Renal effects of neuropeptide Y

 Neuropeptide Y (NPY) is a co-transmitter of the sympathetic nervous system including the renal nerves. The kidney expresses NPY receptors, which can also be activated by peptide YY (PYY), a circulating hormone released from gastrointestinal cells. Five subtypes of NPY receptors have been cloned, among which Y₁, Y₂ and Y₅ appear to be involved in the regulation of renal function. NPY produces potent renal vasoconstriction in vitro in isolated interlobar arteries and in the isolated perfused kidney and in vivo upon intrarenal or systemic administration via a Y₁ receptor. Nevertheless glomerular filtration rate is altered only little if at all by NPY, indicating a greater effect on the vas efferens than the vas afferens. NPY can inhibit renin release via Y₁-like receptors. NPY can stimulate Na⁺/K⁺ adenosine triphosphatase (Na⁺/K⁺-ATPase) in proximal tubules via Y₂ receptors and can antagonize the effects of vasopressin on isolated collecting ducts. It can also act prejunctionally to inhibit noradrenaline release via Y₂ receptors. Despite the profound reductions of renal blood flow, systemic NPY infusion can cause diuresis and natriuresis; this is largely independent of pressure natriuresis mechanisms and is possibly mediated by an extrarenal Y₅ receptor. Studies with the converting enzyme inhibitor ramiprilat and the bradykinin receptor antagonist icatibant indicate that bradykinin mediates, at least partly, diuretic NPY effects. NPY antagonists enhance basal renal blood flow but do not alter basal diuresis or natriuresis indicating that renovascular, but not tubular, NPY receptors may be tonically activated by endogenous NPY.     

Different Regulation of Atrial ANP Release through Neuropeptide Y2 and Y4 Receptors

Neuropeptide Y (NPY) receptors are present in cardiac membranes. However, its physiological roles in the heart are not clear. The aim of this study was to define the direct effects of pancreatic polypeptide (PP) on atrial dynamics and atrial natriuretic peptide (ANP) release in perfused beating atria. Pancreatic polypeptides, a NPY Y₄ receptor agonist, decreased atrial contractility but was not dose-dependent. The ANP release was stimulated by PP in a dose-dependent manner. GR 23118, a NPY Y₄ receptor agonist, also increased the ANP release and the potency was greater than PP. In contrast, peptide YY (3-36) (PYY), an NPY Y₂ receptor agonist, suppressed the release of ANP with positive inotropy. NPY, an agonist for Y_{1, 2, 5} receptor, did not cause any significant changes. The pretreatment of NPY (18-36), an antagonist for NPY Y₃ receptor, markedly attenuated the stimulation of ANP release by PP but did not affect the suppression of ANP release by PYY. BIIE0246, an antagonist for NPY Y₂ receptor, attenuated the suppression of ANP release by PYY. The responsiveness of atrial contractility to PP or PYY was not affected by either of the antagonists. These results suggest that NPY Y₄ and Y₂ receptor differently regulate the release of atrial ANP.     

Mechanisms underlying pre- and postjunctional effects of neuropeptide Y in sympathetic vascular control

The effects of porcine neuropeptide Y (NPY) regarding sympathetic vascular control were studied in vitro on isolated rat blood vessels. The 10(-9)M NPY enhanced (about two-fold) the contractile responses to transmural nerve stimulation (TNS), noradrenaline (NA) and adrenaline (about two-fold) in the femoral artery. Higher concentrations of NPY (greater than 10(-8)M) caused an adrenoceptor-resistant contraction per se. The TNS-evoked [3H]NA efflux was significantly reduced by NPY in a concentration-dependent manner (threshold 10(-9)M). The calcium antagonist, nifedipine, abolished the contractile effects of NPY and the NPY-induced enhancement of NA contractions but did not influence the prejunctional inhibition of [3H]NA release. Receptor-binding studies showed that the ratio of alpha 1-to alpha 2-adrenoceptors in the femoral artery was 30:1. The NPY did not cause any detectable change in the number of alpha 1-or alpha 2-adrenoceptor binding sites or in the affinity of alpha 2-binding sites, as revealed by prazosin- and clonidine-binding, respectively. The NPY also inhibited the TNS-evoked [3H]NA release (by 42-86%) in the superior mesenteric and basilar arteries and in femoral and portal veins. The NPY still depressed TNS-evoked [3H]NA secretion from the portal vein in the presence of phentolamine. The NPY caused a clear-cut contraction in the basilar artery, increased the contractile force of spontaneous contractions in the portal vein, while only weak responses were observed in the superior mesenteric artery and femoral vein. The NA-induced contraction was markedly enhanced by NPY in the superior mesenteric artery, only slightly enhanced in the portal vein and uninfluenced in the femoral vein. In conclusion, in all blood vessels tested, NPY depresses the TNS-evoked [3H]NA secretion via a nifedipine-resistant action. Furthermore, NPY exerts a variable, Ca2+-dependent vasoconstrictor effect and enhancement of NA and TNS contractions.     

Neuropeptide Y in cerebrovascular function: comparison of membrane potential changes and vasomotor responses evoked by NPY and other vasoconstrictors in the guinea pig basilar artery

The membrane depolarization and vasomotor response evoked by NPY and other vasoconstrictors were compared in guinea pig basilar artery. Concentrations below the pD2 value of amines and PGF2 alpha induced contractions without significant membrane depolarization, while higher agonist concentrations depolarized the membrane slightly. Potassium-induced contractions were paralleled by strong depolarization. NPY evoked a slow depolarization which correlated to vasoconstriction over a wide concentration range. The mechanism of activation did not appear to involve the endothelium. The results suggest that NPY induces prolonged cerebrovascular smooth muscle tone by evoking longlasting depolarization, at least partly in conjunction with activation of voltage-operated calcium channels.     

Characterization of the contractile effect of neuropeptide Y in feline cerebral arteries

The action of neuropeptide Y (NPY), which coexists with noradrenaline (NA) in perivascular sympathetic nerves, has been examined on feline cerebrovascular smooth muscle using a sensitive in vitro system. The direct cerebrovascular responses of peptides with structural similarities with NPY, peptide YY (PYY), avian (APP), and bovine (BPP) and human (HPP) pancreatic polypeptides, have been compared with that of NPY on isolated feline cerebral arteries. The relative potency for contractions induced by the peptides is: NPY, PYY greater than APP greater than BPP, HPP. The alpha-adrenoceptor antagonist rauwolscine, which blocked the response to noradrenaline (NA), had no effect on NPY-induced contractions. Neuropeptide Y significantly potentiated contractions induced by 10(-6) M NA, but not by 10(-5) M. Withdrawal of Ca2+ from the extracellular medium for 30 min reduced the contractile response to NPY in cerebral vessels by about 80%. Subsequent readdition of Ca2+ caused reproducible contractions which were inhibited by the calcium entry blocker nimodipine. Nimodipine also relaxed isolated middle cerebral artery segments contracted by NPY and NA in a concentration-dependent manner. The data suggest that NPY mediates contraction of cerebrovascular smooth muscle via a mechanism that is dependent on the concentration of extracellular calcium.     

Cholinergic modulation of non-adrenergic, non-cholinergic relaxation in isolated, small coronary arteries from lambs

The presence of cholinergic innervation of small coronary arteries in the lamb was investigated by measuring choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activities and by performing in vitro experiments in a microvascular myograph to establish whether or not there is a cholinergic component in the response to electrical field stimulation (EFS). ChAT-specific activity was present in proximal coronary segments, but was significantly higher in small coronary arteries. AChE-positive ganglia and fibres were distributed within the adventitia and outer third of the media in proximal coronary segments, and dense perivascular nerve plexuses were observed in small coronary arteries. Acetylcholine induced contractions in all preparations examined and relaxations in 20% of the segments contracted with the thromboxane analogue U46619. EFS did not induce neurogenic contractions in lamb small coronary arteries. In the presence of the α-adrenoceptor antagonist, phentolamine, EFS caused frequency-dependent reproducible relaxations that were enhanced by the blocker of cholinergic transmission, botulinum neurotoxin. An inhibitor of AChE, physostigmine, had no significant effect on the relaxations caused by EFS, while both the muscarinic receptor antagonist, atropine, and the muscarinic M₂-receptor antagonist, AFDX 116, enhanced these responses. Blockade of sympathetic neurotransmission with guanethidine or incubation with the P₂-receptor antagonist, suramin, abolished the relaxations induced by EFS, whereas propranolol was without effect. Low-frequency EFS caused less relaxation in preparations activated by acetylcholine than in those contracted with U46619, while sensitivity and maximal relaxation induced by adenosine 5′-triphosphate (ATP) were not different in U46619- and acetylcholine-contracted arteries. The presence of the enzymes necessary for both biosynthesis and degradation of acetylcholine and the finding that blockers of cholinergic neurotransmission enhance EFS-induced relaxations suggest that small coronary arteries are cholinergically innervated. Received: 12 November 1998 / Received after revision: 15 March 1999 / Accepted: 24 March 1999     

Effect of temperature on guinea pig urinary bladder contraction mediated via P2X-receptors

In vitro experiments showed that P2X-receptor agonist α,β-methylene-ATP and electrical field stimulation in the presence of muscarinic and α-adrenoreceptors blockers induced contractile responses of isolated guinea pig bladder, which were more pronounced at 30°C than at 37°C or 42°C. P2x-receptor antagonist pyridoxal-6-phosphate-2′,4′-disulfonic acid, produced a more potent inhibitory effect on contractions induced by electrical field stimulation at 30°C in comparison with that at 37°C or 42°C while the contractions induced by α,β-methylene-ATP were similarly suppressed at all examined temperatures. Translated fromByulleten’s Eksperimental’noi Biologii i Meditsiny, Vol. 130, No. 10, pp. 407–410, October, 2000     

Chronic sympathetic activation promotes downregulation of β-adrenoceptor-mediated effects in the guinea pig heart independently of structural remodeling and systolic dysfunction

It is uncertain if downregulation of β-adrenoceptor signaling pathway is promoted by an enhanced adrenergic tone at an early stage of cardiac disease, or it develops secondary to detrimental local myocardial changes in advanced heart failure. We examined the integrity of β-adrenoceptor signaling pathway upon chronic infusion of isoproterenol, a β-adrenoceptor agonist, at a dose producing no structural left ventricular (LV) remodeling and systolic dysfunction. Subcutaneous isoproterenol infusion (400 μg kg⁻¹ h⁻¹ over 16 days) to guinea pigs using osmotic minipumps produced no change in cardiac weights, LV internal dimensions, myocyte cross-sectional area, extent of interstitial fibrosis, and basal contractile function. Isolated, perfused heart preparations from isoproterenol-treated guinea pigs exhibited attenuated responsiveness to acute β-adrenoceptor stimulation, as evidenced by reduced LV developed pressure increase, less shortening of LV epicardial monophasic action potential and effective refractory period, and less myocardial cyclic adenosine monophosphate elevation, in response to isoproterenol exposure, when compared to saline-treated controls. Pharmacological responses to forskolin, an activator of the adenylate cyclase catalytic subunit, were well preserved in isoproterenol-treated hearts. Downregulation of β-adrenoceptor-mediated effects upon chronic isoproterenol infusion was associated with markedly reduced stimulatory G-protein α-subunit (G_sα) myocardial expression levels. No change in expression levels of β-adrenoceptors, G-protein-coupled receptor kinase 2, inhibitory G-protein α-subunit (G_iα2), and Ca_v1.2 and K_v7.1 ion channels was determined in isoproterenol-treated hearts. We therefore conclude that sustained adrenergic overstimulation may promote downregulation of myocardial β-adrenoceptor-mediated effects independently of structural LV remodeling and systolic failure, an effect attributed to β-adrenoceptor uncoupling from adenylate cyclase due to reduced G_sα-protein expression.     

Calcium-contraction relationship in rat mesenteric arterial smooth muscle. Effects of exogenous and neurogenic noradrenaline

 We evaluated the relationship between intracellular calcium concentration ([Ca²⁺]_i) and vasoconstriction during the presence of exogenous noradrenaline (NA) and sympathetic nerve stimulation. An imaging technique was used to determine calcium/tension relationships in isolated rat mesenteric resistance arteries that had been mounted for recording of isometric tension development and loaded with Fura-2/AM. Experiments were performed after depletion of vasodilator neuropeptides and in the continuous presence of 1 μM propranolol, 3 μM indomethacin, and 30 μM nitro-l-arginine. NA (10 μM) was shown first to induce a further increase in tension, but not [Ca²⁺]_i, during the contraction induced by 125 mM K⁺. Subsequently, calcium/tension relationships were determined during stimulation with graded increases in extracellular [K⁺] (5.9–125 mM K⁺), cumulative administration of NA (0.2–10 μM) and electrical field stimulation of perivascular nerves (EFS, 1–16 Hz). A basal calcium/tension relationship without the calcium-sensitizing property of NA was constructed using a cumulative concentration/response curve of 5.9–125 mM K⁺ in arteries after prior exposure to the irreversible α-adrenoceptor antagonist phenoxybenzamine (POB). K⁺ series before and during α-blockade were also studied using the combination of the α₁-antagonist prazosin and α₂-antagonist yohimbine yielding comparable results as with POB. Calcium/tension curves obtained in the presence of NA, K⁺ and during EFS all were shifted to the left compared with the basal condition and all showed a similar slope indicating that neurogenically released NA is equally capable of inducing calcium sensitization in smooth muscle of mesenteric resistance arteries as exogenously applied NA. In the presence of exogenous and endogenous NA we not only observed an elevated contractile response for a given increase in [Ca²⁺]_i, but also an attenuated rise in [Ca²⁺]_i for a given intensity of stimulation. This suggests that the agonist-induced calcium-sensitization is accompanied by a reduction of the rise in [Ca²⁺]_i. Received: 24 October 1997 / Received after revision: 26 January 1998 / Accepted: 26 February 1998     

Tail arteries from chronically spinalized rats have potentiated responses to nerve stimulation in vitro

Patients with severe spinal cord lesions that damage descending autonomic pathways generally have low resting arterial pressure but bladder or colon distension or unheeded injuries may elicit a life-threatening hypertensive episode. Such episodes (known as autonomic dysreflexia) are thought to result from the loss of descending baroreflex inhibition and/or plasticity within the spinal cord. However, it is not clear whether changes in the periphery contribute to the exaggerated reflex vasoconstriction. The effects of spinal transection at T7–8 on nerve- and agonist-evoked contractions of the rat tail artery were investigated in vitro . Isometric contractions of arterial segments were recorded and responses of arteries from spinalized animals ('spinalized arteries') and age-matched and sham-operated controls were compared. Two and eight weeks after transection, nerve stimulation at 0.1–10 Hz produced contractions of greater force and duration in spinalized arteries. At both stages, the α-adrenoceptor antagonists prazosin (10 n m ) and idazoxan (0.1 μ m ) produced less blockade of nerve-evoked contraction in spinalized arteries. Two weeks after transection, spinalized arteries were supersensitive to the α₁-adrenoceptor agonist phenylephrine, and the α₂-adrenoceptor agonist, clonidine, but 8 weeks after transection, spinalized arteries were supersensitive only to clonidine. Contractions of spinalized arteries elicited by 60 m m K⁺ were larger and decayed more slowly at both stages. These findings demonstrate that spinal transection markedly increases nerve-evoked contractions and this can, in part, be accounted for by increased reactivity of the vascular smooth muscle to vasoconstrictor agents. This hyper-reactivity may contribute to the genesis of autonomic dysreflexia in patients.     

Chronic decentralization potentiates neurovascular transmission in the isolated rat tail artery, mimicking the effects of spinal transection

Spinal cord transection produces a marked increase in the response of the isolated rat tail artery to sympathetic nerve stimulation, possibly as a result of a decrease in ongoing sympathetic activity. We have tested the effects of removing ongoing nerve activity on neurovascular transmission by cutting the preganglionic input to postganglionic neurones supplying the tail artery (decentralization). Isometric contractions to nerve stimulation were compared between decentralized arteries and those from age-matched and sham-operated controls. Nerve-evoked responses of decentralized arteries were much larger than those of control arteries at 2 and 7 weeks post operatively. The extent of blockade of nerve-evoked contraction by α-adrenoceptor antagonists prazosin (10 n m ) or idazoxan (0.1 μ m ) was reduced. Decentralized arteries were transiently supersensitive to the α₁-adrenoceptor agonist phenylephrine and the α₂-adrenoceptor agonist clonidine; the unchanged sensitivity to methoxamine and phenylephrine after 2 weeks indicated no effect on the neuronal noradrenaline uptake transporter. Decentralized arteries were hypersensitive to α,β methylene-ATP, but the P2-purinoceptor antagonist suramin (0.1 m m ) did not reduce nerve-evoked contractions. Enlarged responses to 60 m m K⁺ after both 2 and 7 weeks were correlated with the response of the arteries to nerve stimulation, suggesting that increased postjunctional reactivity contributes to the enhanced contraction. Comparison between data from decentralized arteries and our previous data from spinalized animals showed that the two lesions similarly potentiate nerve-evoked contractions and have similar but not identical postjunctional effects. The enhanced vascular responses following a reduction in tonic nerve activity may contribute to the hypertensive episodes of autonomic dysreflexia in spinally injured patients.     

Specific adrenergic responses of smooth muscles in the vascular wall of guinea pig pulmonary arteries during ovalbumin sensitization

The adrenergic contractile responses of smooth muscles in the vascular wall of guinea pig pulmonary arteries were studied during ovalbumin sensitization. Sensitization was followed by inhibition of contractile responses to an α-adrenoceptor agonist mesatone, prevented endothelium-derived relaxation, and potentiated the contractile response to isoproterenol. Administration of a β-adrenoceptor agonist isoproterenol potentiated the increase in mechanical strain of smooth muscles in the pulmonary artery precontracted with high-potassium Krebs solution. Removal of the endothelium had no effect on the contractile response of smooth muscle segments from the pulmonary artery of intact and sensitized guinea pigs to β-adrenergic influences. The contractile responses of smooth muscles of the pulmonary artery are associated with activity of the cAMP-dependent signal system and play a role in the pathogenesis of ventilation-perfusion disturbances during atopic inflammation. __________ Translated from Byulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 145, No. 6, pp. 617–620, June, 2008     

Prostaglandins and neurotransmission at the guinea pig and rabbit urinary bladder

High concentrations of prostaglandins (PGE₁, PGE₂, or PGE₂) (2×10^–6 M) produced a slow contraction of longitudinal strips of detrusor muscle taken from the bladders of guinea pigs and rabbits. At a lower concentration (10^–6 M) prostaglandins enhanced contractions produced by field stimulation of nerves in guinea pig but not rabbit strips. The contractions were not affected by indomethacin. Contractions of guinea pig strips in response to acetylcholine at 10^–4 M were enhanced by prostaglandins and unaffected by indomethacin. Membrane potentials of smooth muscle cells recorded with micro electrodes, were unchanged up to 10^–6 M PGE₂. Above this the cells were depolarized with an increase in frequency of spontaneous action potentials. Synchronous recording of electrical and mechanical activity with the double sucrose gap indicated a decrease in amplitude of the evoked excitatory junction potential and action potential even when the contraction was enhanced in the presence of PGE₂. Responses to repeated stimulation at 10 Hz for 1 min were progressively depressed. This trend was slightly reduced by PGE₂ but unaffected by indomethacin. It is concluded that prostaglandins are not normally released by the nerves to the urinary bladder but are able to facilitate contraction in the guinea pig. This effect is probably on the excitatory-contraction coupling, possibly by mobilizing Ca²⁺. Some modification of transmitter release by the nerves may also occur.     

Stimulation of α1-adrenoceptors reduces glutamatergic synaptic input from primary afferents through GABAA receptors and T-type Ca channels

Activation of the descending noradrenergic system inhibits nociceptive transmission in the spinal cord. Although both α₁- and α₂-adrenoceptors in the spinal cord are involved in the modulation of nociceptive transmission, it is not clear how α₁-adrenoceptors regulate excitatory and inhibitory synaptic transmission at the spinal level. In this study, inhibitory and excitatory postsynaptic currents (IPSCs and EPSCs, respectively) were recorded from lamina II neurons in rat spinal cord slices. The specific α₁-adrenoceptor agonist phenylephrine significantly increased the frequency of GABAergic spontaneous IPSCs in a concentration dependent manner, and this effect was abolished by the α₁-adrenoceptor antagonist 2-(2,6-dimethoxyphenoxy)ethylaminomethyl-1,4-benzodioxane (WB4101). Phenylephrine also significantly reduced the amplitude of monosynaptic and polysynaptic EPSCs evoked from primary afferents. The inhibitory effect of phenylephrine on evoked monosynaptic glutamatergic EPSCs was largely blocked by the GABA_A receptor antagonist picrotoxin and, to a lesser extent, by the GABA_B receptor antagonist CGP55845. Furthermore, blocking T-type Ca²⁺ channels with amiloride or mibefradil diminished the inhibitory effect produced by phenylephrine or the GABA_A receptor agonist muscimol on monosynaptic EPSCs evoked from primary afferents. Collectively, these findings suggest that activation of α₁-adrenoceptors in the spinal cord increases synaptic GABA release, which attenuates glutamatergic input from primary afferents mainly through GABA_A receptors and T-type Ca²⁺ channels. This mechanism of presynaptic inhibition in the spinal cord may be involved in the regulation of nociception by the descending noradrenergic system.     

Molecular mechanisms of vasoconstrictor action of imidazo[1,2-α]benzimidazole derivative RU-117 possessing local anesthetic properties

We studied the mechanisms of action of imidazobenzimidazole derivative RU-1117 on calcium homeostasis in myocytes isolated from rat thoracic aorta. In therapeutic concentrations, RU-1117 increased the content of free calcium ions due to their mobilization from intracellular depot via the IP₃-dependent mechanisms. Antagonists of angiotensin II AT₁-receptors irbesartan and, to greater extent, eprosartan abolished the calcium-mobilizing action of RU-1117. Selective antagonist of endothelin ET_A-receptors sitaxsentan and α1-adrenoceptor agonist prazosin produced no effect on calcium mobilization caused by novel local anesthetic RU-1117. __________ Translated from Byulleten’ Eksperimental’noi Biologii i Meditsiny, Vol. 143, No. 5, pp. 531–534, May, 2007     

Pregnancy is associated with altered response to neuropeptide Y in uterine artery

Pregnancy is associated with a significant increase in uterine blood flow which contributes to optimal fetal development. Although neuropeptide Y (NPY) is considered to be an important regulator of uterine blood flow, it is not known whether: (i) products from the vascular endothelium modulate NPY action in the uterine artery; (ii) pregnancy changes the responsiveness of the uterine artery to NPY, or (iii) NPY interacts with noradrenaline and acetylcholine on the uterine artery, with pregnancy regulating this possible interaction. In the present study, NPY induced a concentration-dependent contraction of guinea pig uterine arterial rings both intact and denuded of endothelium. Pregnancy significantly decreased the potency of NPY to contract uterine artery with and without endothelium. In all preparations, addition of N(G)-monomethyl-l-arginine acetate (l-NMMA), indomethacin and diethylcarbamazine did not modify the effect of NPY. In the presence of NPY concentration-response curves for acetylcholine and noradrenaline were significantly shifted to the right and left respectively. This effect of NPY was independent of endothelial condition or pregnancy status. The receptor reserve (K(A)/EC(50)) for acetylcholine was decreased and for noradrenaline was increased in the presence of NPY, although no changes in the dissociation constants of the neurotransmitter-receptor complexes were observed. Thus, this study has shown that: (i) NPY induces contraction of guinea pig uterine arteries acting on receptors localized in smooth muscle; (ii) pregnancy alters the response of guinea pig uterine arteries to NPY in such a way as to promote vasorelaxation, and (iii) NPY modulates the effect of neurotransmitters on guinea pig uterine arteries, but pregnancy is not associated with the changes at the level of NPY-neurotransmitter interaction.     

Blockade of nitric oxide decreases the renal vasodilatory effect of neuropeptide Y in the insulin-treated diabetic rat

 The effect of 50 days of streptozotocine-induced diabetes mellitus (blood glucose 20 mmol/l) on contraction and relaxation of isolated renal and intrarenal arteries in rats were examined. Strong and similar contractions were induced by potassium (60 mM), 5-hydroxytryptamine (5-HT) and endothelin-1 (ET-1) in renal and intrarenal arteries in diabetic and control rats. The vasodilatory reactivity, after precontraction with 5-HT, of neuropeptide Y (NPY) was similar to that of acetylcholine (ACh), calcitonin gene-related peptide (CGRP) and vasoactive intestinal polypeptide (VIP) and was similar in diabetic and control rats. The relaxing effect of NPY was decreased (40%) only in the diabetic group by blockade of nitric oxide synthase with N ^G-nitro-L-arginine methyl ester (10^–4 M) and by blockade (50%) of NPY with α-trinositol (10^–6 M). In conclusion, the present study showed that diabetes mellitus in the rat is associated with normal vasoconstrictive and vasodilatory capacities. However, the vasodilatory response to NPY was largely eliminated by blockade of nitric oxide synthesis only in the diabetic animals. This indicates that the vasodilatory effect of NPY in diabetes mellitus may be dependent on nitric oxide synthesis. Received: 12 November 1996 / Received after revision: 10 March 1997 / Accepted: 7 April 1997     

Activation of α2-adrenoreceptors suppresses the excitability of C1 spinal neurons having convergent inputs from tooth pulp and superior sagittal sinus in rats

The aim of the present study was to test the hypothesis that activation of α₂-adrenoreceptors modulates the excitability of C1 neurons having convergent inputs from both the tooth pulp (TP) and the superior sagittal sinus (SSS), by using the microiontophoretic techniques of drug application and immunohistochemical approaches. Extracellular single-unit recordings were made from 38 C1 neurons responding to electrical stimulation of TP under pentobarbital-anesthetized rats. Seventy-one percent of C1 neurons (27/38) that responded to TP stimulation also responded to electrical stimulation of the SSS. In these neurons, l-glutamate-evoked C1 neuronal discharge firings were increased in a dose-dependent manner. The mean glutamate-evoked firing rates were dose-dependently inhibited after microiontophoretic application of clonidine (α₂-adrenoreceptor/imidazoline I₁ receptor agonist). The inhibition of glutamate-evoked C1 mean firings by clonidine was antagonized by the co-application of idazoxan (α₂-adrenoreceptor/imidazoline I₂ receptor antagonist), yohimbine (α₂-adrenoreceptor) but not the α₁-adrenoreceptor antagonist, prazosin with affinity for α_2B- and α_2C-adrenoreceptors. The mean spontaneous discharge frequencies were significantly inhibited by the microiontophoretic application of clonidine and this inhibition was reversed by the co-application of idazoxan, yohimbine. Microiontophoresis of clonidine also resulted in a reduction of TP-/SSS-evoked activity and this effect was reversed by the co-application of yohimbine. Immunoreactivity for α_2A-adrenoreceptor was found in the superficial layers of I–III in the C1 region. These results suggest that α₂-adrenoreceptor agonist clonidine inhibits the excitability of C1 neurons having convergent inputs from TP and SSS afferents, and that the activation of α_2A-adrenoreceptors onto C1 dorsal horn neurons may contribute as a useful therapeutic target for the alleviation of trigeminal referred pain associated with migraine and tooth pain.     

Activation of contraction of arterial smooth muscle in the presence of nitrate and other anions

Contraction strength was measured in ear artery and aorta of the rabbit and coronary artery of the pig during replacement of chloride with anions of the lyotropic series. Contractions induced by depolarization with elevated external potassium were potentiated in the presence of foreign anions, while those induced by noradrenaline were affected only at higher concentrations of the drug. Drug induced contractions in calcium-free solutions were not affected by the anions. In the presence of nitrate and excess K the change of membrane potential per tenfold change in (K)₀ was 56 mV in chloride and 50 mV in nitrate solution. Enhancement of contraction strength by nitrate was not associated with alterations in membrane potential. Stimulation of aorta with 29.5 mM K resulted in a larger net uptake of⁴⁵Ca in nitrate than in chloride. K-stimulated⁴⁵Ca efflux was greater in the ear artery in NO₃ solution than Cl solution while⁴⁵Ca efflux induced by noradrenaline was not effected by anion replacement. These results suggest that in these arteries anions potentiate contraction by enhancing the influx of external calcium, and do not modify the release of internal stores of calcium.     

Effects of prostanoids on isolated feline cerebral arteries

The effects of various prostanoids on isolated feline basilar arteries (BA) were studied. Contractions were induced with the following order of potency: U 46619 ? U44069 > PGB₂ > PGF_2a? PGA, ? PGB, ≥ PGA, ? PGE₂= PGD₂ > PGF_1a? TXB₂? PGE₁? PGD₁. Distinct bimodal responses with a relaxation at low concentrations followed by a contraction at high concentrations, were induced by PGA₁, PGA₂, PGD, and PGE₂. None of the tested prostanoids relaxed K⁺-contracted arteries, and a sizable relaxant effect in PGF_2a-contracted arteries could be induced only by PGE₁. As judged by the relative order of potency, PG-induced contractions of the feline BA seem to be mediated by a thromboxane sensitive receptor. PGF_2a-induced contractions apparently do not involve the release of noradrenaline from perivascular nerves since phentolamine failed to affect contractions induced by this agent.