VNUT and VMAT2 segregate within sympathetic varicosities and localize near preferred Cav2 isoforms in the rat tail artery

ATP and norepinephrine (NE) are coreleased from peripheral sympathetic nerve terminals. Whether they are stored in the same vesicles has been debated for decades. Preferential dependence of NE or ATP release on Ca²⁺ influx through specific voltage-gated Ca²⁺ channel (Cav2) isoforms suggests that NE and ATP are stored in separate vesicle pools, but simultaneous imaging of NE and ATP containing vesicles within single varicosities has not been reported. We conducted an immunohistochemical study of vesicular monoamine transporter 2 (VMAT2/SLC18A2) and vesicular nucleotide translocase (VNUT/SLC17A9) as markers of vesicles containing NE and ATP in sympathetic nerves of the rat tail artery. A large fraction of varicosities exhibited neighboring, rather than overlapping, VNUT and VMAT2 fluorescent puncta. VMAT2, but not VNUT, colocalized with synaptotagmin 1. Cav2.1, Cav2.2, and Cav2.3 are expressed in nerves in the tunica adventitia. VMAT2 preferentially localized adjacent to Cav2.2 and Cav2.3 rather than Cav2.1. VNUT preferentially localized adjacent to Cav2.3 > Cav2.2 >> Cav2.1. With the use of wire myography, inhibition of field-stimulated vasoconstriction with the Cav2.3 blocker SNX-482 (0.25 µM) mimicked the effects of the P2X inhibitor suramin (100 µM) rather than the α-adrenergic inhibitor phentolamine (10 µM). Variable sensitivity to SNX-482 and suramin between animals closely correlated with Cav2.3 staining. We concluded that a majority of ATP and NE stores localize to separate vesicle pools that use different synaptotagmin isoforms and that localize near different Cav2 isoforms to mediate vesicle release. Cav2.3 appears to play a previously unrecognized role in mediating ATP release in the rat tail artery. NEW & NOTEWORTHY Immunofluorescence imaging of vesicular nucleotide translocase and vesicular monoamine transporter 2 in rat tail arteries revealed that ATP and norepinephrine, classical cotransmitters, localize to well-segregated vesicle pools. Furthermore, vesicular nucleotide translocase and vesicular monoamine transporter 2 exhibit preferential localization with specific Cav2 isoforms. These novel observations address long-standing debates regarding the mechanism(s) of sympathetic neurotransmitter corelease.

Mechanism of neurally induced monkey mesenteric artery relaxation and contraction

Physiological importance in vasodilator innervation alleviating noradrenergic neurogenic vasoconstriction has not been clarified. Isolated monkey mesenteric artery strips denuded of the endothelium responded to nerve stimulation by electrical pulses or nicotine with a contraction, which was potentiated by Ng-nitro-L-arginine, a nitric oxide synthesis inhibitor, but not by the D-enantiomer. The potentiation was abolished by L-arginine. NG-Nitro-L-arginine did not potentiate the response to exogenous norepinephrine nor did it increase the release of [3H]norepinephrine from adrenergic nerves electrically stimulated. The contraction was reversed by treatment with phentolamine and guanethidine to a relaxation, which was abolished by NG-nitro-L-arginine. The inhibition was reversed by L- but not D-arginine. The relaxant response was not influenced by atropine, timolol, or indomethacin. These findings strongly suggest the importance of reciprocal nitric oxide-related (nitroxidergic) vasodilator and noradrenergic vasoconstrictor innervation in the regulation of monkey arterial tone.

Cholinergic constriction in the general circulation and its role in coronary artery spasm

The release of acetylcholine from autonomic nerves in those tissues that receive a cholinergic innervation is widely believed to dilate blood vessels. Exogenously administered acetylcholine in vivo does dilate vascular beds and produce hypotension; however, this latter effect is indirect and probably the result of liberation of endothelium-derived relaxing factor (EDRF) from endothelial cells. Some blood vessels contain a substantial population of medial constrictor receptors for acetylcholine, and the implications of this presence for vascular control systems has been largely ignored, although it needs to be considered. A survey of the evolution of vasomotor control systems indicates that acetylcholine serves principally as an excitatory transmitter to blood vessels. Neurally mediated cholinergic constriction and not dilation is found in fish, amphibians, reptiles, and birds, with responses initiated by medial muscarinic receptors. Acetylcholine constricts many vascular preparations from these lower animals, but some vessels relax, reflecting the emergence of an EDRF responsive to acetylcholine. An examination of cholinergic responses in mammalian vessels reveals that cholinergic (neurogenic) dilation is limited to a very few vascular beds and to only a few species. Both experimental evidence and evolutionary considerations support the likelihood that cholinergic (neural) constriction operates in some vascular regions in mammals and, in particular, in the coronary circulation of some species, including humans. In fact, constriction, and not dilation, may be the dominant vascular response to activation of the cholinergic axis in most mammals, including humans. The complications and contradictions introduced by the simultaneous presence of both EDRF and a cholinergic constrictor innervation involving medial muscarinic receptors are discussed. A variety of evidence is also presented that implicates cholinergic constriction in at least some instances of coronary artery spasm and sudden death.

Pharmacological evidence for A1 and A2 adenosine receptors in the skin microcirculation

To characterize adenosine-mediated vascular responses, synthetic A1 and A2 receptor agonists (N-ethyl carboxamido adenosine [NECA], 2-chloro adenosine [2CA], or cyclohexyl adenosine [CHA]), the parent compound (adenosine [ADO]), an uptake inhibitor (dipyridamole [DIPYRID]) or a nonselective, competitive antagonist (8-phenyl theophylline [8pTHEO]) were topically applied to 20-60 microns arterioles in the subcutaneous microcirculation of the hamster. Blood flow was calculated from arteriolar diameter and red blood cell velocity using intravital microscopy. At greater than 10(-8) M, the potency order for vasodilation (maximum, 170-190% of control) was NECA greater than 2CA greater than ADO; these responses were attenuated by 10(-5) M 8pTHEO. From 10(-8) to 10(-6) M, 2CA evoked vasodilation whereas ADO, which has an identical affinity at A1 and A2 receptors, evoked lesser responses. ADO-induced vasodilation was potentiated by 10(-5) M DIPYRID; this response was similar to that evoked by 2CA alone or 2CA + DIPYRID. In contrast to ADO, 2CA is a poor substrate for cellular uptake, which suggests that uptake reduces the A2 effect of exogenous ADO. From 10(-10) to 10(-8) M, CHA and ADO were equipotent antagonized by 8pTHEO. Norepinephrine was a more potent vasoconstrictor and 8pTHEO did not alter these responses. Since ADO is a metabolic substrate and a nonselective receptor agonist, while CHA is A1-selective and a poor substrate for cellular uptake, neither A2 activation nor cellular uptake altered expression of the A1 effect of exogenous ADO. Furthermore, DIPYRID had no effect on the A1 response.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of intravertebral angiotensin II on cardiac output and its distribution in conscious dogs

Intravertebral infusion of angiotensin II (Ang II) increases mean arterial pressure (MAP), but the contribution of cardiac output (CO) and total peripheral resistance (TPR) to this increase is unclear. In the present study, the effects of Ang II infusion on CO and regional blood flow was determined by the microsphere technique in eight conscious, chronically catheterized dogs. Ang II was infused into both vertebral arteries at 0.33 and 1.0 ng/kg/min, and intravenously at 0.66, 2.0 and 5.0 ng/kg/min. Intravertebral infusion of Ang II at 0.33 ng/kg/min increased MAP by increasing CO without changing TPR or peripheral plasma Ang II concentration. MAP also was increased with intravertebral infusion of Ang II at 1.0 ng/kg/min, but this resulted from small increases in both CO and TPR. In contrast, intravenous infusion of Ang II at 2.0 and 5.0 ng/kg/min increased MAP by increasing TPR in association with a decrease in CO. The increase in CO with intravertebral infusion of Ang II at 0.33 ng/kg/min was distributed primarily to the muscles, kidneys, heart, and brain. Intravenous infusion of Ang II at 5.0 ng/kg/min and, to a lesser extent, 2.0 ng/kg/min decreased blood flow to the skin, splanchnic region, and kidneys. These data indicate that the increase in MAP produced by a low intravertebral dose of Ang II results from an increase in CO, which is distributed primarily to the muscle, kidney, heart, and brain. In contrast, the increase in MAP produced by a higher intravertebral dose of Ang II results from increases in CO and TPR. This latter action is apparently due to a peripheral action of Ang II to increase resistance in the skin, splanchnic, and renal circulations.

A dual function for adenosine 5'-triphosphate in the regulation of vascular tone. Excitatory cotransmitter with noradrenaline from perivascular nerves and locally released inhibitory intravascular agent

A dual function for adenosine 5'-triphosphate in the regulation of vascular tone is considered. Adenosine 5'-triphosphate can cause vasodilation, acting via P2-purinoceptors located on vascular endothelial cells to release an endothelium-derived relaxing factor which diffuses to the vascular smooth muscle and induces vasodilation. The main source of intraluminal adenosine 5'-triphosphate is likely to be endothelial cells, and its release can be measured during pathophysiological conditions such as ischemia and hypoxia, in amounts likely to be sufficient to activate endothelial P2-purinoceptors. Adenosine 5'-triphosphate can also be released during intravascular platelet aggregation and from intact and damaged vascular smooth muscle cells, and so may play a role in the complex physiological mechanisms controlling local vascular tone under normoxic conditions and during vessel injury. Evidence is also presented for adenosine 5'-triphosphate acting as an excitatory cotransmitter with noradrenaline from sympathetic perivascular nerves, to cause vasoconstriction via excitatory P2-purinoceptors located on vascular smooth muscle. The postjunctional mechanical and electrical responses of a number of blood vessels following perivascular nerve stimulation contain a component that is resistant to blockade of the alpha-adrenoceptor. This nonadrenergic response is mimicked by adenosine 5'-triphosphate and can be blocked by selective desensitization of the P2-purinoceptor by alpha,beta-methylene adenosine 5'-triphosphate. Vesicular storage of adenosine 5'-triphosphate and its release from sympathetic perivascular nerves has also been demonstrated. The functional significance of adenosine 5'-triphosphate acting intraluminally as a vasodilator and extraluminally as a vasoconstrictor neuronal agent in the control of vascular tone is discussed.

Pathophysiology of migraine: a new hypothesis

Cerebral blood flow is thought to decrease during the preheadache phase of migraine and increase during the headache phase. Most investigations of the pathophysiology of migraine have been concerned with the factors that trigger the preheadache phase. The present hypothesis proposes that, regardless of the triggering factors associated with vasospasm, this will be followed by a common event--namely, reactive hyperaemia due to hypoxia. It is suggested that adenosine triphosphate (ATP), perhaps released from "purinergic" nerves, and its breakdown products adenosine monophosphate (AMP) and adenosine are strong contenders for agents mediating this vasodilatation. ATP is a potent dilator of cerebral vessels and its breakdown products are also dilators. High concentrations of AMP and adenosine have been collected in cerebrospinal fluid during vasodilatation following ischaemia or hypoxia. The presence of ATP and its breakdown products could also explain pain in migraine. These substances stimulate primary afferent nerve terminals in the skin and produce pain in human skin blisters; and nerve profiles that resemble afferent terminals in their ultrastructure have been described in the adventitia of cerebral arteries and in the subarachnoid meshwork. The asymmetrical nature of migraine headaches, the changes in platelet aggregation, and the responses to several therapeutic procedures are consistent with this hypothesis. Some analogues of ATP might have beneficial effects on migraine headache.