Properties of different Ca2+ pools in permeabilized rat thymocytes

Heme-arginate suppresses phospholipase C and oxidative stress in the mesenteric arterioles of mineralcorticoid-induced hypertensive rats

Induction of heme-oxygenase (HO) is an important cellular defense mechanism against oxidative and inflammatory insults. We analyzed the effects of the HO inducer, heme-arginate, on the phospholipase C (PLC)/inositol-triphosphate (IP(3)) pathway in the mesenteric arterioles of uninephrectomized (UnX) deoxycorticosterone acetate (DOCA)-salt hypertensive rats, which is a volume-overload model characterized by elevated endothelin (ET-1) and mineralocorticoid-induced oxidative/inflammatory insults. Our study included the following groups: (A) controls [(i) surgery-free Sprague-Dawley (SD) rats, (ii) UnX-Sham, (iii) UnX-Salt (0.9% NaCl+0.2% KCl) and (iv) UnX-DOCA)]; (B) UnX-DOCA-salt hypertensive rats; (C) UnX-DOCA-salt+heme-arginate; (D) UnX-DOCA-salt+heme-arginate+chromium mesoporphyrin (CrMP), the HO inhibitor; (E) UnX-DOCA-salt+CrMP (F); SD+heme-arginate, (G) UnX-DOCA-salt+vehicle dissolving heme-arginate and CrMP and (H) normal-SD+heme-arginate. Quantitative reverse transcriptase PCR, western blot, enzyme immunoassay and spectrophotometric analyses were used. Heme-arginate enhanced mesenteric arteriole HO-1, HO activity, cyclic guanosine monophosphate (cGMP) and anti-oxidants including bilirubin, ferritin, superoxide dismutase with potentiation of the total anti-oxidant capacity. Correspondingly, oxidative/inflammatory mediators such as 8-isoprostane, nuclear-factor kappaB (NF-kappaB) and ET-1 were markedly reduced. Furthermore, heme-arginate suppressed PLC activity, attenuated IP(3) and reduced resting intracellular calcium. The effects of heme-arginate were nullified by the HO inhibitor, with aggravation of oxidative/inflammatory insults. In heme-arginate-treated SD rats, the HO system was potentiated to a lesser magnitude and the suppression of ET-1, PLC, IP(3) and NF-kappaB were less accentuated, suggesting greater selectivity of HO against the ET-1-PLC-IP(3)-NF-kappaB destructive axis in the pathological condition of mineralocorticoid-induced hypertension. Given that ET-1 stimulates PLC and IP(3), which in turn activates NF-kappaB, the concomitant reduction of ET-1, PLC, IP(3) and NF-kappaB alongside the corresponding decline of resting intracellular calcium may account for the reduction of blood pressure and attenuation of oxidative/inflammatory injury by heme-arginate.

Crosstalk between the heme oxygenase system, aldosterone, and phospholipase C in hypertension

BACKGROUND

Aldosterone is a mineral corticoid hormone that is produced in response to angiotensin-II, and like angiotensin-II, stimulates inflammation, oxidative stress, and fibrosis by activating nuclear factor-kappaB and activating protein-1. Recent evidence, however, indicates that aldosterone stimulates phospholipase C and activates nuclear factor-kappaB and activating protein-1. Although the heme oxygenase system is cytoprotective, its effects on aldosterone-phospholipase C signaling in deoxycorticosterone acetate (DOCA-salt) hypertension, a model of aldosteronism, and spontaneously hypertensive rat, a genetic model of human essential hypertension, have not been fully characterized.

METHODS

In the present study, the heme oxygenase inducer, hemin, was given to spontaneously hypertensive and deoxycorticosterone acetate hypertensive rats, and the effects on blood pressure, aldosterone, nuclear factor-kappaB, activating protein-1, phospholipase C, and inositol 1,4,5-triphosphate were examined.

RESULTS

Hemin therapy restored physiological blood pressure to spontaneously hypertensive rats (209.9 +/- 0.9 to 127.3 +/- 0.85 mmHg, n = 10, P < 0.01) and to deoxycorticosterone acetate salt hypertensive rats (195.7 +/- 1.8 vs.132.5 +/- 2.1 mmHg; P < 0.01, n = 10), but had no effect on age-matched normotensive Wistar-Kyoto or Sprague-Dawley strains. The antihypertensive effect was accompanied by enhanced heme oxygenase activity, upregulated cyclic guanosine monophosphate-protein kinase G signaling, increased superoxide dismutase activity, and the potentiation of total antioxidant capacity, whereas aldosterone, activating protein-1, and nuclear factor-kappaB were reduced. Furthermore, hemin suppressed phospholipase C activity, attenuated inositol 1,4,5-triphosphate, and reduced resting intracellular calcium in the aorta.

CONCLUSION

Collectively, our results suggest that the concomitant depletion of aldosterone, phospholipase C-inositol 1,4,5-triphosphate activity, resting intracellular calcium and the corresponding decline of inflammatory, and oxidative insults may account for the antihypertensive effects of hemin in deoxycorticosterone acetate hypertension and spontaneously hypertensive rats.

Targeted polyphosphatase expression alters mitochondrial metabolism and inhibits calcium-dependent cell death

Polyphosphate (polyP) consists of tens to hundreds of phosphates, linked by ATP-like high-energy bonds. Although polyP is present in mammalian mitochondria, its physiological roles there are obscure. Here, we examine the involvement of polyP in mitochondrial energy metabolism and ion transport. We constructed a vector to express a mitochondrially targeted polyphosphatase, along with a GFP fluorescent tag. Specific reduction of mitochondrial polyP, by polyphosphatase expression, significantly modulates mitochondrial bioenergetics, as indicated by the reduction of inner membrane potential and increased NADH levels. Furthermore, reduction of polyP levels increases mitochondrial capacity to accumulate calcium and reduces the likelihood of the calcium-induced mitochondrial permeability transition, a central event in many types of necrotic cell death. This confers protection against cell death, including that induced by beta-amyloid peptide, a pathogenic agent in Alzheimer's disease. These results demonstrate a crucial role played by polyP in mitochondrial function of mammalian cells.

Modulation of intracellular Ca(2+) concentration by vitamin B12 in rat thymocytes

We have studied several novel effects of vitamin B12 (cyanocobalamin) on cellular Ca(2+) homeostasis in rat thymocytes. We determined the effect of various concentrations of vitamin B12 on intracellular Ca(2+) concentration ([Ca(2+)]i) and parameters of Ca(2+)in signaling using the fluorescent dye Fura-2. The basal [Ca(2+)]i in Ca(2+)-containing media was 115 +/- 5 nM but in vitamin B12 (10 nM)-treated thymocytes [Ca(2+)]i was decreased to 60 +/- 15 nM (mean +/- SEM) during the first 5 min. The decline in [Ca(2+)]i was accompanied by an increase in the endoplasmic reticulum Ca(2+) store, presumably as a result of Ca-ATPase activation. At the same time 100 nM-10 mM B12 induced the accumulation of Ca(2+) in mitochondria. Somewhat higher concentrations of B12 (1-10 microM) had no effect on [Ca(2+)]i. A further increase in B12 concentration with range from 50 microM to 1 mM caused a dose-dependent elevation of [Ca(2+)]i from the basal level (115 +/- 5 nM) up to 200 +/- 50 nM in thymocytes, and this elevation was partially blocked in Ca(2+)-free media. This high concentration of vitamin B12 caused a gradual decrease of endoplasmic reticulum Ca(2+) stores by means of Ca-ATPase inhibition. The B12-induced increase in [Ca(2+)]i was not observed after depletion of intracellular Ca(2+) stores, induced by addition of 2',5'-di(tert-butyl)-1,4-benzohydroquinone (BHQ), an inhibitor of endoplasmic reticulum Ca (2+)-ATPase, concanavalin A, or arachidonic acid. These studies show that vitamin B12 regulates [Ca(2+)]i via several different mechanisms at different B12 concentrations. Participation of G proteins and calmodulin activity in B12-mediated [Ca(2+)]i increase is discussed.

Ionophoretic properties of ferutinin

The influence of the natural terpenoid ferutinin (4-oxy-6-(4-oxybenzoyloxy) dauc-8,9-en), isolated from the plant Ferula tenuisecta, on ion permeability of biological and artificial membranes was investigated. It was shown that ferutinin, in the concentration range 1-50 microM, increases the permeability of thymocytes, mitochondria, sarcoplasmic reticulum, liposomes and bilayer lipid membranes (BLM) for Ca2+. Ferutinin establishes a transmembrane potential in BLM equal to the Nernst's potential. The permeability ratio for Na+/Ca2+ is 0.41. The dependence of BLM conductivity on ferutinin concentration is linear. The stoichiometry of the ferutinin:Ca2+ complex is 2, assuming the formation of a structure with participation of two terpenoid molecules and one Ca2+ ion.

Effect of the sulfhydryl reagent thimerosal on cytosolic free Ca2+ and membrane potential of thymocytes

The sulfhydryl reagent thimerosal at concentrations 5-100 microM has been found to induce a variety of changes in ion transport in rat thymocytes. In particular, [Ca2+]i increases about 10-fold from the basal level. The [Ca2+]i response to thimerosal displays a two-stage time course, with the main [Ca2+]i rise during the second stage. Evidence has been obtained for the depletion of intracellular Ca2+ pools in thimerosal-treated cells, however, Ca2+ mobilization from intracellular stores does not contribute significantly into [Ca2+]i rise. Thimerosal elicits permeability not only for Ca2+, but also for Mn2+ and Ni2+, which is Ca(2+)-dependent. We failed to get any evidence on thimerosal-induced inhibition of the plasma membrane Ca(2+)-ATPase. The induction of Ca2+ influx, rather than inhibition of Ca(2+)-ATPase, accounts for the disturbance of [Ca2+]i homeostasis in thimerosal-treated cells. Thimerosal also elicits changes in monovalent ion fluxes resulting in marked depolarization. The latter seems unrelated to the changes in [Ca2+]i and is suggested to be mediated both by increased permeability for Na+ and a decreased one for K+. Thimerosal significantly stimulates AA release from thymocytes. Evidence has been presented that AA metabolite(s), probably, LO product(s), may mediate the changes in the transport of mono- and divalent cations elicited by the sulfhydryl reagent. Prolonged treatment of thymocytes with thimerosal resulted in cell death.

The relationship between mitogen-induced changes in the cytoplasmic pH and free Ca2+ concentration in rat thymocytes

The relationship between pHi and [Ca]i signals generated in rat thymocytes by the mitogen Con A has been investigated. It is shown that the mitogen-induced [Ca]i rise is dependent on Na+/H+ exchange or some other Na(+)-sensitive process. This conclusion is based on the following findings: (i) [Ca]i response to Con A weakens upon decreasing the concentration of extracellular Na+, or inhibiting Na+/H+ exchange; (ii) agents that alkalinize the cytoplasm (the phorbol ester TPA, the Na+/H+ ionophore monensin and NH4Cl) cause an increase in [Ca]i (Klip, A., Rothstein, A. and Mack, E. (1984) Biochem. Biophys. Res. Commun. 124, 14-22; Grinstein, S. and Goetz, J.D. (1985) Biochim. Biophys. Acta 819, 267-270); (iii) The effects of Con A, TPA and monensin on [Ca]i are not additive. The last observation suggests that all these agents activate the same Na+/H+ (Na+ and/or H+)-dependent system of Ca2+ transport. It is found that the pH i and [Ca]i responses in rat thymocytes are sensitive to changes in the intracellular levels of cyclic nucleotides, ATP and in temperature. These regulatory effects on the ionic signals are different for Con A, TPA and monensin. In particular, both the stimulation of Na+/H+ antiport and the [Ca]i rise brought about by Con A or TPA are inhibited upon elevating the cellular cAMP. In contrast, the monensin-induced [Ca]i signal is almost independent of cAMP but is highly sensitive to changes in cGMP and temperature. Reducing the ATP level eliminates both the pHi and [Ca]i responses to Con A but not to monensin. These different characteristics of [Ca]i signals elicited by the mitogen and the Na+/H+ ionophore indicate that these agents use different mechanisms to activate the Na+/H(+)-dependent Ca2+ transporting system. A [Ca]i response to monensin has been obtained in some other cell types, namely, in lymphoblastoid Raji cells, Ehrlich ascites tumor cells and also in platelets.

Stimulation of respiration by mitogens in rat thymocytes is independent of mitochondrial calcium

The role of calcium in the control of respiration by the mitogen concanavalin A (ConA) was investigated in rat thymocytes. ConA induced an increase in both mitochondrial respiration and the mitochondrial calcium pool. The stimulation of respiration was shown to be independent of the increase in mitochondrial calcium: the calcium pool declined after 3 min, whereas the respiration increase was persistent, and was not affected by depletion of the calcium pool or by buffering intracellular Ca2+ transients with quin2. The mitogen phytohaemagglutinin stimulated respiration to the same extent as ConA, but did not increase the mitochondrial calcium pool. In addition, respiration was unaffected by changes in the mitochondrial calcium pool induced by increasing or decreasing extracellular calcium. These results indicate that control of respiration is not located in the Ca2+-sensitive mitochondrial dehydrogenases. The ConA-induced increase in respiration could be blocked by oligomycin, suggesting control by cytoplasmic ATP turnover, and was not associated with detectable changes in NAD(P)H fluorescence, indicating a balance between increased electron transfer and increased supply of reduced substrates.

Mitogenic stimulation transiently increases the exchangeable mitochondrial calcium pool in rat thymocytes

Exchangeable calcium pools were measured in rat thymocytes by 45Ca labelling and selective depletion of intracellular pools with oligomycin in the presence or absence of rotenone. The mitochondrial pool increased by 150% after 3 min of treatment with the mitogen concanavalin A, and decreased to zero 10 min after mitogen addition. No significant change in the ATP-dependent pool could be detected.