Separation of two antagonistic biologically active materials from fraction II

Insulin treatment significantly altered the elution profile of deproteinized muscle extracts chromatographed on Sephadex G-25 columns, particularly in fraction II, which contains the insulin mediator. Further purification of fraction II by high-voltage paper electrophoresis at pH 1.9 and 3.5 resulted in two active fractions. Fraction 1 leads to 4 stimulated the cyclic AMP-dependent protein kinase and inhibited glycogen synthase phosphoprotein phosphatase, and may be a novel substance. Fractions 1 leads to 6 and 3 leads to 6 inhibited the cyclic AMP-dependent protein kinase and stimulated glycogen synthase phosphatase. It is proposed that the insulin mediator is present in fractions 1 leads to 6 and 3 leads to 6.

Generation by insulin of a chemical mediator that controls protein phosphorylation and dephosphorylation

Deproteinized skeletal muscle extracts free of major nucleotides from control and insulin-treated rats were fractionated and assayed for inhibition of protein phosphorylation by cyclic adenosine monophosphate (AMP)-dependent and -independent protein kinases. A differential effect of insulin on a particular fraction was observed on cyclic AMP-dependent protein kinase but not on cyclic AMP-independent protein kinases. This fraction that inhibited cyclic AMP-dependent protein kinase also stimulated glycogen synthase phosphoprotein phosphatase. It is proposed that this fraction may contain a mediator substance generateed in the presence of insulin.

Thermochemical decomposition of water based on reactions of chromium and barium compounds

A potentially useful thermochemical cycle developed for the production of hydrogen and oxygen from water consists of three chemical reactions that take place in the temperature range from 400 degrees to 1200 degrees K. The oxidation and reduction of chromium compounds by barium hydroxide and the hydrolytic disproportionation of barium chromate(IV) and barium chromate(V), the reactions which constitute the proposed cycle, have been demonstrated.