Regulation of intestinal mucosa guanylate cyclase by hemin, heme and protoporphyrin IX

Hemin induces active chloride secretion in Caco-2 cells

Enterocytes maintain fluid-electrolyte homeostasis by keeping a tight barrier and regulating ion channels. Carbon monoxide (CO), a product of heme degradation, modulates electrolyte transport in kidney and lung epithelium, but its role in regulating intestinal fluid-electrolyte homeostasis has not been studied. The major source of endogenous CO formation comes from the degradation of heme via heme oxygenase. We hypothesized that heme activates electrolyte transport in intestinal epithelial cells. Basolateral hemin treatment increased baseline Caco-2 cell short-circuit currents (I(sc)) twofold (control = 1.96 +/- 0.14 microA/cm(2) vs. hemin = 4.07 +/- 0.16 microA/cm(2), P < 0.01); apical hemin had no effect. Hemin-induced I(sc) was caused by Cl- secretion because it was inhibited in Cl- -free medium, with ouabain, 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), or DIDS. Apical electrogenic Na+ channel inhibitor benzamil had no effect on hemin-induced I(sc). Hemin did not alter the ability of Caco-2 cells to respond maximally to forskolin, but a soluble guanylate cyclase inhibitor, [1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ) inhibited the effects of hemin. A CO-releasing molecule, tricarbonyldichlororuthenium II, induced active Cl- secretion that was also inhibited with ODQ. We conclude that hemin induces active Cl- secretion in Caco-2 cells via a cGMP-dependent pathway. These effects are probably the consequence of CO formation. Heme and CO may be important regulators of intestinal fluid-electrolyte homeostasis.

Maturational differences in soluble guanylate cyclase activity in ovine carotid and cerebral arteries

Basal cGMP concentrations are greater in immature than in mature cranial arteries, which may help explain why cerebrovascular resistance is lower in neonates than in adults. The present studies explore the hypothesis that this difference derives from age-related differences in soluble guanylate cyclase (sGC) activity. Maturation depressed (p < 0.01) maximal sGC activity (pmol cGMP/mg/min) in both carotid (from 11.10 +/- 0.50 to 3.60 +/- 0.20) and cerebral (from 3.10 +/- 0.31 to 1.45 +/- 0.08) arteries. Western blot analysis of relative sGC abundance (relative to sGC expression in adult kidney) found that sGC abundance was significantly greater (p < 0.05) in newborn carotid (0.38 +/- 0.04) and cerebral arteries (0.37 +/- 0.06) than in adult arteries (0.25 +/- 0.05 and 0.17 +/- 0.03, respectively). Basal Km values in carotid and cerebral arteries did not differ significantly between newborns (3- to 7-d old) and adults. Activation of sGC with nitrosylated heme significantly reduced Km values 3- to 5-fold in both types of artery and in both age groups. Within artery type, maturation had no significant effect on activated Km. Between artery types, activated Km values were greater (p < 0.05) in cerebral (200 +/- 40 microM) than in carotid (80 +/- 10 microM) arteries. Together, these data suggest that variations in sGC substrate affinity contribute to observed differences in sGC activity between artery types but not those between age groups. In contrast, variations in enzyme abundance, and possibly also enzyme-specific activity, appear responsible for differences in sGC activity associated with both age and artery type.

Nucleotide regulation of heat-stable enterotoxin receptor binding and of guanylate cyclase activation

Certain nucleotides were found to regulate the binding of the Escherichia coli heat-stable enterotoxin (STa) to its receptor in pig intestinal brush border membranes. ATP and adenine nucleotide analogues inhibited 125I-STa binding, while guanine nucleotide analogues stimulated binding, with maximal effects at 0.5-1.0 mM. The strongest inhibitors were adenosine 5'-[beta gamma-imido]triphosphate (App[NH]p) (36%) and adenosine 5'-[beta-thio]diphosphate (ADP[S]) (41%). Inhibition did not require Mg2+, and was blocked by p-chloromercuribenzenesulphonate (PCMBS). Stimulation of binding required Mg2+, was not prevented by PCMBS and was maximal with GDP[S] (41%). While App[NH]p and MgGDP[S] appeared to be acting at different sites, they also interfered with each other. These nucleotides exerted only inhibitory effects on STa-stimulated guanylate cyclase activity, in contrast with the stimulatory effects of adenine nucleotides on atrial natriuretic peptide (ANP)-stimulated guanylate cyclase. Inhibition by low concentrations of MgApp[NH]p and MgATP was weaker above 0.1 mM, while MgGDP[S] and magnesium guanosine 5'-[gamma-thio]triphosphate (MgGTP[S]) inhibited in a single phase. Inhibition by MgApp[NH]p, at all concentrations, was competitive with the substrate (MgGTP), as was that by MgGDP[S] and MgGTP[S]. Whereas membrane guanylate cyclases usually show positively co-operative kinetics with respect to the substrate, STa-stimulated activity exhibited Michaelis-Menten kinetics with respect to MgGTP. This changed to positive co-operativity when Lubrol PX was the activator, or when the substrate was MnGTP. These results suggest the presence of both a regulatory and a catalytic nucleotide-binding site, which do not interact co-operatively with STa activation.

Age-dependent changes in affinity-labeled receptors for Escherichia coli heat-stable enterotoxin in the swine intestine

Intestinal brush border membranes from 1-day-old and 4-week-old (day of weaning) pigs were affinity labeled with an Escherichia coli heat-stable enterotoxin (STa) by cross-linking 125I-STa to receptor proteins with disuccinimidyl suberate. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography revealed that a radioactive protein with a relative molecular weight of 137,000 to 145,000 was present in both age groups. A strongly radioactive protein with an apparent Mr of 90,000 was present in the 1-day-old animals but not in those that were 4 weeks old. The major radioactive protein present in the older pigs had an Mr of 64,000 to 67,000, but this protein was missing or very weakly radioactive in the younger pigs. There was no significant difference between the groups in receptor affinity for STa, although the receptor density in the older animals was marginally significantly greater. STa-stimulated guanylate cyclase activity in membranes from 1-day-old pigs was only one-sixth that in 4-week-old pigs, although the basal and Lubrol PX-stimulated activities were similar.

Solubilization and reprecipitation from intestinal brush border membranes of a complex containing guanylate cyclase activatable by the heat-stable enterotoxin

Extraction of pig intestinal brush border membranes with the zwitterionic detergent 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate (Chaps) in the presence of 0.5 M KCl yielded a solution which contained 60-70% of the receptor for the Escherichia coli heat-stable enterotoxin (STa) and of the Lubrol PX-activated guanylate cyclase activity present in the membrane. When the supernatant solution was diluted fivefold with 10 mM Hepes buffer (pH 7.4) and kept at 4 degrees C overnight, a precipitate formed. Centrifugation yielded a pellet (P2) which contained 25-30% of both the cyclase and the receptor in the original membranes, with a 2.5- to 3-fold enrichment of both. The process could be repeated for further enrichment (P4). The addition of MgCl2 to the diluted extract affected both basal and STa-stimulated activity of P2; 1 mM was optimal. P2 resembled membranes with respect to competitive inhibition of 125I-STa binding by STa, and the concentration-dependent activation of cyclase by STa. Guanylate cyclase in resolubilized P2 was also activated by STa. Most of the enzymes interfering with guanylate cyclase determinations were removed, as were the brush border marker enzymes sucrase and gamma-glutamyltransferase, and a GTP-binding protein that is a pertussis toxin substrate. Specific cross-linking of 125I-STa to receptors in the membrane was preserved in P2 and P4, the three proteins showing the strongest radioactivity having relative molecular masses of 55,000-60,000, 70,000-80,000, and 135,000-140,000. P2 and P4 appear to contain a complex of membrane proteins with certain functional properties intact.

Activation of intestinal brush border guanylate cyclase by aromatic disulphide compounds

Guanylate cyclase in pig intestinal brush border membranes was stimulated by certain aromatic disulphides. The most effective were 6-thioguanine disulphide [(TGS)2], 6-mercaptopurine disulphide, 6,6'-dithiodinicotinic acid, 5,5'-dithiobis-(2-nitrobenzoic acid) and 5-carboxy-2-thiouracil disulphide. (TGS)2 stimulated activity 15-fold when present at 0.1 mM. The optimum concentration for each disulphide was different, and higher concentrations were inhibitory. There was no activation by alkyl disulphides or by N-ethylmaleimide. Activation by 50 microM-(TGS)2 was partially reversed by later addition of 0.1 mM-dithiothreitol, whereas activation by the Escherichia coli heat-stable enterotoxin STa was relatively unaffected. Pretreatment of the membranes with (TGS)2 produced a concentration-dependent inhibition of STa-stimulated activity, while stimulating basal activity, until the activities were equal at 50 microM. Activity was [Mg2+]-dependent, the optimal [Mg2+] progressively increasing as the enzyme was stimulated by (TGS)2, STa and Lubrol PX respectively. However, (TGS)2 pretreatment prevented the shift to higher [Mg2+]optima induced by STa or Lubrol alone. Substitution of Mn2+ for Mg2+ in the reaction elevated basal activity and eliminated by activation (TGS)2. (TGS)2 only inhibited Mn2(+)-dependent activity (both basal and stimulated). The affinity of 125I-STa for its receptor was slightly increased by (TGS)2. We propose that (TGS)2 undergoes thiol-disulphide exchange with at least three different protein thiols of decreasing reactivity. The first is associated with Mg2(+)-dependent activation, the second is associated with a tonic inhibition of activity and the third is associated with the catalytic activity, although probably not at the active site.

Mechanism of dimethylsulfoxide protection against the teratogenicity of secalonic acid D in mice

Dimethylsulfoxide (DMSO) is known to antagonize the teratogenic effects of secalonic acid D (SAD) in mice. To establish the optimum protective dose of DMSO, pregnant CD-1 mice were treated, i.p., with 30 mg/kg of SAD in 5% (w/v) NaHCO3, containing 0, 10, 20, or 30% (v/v) DMSO on day 11 of gestation. Data indicate that at 10% and 20% levels, DMSO affords an apparent dose-related protection against SAD-induced cleft palate, whereas 30% DMSO enhanced fetal resorption with no reduction in the incidence of cleft palate. Ultraviolet spectra and TLC mobility indicated that DMSO at 20% did not directly interact with SAD. Distribution and elimination of 14C-SAD was studied in fetal and maternal tissues from pregnant mice at 24 and 48 hr after exposure to 30 mg/kg of 14C-SAD, i.p., in NaHCO3 (control) or in 20% DMSO. Compared with those not receiving DMSO, maternal exposure to DMSO: 1) significantly reduced (42-75%) radioactivity in fetal heads and bodies, placenta, and maternal tissues other than liver; 2) significantly increased (up to 222%) the radioactivity in maternal liver; and 3) significantly reduced (44-58%) fecal and urinary elimination of SAD-derived radioactivity. These results suggest that the antiteratogenic effect of DMSO against SAD may be at least partly mediated by increased SAD (or its metabolites) retention by maternal liver leading to reduced SAD uptake by the fetus.

Hemin increases aerobic capacity of cultured regenerating skeletal myotubes

Regeneration of damaged, mature muscle occurs by differentiation of satellite cells. In culture, satellite cell myoblasts proliferate, align, and fuse to form cross-striated, contracting myotubes. The biochemical changes and the factors that regulate differentiation in satellite cells have not been investigated previously. We report here that no significant differences in glucose uptake rate or glucose oxidation rate were observed between regenerating myoblasts and myotubes, whereas the aerobic oxidation of palmitic acid increased 7.3-fold between these differentiation states. Specific activities of enzymes of critical importance in aerobic metabolism or in production of ATP were increased 2- to 3.5-fold during fusion. Addition of 20 microM hemin to regenerating muscle cultures potentiated the aerobic capacity as evidenced by a 23.6% increase in palmitate oxidation rate. Hemin also increased the specific activities of all nonheme enzymes investigated with the exception of phosphofructokinase. This augmentation of aerobic metabolism together with the time frame of active muscle differentiation suggests a complex role for hemin in myogenesis.