Actions of adenine dinucleotides in the guinea-pig taenia coli: NAD acts indirectly on P1-purinoceptors; NADP acts like a P2-purinoceptor agonist

History of Geoff Burnstock's research on P2 receptors

Geoffrey Burnstock is a purinergic signalling legend who's discoveries and conceptualisation created and shaped the field. His scientific achievements were extraordinary and sustained. They included his demonstration that ATP can act as a neurotransmitter and hence extracellular signalling molecule, which he championed despite considerable initial opposition to his proposal that ATP acts outside of its role as an energy source inside cells. He led on purine receptor classification: initially of the P1 and P2 receptor families, then the P2X and P2Y receptor families, and then subtypes of P2X and P2Y receptors. This was achieved across several decades as he conceptualised and made sense of the emerging and growing evidence that there were multiple receptor subtypes for ATP and other nucleotides. He made discoveries about short term and long term/trophic purinergic signalling. He was a leader in the field for over 50 years. He inspired many and was a great colleague and mentor. I had the privilege of spending over 10 years (from 1985) with Geoff at the Department of Anatomy and Developmental Biology, University College London. This review is a personal perspective of some of Geoff's research on P2 receptors carried out during that time. It is a tribute to Geoff who I regarded with enormous respect and admiration.

Extracellular ATP and β-NAD alter electrical properties and cholinergic effects in the rat heart in age-specific manner

Extracellular ATP and nicotinamide adenine dinucleotide (β-NAD) demonstrate properties of neurotransmitters and neuromodulators in peripheral and central nervous system. It has been shown previously that ATP and β-NAD affect cardiac functioning in adult mammals. Nevertheless, the modulation of cardiac activity by purine compounds in the early postnatal development is still not elucidated. Also, the potential influence of ATP and β-NAD on cholinergic neurotransmission in the heart has not been investigated previously. Age-dependence of electrophysiological effects produced by extracellular ATP and β-NAD was studied in the rat myocardium using sharp microelectrode technique. ATP and β-NAD could affect ventricular and supraventricular myocardium independent from autonomic influences. Both purines induced reduction of action potentials (APs) duration in tissue preparations of atrial, ventricular myocardium, and myocardial sleeves of pulmonary veins from early postnatal rats similarly to myocardium of adult animals. Both purine compounds demonstrated weak age-dependence of the effect. We have estimated the ability of ATP and β-NAD to alter cholinergic effects in the heart. Both purines suppressed inhibitory effects produced by stimulation of intracardiac parasympathetic nerve in right atria from adult animals, but not in preparations from neonates. Also, ATP and β-NAD suppressed rest and evoked release of acetylcholine (ACh) in adult animals. β-NAD suppressed effects of parasympathetic stimulation and ACh release stronger than ATP. In conclusion, ATP and β-NAD control the heart at the postsynaptic and presynaptic levels via affecting the cardiac myocytes APs and ACh release. Postsynaptic and presynaptic effects of purines may be antagonistic and the latter demonstrates age-dependence.

Coupling of P2Y receptors to Ca2+ mobilization in mesenchymal stromal cells from the human adipose tissue

The purinergic transduction was examined in mesenchymal stromal cells (MSCs) from the human adipose tissue, and several nucleotides, including ATP, UTP, and ADP, were found to mobilize cytosolic Ca²⁺. Transcripts for multiple purinoreceptors were detected in MSC preparations, including A₁, A_2A, A_2B, P2Y₁, P2Y₂, P2Y₄, P2Y₆, P2Y₁₁, P2Y₁₃, P2Y₁₄, P2X₂, P2X₄, and P2X₇. Cellular responses to nucleotides were insignificantly sensitive to bath Ca²⁺, pointing at a minor contribution of Ca²⁺ entry, and were suppressed by U73122 and 2-APB, implicating the phosphoinositide cascade in coupling P2Y receptors to Ca²⁺ release. While individual cells were sensitive to several P2Y agonists, responsiveness to a given nucleotide varied from cell to cell, suggesting that particular MSCs could employ different sets of purinoreceptors. Caged Ca²⁺ stimulated Ca²⁺-induced Ca²⁺ release (CICR) that was mediated largely by IP₃ receptors, and resultant Ca²⁺ transients were similar to nucleotide responses by magnitude and kinetics. A variety of findings hinted at CICR to be a universal mechanism that finalizes Ca²⁺ signaling initiated by agonists in MSCs. Individual MSCs responded to nucleotides in an all-or-nothing manner. Presumably just CICR provided invariant Ca²⁺ responses observed in MSCs at different nucleotide concentrations. The effects of isoform specific agonists and antagonists suggested that both P2Y₁ and P2Y₁₃ were obligatory for ADP responses, while P2Y₄ and P2Y₁₁ served as primary UTP and ATP receptors, respectively. Extracellular NAD⁺ stimulated Ca²⁺ signaling in each ATP-responsive MSC by involving P2Y₁₁. The overall data indicate that extracellular nucleotides and NAD⁺ can serve as autocrine/paracrine factors regulating MSC functions.

Effects of exogenous nicotinamide adenine dinucleotide (NAD+) in the rat heart are mediated by P2 purine receptors

Background

Recently, NAD+ has been considered as an essential factor, participating in nerve control of physiological functions and intercellular communication. NAD+ also has been supposed as endogenous activator of P1 and P2 purinoreceptors. Effects of extracellular NAD+ remain poorly investigated in cardiac tissue. This study aims to investigate the effects of extracellular NAD+ in different types of supraventricular and ventricular working myocardium from rat and their potential mechanisms.

Methods

The standard technique of sharp microelectrode action potential recording in cardiac multicellular preparations was used to study the effects of NAD+.

Results

Extracellular NAD+ induced significant changes in bioelectrical activity of left auricle (LA), right auricle (RA), pulmonary veins (PV) and right ventricular wall (RV) myocardial preparations. 10–100 μM NAD+ produced two opposite effects in LA and RA – quickly developing and transient prolongation of action potentials (AP) and delayed sustained AP shortening, which follows the initial positive effect. In PV and RV only AP shortening was observed in response to NAD+ application. In PV preparations AP shortening induced by NAD+ may be considered as a potential proarrhythmic effect. Revealed cardiotropic effects of NAD+ are likely to be mediated by P2 purine receptors, since P1 blocker DPCPX failed to affect them and P2 antagonist suramin abolished NAD + −induced alterations of electrical activity. P2X receptors may be responsible for NAD + −induced short-lasting AP prolongation, while P2Y receptors mediate persistent AP shortening. The latter effect is partially removed by PLC inhibitor U73122 showing the potential involvement of phosphoinositide signaling pathway in mediation of NAD+ cardiotropic effects.

Conclusions

Extracellular NAD+ is supposed to be a novel regulator of cardiac electrical activity. P2 receptors represent the main target of NAD+ at least in the rat heart.

Effects of NAD at purine receptors in isolated blood vessels

Nicotinamide adenine dinucleotide (NAD) belongs to the family of naturally occurring adenine dinucleotides, best known for their various intracellular roles. However, there is evidence that they can also be released from cells to act as novel extracellular signalling molecules. Relatively little is known about the extracellular actions of NAD, especially in the cardiovascular system. The present study investigated the actions of NAD in the rat thoracic aorta, porcine coronary artery and porcine mesenteric arteries, mounted in organ baths for isometric tension recording. In the rat thoracic aorta and porcine coronary artery, NAD caused endothelium-independent concentration-dependent vasorelaxations which were unaffected by palmitoylCoA, a P2Y1 receptor antagonist, but which were blocked by CGS15943, a non-selective adenosine receptor antagonist. In the porcine coronary artery, NAD-evoked relaxations were abolished by SCH58261, a selective A2A receptor antagonist. In the rat thoracic aorta, NAD-evoked relaxations were attenuated by A2A receptor antagonism with SCH58261 but were unaffected by an A2B receptor antagonist, MRS1754. In contrast, in the porcine mesenteric artery, NAD-evoked endothelium-independent contractions, which were unaffected by a P2 receptor antagonist, suramin, or by NF449, a P2X1 receptor antagonist, but were attenuated following P2X receptor desensitisation with αβ-meATP. In conclusion, the present results show that NAD can alter vascular tone through actions at purine receptors in three different arteries from two species; its molecular targets differ according to the type of blood vessel.

Purinergic signalling in the gastrointestinal tract and related organs in health and disease

Purinergic signalling plays major roles in the physiology and pathophysiology of digestive organs. Adenosine 5'-triphosphate (ATP), together with nitric oxide and vasoactive intestinal peptide, is a cotransmitter in non-adrenergic, non-cholinergic inhibitory neuromuscular transmission. P2X and P2Y receptors are widely expressed in myenteric and submucous enteric plexuses and participate in sympathetic transmission and neuromodulation involved in enteric reflex activities, as well as influencing gastric and intestinal epithelial secretion and vascular activities. Involvement of purinergic signalling has been identified in a variety of diseases, including inflammatory bowel disease, ischaemia, diabetes and cancer. Purinergic mechanosensory transduction forms the basis of enteric nociception, where ATP released from mucosal epithelial cells by distension activates nociceptive subepithelial primary afferent sensory fibres expressing P2X3 receptors to send messages to the pain centres in the central nervous system via interneurons in the spinal cord. Purinergic signalling is also involved in salivary gland and bile duct secretion.

P2Y(1) knockout mice lack purinergic neuromuscular transmission in the antrum and cecum

BACKGROUND

Pharmacological studies using selective P2Y(1) antagonists, such as MRS2500, and studies with P2Y(1)(-/-) knockout mice have demonstrated that purinergic neuromuscular transmission is mediated by P2Y(1) receptors in the colon. The aim of the present study was to test whether P2Y(1) receptors are involved in purinergic neurotransmission in the antrum and cecum.

METHODS

Microelectrode recordings were performed on strips from the antrum and cecum of wild type animals (WT) and P2Y(1)(-/-) mice.

KEY RESULTS

In the antrum, no differences in resting membrane potential and slow wave activity were observed between groups. In WT animals, electrical field stimulation elicited a MRS2500-sensitive inhibitory junction potential (IJP). In P2Y(1)(-/-) mice, a nitrergic IJP (N(ω) -nitro-l-arginine-sensitive), but not a purinergic IJP was recorded. This IJP was equivalent to the response obtained in strips from WT animals previously incubated with MRS2500. Similar results were obtained in the cecum: 1- the purinergic IJP (MRS2500-sensitive) recorded in WT animals was absent in P2Y(1)(-/-) mice 2- nitrergic neurotransmission was preserved in both groups. Moreover, 1- spontaneous IJP (MRS2500-sensitive) could be recorded in WT, but not in P2Y(1)(-/-) mice 2- MRS2365 a P2Y(1) agonist caused smooth muscle hyperpolarization in WT, but not in P2Y(1) (-/-) animals, and 3- β-NAD caused smooth muscle hyperpolarization both in WT and P2Y(1)(-/-) animals.

CONCLUSIONS & INFERENCES

1- P2Y(1) receptor is the general mechanism of purinergic inhibition in the gastrointestinal tract, 2- P2Y(1)(-/-) mouse is a useful animal model to study selective impairment of purinergic neurotransmission and 3- P2Y(1)(-/-) mouse might help in the identification of purinergic neurotransmitter(s).

P2Y1 purinoreceptors are fundamental to inhibitory motor control of murine colonic excitability and transit

Activation of enteric inhibitory motor neurons causes inhibitory junctional potentials (IJPs) and muscle relaxation in mammalian gastrointestinal (GI) muscles, including humans. IJPs in many GI muscles are bi-phasic with a fast initial hyperpolarization (fIJP) due to release of a purine neurotransmitter and a slower hyperpolarization component (sIJP) due to release of nitric oxide. We sought to characterize the nature of the post-junctional receptor(s) involved in transducing purinergic neural inputs in the murine colon using mice with genetically deactivated P2ry1. Wild-type mice had characteristic biphasic IJPs and pharmacological dissection confirmed that the fIJP was purinergic and the sIJP was nitrergic. The fIJP was completely absent in P2ry1(−/−) mice and the P2Y1 receptor antagonist MRS2500 had no effect on electrical activity or responses to electrical field stimulation of intrinsic nerves in these mice. Contractile experiments confirmed that purinergic responses were abolished in P2ry1(−/−) mice. Picospritzing of neurotransmitter candidates (ATP and its primary metabolite, ADP) and β-NAD (and its primary metabolite, ADP-ribose, ADPR) caused transient hyperpolarization responses in wild-type colons, but responses to β-NAD and ADPR were completely abolished in P2ry1(−/−) mice. Hyperpolarization and relaxation responses to ATP and ADP were retained in colons of P2ry1(−/−) mice. Video imaging revealed that transit of fecal pellets was significantly delayed in colons from P2ry1(−/−) mice. These data demonstrate the importance of purinergic neurotransmission in regulating colonic motility and confirm pharmacological experiments suggesting that purinergic neurotransmission is mediated via P2Y1 receptors.

Purinergic neuromuscular transmission is absent in the colon of P2Y(1) knocked out mice

Purinergic and nitrergic co-transmission is the dominant mechanism responsible for neural-mediated smooth muscle relaxation in the gastrointestinal tract. The aim of the present paper was to test whether or not P2Y(1) receptors are involved in purinergic neurotransmission using P2Y(1)(−/−) knock-out mice. Tension and microelectrode recordings were performed on colonic strips. In wild type (WT) animals, electrical field stimulation (EFS) caused an inhibitory junction potential (IJP) that consisted of a fast IJP (MRS2500 sensitive, 1 μm) followed by a sustained IJP (N(ω)-nitro-L-arginine (L-NNA) sensitive, 1 mm). The fast component of the IJP was absent in P2Y(1)(−/−) mice whereas the sustained IJP (L-NNA sensitive) was recorded. In WT animals, EFS-induced inhibition of spontaneous motility was blocked by the consecutive addition of L-NNA and MRS2500. In P2Y(1)(−/−) mice, EFS responses were completely blocked by L-NNA. In WT and P2Y(1)(−/−) animals, L-NNA induced a smooth muscle depolarization but ‘spontaneous' IJP (MRS2500 sensitive) could be recorded in WT but not in P2Y(1)(−/−) animals. Finally, in WT animals, 1 μm MRS2365 caused a smooth muscle hyperpolarization that was blocked by 1 μm MRS2500. In contrast, 1 μm MRS2365 did not modify smooth muscle resting membrane potential in P2Y(1)(−/−) mice. β-Nicotinamide adenine dinucleotide (β-NAD, 1 mm) partially mimicked the effect of MRS2365. We conclude that P2Y(1) receptors mediate purinergic neurotransmission in the gastrointestinal tract and β-NAD partially fulfils the criteria to participate in rodent purinergic neurotransmission. The P2Y(1)(−/−) mouse is a useful animal model to study the selective loss of purinergic neurotransmission.

NADPH-induced contractions of mouse aorta do not involve NADPH oxidase: a role for P2X receptors

Reactive oxygen species elicit vascular effects ranging from acute dilatation because of hydrogen peroxide-mediated opening of K(+) channels to contraction arising from superoxide-dependent inactivation of endothelium-derived nitric oxide. Given that NADPH oxidases are major sources of superoxide in the vascular wall, this study examined the effects of exogenous NADPH, a substrate of these enzymes, on superoxide generation and isometric tone in mouse isolated aortic rings. NADPH caused concentration-dependent increases in superoxide generation (measured by lucigenin-enhanced chemiluminescence) and vascular tone (isometric tension recordings). However, surprisingly, whereas oxidized NADP(+) was unable to support superoxide production, it was equally as effective as reduced NADPH at stimulating vasocontraction. In addition, an NADPH oxidase inhibitor, diphenyleneiodonium, markedly attenuated NADPH-induced superoxide production, yet had no effect on vasocontractions to NADPH. In contrast, a broad specificity P2X receptor antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, as well as the P2X1 selective antagonist, NF023, markedly attenuated both endothelium-dependent and -independent vasocontractions to NADPH, as did the P2X-desensitizing agent alpha,beta-methylene-ATP. Importantly, alpha,beta-methylene-ATP had no effect on superoxide production induced by NADPH. In conclusion, these findings suggest little role for NADPH oxidase-derived superoxide in the contractile effects of NADPH in the mouse aorta. Rather, NADPH seems to act as an agonist at two distinct P2X receptor populations; one located on the endothelium and the other on smooth muscle layer, both of which ultimately lead to contraction.

Calcium mobilization by nicotinic acid adenine dinucleotide phosphate (NAADP) in rat astrocytes

Nicotinic acid adenine dinucleotide phosphate (NAADP) has been shown to release intracellular Ca(2+) in several types of cells. We have used Ca(2+)-sensitive fluorescent dyes (Fura-2, Fluo-4) to measure intracellular Ca(2+) in astrocytes in culture and in situ. Bath-applied NAADP elicited a reversible and concentration-dependent Ca(2+) rise in up to 90% of astrocytes in culture (EC(50)=7 microM). The NAADP-evoked Ca(2+) rise was maintained in the absence of extracellular Ca(2+), but was suppressed after depleting the Ca(2+) stores of the ER with ATP (20 microM), with cyclopiazonic acid (10 microM) or with ionomycin (5 microM). P(2) receptor antagonist pyridoxalphosphate-6-azophenyl-2'4'-disulfonic acid (PPADS, 100 microM), IP(3) receptor blocker 2-aminoethoxydiphenyl borate (2-APB, 100 microM) and PLC inhibitor U73122 (10 microM) also reduced or suppressed the NAADP-evoked Ca(2+) rise. NAADP still evoked a Ca(2+) response after application of glycyl-l-phenylalanine-beta-naphthylamide (GPN, 200 microM), which permeabilizes lysosomes, or preincubation with H(+)-ATPase inhibitor bafilomycin A1 (4 microM) and of p-trifluoromethoxy carbonyl cyanide phenylhydrazone (FCCP, 2 microM), that impairs mitochondrial Ca(2+) handling. In acute brain slices, NAADP (10 microM) evoked Ca(2+) transients in cerebellar Bergmann glial cells and in hippocampal astrocytes. Our results suggest that NAADP recruits Ca(2+) from inositol 1,4,5-trisphosphate-sensitive Ca(2+) stores in mammalian astrocytes, at least partly by activating metabotropic P(2)Y receptors.

The effects of different culture media, glucose, pyridine nucleotides and adenosine on the activity of 11beta-hydroxysteroid dehydrogenase in rat Leydig cells

11Beta-hydroxysteroid dehydrogenase (11betaHSD) reversibly converts glucocorticoids into inert 11-ketosteroids. The direction of the reaction has been found to vary with the cell type and sub-cellular preparation used. We have investigated if the directionality of 11betaHSD can be influenced by the nature of the culture medium and compounds added during incubation of rat testis Leydig cells. We found that when the cells were cultured in Dulbecco's Modified Eagle Medium (DMEM) that the dehydrogenase (11betaDH) activity was higher than the reductase (11KSR) activity (11betaDH:11KSR ratio approximately 2:1). When glucose was omitted from the DMEM a higher 11betaDH:11KSR ratio (approximately 33:1) was obtained. However, when the cells were cultured in a combination of DMEM/Ham's F12 (1:1, v/v), a ninefold increase in 11KSR activity was obtained whereas 11betaDH activity was inhibited by 64% compared with cells incubated in DMEM alone. Consequently, the predominant activity changed from a dehydrogenase to a reductase (11betaDH:11KSR ratio 1:15). Addition of the individual components of the Ham's F12 medium to DMEM showed that only pyruvate and/or the amino acids were able to mimic the effects of DMEM/Ham's F12. Similar differential effects were found when NAD+, NADH or adenosine were added to the Leydig cells incubated in DMEM (three to fivefold increases and 20-50% decreases in 11KSR and 11betaDH activities, respectively). In contrast, NADP+ was found to increase 11betaDH activity (up to threefold) but NADPH had no effect on 11KSR activity. Cells incubated with DMEM/Ham's F12, NAD+, NADP+ and adenosine were found to have higher ATP levels (four to sixfold) than those incubated in DMEM alone. These results illustrate that the relative 11betaDH and 11KSR activities of 11betaHSD in Leydig cells are markedly and differentially altered by the nature of the incubation medium and compounds added.

Comparative effects of adenosine-5'-triphosphate and related analogues on insulin secretion from the rat pancreas

Adenosine tri- and diphosphate (ATP and ADP) and their structural analogues stimulate insulin secretion from the isolated perfused rat pancreas, an effect mediated by P2Y-purinoceptor activation. Concerning the base moiety of the nucleotide, it was previously shown that purine but not pyrimidine nucleoside triphosphates were active and that substitution on purine C2 with the 2-methylthio group greatly enhanced the potency. In this study, we further analyze the consequences of ribose and polyphosphate chain modifications. Modifications in 2' and 3' position on the ribose led to a decrease in insulin response when bulky substitutions were made: indeed, 2'-deoxy ATP was similar in activity to ATP, whereas arylazido-aminopropionyl ATP (ANAPP3) was weakly effective and trinitrophenyl ATP (TNP-ATP) was inactive. Substitution on the gamma phosphorus of the triphosphate chain led to a decrease (gamma-anilide ATP) or no change (gamma-azido ATP) in potency; the replacement of the bridging oxygen between beta and gamma phosphorus by a peroxide group did not significantly change the activity, whereas substitution by a methylene group completely abolished stimulation of insulin secretion. As for the phosphorothioate analogues, adenosine-5'-O-(3-thiotriphosphate) (ATP gamma S) induced an insulin response similar to that produced by ATP, whereas adenosine-5'-O-(2-thiodiphosphate) (ADP beta S) was about 100-fold more potent than ATP, as previously shown. In conclusion, two structural features seem to have a strategic importance for increasing the insulin secretory activity of ATP analogues: substitution at the C2 position on the adenine ring of ATP and modifications of the polyphosphate chain at the level of the beta phosphorus.

Pre- and postjunctional effects of diadenosine polyphosphates in the guinea-pig vas deferens

The pre- and postjunctional activities of a number of diadenosine polyphosphates were examined in the guinea-pig isolated vas deferens at the level of the membrane potential, using a modified sucrose-gap technique. P1,P3-Di(adenosine 5')triphosphate (Ap3A), P1,P4-di(adenosine 5')tetraphosphate (Ap4A) and P1,P5-di(adenosine 5')pentaphosphate (Ap5A) all caused concentration-dependent depolarization of the smooth muscle membrane. The potency order was: Ap5A > Ap4A > or = Ap3A. P1,P2-Di(adenosine 5')pyrophosphate (Ap2A) did not evoke depolarization even at the highest concentration tested (1 mM). All the dinucleotides caused a reduction in the amplitude of evoked excitatory junction potentials (e.j.ps). The potency order was: Ap5A = Ap4A > Ap3A > Ap2A. The depolarizations evoked by the dinucleotides were markedly reduced by the selective P2X-purinoceptor antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 microM), as was the amplitude of the fully facilitated e.j.p. The inhibition of the e.j.p. evoked by Ap3A and Ap2A was reduced by the P1-purinoceptor antagonist, 8-p-sulphophenyltheophylline (8-pSPT, 50 microM), but that evoked by Ap5A and Ap4A was not. Thus, Ap3A, Ap4A and Ap5A evoke depolarization of the guinea-pig vas deferens via P2X-purinoceptors, and additionally Ap2A and Ap3A exert a prejunctional effect via P1-purinoceptors. The prejunctional activity of Ap4A and Ap5A is mediated via an undefined purinoceptor, which is neither P1 nor P2X.

Modulation by nicotinamide adenine dinucleotide of sympathetic and sensory-motor neurotransmission via P1-purinoceptors in the rat mesenteric arterial bed

1. The pharmacological actions of the purine nucleotides beta-nicotinamide adenine dinucleotide (NAD), beta-nicotinamide adenine dinucleotide phosphate (beta-NADP), adenosine 5'-diphosphoribose (ADP-ribose), the vitamin nicotinamide and structural analogues of NAD and NADP were tested in the isolated perfused mesenteric arterial bed of the rat. Prejunctional effects of NAD were tested against sympathetic vasoconstriction at basal tone, and against sensory-motor vasodilatation at raised tone. 2. NAD and NADP had no vasoconstrictor action but were weak vasodilators of the raised-tone mesenteric arterial bed. A rank order of vasodilator potency of ADP >> ADP-ribose >> NADP > or = NAD = adenosine was observed. The P1-purinoceptor antagonist, 8-para-sulphophenyltheophylline (8-pST; 3 microM) inhibited vasodilator responses to NAD (pKB of 6.61 +/- 0.21, n = 7) and adenosine (pKB of 5.78 +/- 0.14, n = 6), but not those elicited by NADP, ADP and ADP-ribose. Nicotinamide, and analogues of NAD and NADP, namely nicotinamide-1,N6-ethenoadenine dinucleotide phosphate, beta-nicotinamide mononucleotide, nicotinamide hypoxanthine dinucleotide phosphate, nicotinamide hypoxanthine dinucleotide, nicotinamide guanine dinucleotide, and nicotinamide-1, N6-ethenoadenine dinucleotide had no vasoconstrictor or vasodilator actions (at doses of up to 50 nmol). 3. At basal tone, electrical field stimulation (EFS) (32 Hz, 1ms, 90 V, 5 s) at 2 min intervals elicited reproducible vasoconstrictor responses due to activation of sympathetic nerves. NAD and adenosine (10-100 microM) inhibited these responses in a concentration-dependent manner with similar potencies. Nicotinamide had no effect on sympathetic vasoconstriction at concentrations of up to 0.1 mM. Postjunctional effects of NAD (100 microM), as tested on constrictor responses to NA (5 nmol), accounted for approximately 60% inhibition at this concentration.4. In preparations in which tone had been raised with methoxamine (10-40 microM), EFS (8 Hz, 0.1ms,60 V, for 30 s) elicited vasodilatation due to activation of sensory-motor nerves. This vasodilatation was inhibited by NAD and adenosine (O.1-100 microM) in a similar concentration-dependent manner: pD2 values were 6.2 +/- 0.10 (n = 11) and 6.1 +/- 0.15 (n = 6) for NAD and adenosine respectively. Nicotinamide had no effect on sensory-motor vasodilatation at concentrations of up to 0.1 mM.5. Inhibition of sympathetic constriction by NAD and adenosine was antagonized by 8-pSPT (3 microM).Inhibitory effects of NAD and adenosine on sensory-motor vasodilatation were similarly antagonized by 8-pSPT (1 microM), pKB values were 6.72 +/- 0.21 for NAD and 6.36 +/- 0.22 for adenosine, resulting in parallel rightward shifts in the concentration-inhibitory effect curves.6. The adenosine deaminase inhibitor, pentostatin (1 microM), augmented the inhibitory effects of NAD and adenosine. Concentration-inhibitory effect curves for NAD and adenosine on sympathetic vasoconstriction and sensory-motor vasodilatation were shifted to the left without a change in the maximum.7. It is concluded that NAD can act as a modulator of sympathetic and sensory-motor transmission in rat mesenteric arteries via P1-purinoceptors possibly via direct actions but with a contribution of adenosine formed following breakdown of NAD or released pre- and/or post junctionally. Structure activity relationships of NAD, NADP, ADP and ADP-ribose showed that the P1-purinoceptor activity of NAD is abolished after removal of nicotinamide, or ribose plus nicotinamide, to yield the structurally-related ADP-ribose and ADP respectively, or when there is phosphorylation of the 2'-hydroxyl group of NAD to yield NADP.

Pivotal role of phosphate chain length in vasoconstrictor versus vasodilator actions of adenine dinucleotides in rat mesenteric arteries

1. The isolated perfused rat mesenteric arterial bed was used to examine the activity of the adenine dinucleotides: beta-nicotinamide adenine dinucleotide (NAD); beta-nicotinamide adenine dinucleotide phosphate (NADP); flavin adenine dinucleotide (FAD); and of the alpha,omega-diadenosine polyphosphates: adenylyl adenosine (AP1A); P1,P2-diadenosine pyrophosphate (AP2A); P1,P3-diadenosine triphosphate (AP3A); P1,P4-diadenosine tetraphosphate (AP4A); P1,P5-diadenosine pentaphosphate (AP5A); P1,P6-diadenosine hexaphosphate (AP6A). Responses were compared with those of ADP, ATP, 2-methylthio-ATP (2-meSATP) and alpha,beta-methylene ATP (alpha,beta-meATP). 2. In basal tone preparations mono- and dinucleotides elicited vasoconstriction with the order of potency: alpha,beta-meATP > or = AP5A > or = AP6A > or = AP4A > or = 2-meSATP >> ATP >> ADP. The dinucleotides NAD, NADP, FAD, AP1A, AP2A and AP3A had no effect. 3. The P2X-purinoceptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (30 microM) virtually abolished vasoconstrictor responses to AP4A, AP5A and AP6A. 4. Auto- and cross-desensitization of vasoconstrictor responses to AP4A, AP5A, AP6A, ATP and alpha,beta-meATP were observed. 5. In raised tone preparations nucleotides elicited endothelium-dependent vasodilatation with the order of potency: 2-meSATP = ADP > ATP > AP3A > AP2A > AP1A = NADP = FAD > NAD. The nucleotides AP4A, AP5A, AP6A and alpha,beta-meATP had no vasodilator effects. 6. It is concluded that the alpha,omega-adenine dinucleotides AP4A, AP5A and AP6A elicit vasoconstriction, but not vasodilatation, in the rat mesenteric arterial bed via P2x-purinoceptors. In contrast, the dinucleotides NADP, FAD, AP1A, AP2A and AP3A elicit vasodilatation, but not vasoconstriction, via endothelial P2Y-purinoceptors. 7. It is suggested that there is a crucial relationship between the structure of the alpha,omega-diadenosine polyphosphates and their activity at P2X- and P2Y-purinoceptors with a pivotal role played by the polyphosphate chain. Molecules with four or more phosphates are vasoconstrictors, while those with three or less phosphates are vasodilators.

Pharmacological actions of the coenzymes NAD(H) and NADP(H) on the rat anococcygeus muscle

1. The pharmacological actions of the oxidized and reduced forms of nicotinamide-adenosine dinucleotide (NAD, NADH) and nicotinamide-adenosine dinucleotide phosphate (NADP, NADPH) were studied on rat isolated anococcygeus muscles. 2. The actions of the two nucleotides were different, but there were no apparent qualitative differences between the oxidized and reduced forms of each. 3. In fully relaxed anococcygeus muscles, NADP(H) produced transient contractions that were subject to desensitization, but NAD(H) had no effect. 4. NADP(H) slightly enhanced contractions elicited by noradrenergic nerve stimulation. In contrast, noradrenergic contractions were inhibited by NAD(H). NADH reduced the stimulation-induced release of noradrenaline, but enhanced contractions elicited by exogenous noradrenaline. 5. In anococcygeus muscles partly contracted with guanethidine, NAD(H) produced a further sustained increase in tone; in contrast, NADP(H) mainly produced transient relaxations to which there was immediate desensitization. 6. Relaxations of anococcygeus muscle elicited by nitrergic nerve stimulation were not affected by NAD. In contrast, NADP(H) reduced them. 7. The actions of NAD(H) were generally the same as those of adenosine and can be attributed to activation of P1-purinoceptors since they were blocked by the selective antagonist 8-sulphophenyltheophylline. 8. The actions of NADP resembled those of the P2-purinoceptor agonist ATP to some extent, but there were some differences. As suggested by others, NADP may act on a unique receptor.

Activation of P1- and P2Y-purinoceptors by ADP-ribose in the guinea-pig taenia coli, but not of P2X-purinoceptors in the vas deferens

1. The activity of adenosine 5'-diphosphoribose (ADP-ribose), a ribosylated purine nucleotide, was investigated on the carbachol-contracted taenia coli, a tissue possessing P1- (A2) and P2Y-purinoceptors and on the guinea-pig vas deferens which possesses P2X-purinoceptors. 2. In the vas deferens, where ATP (1 microM-1 mM) produced concentration-dependent contractions, ADP-ribose was without effect at concentrations up to 1 mM. 3. In the taenia coli, ADP-ribose (0.1 microM-1 mM) produced concentration-dependent relaxations with a potency similar to that of adenosine, but less than that of ATP. The pD2 values for ADP-ribose, adenosine and ATP were 4.5 +/- 0.07 (27), 4.4 +/- 0.10 (9) and 5.5 +/- 0.14 (21), respectively. The time-course of the relaxations elicited by ADP-ribose was found to be significantly longer than that for ATP and significantly shorter than that for adenosine. 4. The P1-purinoceptor antagonist, 8-phenyltheophylline (5 microM), produced parallel rightward shifts in the concentration-response curves of the relaxations of the taenia coli elicited by ADP-ribose and adenosine but not ATP. 5. Dipyridamole (0.3 microM), a purine nucleoside uptake inhibitor, potentiated the responses to adenosine and ADP-ribose in the taenia coli. These potentiations were sensitive to 8-phenyltheophylline (5 microM). 6. Reactive blue 2, a P2Y-purinoceptor antagonist, antagonized the inhibitory responses of ADP-ribose and ATP in the taenia coli, without significantly altering the inhibitory responses of either adenosine or noradrenaline.7. In the presence of the potassium channel blocker, apamin (0.3 microM), the inhibitory responses of ADP-ribose were severely attenuated, and the inhibitory responses of ATP in the taenia coli were converted to transient contractions. Further addition of 8-PT blocked the residual responses of ADPribose.8. The P2-purinoceptor antagonist, suramin (500 microM), antagonized responses to ATP and ADP-ribose,but not adenosine. Further addition of 8-PT antagonized the residual responses to ADP-ribose, but not to ATP.9. It is concluded that ADP-ribose has a mixed pharmacological profile, evoking both PI (A2)-purinoceptor-mediated responses and P2Y-purinoceptor-mediated responses, while being inert at P2Xpurinoceptors.It is suggested that ADP-ribose may provide a useful starting point for the generation of structural analogues which have specific activity at the P2Y-purinoceptor.

Receptors for adenosine and adenine nucleotides

1. A summary is provided of current classifications of receptors for the adenine nucleosides and nucleotides. 2. The origin of the division of nucleoside (P1) and nucleotide (P2) receptors is discussed as well as the subdivisions of these into A1 and A2, determined by structure activity considerations, and P2x and P2y. 3. Further suggestions have been made recently for identifying A1a and A1b, A2a and A2b subtypes of the P1 receptor family, and for proposing distinctive nucleotide receptors on platelets (P2t) and mast cells (P2z). 4. In addition, an A3 site may exist with properties intermediate between A1 and A2, and a novel P2s receptor on some smooth muscle systems and a P3 site.

Pharmacological activity of adenine dinucleotides in the periphery: possible receptor classes and transmitter function

1. The pharmacological actions of adenine dinucleotides, in particular beta-nicotinamide adenine dinucleotide (NAD), beta-nicotinamide adenine dinucleotide phosphate (NADP) and a homologous series of alpha,omega-adenine dinucleotide polyphosphates has been reviewed. 2. It is apparent that many actions of NAD can be explained in terms of activation of P1-purinoceptors, but actions of NADP cannot be explained in terms of activation of P1- or P2-purinoceptors. 3. Similarly, pharmacological activities of P1,P3-diadenosine triphosphate and P1,P4-diadenosine tetraphosphate are not in keeping with activation of P1- or P2-purinoceptors. 4. In the vas deferens and urinary bladder, P1,P4-diadenosine tetraphosphate, P1,P5-diadenosine pentaphosphate and P1,P6-diadenosine hexaphosphate act on P2x-purinoceptors and can cause desensitization of these receptors. 5. It is suggested that classes of receptors for adenine dinucleotides exist which are distinct from either P1- or P2-purinoceptors. 6. It is also suggested that in view of the finding of high concentrations of alpha,omega-adenine dinucleotide polyphosphates in adrenal medullary chromaffin cells, and of the involvement of the P2x-purinoceptor in the vas deferens and urinary bladder with purinergic neuromuscular transmission, that alpha,omega-adenine dinucleotide polyphosphates may yet be discovered in autonomic neurones and serve as neurotransmitters.

Subset of neurons characterized by the presence of NADPH-diaphorase in human substantia innominata

The substantia innominata encompasses an area of the basal forebrain that is ventral to the lenticular nucleus and anterior commissure, medial to the claustrum and external capsule, and lateral to the hypothalamus. The nucleus basalis of Meynert consists primarily of large acetylcholinesterase (AchE)-positive neurons embedded within the substantia innominata. Damage to these neurons may be important in the pathogenesis of cortical dysfunction in Alzheimer's disease. In order to characterize other neuronal elements in the substantia innominata and their relationship to the nucleus basalis, we chose to study a biochemically distinct neuronal subset containing the enzyme nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d). The substantia innominata was blocked from six normal brains obtained postmortem and fixed in neutral-buffered formalin at 4 degrees C for 48 hours. Free-floating 50-micron sections from several levels were stained for NADPH-d or AchE activities. Selected sections were double stained for NADPH-d and AchE. NADPH-d activity was present in a network of pleomorphic neurons that extended through all levels of the substantia innominata and into the striatum and amygdala. NADPH-d neurons were particularly numerous at the level of the anterior commisure and were closely associated with the cholinergic neurons of the nucleus basalis. They were not seen in the ventral pallidum, or the vertical limb of the diagonal band of Broca or in the islands of Calleja. The cell bodies of NADPH-d neurons were quite varied in shape, ranging from ovoid to fusiform, and about half the cells were bipolar. Where neuronal density was high, their dendrites formed an interlacing pattern. NADPH-d-positive fibres were seen coursing through the external capsule, hypothalamus, and amygdala. This novel set of neurons in the substantia innominata may be part of a more extensive network that interacts with the magnocellular basal forebrain system at the level of the nucleus basalis. Whether other neurotransmitters are present within these neurons and whether NADPH-d neurons are involved in Alzheimer's disease remain to be elucidated.

Neuropeptide Y, somatostatin, and reduced nicotinamide adenine dinucleotide phosphate diaphorase in the human striatum: a combined immunocytochemical and enzyme histochemical study

Neuropeptide Y and somatostatin immunoreactive neurons and processes were examined in human striatum using both immunofluorescence and avidin biotin immunoperoxidase methods. Reduced nicotinamide adenine dinucleotide phosphate diaphorase activity was histochemically determined by the reduction of nitro blue tetrazolium. Immunofluorescence using a monoclonal anti-somatostatin antibody and a polyclonal anti-neuropeptide Y antibody, followed by diaphorase histochemistry, showed that these three neurochemical markers are co-localized in a single population of medium-sized aspiny intrinsic neurons. Cells were evenly distributed in clusters throughout the striatum, but fiber density was higher in the nucleus accumbens and ventromedial regions of the caudate and putamen. Double-stained reduced nicotinamide adenine dinucleotide phosphate diaphorase-acetylcholinesterase sections demonstrated that these neurons are located in zones of high acetylcholinesterase activity, often at the interface of these zones with regions of low enzyme activity. These biochemically distinctive neurons are uniquely situated to modulate activity between striatal compartments. Our findings provide new information about the modular organization of the striatum and extend these observations in human brain.

Relationship of metabolism of 2'-, 3'- and 5'-adenine nucleotides to presynaptic inhibition of transmitter release in rat vas deferens

In the isolated rat vas deferens stimulated at 0.2 Hz, a series of 2'-, 3'-, and 5'-substituted adenine nucleotides all inhibited the twitch responses, their actions being potentiated by the nucleoside transport inhibitors, HNBTGR, NBMPR and dipyridamole. The metabolism of these nucleotides was examined utilizing HPLC analysis of the bathing medium after exposure to 30 microM nucleoside or nucleotide for 5 min. 5'-AMP, 5'-ADP, 5'-ATP, and NAD+ were all partially hydrolysed to adenosine, the relative extent of this being 5'-AMP greater than 5'-ADP = 5'-ATP greater than NAD+. However, the other nucleotides examined were not detectably converted to adenosine or to adenosine deamination products. These results indicate that the 2'-, 3'- and 5'-substituted nucleotides studied act at a P1-purinoceptor in rat vas deferens to inhibit neurotransmission and, with the exception of 5'-AMP, 5'-ADP, 5'-ATP and NAD+, all appear to act directly at this receptor. However, the 5'-adenine nucleotides (AMP, ADP and ATP) and NAD+ all appear to act at least partially indirectly subsequent to their hydrolysis to adenosine.

PACPX--a substituted xanthine--antagonizes both the A1 and A2 subclasses of the P1-purinoceptor: antagonism of the A2 subclass is competitive but antagonism of the A1 subclass is not

1,3-Dipropyl-8-(2-amino-4-chlorophenyl)xanthine (PACPX) was examined for its ability to antagonize adenosine acting on the A1 and A2 subclasses of the P1-purinoceptor. A1-purinoceptors were studied in the isolated, driven left atria of the guinea-pig, and A2-purinoceptors in the isolated, carbachol-contracted taenia coli of the guinea-pig. PACPX antagonized the actions of adenosine in both types of preparation and was a more potent antagonist than 8-phenyltheophylline. The antagonism at the A2-purinoceptor was competitive with a pA2 of 5.95. The antagonism at the A1-purinoceptor was not competitive, although antagonism at the A1-purinoceptor was greater than that at the A2-purinoceptor, based on a comparison of pD2 values. The manner of antagonism of PACPX on the A1-purinoceptors of the heart was different from that found for the A1-receptors in bovine brain, implying that there is a fundamental difference between these central and peripheral A1 subclasses of P1-purinoceptor.