Reassessment of the blocking activity of prazosin at low and high concentrations on sympathetic neurotransmission in the isolated mesenteric vasculature of rats

Effect of chronic vitamin E deficiency on sympathetic and sensorimotor function in rat mesenteric arteries

1. Mesenteric arterial beds from male rats deprived of vitamin E for 12 months postweaning were isolated and perfused at 5 ml min-1. 2. The basal perfusion pressure of vitamin E-deficient preparations was significantly higher (34.0 +/- 1.9 mmHg, n = 15) than in age-matched controls (26.1 +/- 2 mmHg, n = 14; P < 0.01). 3. At basal tone, vasoconstrictor responses to electrical field stimulation (EFS) were not attenuated by vitamin E deficiency; at high stimulation frequencies, responses were enhanced. According to dose-response curves, exogenous noradrenaline was significantly more efficacious in preparations from vitamin E-deficient rats (P < 0.05). 4. In preparations with tone raised by methoxamine (6-20 microM) and in the presence of guanethidine (5 microM), EFS of perivascular sensorimotor nerves elicited frequency-dependent vasodilatation which was significantly attenuated by vitamin E deficiency. There was no difference in relaxation to calcitonin gene-related peptide (CGRP; 1.5 x 10(-11) mol), or to the sensory neurotoxin capsaicin (5 x 10(-11) mol). 5. Immunohistochemical analysis of CGRP-containing nerves in the superior mesenteric artery showed no differences in density of innervation. 6. In conclusion, chronic vitamin E deficiency impairs sensorimotor vasodilatation in rat mesenteric arteries; this does not appear to be due to changes in postjunctional receptors, or to a depletion of transmitter (CGRP) content of the superior mesenteric artery. Sensorimotor nerves appear to be more vulnerable than sympathetic nerves to chronic vitamin E deficiency.

Alpha-adrenoceptor modulation of norepinephrine and ATP release in isolated kidneys of spontaneously hypertensive rats

The present study investigates sympathetic cotransmission and its alpha-adrenoceptor-mediated modulation in kidneys of spontaneously hypertensive rats (SHR, 12 to 14 weeks) and age-matched normotensive Wistar-Kyoto rats (WKY). In the presence of cocaine and corticosterone, renal nerve stimulation at 1 Hz (30 seconds) induced a greater outflow of norepinephrine in SHR (4.2 +/- 0.2 pmol/g kidney) than in WKY (3.0 +/- 0.2 pmol/g kidney). The alpha 2-adrenoceptor antagonist rauwolscine (0.01 to 1 mumol/L) increased the stimulation-induced norepinephrine outflow to a greater extent in SHR than in WKY. In contrast, the alpha 1-adrenoceptor antagonist prazosin (0.03 to 3 mumol/L) increased the stimulation-induced norepinephrine outflow to a greater extent in WKY than in SHR. This difference was not observed in the presence of the P1-purinoceptor antagonist 8-(p-sulfophenyl)theophylline (100 mumol/L). Stimulation at 4 Hz (30 seconds) induced an outflow of ATP (SHR, 12.7 +/- 3.3 pmol/g kidney; WKY, 16.7 +/- 2.1 pmol/g kidney; perfusion solution without cocaine and corticosterone). Prazosin (0.03 mumol/L) markedly reduced pressor responses to stimulation and inhibited the induced ATP outflow by 60% to 70%. When prazosin (0.03 mumol/L) was present, rauwolscine (0.1 mumol/L) increased the induced outflow of norepinephrine and ATP and markedly enhanced prazosin-resistant pressor responses. These pressor responses were abolished by the P2-purinoceptor antagonist suramin (300 mumol/L). The results demonstrate an increased alpha 2-adrenoceptor-mediated automodulation of norepinephrine release in SHR kidneys caused by increased intrasynaptic norepinephrine levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasoconstrictor and vasodilator responses to various agonists in the rat perfused mesenteric arterial bed: selective inhibition by PPADS of contractions mediated via P2x-purinoceptors

1. The effect of pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) on vasoconstrictor and/or vasodilator responses to various agonists and electrical field stimulation was investigated in the rat mesenteric arterial bed at basal tone and at tone raised by methoxamine (15-50 microM). 2. At basal tone, nucleotides produced vasoconstriction with the following rank order of potency: alpha,beta-methylene ATP >> 2-methylthio ATP > or = ATP = UTP. PPADS (0.3-10 microM) concentration-dependently antagonized alpha, beta-methylene ATP-, 2-methylthio ATP- and ATP-induced responses. UTP-, noradrenaline- and nerve-mediated (4-32 Hz) increases in perfusion pressure remained unaffected by 10 microM PPADS. 3. In raised tone preparations, nucleotides produced vasodilations, their rank order of potency being 2-methylthio ATP > ATP > UTP. Responses to 2-methylthio ATP were slightly antagonized, whereas ATP- and UTP-induced responses remained unaffected by 10 microM PPADS. In addition, acetylcholine- and adenosine-elicited relaxations were not influenced by 10 microM PPADS. 4. The present results confirm the previously described selective P2x antagonism by PPADS, this compound being ineffective at muscarinic M3- and adenosine P1-receptors as well as at alpha 1-adrenoceptors. There was some inhibition of P2y-purinoceptors but at a much higher concentration than required for inhibition of P2x-purinoceptors. 5. In addition, this study provides evidence for the ineffectiveness of PPADS at both vasoconstriction- and vasodilatation-mediating P2u-purinoceptors.

Release of endogenous ATP from the vasa deferentia of the rat and guinea-pig by the indirect sympathomimetic tyramine

1. Adenosine 5'triphosphate (ATP) as well as [3H]-noradrenaline ([3H]-NA) is released by perfusion of the vas deferens with the indirect sympathomimetic tyramine (100 microM); this result is consistent with the concept of sympathetic cotransmission. 2. While tyramine produced a strong contraction in the vas deferens of the rat, it had little mechanical action in the guinea-pig vas deferens. This appears to be largely because tyramine induces considerably lower levels of release of both ATP and NA from the guinea-pig vas deferens compared to that of the rat. Furthermore, NA released by tyramine appears to release ATP from a secondary pool in the rat vans deferens, but not that of the guinea-pig, since prazosin reduced the tyramine-induced release of ATP in the rat vas deferens. 3. alpha,beta-Methylene ATP (alpha,beta-meATP) increased both the spontaneous release of ATP and the tyramine-evoked efflux of ATP and [3H]-NA. The basal and tyramine-induced efflux of [3H]-NA was also enhanced by the alpha 1-adrenoceptor antagonist, prazosin, suggesting that prejunctional alpha 1-adrenoceptors may modulate neurotransmitter release.

Characterization of the isolated rat mesenteric vascular-intestinal loop preparation

The isolated mesenteric vascular-intestinal loop preparation has been shown to be an important model of resistance vessel hemodynamics reflective of both physiological and pathological conditions. The present experiments were performed to characterize the isolated rat mesenteric vascular-intestinal loop preparations. The mesenteric vasculature with intestine was perfused with modified Krebs-Ringer bicarbonate solution at a constant flow of 5 mL/min. To examine spontaneous norepinephrine overflow, the perfusate and superfusate were collected for 4-min periods from 36 to 60 min after the start of the perfusion. The norepinephrine overflow from the mesenteric vasculature was also determined in response to field stimulation (FS). Basal spontaneous endogenous norepinephrine overflow was relatively constant. The spontaneous norepinephrine overflow was increased by guanethidine (10(-5) M) but was not influenced by tetrodotoxin (3 x 10(-7) M), cocaine (10(-6) M), or prazosin (10(-6) M) treatment. Field stimulation (4-12 Hz) caused frequency-dependent pressor responses and increases in norepinephrine overflow. When norepinephrine overflow was expressed as amount/stimulus (picogram/stimulus), it was frequency independent. The pressor responses to FS were abolished by guanethidine (10(-5) M), tetrodotoxin (3 x 10(-7) M), and prazosin (3 x 10(-8) M). Prazosin at 10(-6) M significantly augmented the FS-induced endogenous norepinephrine overflow. Thus, these results indicate that the isolated rat mesenteric vascular-intestinal loop preparation is an excellent model for demonstrating resistance changes in isolated vascular beds while simultaneously measuring endogenous catecholamine overflow.

Roles of P2-purinoceptors in the cardiovascular system

Characterization of P2-purinoceptor subtypes has facilitated understanding of the many diverse effects produced by purine nucleotides. P2X-Purinoceptors are located on vascular smooth muscle where they mediate vasoconstriction resulting from ATP released as a cotransmitter with noradrenaline from sympathetic nerves. P2Y-Purinoceptors are usually located on the vascular endothelium where they have a role as mediators of vascular relaxation by locally produced ATP. In some vessels, P2Y-purinoceptors are also located on the smooth muscle, perhaps in association with purinergic or sensory nerves, where they can elicit direct relaxation to neuronally released ATP. The net effect of ATP and its analogues on isolated vessels or on vascular beds will be the results of actions mediated by P2X- and P2Y-purinoceptor subtypes, although changes in vascular tone and in integrity of nerves and endothelial cells may alter the balance of the response. Such changes have been observed in diseased states (e.g., atherosclerosis) and may have important implications for the involvement of P2-purinoceptors in, for example, vasospasm. The development of selective and potent antagonists to P2X- and P2Y-purinoceptors has so far remained elusive, and their therapeutic potential can only be guessed.

Roles of neuropeptide Y and noradrenaline in sympathetic neurotransmission to the thoracic vena cava and aorta of guinea-pigs

The roles of neuropeptide Y (NPY) and noradrenaline (NA) in sympathetic neurotransmission to large arteries and veins were studied in vitro using the thoracic portions of the aorta and inferior vena cava from guinea-pigs. Both vessels are densely innervated by axons containing NA and NPY. Repetitive transmural stimulation at 2-30 Hz produced contractions of the aorta, which were abolished by prazosin. NPY did not have significant postsynaptic or presynaptic effects on vascular tone of the aorta. Transmural stimulation of the vena cava produced long-lasting contractions which were enhanced by alpha- and beta-adrenoceptor antagonists, and were blocked by guanethidine. Precontracted venae cavae responded to sympathetic stimulation with beta-adrenoceptor-mediated relaxation, followed by contraction. alpha-Adrenoceptor blockade delayed the onset of neurogenic contractions. NPY was a potent contractile agent of the vena cava (EC50 approximately 1.5 x 10(-8) M). A high concentration (3 x 10(-6) M) of NPY, or the specific NPY Y1 receptor agonist, [Leu31, Pro34]NPY, caused parallel, and reversible, desensitization of contractions produced by sympathetic nerve stimulation, and by low concentrations of exogenous NPY. This provides good evidence that NPY is the mediator of the non-adrenergic sympathetic contractions of the vena cava. Furthermore, these results demonstrate that differential location or coupling of postsynaptic receptors for NA and NPY in the aorta and vena cava, leads to differential participation by these substances in sympathetic vasomotor responses. This is likely to be related to the different functions of these two parts of the systemic circulation.

A new method for simultaneous measurement of endogenous noradrenaline release and vascular responsiveness in perfused rabbit carotid arteries in vitro

1. A new preparation for large arteries was tested for the simultaneous measurement of noradrenaline release and vasoconstrictor response induced by nerve stimulation in vitro. 2. Preparations of rabbit carotid artery with an outflow cannula were perfused with a modified Krebs-Ringer bicarbonate solution containing propranolol (3 x 10(-6) M) and cocaine (10(-6) M), to block beta-adrenoceptors and uptake-1, respectively. 3. The perfused carotid arteries were subjected to both transmural field stimulations (TFS) and exogenous noradrenaline administrations. To evaluate the preparation, we carried out the procedures in two control groups and a group with yohimbine (10(-7) M) treatment, respectively. 4. TFS over the range of 1.5-24 Hz caused a frequency-dependent pressor response. Yohimbine significantly increased the TFS-induced pressor responses. 5. TFS also caused frequency-dependent endogenous noradrenaline release. The endogenous noradrenaline release was significantly enhanced by yohimbine treatment. When noradrenaline release was expressed as release per stimulus, it was frequency-dependent in the group with yohimbine treatment but not in control groups. 6. Exogenous noradrenaline induced a dose-dependent pressor response. The exogenous noradrenaline-induced vasoconstrictor response was significantly diminished by yohimbine treatment. 7. It is concluded that this preparation can be effectively used to study drug effects on noradrenergic neurotransmission in relatively large arteries under various pathophysiological states by simultaneously measuring catecholamine release and vascular responsiveness.

Dual control of local blood flow by purines

The potent and widespread vascular actions of purine nucleotides and nucleosides have long been recognized. A dual function for ATP in the regulation of vascular tone is considered. ATP acts as an excitatory cotransmitter with noradrenaline from sympathetic perivascular nerves, to cause vasoconstriction via P2X-purinoceptors located on vascular smooth muscle. In contrast, ATP can act via P2Y-purinoceptors located on vascular endothelial cells to release EDRF, which diffuses to the vascular smooth muscle and produces vasodilatation. The main source of intraluminal ATP is likely to be endothelial cells, and its release can be measured during conditions such as changes in flow and hypoxia, in amounts sufficient to activate endothelial P2Y-purinoceptors. In some vessels, ATP acts directly on P2Y-purinoceptors located in the vascular smooth muscle to produce vasodilatation; the possibility that the origins of this ATP are nonsympathetic purinergic or sensory-motor nerves is discussed. ATP 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, during changes in blood flow and during vessel injury.

Postjunctional synergism of noradrenaline and adenosine 5'-triphosphate in the mesenteric arterial bed of the rat

The ability of purines to modify responses to exogenous noradrenaline (NA) was investigated using the isolated perfused rat mesenteric bed. ATP, at subthreshold doses and above-threshold doses, produced a potentiation of vasoconstrictor responses to NA while adenosine was without effect. The stable analogue of ATP, alpha,beta-methylene ATP alpha,beta-meATP, at subthreshold and above-threshold doses also enhanced pressor responses to NA (to a greater extent than ATP). This potentiation caused a shift to the left of the dose-response curve, with no increase in the maximum response. Pressor responses to 5-hydroxytryptamine (5-HT) and to potassium chloride (KCl), however, were not affected by alpha,beta-meATP. Conversely, suprathreshold doses of NA potentiated contractions evoked by alpha,beta-meATP, but no potentiation was observed using subthreshold doses of NA. These results demonstrate a postjunctional synergistic action between NA and ATP which appeared to be specific for the alpha 1-adrenoceptors and the P2x-purinergic receptors since: (i) the potentiation of the contractile response to NA by ATP was mimicked by alpha,beta-meATP but not by adenosine and (ii) pressor responses to 5-HT or to KCl were not affected by alpha,beta-meATP. Possible mechanisms for this postjunctional synergism are discussed.

Effect of moderate cooling on endogenous noradrenaline release from the mesenteric vasculature of rats

1. Experiments were performed to investigate whether moderate cooling can reduce exocytotic noradrenaline release in the deep circulation of rats. 2. The release of endogenous noradrenaline from the entire mesenteric vasculature of the isolated intestine of rats was determined along with the perfusion pressure response to transmural field stimulation (TFS) (4-14 Hz) at 37 degrees C or 24 degrees C. 3. Although moderate cooling significantly augmented the perfusion pressure responses to TFS, it markedly depressed TFS-induced endogenous noradrenaline release. Moderate cooling also significantly reduced the perfusion pressure responses to exogenous noradrenaline. 4. Treatment of mesenteric vascular preparations with cocaine (10(-6) M) plus corticosterone (4 x 10(-5) M) to inhibit neuronal and extraneuronal noradrenaline uptake, respectively, significantly increased the TFS-induced pressor response at 37 degrees C, but not at 24 degrees C. 5. However, treatment with cocaine in combination with corticosterone significantly increased the TFS-induced release of noradrenaline at both temperatures. The enhancement produced by these uptake inhibitors on the TFS-induced noradrenaline release at 37 degrees C was significantly smaller than that observed at 24 degrees C. 6. These results indicate that moderate cooling can depress the exocytotic noradrenaline release in the mesenteric vasculature. However, it appears that reduced re-uptake by moderate cooling probably overcomes the inhibitory effect of cooling on the release of noradrenaline. 7. Thus, it is suggested that the increased pressor responses to TFS by moderate cooling are due, at least in part, to reduced uptake at lower temperatures.