The mechanism of the relaxant effect of 2-2'-pyridylisatogen on the isolated taenia of the guinea-pig caecum

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Role of spin trapping and P2Y receptor antagonism in the neuroprotective effects of 2,2'-pyridylisatogen tosylate and related compounds

2,2'-Pyridylisatogen tosylate (PIT) is both an allosteric modulator of P2Y receptors, and an immine oxide, acting as a spin trap for free radicals. PIT (10 mg kg(-1), i.p.) was found to be a powerful neuroprotective agent in protecting against the lesions induced by 15 micro g S-bromo-willardiine injected into the cortex or white matter of 5-day-old mice pups. As the multiple effects of PIT may induce both beneficial and deleterious effects, a reanalysis of the structure-activity relationship was undertaken. PIT (50 micro M) and 2,3'-pyridylisatogen were potent antagonists of responses to ATP in the taenia preparation of the guinea-pig caecum, but 2,3'-nitrophenylisatogen was not. The reactive immine oxide group could be substituted by a keto moiety (N-(2'-pyridyl)phthalide) while maintaining antagonism of responses to ATP, equivalent to PIT. Thus, antagonism of P2Y receptors was not restricted to the isatogen nucleus. Other spin traps did not antagonise P2Y receptors, although dimethyl-pyrroline-N-oxide (DMPO) increased the sensitivity of responses to ATP. Both N-(2'-pyridyl)phthalide and 2,3'-nitrophenylisatogen was less neuroprotective than PIT (10 mg kg(-1), i.p.) in protecting against the S-bromo-willardiine-induced lesions in mice, implying that both antagonism of P2Y receptors and the immine oxide moiety may be important for the neuroprotective effects of PIT. However, the usefulness of the neuroprotection was limited because, in motoneurones obtained from rat embryos, PIT (10-100 micro M) exacerbated cell death.

Antagonists and the purinergic nerve hypothesis: 2, 2'-pyridylisatogen tosylate (PIT), an allosteric modulator of P2Y receptors. A retrospective on a quarter century of progress

2,2'-Pyridylisatogen tosylate (PIT) is a selective antagonist of P2Y responses in smooth muscle and does not antagonise the effects of adenosine. Responses to purinergic nerve stimulation are resistant to PIT. PIT is an allosteric modulator of responses to ATP in recombinant P2Y(1) receptors expressed in Xenopus oocytes with potentiation of ATP at low concentrations (0.1-10 microM) and antagonism at higher ones (>10 microM). A radioligand binding profile showed that PIT did not interact with any other receptors, with the exception of low affinity for the adenosine A(1) receptor (pK(i), 5.3). The compound recognises purine sites and then may cause irreversible binding to sulfhydryl groups following prolonged incubation or high concentrations. PIT is a potent spin trapper.

Prominent sympathetic purinergic vasoconstriction in the rabbit splenic artery: potentiation by 2,2'-pyridylisatogen tosylate

1. Vasoconstrictions induced by transmural electrical field stimulation were frequency-dependent from 2 to 32 Hz in the rabbit isolated splenic artery. All contractions were abolished in the presence of tetrodotoxin 1 microM or guanethidine 100 microM. Stimulation at a frequency of more than 32 Hz induced both neurogenic and myogenic responses. 2. Prazosin (1 microM) did not significantly affect vascular contractions to electrical stimulation. Desensitization of P2X-purinoceptors with alpha, beta-methylene ATP (alpha, beta-meATP, 3 microM) abolished the contractions to stimulation at 2-8 Hz and inhibited more than 80% of the vascular response at 16 Hz, but it did not significantly change the responses at 32 Hz. Contractile responses at 32 Hz were inhibited by a combination of prazosin and alpha, beta-meATP. Effects of pyridoxal-phosphate-6-azophenyl-2', 4'-disulphonic acid tetrasodium salt (a selective P2X-purinoceptor antagonist) and suramin (a competitive P2-purinoceptor antagonist) on the neurogenic responses were investigated in this study. 3. 2,2'-Pyridylisatogen tosylate (PIT, 0.3-3 microM) significantly potentiated the vasoconstrictions to electrical stimulation at 2-32 Hz in a concentration-dependent manner. Potentiated responses were restored to the control level 30 min after washing. Concentration-dependent response curves for noradrenaline (NA) or alpha, beta-meATP were not significantly changed by 3 microM PIT, and vasoconstriction by adenosine 5'-triphosphate (ATP, 300 microM) was unaffected by PIT. Coomassie brilliant blue-G (1 microM), which shares the potentiating effect on a recombinant P2Y-purinoceptor with PIT (King et al., 1996), did not inhibit or potentiate the purinergically-mediated component of the response to sympathetic nerve stimulation. The selective alpha 2-adrenoceptor antagonist yohimbine (1 microM) also potentiated the vascular responses to electrical stimulation. 4. The present results indicate that ATP evokes postjunctional contractile responses at low and high frequency electrical stimulation of sympathetic nerves supplying the rabbit splenic artery. PIT potentiates the responses to sympathetic (purinergic) nerve stimulation; this appears to be mainly via prejunctional rather than postjunctional actions.

Potentiation by 2,2'-pyridylisatogen tosylate of ATP-responses at a recombinant P2Y1 purinoceptor

1. 2,2'-Pyridylisatogen tosylate (PIT) has been reported to be an irreversible antagonist of responses to adenosine 5'-triphosphate (ATP) at metabotropic purinoceptors (of the P2Y family) in some smooth muscles. When a recombinant P2Y1 purinoceptor (derived from chick brain) is expressed in Xenopus oocytes, ATP and 2-methylthioATP (2-MeSATP) evoke calcium-activated chloride currents (ICl,Ca) in a concentration-dependent manner. The effects of PIT on these agonist responses were examined at this cloned P2Y purinoceptor. 2. PIT (0.1-100 microM) failed to stimulate P2Y1 purinoceptors directly but, over a narrow concentration range (0.1-3 microM), caused a time-dependent potentiation (2-5 fold) of responses to ATP. The potentiation of ATP-responses by PIT was not caused by inhibition of oocyte ecto-ATPase. At high concentrations (3-100 microM), PIT irreversibly inhibited responses to ATP with a IC50 value of 13 +/- 9 microM (pKB = 4.88 +/- 0.22; n = 3). PIT failed to potentiate inward currents evoked by 2-MeSATP and only inhibited the responses to this agonist in an irreversible manner. 3. Known P2 purinoceptor antagonists were tested for their ability to potentiate ATP-responses at the chick P2Y1 purinoceptor. Suramin (IC50 = 230 +/- 80 nM; n = 5) and Reactive blue-2 (IC50 = 580 +/- 130 nM; n = 6) reversibly inhibited but did not potentiate ATP-responses. Coomassie brilliant blue-G (0.1-3 microM) potentiated ATP-responses in three experiments, while higher concentrations (3-100 microM) irreversibly inhibited ATP-responses. The results indicated that potentiation and receptor antagonism were dissociable and not a feature common to all known P2 purinoceptor antagonists. 4. In radioligand binding assays, PIT showed a low affinity (pKi < 5) for a range of membrane receptors, including: alpha 1, alpha 2-adrenoceptors, 5-HT1A, 5-HT1B, 5-HT2, 5-HT3, D1, D2, muscarinic, central benzodiazepine, H1, mu-opioid, dihydropyridine and batrachotoxin receptors. PIT showed some affinity (pKi = 5.3) for an adenosine (A1) receptor. 5. In guinea-pig isolated taenia caeci, PIT (12.5-50 microM) irreversibly antagonized relaxations to ATP (3-1000 microM); PIT also directly relaxed the smooth muscle and histamine was used to restore tone. Relaxations to nicotine (10-100 microM), evoked by stimulating intrinsic NANC nerves of taenia caeci preparations in the presence of hyoscine (0.3 microM) and guanethidine (17 microM), were not affected by PIT (50 microM, for 25-60 min). 6. These experiments indicate that PIT causes an irreversible antagonism of ATP receptors but, for recombinant chick P2Y1 purinoceptors, this effect is preceded by potentiation of ATP agonism. The initial potentiation by PIT (and by Coomassie brilliant blue-G) of ATP-responses raises the possibility of designing a new class of modulatory drugs to enhance purinergic transmission at metabotropic purinoceptors.

Prompt effect of progesterone on the adrenergic response of smooth muscles

The contractile effects of epinephrine on the uterus and ductus deferens of the rabbit and the ductus deferens of the monkey were inhibited by the preincubation with progesterone (6.4 X 10(-5) M) for 1 or 3 min in Locke-Ringer solution. Epinephrine relaxed the guinea pig uterus and taenia caecum. The relaxant effects were enhanced by preincubation with progesterone. Their effects were in a dose-dependent manner. There was no apparent change in the number and affinity of alpha-adrenergic receptors in the uterus of rabbits and the ductus deferens of guinea pigs during the incubation with progesterone. Progesterone has no direct effect on alpha-adrenergic receptors. All smooth muscles yielded reproducible contractile reactions to Ca2+ when maintained in depolarizing Tyrode's solution containing K+ (40 mmol/l). Their concentration-response curves were inhibited by preincubation with progesterone (6.4 X 10(-5) M), and they were shifted to the right in a concentration-dependent manner. Established Ca2+-induced contractions were rapidly relaxed by the addition of progesterone (6.4 X 10(-5) M). It suggests that progesterone directly affects the plasma membrane and inhibits the voltage-dependent Ca2+ channel and then inhibits smooth muscle contraction.

The mechanism of inhibition by 2,2'-pyridylisatogen tosylate of NADPH-linked enzyme activities in microsomes isolated from rat liver

Microsomal preparations isolated from rat liver were used to study the action of 2.2'-pyridylisatogen tosylate (PIT) on aniline hydroxylation, cytochrome c reduction and NADPH oxidation. PIT was found to inhibit both the NADPH-dependent (5-100 microM, PIT) and the NADPH-independent (0.05-2.5 mM, PIT) hydroxylation of aniline, but had no significant effect on either the NADPH-dependent oxidation of hexobarbital, or the NADPH-independent hydrolysis of glucose-6-phosphatase. PIT was also found to inhibit cytochrome c reductase competitively (Ki = 35 microM) and to stimulate NADPH oxidation (ED50 = 6.5 microM) PIT and aniline were both found to bind to the microsomal haemoprotein cytochrome P-450 and produce Type II spectral changes. It is proposed that PITs ability to bind to the haemoprotein and its ability to accept electrons from the microsomal NADPH-cytochrome c reductase system leads to the inhibition of aniline hydroxylase activity.

"Calcium antagonists": a class of drugs with a bright future. Part II. Determination of basic pharmacological properties

This minireview discusses some simple pharmacological tests useful in detecting biological activity (screening), characterizing mechanisms of action and predicting possible therapeutic applications for calcium antagonists in general and calcium slow channel blockers in particular. In smooth muscle preparations these agents inhibit mechanical effects evoked by K+-depolarization which selectively opens voltage-operated calcium channels (VOC) to allow extracellular Ca++ into the cytosol. In contrast, any inhibition of receptor-mediated responses by calcium antagonists appears to depend on the transduction system and the specific cellular mechanism (e.g. VOC opening consequent to partial depolarization) activated by the receptor and, evidently, on ancillary pharmacological properties of the studied compound. For instance, whereas calcium slow channel blockers antagonize contractions produced by norepinephrine and K+-depolarization in the rat isolated portal vein, they inhibit effectively only the latter response in the rabbit aorta. This apparent discrepancy may be accounted for by the different pool of Ca++ mobilized in the two tissues by norepinephrine. Agents (e.g. diphenylalkylamines, calmodulin blockers) that impair the interaction of Ca++ with intracellular proteins produce effects which are less specific than those of slow channel blockers. Currently, the pharmacological profile of calcium antagonists can be appropriately defined by studying their effects on radioligand (dihydropyridine) binding, radioactive calcium movements through biological membranes, electrophysiological parameters in cardiac and vascular smooth muscle and on various in vivo cardiovascular preparations. Together, these approaches allow a functional classification of new calcium antagonists in relation to already known compounds and some hypotheses on their potential clinical applications. Finally, desirable pharmacokinetics and pharmacological properties for novel calcium antagonists are mentioned. This point will be further explored in the forthcoming minireview which will deal with the clinical applications of calcium antagonists.

Assessment of "Ca2+ -antagonist" effects of drugs in K+ -depolarized smooth muscle. Differentiation of antagonist subgroups

1. Taenia preparations from the guinea-pig caecum yielded reproducible concentration-response curves to Ca2+ (EC50 134 +/- 8 mumol/1) when maintained in depolarizing Tyrode solution containing K+ (40 mmol/1). Drugs which are claimed to be "Ca2+ -antagonists" displaced the curves to the right without depression of the maximum response. In this test nifedipine, verapamil, diltiazem, pimozide, cinnarizine, flunarizine and fendiline appeared qualitatively similar but had different potencies. 2. The antagonist effects of nifedipine, verapamil and diltiazem were readily reversed by washout of the drugs from the bathing fluid, but the effects of the other drugs were not. 3. Cinnarizine, flunarizine, pimozide and fendiline were only weakly active as relaxants of Ca2+ (100 mumol/1)-induced contractions, when compared with their antagonist activity when applied initially in Ca2+ -free media. As the presence of Ca2+ (100 mumol/1) in the K+ -Tyrode reduced the antagonist effects of cinnarizine and pimozide, but not that of verapamil and diltiazem, the weak activity of some of the antagonists as relaxants of Ca2+ -induced contractions can be attributed to a protective effect of Ca2+ during the incubation period with the antagonist. 4. The problems associated with the assessment of the potency of drugs as "Ca2+ -antagonists" are discussed and it is proposed that three subgroups of drugs may exist within the overall classification.

The mechanism of the relaxant effect of 2-2'-pyridylisatogen on the isolated taenia of the guinea-pig caecum

1 2-2'-Pyridylisatogen tosylate (PIT) slowly relaxed taenia caeci preparationg of the guinea-pig in a concentration-dependent manner (threshold 2.5 muM). The relaxant effect did not show tachyphylaxis.2 The relaxation was not affected by tetrodotoxin (0.3 muM), guanethidine (17 muM) nor by a combination of phentolamine (36 muM) and propranolol (4 muM)3 In taenia caeci preparations suspended in K(+)-depolarizing, Ca(2+)-free Ringer, addition of Ca(2+) (0.1 to 30 mM) resulted in a slow contraction. PIT (50 muM) and papaverine (15 muM) antagonized these contractions, whereas indomethacin (28 muM) was ineffective.4 Although PIT (50 muM for 30 min) caused a relaxation of the taenia, and, when the tone of the preparations was restored with carbachol, antagonized adenosine 5'-triphosphate (ATP)-induced relaxations, relaxation of the taenia with papaverine (30 muM for 5 min) did not antagonize ATP-induced relaxations. It is concluded that the relaxant and ATP-receptor blocking actions of PIT are independent properties of the compound.