The influence of isatin on guanylyl cyclase of rat heart membranes

[Potentiation of NO-dependent activation of soluble guanylyl cyclase by 5-nitroisatin and antiviral preparatation arbidol]

Isatin (indole-dione) is an endogenous indole that exibits a wide range of biological and physiological activity. The influence of isatin derivatives 5-nitroisatin and arbidol (an antiviral preparatation) on spermine NONO-induced activation of human platelet soluble guanylyl cyclase was investigated. 5-nitroistnin and arbidol had no effect on basal activity, but synergistically increased in a concentration-dependent manner the spermine NONO-induced activation of this enzyme. 5-Nitroisatin and arbidol, like YC-1, sensitized guanylyl cyclase towards nitric oxide (NO) and produced a leftward shift of the spermine NONO concentration response curve. At the same time both compounds used did not influence the activation of guanylyl cyclase by YC-1 and did not change the synergistic increase of spermine NONO-induced activation of soluble guanylyl cyclase in the presence of YC-1. This suggests that 5-nitroisanin and arbidol did not compete with YC-1. Addition of isatin did not change the synergistic increase in the spermine NONO-induced guanylyl cyclase activation by 5-nitroisatin and arbidol and did not influence a leftward shift of spermine NONO concentration response curve produced by these compounds. These data suggest lack of competitive interaction between isatin and both its derivatives used.

[Inhibition of no-dependent soluble human platelet guanylate cyclase by isatin]

Isatin (indole-dione-2,3) is an endogenous indole that exhibits a wide spectrum of biological and pharmacological activities. Physiologically relevant concentrations of isatin (ranged from 1 nM to 10 M) did not influence basal activity of soluble human platelet guanylate cyclase (sGC), but caused a bell-shaped inhibition of the NO-activated enzyme. Inhibition of the NO-dependent activation by isatin did not depend on a chemical nature of the NO donors. The inhibitory effects of ODC (a heme-dependent inhibitor of sGC) and isatin were non-additive suggesting that the inhibitory effect of isatin may involve the heme binding domain (possibly heme iron) and experiments with hemin revealed some isatin-dependent changes in its spectrum. Isatin also inhibited sGC activation by the allosteric activator YC-1. It is suggested that the bell shaped inhibition of the NO-dependent activation of sGC by isatin may be attributed to complex interaction of isatin with the heme binding domain and the allosteric YC-1-binding site of sGC.

Pharmacological activity of novel 2-hydroxyacetophenone isatin derivatives on cardiac and vascular smooth muscles in rats

Isatin (1H-indole-2,3 dione) is an endogenous compound with biological activities. Many of its derivatives have pharmacological effects, including inhibition of cyclic guanosine monophosphate levels in cardiac tissue; sedative-hypnotic profiles; anticonvulsant, analgesic, antithermic, and anti-inflammatory activities; and anxiolytic, antimicrobial, and proapoptotic effects. Carbamates derived from isatin have a vasorelaxant profile. This study investigated the activity of 2 novel 2-hydroxyacetophenone derivatives of isatin (named MB101 and MB130) on the contractility of rat aorta and papillary muscles. Both compounds induced a concentration-dependent relaxation (5-100 μM) in the endothelium-intact aorta that was abolished by N-nitro-L-arginine methyl ester. Atropine, a muscarinic receptor antagonist, significantly prevented vasodilatation of 100 μM MB101. In contrast, atropine caused no significant alteration in MB130-induced vasorelaxation. Naloxone, a nonselective opioid receptor antagonist, completely prevented the relaxing effect of MB101 and MB130 at all concentrations. In papillary muscles, only MB130 induced a significant depression, and this contractile response was not altered by propranolol and atropine. Both the compounds reduced systolic and diastolic pressures in a dose-dependent manner in anesthetized rats. The 2-hydroxyacetophenones produced direct effects on vascular tonus through either muscarinic or opioid pathways. MB130 produced cardiac depression by opioid receptors and bradykinin because pretreatment HOE140 or with naloxone, an antagonist of type-2, bradykinin were able to partially block the decrease in twitch amplitude in papillary muscles induced by MB130. These findings provide information for designing new strategies for the treatment of cardiovascular disorders.

B-type natriuretic peptide at early reperfusion limits infarct size in the rat isolated heart

Natriuretic peptides are regulatory autacoids in the mammalian myocardium whose functions, mediated via particulate guanylyl cyclase/cGMP, may include cytoprotection against ischaemia-reperfusion injury. Previous work has identified that B-type natriuretic peptide (BNP) limits infarct size when administered prior to and during coronary occlusion through a K(ATP) channel-dependent mechanism. The present study examined the hypothesis that the protection afforded by BNP is mediated specifically at reperfusion in a postconditioning-like manner. Langendorff-perfused rat hearts were subjected to 35 min coronary artery occlusion and 120 min reperfusion, and infarct size was determined by tetrazolium staining. Postconditioning was effected by applying six 10-second periods of global ischaemia at the onset of reperfusion.Treatment with either BNP 10 nM or the NO donor S-nitroso-N-acetylpenicillamine (SNAP) 1-10 microM was commenced 5 min prior to reperfusion and continued until 10 min after reperfusion. Control infarct size (% of ischaemic risk zone) was 40.8 +/- 3.7%.BNP at reperfusion induced a significant limitation of infarct size (BNP 22.9 +/- 4.1% P<0.05 vs. control). Co-treatment at reperfusion with BNP and the K(ATP) channel blockers 5-hydroxydecanote (5HD, 100 microM), glibenclamide (Glib; 10 microM) or HMR1098 (10 microM) abolished the infarct-limiting effect of BNP (BNP + 5HD 41.0 +/- 3.9%, BNP + Glib 39.8 +/- 5.6%, BNP + HMR 1098 46.0 +/- 7.1%,P < 0.05 vs. BNP). BNP given together with L-NAME (100 microM) at reperfusion resulted in a marked loss of protection (BNP + L-NAME 53.1 +/- 3.8% P < 0.001 vs. BNP). In a second series of experiments, SNAP (1-10 microM) given at reperfusion was found not to be protective (SNAP 1 microM 30.2 +/- 4.9%, SNAP 2 microM 27.5 +/- 9.5%, SNAP 5 microM 39.2 +/- 5.7%, SNAP 10 microM 33.7 +/- 6.4%, not significant vs. control). In a third series of experiments, postconditioning significantly limited infarct size (14.9 +/- 3.6 % vs. control 34.5 +/- 4.9%, P < 0.01) and this effect of postconditioning was abolished in the presence of isatin (100 microM), a non-specific blocker of particulate guanylyl cyclases (35.1 +/- 6%, P < 0.05 vs. postconditioning). In conclusion, pharmacological activation of pGC by BNP can effectively induce protection against reperfusion injury, by mechanisms involving K(ATP) channel opening and endogenous NO synthase activation. Furthermore, endogenous activation of pGC could play a role in the mechanism of postconditioning.

Atrial natriuretic peptide administered just prior to reperfusion limits infarction in rabbit hearts

We investigated whether atrial natriuretic peptide (ANP) given just prior to reperfusion reduces infarction in rabbit hearts and whether protection is related to activation of protein kinase G (PKG). Isolated rabbit hearts were subjected to a 30-min period of regional ischemia; treated hearts received a 20-min infusion of ANP (0.1 microM) starting 5 min before 2 h of reperfusion. ANP infusion decreased infarction from 31.5+/-2.4% of the risk zone in untreated hearts to 12.5+/-2.0% (P<0.001). To explore mechanisms of protection ischemic hearts were treated simultaneously with ANP and isatin, a blocker of the natriuretic peptide receptor, shortly before reperfusion. ANP's protective effect was aborted (36.8+/-2.9% infarction). There is no acceptable blocker of protein kinase G that can be used in intact organs. However, 8-(4-chlorophenylthio)-guanosine 3', 5'-cyclic monophosphate (10 microM), a cell-permeable cGMP analog that directly activates PKG, was infused from 5 min before to 15 min after reperfusion. The PKG activator mimicked ANP's protection with only 18.2+/-3.6% infarction (P<0.001). 5-Hydroxyde-canoate (5-HD), a putative mitochondrial KATP channel (mKATP) inhibitor, abrogated ANP's protection (34.4+/-2.6% infarction). Unexpectedly, 1H-[1,2,4]oxadiazole- [4,3-a]quinoxalin-1-one (ODQ), a blocker of soluble guanylyl cyclase also prevented ANP's infarct-sparing effect. It is unclear whether this observation implicated participation of soluble guanylyl cyclase in the mechanism or simply a lack of selectivity of ODQ. Finally the reperfusion injury salvage kinases (RISK), phosphatidylinositol 3-kinase and extracellular signal-regulated kinase, were implicated in ANP's mechanism since either wortmannin or PD98059 infused at reperfusion prevented ANP's infarct-sparing effect. ANP administered just prior to reperfusion protects hearts against infarction, likely by activation of PKG, opening of mKATP, and stimulation of downstream kinases.

Bremazocine increases C-type natriuretic peptide levels in aqueous humor and enhances outflow facility

A relatively selective agonist of kappa opioid receptors (KOR), bremazocine (BRE), lowers intraocular pressure in rabbits, in part, by increasing natriuretic peptide levels in aqueous humor and by enhancing total outflow facility (TOF). Natriuretic peptide (NP) levels [atrial NP (ANP), brain NP (BNP), and C-type NP (CNP)] were measured in aqueous humor of rabbits either by radioimmunoassay or enzyme immunoassay. TOF was determined in rabbits by two-level constant pressure perfusion of the anterior chamber. Experimental regimens included topical treatment with BRE in the presence or absence of KOR antagonist (norbinaltorphimine), protein kinase C inhibitor (chelerythrine), and natriuretic peptide receptor antagonist (isatin). The rank order of basal NP levels in aqueous humor of rabbits was BNP CNP > ANP. Topical administration of BRE (1-100 microg) caused dose-related elevations of CNP levels in aqueous humor that were inhibited by topical pretreatment with either norbinaltorphimine (100 microg, bilaterally) or chelerythrine (10 microg, bilaterally). Topically administered BRE (100 microg) also elevated levels of ANP and BNP in aqueous humor and evoked an 80% increase in TOF. The increase in TOF was antagonized by topical pretreatment with either norbinaltorphimine (100 microg, bilaterally) or isatin (100 microg, bilaterally). Bremazocine induced an increase in NP (ANP, BNP, and CNP) levels and TOF in rabbits by activating KOR. The increase in CNP levels elicited by BRE was inhibited by norbinaltorphimine and chelerythrine; therefore, this event is most likely mediated by a KOR-linked activation of protein kinase C. These data provide evidence that the increase in TOF elicited by BRE was mediated by a KOR-activated paracrine effect of NPs on tissues within ocular outflow tract(s).

Baclofen inhibits ANP-mediated cyclic GMP synthesis in the rat cervical spinal cord

The neurotransmitter gamma-aminobuteric acid (GABA) is believed to have a controlling action on spinal locomotor networks. In spasticity, spinal locomotor networks are thought to play a role. A well known drug in the treatment of spasticity is the GABA(B) agonist Baclofen. We report an inhibitory effect of Baclofen on the ANP-mediated cGMP synthesis in the superficial dorsal horn (laminae I-III) of the rat cervical spinal cord. This inhibitory effect of Baclofen could not be detected after incubation with the NO donor SNP. The clinical effect of Baclofen on the reduction of spasticity might be explained by an enhancement of GABAergic inhibition of ANP mediated cGMP concentration in the spinal cord dorsal horn, thus reducing afferent input.

The action of isatin (2,3-dioxoindole) an endogenous indole on brain natriuretic and C-type natriuretic peptide-induced facilitation of memory consolidation in passive-avoidance learning in rats

Isatin is an endogenous indole which has been shown to counteract some of the effects of atrial natriuretic peptide (ANP) both in vitro and in vivo. The present study was designed to determine whether it could antagonise in vivo effects of the related peptides brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP). The model used was consolidation of memory in a one-trial step-through passive-avoidance paradigm in the rat. Previous studies have shown that all three peptides (1 microg intracerebroventricular) can consolidate such learning and increase latency to entry a dark box. Isatin was given intraperitoneally at doses of 5, 10 and 50 mg/kg before the peptide, or a saline control. Both BNP and CNP significantly increased the latency of entry. Isatin alone had no effect. Isatin reduced the effect of both BNP and CNP; this was significant for its effect on BNP at 50 mg/kg and on CNP at both 10 and 50 mg/kg. These results show that isatin can inhibit behavioural effects of BNP and CNP as well as ANP.