Roles of extrajunctional receptors in the response of guinea-pig mesenteric and rat tail arteries to adrenergic nerves

Effects of high-fat diet on sympathetic neurotransmission in mesenteric arteries from Dahl salt-sensitive rat

Obesity hypertension is driven by sympathetic neurotransmission to the heart and blood vessels. We tested the hypothesis that high-fat diet (HFD)-induced hypertension is driven by sympathetic neurotransmission to mesenteric arteries (MA) in male but not female Dahl salt-sensitive (Dahl ss) rat. Rats were fed a control diet (CD; 10 kcal% from fat) or HFD (60 kcal% from fat) beginning at 3 weeks (wk) of age; measurements were made at 10-, 17- and 24-wk. Body weight increased with HFD, age and sex. Mean arterial pressure (MAP) was higher in HFD versus CD rats from both sexes at 17- and 24-wk. MA constriction measured using pressure myography, and electrical field stimulation (EFS, 0.2-30 Hz) was greater in HFD versus CD in males at 17-wk; this was not due to changes in α2 autoreceptor or norepinephrine transporter (NET) function. Prazosin (α1-AR antagonist) and suramin (P2 receptor antagonist) inhibited neurogenic MA constriction equally in all groups. Arterial reactivity to exogenous norepinephrine (NE; 10^-8 - 10^-5 M) was lower in HFD versus CD at 10-wk in males. Female MA reactivity to exogenous ATP was lower at 24-weeks compared to earlier time points. HFD did not affect tyrosine hydroxylase (TH) or the vesicular nucleotide transporter (VNUT) nerve density in MA from both sexes. NE content was lower in MA but higher in plasma at 24-wk compared to 10- and 17-wk in both sexes. In conclusion, HFD-induced hypertension is not driven by increased sympathetic neurotransmission to MA in male and female Dahl ss rats.

Inhibition of KATP channels in the rat tail artery by neurally released noradrenaline acting on postjunctional alpha2-adrenoceptors

In rat tail artery, activation of postjunctional alpha(2)-adrenoceptors by noradrenaline (NA) released from sympathetic axons produces a slow depolarization (NAD) of the smooth muscle through a decrease in K(+) conductance. In this study we used intracellular recording to investigate whether the K(+) channel involved is the ATP-sensitive K(+) (K(ATP)) channel. Changes in membrane resistance were monitored by measuring the time constant of decay of excitatory junction potentials. The K(ATP) channel blockers, glibenclamide (10 microm) and PNU 37883A (5 microm), depolarized the smooth muscle and increased membrane resistance. Conversely, the K(ATP) channel openers, pinacidil (0.1 and 0.5 microm) and levcromakalim (0.1 microm), hyperpolarized the smooth muscle and decreased membrane resistance. Activation of K(ATP) channels with calcitonin gene-related peptide (CGRP; 10 nM) also hyperpolarized the smooth muscle and decreased membrane resistance. The NAD was abolished by both glibenclamide and PNU 37883A but was potentiated by CGRP. However, unlike CGRP, the directly acting K(ATP) channel openers, pinacidil and levcromakalim, inhibited the NAD. The effects of other K(+) channel blockers were also determined. A high concentration of Ba(2+)(1 mM), which would be expected to block K(ATP) channels, abolished the NAD, whereas teteraethylammonium (1 mM) and 4-aminopyridine (1 mM) increased its amplitude. Apamin (0.5 microm) and a lower concentration of Ba(2+) (0.1 mM) did not affect the NAD. These findings indicate that activation of alpha(2)-adrenoceptors by neurally released NA depolarizes the membrane of vascular smooth muscle by inhibiting K(ATP) channels open in the resting membrane.

Rho kinase inhibitors reduce neurally evoked contraction of the rat tail artery in vitro

The effects of Rho kinase inhibitors (Y27632, HA-1077) on contractions to electrical stimulation and to application of phenylephrine, clonidine or alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-mATP) were investigated in rat tail artery in vitro. In addition, continuous amperometry and intracellular recording were used to monitor the effects of Y27632 on noradrenaline (NA) release and postjunctional electrical activity, respectively. Y27632 (0.5 and 1 microM) and HA-1077 (5 microM) reduced neurally evoked contractions. In contrast, the protein kinase C inhibitor, Ro31-8220 (1 microM), had little effect on neurally evoked contraction. In the absence and the presence of Y27632 (0.5 microM), the reduction of neurally evoked contraction produced by the alpha-adrenoceptor antagonists prazosin (10 nM) and idazoxan (0.1 microM) was similar. The P2-purinoceptor antagonist, suramin (0.1 mM), had no inhibitory effect on neurally evoked contraction in the absence or the presence of Y27632 (1 microM). In the presence of Y27632, desensitization of P2X-purinoceptors with alpha,beta-mATP (10 microM) increased neurally evoked contractions.Y27632 (1 microM) and H-1077 (5 microM) reduced sensitivity to phenylephrine and clonidine. In addition, Y27632 reduced contractions to alpha,beta-mATP (10 microM). Y27632 (1 microM) had no effect on the NA-induced oxidation currents or the purinergic excitatory junction potentials and NA-induced slow depolarizations evoked by electrical stimulation. Rho kinase inhibitors reduce sympathetic nerve-mediated contractions of the tail artery. This effect is mediated at a postjunctional site, most likely by inhibition of Rho kinase-mediated 'Ca2+ sensitization' of the contractile apparatus.

Modulation of the myogenic response by neurogenic influences in rat small arteries

The hypothesis that the amplitude of the myogenic response is modulated by factors released from nerve endings was tested in rat tail small arteries. A pressure myograph in conjunction with direct stimulation of nerve endings by electrical field stimulation (EFS) was used to determine rat small artery contractile reactions. Vessel pretreatment with 10(- 5) M phentolamine abolished EFS-induced reactions completely indicating that they are mediated mainly by an adrenoceptor agonist, probably noradrenaline. In the absence and presence of 10(- 5) M phentolamine, vessel diameter changes in the pressure range from 10 to 120 mmHg were not different. Vessel stimulation by (i) EFS, (ii) noradrenaline, (iii) selective stimulation of alpha1- and alpha2-receptors, (iv) serotonin, or (v) vasopressin significantly reduced the diameter change induced by stepping pressure from 10 to 40 mmHg compared to unstimulated, control vessels. Vessel diameter changes induced by stepping pressure from 40 to 80 and from 80 to 120 mmHg, however, were not different in vessels stimulated with EFS and noradrenaline compared to controls. In conclusion, these data show that factors released from unstimulated adrenergic nerve endings (i.e., not stimulated by EFS) are not involved in the myogenic response. In contrast, factors released upon stimulation of nerve endings can modulate the amplitude of the myogenic response, but only at low pressures. Thus, the pressure range for myogenic blood flow autoregulation is extended to lower pressures. Myogenic autoregulation of blood flow at physiological pressures is unaltered.

An electrophysiological study of excitatory purinergic neuromuscular transmission in longitudinal smooth muscle of chicken anterior mesenteric artery

1. The object of the present study was to clarify the neurotransmitters controlling membrane responses to electrical field stimulation (EFS) in the longitudinal smooth muscle cells of the chicken anterior mesenteric artery. 2. EFS (5 pulses at 20 Hz) evoked a depolarization of amplitude 19.7+/-2.1 mV, total duration 29.6+/-3.1 s and latency 413.0+/-67.8 ms. This depolarization was tetrodotoxin (TTX)-sensitive and its amplitude was partially decreased by atropine (0.5 microM); however, its duration was shortened by further addition of prazosin (10 microM). 3. Atropine/prazosin-resistant component was blocked by the nonspecific purinergic antagonist, suramin, in a dose-dependent manner, indicating that this component is mediated by the neurotransmitter adenosine 5'-triphosphate (ATP). 4. Neither desensitization nor blocking of P2X receptor with its putative receptor agonist alpha,beta-methylene ATP (alpha,beta-MeATP, 1 microM) and its antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic (PPADS, up to 50 microM), had significant effect on the purinergic depolarization. In contrast, either desensitization or blocking of P2Y receptor with its putative agonist 2-methylthioATP (2-MeSATP, 1 microM) and its antagonist Cibacron blue F3GA (CBF3GA, 10 microM) abolished the purinergic depolarization, indicating that this response is mediated through P2Y but not P2X receptor. 5. The purinergic depolarization was inhibited by pertussis toxin (PTX, 600 ng ml(-1)). Furthermore, it was significantly inhibited by a phospholipase C (PLC) inhibitor, U-73122 (10 microM), indicating that the receptors involved in mediating the purinergic depolarization are linked to a PTX-sensitive G-protein, which is involved in a PLC-mediated signaling pathway. 6. Data of the present study suggest that the EFS-induced excitatory membrane response occurring in the longitudinal smooth muscle of the chicken anterior mesenteric artery is mainly purinergic in nature and is mediated via P2Y purinoceptors.

Selective modulation of noradrenaline release by alpha 2-adrenoceptor blockade in the rat-tail artery in vitro

The effects of blocking alpha(2)-adrenoceptors on noradrenaline (NA) and adenosine 5'-triphosphate (ATP) release from postganglionic sympathetic nerves have been investigated in rat-tail artery in vitro. Continuous amperometry was used to measure NA release and intracellularly recorded excitatory junction potentials (e.j.p.'s) were used to measure ATP release. Application of the alpha(2)-adrenoceptor antagonist, idazoxan (1 microm), increased the amplitude of NA-induced oxidation currents evoked by trains of 10 stimuli at 1 and 10 Hz. In cells deep in the media, idazoxan (1 microm) had no effect on the amplitude of e.j.p.'s evoked by trains of 10 stimuli at 1 and 10 Hz. In cells close to the adventitial - medial border, idazoxan produced a small increase in the amplitude of e.j.p.'s evoked at the end of trains of 10 stimuli at 1 Hz. In tissues pretreated with the neuronal NA uptake inhibitor, desmethylimpramine (0.3 microm), idazoxan (1 microm) markedly increased the amplitude of e.j.p.'s in cells deep in the media. The alpha(2)-adrenoceptor agonist, clonidine (0.5 microm), produced similar reductions in the amplitudes of both NA-induced oxidation currents and e.j.p.'s evoked by 10 stimuli at 1 Hz. These effects of clonidine were reversed by the subsequent addition of idazoxan (1 microm). The release of both NA and ATP is inhibited to a similar extent by activation of prejunctional alpha(2)-adrenoceptors by clonidine. In contrast, endogenously released NA more markedly inhibits NA release. These findings provide further support for the differential modulation of NA and ATP release.

Dual alpha(2)-adrenergic agonist and alpha(1)-adrenergic antagonist actions of dexmedetomidine on human isolated endothelium-denuded gastroepiploic arteries

The actions of dexmedetomidine (DEX) on human vascular smooth muscle are unclear. We investigated its effects on isolated, endothelium-denuded human gastroepiploic arteries in vitro and compared them with clonidine (CLO). DEX had little direct effect on resting tension, whereas CLO produced small contractile responses, an effect which is blocked by the alpha(1)-adrenergic antagonist prazosin. DEX markedly enhanced the high K(+) (40 mmol/L)-induced contraction, and this effect was reversed by the alpha(2)-adrenergic antagonists yohimbine and rauwolscine but unaffected by prazosin. However, CLO had little effect on the K(+) contractions. Interestingly, larger concentrations (>10(-7) mol/L) of both alpha(2)-adrenergic stimulants significantly inhibited the contractions elicited by the alpha(1)-adrenergic agonist phenylephrine (10(-6) mol/L) and, to a lesser extent, those elicited by the alpha(1)/alpha(2)-agonist norepinephrine (10(-6) mol/L). These results suggest the possibility that DEX and CLO each have a high affinity for alpha(1)-adrenoceptors in human isolated gastroepiploic arteries, resulting in a reduced efficacy of alpha(1)-adrenergic activation by alpha-agonists. The differing affinities of the drugs for alpha(1)- and alpha(2)-adrenoceptors may help explain their additional actions: 1) DEX enhances the high K(+)-induced contraction presumably through alpha(2)-adrenoceptor activation, and 2) CLO acts on alpha(1)-adrenoceptors as a partial agonist when present alone.

IMPLICATIONS

Dexmedetomidine may not directly affect smooth muscle in human peripheral resistance vessels within the usual range of plasma concentrations (<10(-7) mol/L) achieved in clinical practice. However, in large doses, it could enhance the response to nonadrenergic vasoconstrictor agonists while antagonizing the vasoconstrictor response to alpha(1)-adrenoceptor agonists.

Differential regulation of intracellular Ca2+ signalling induced by high K+ and endothelin-1 in single smooth muscle cells of intact canine basilar artery: detection by means of confocal laser microscopy

Changes in intracellular calcium concentration ([Ca2+]i) in smooth muscle cells play the key role in regulation of vascular smooth muscle tone and pathogenesis of cerebral vasospasm. In this study, we adopted the confocal laser microscopy to detect the fluorescence signals arising from the individual smooth muscle cells of canine basilar artery. Ring preparations were made, loaded with fluo-3 and changes in fluorescence induced by high K+ and endothelin-1 (ET-1) were measured by confocal laser microscopy. In some unstimulated smooth muscle cells Ca2+ waves arising from discrete region of the cell propagated to the whole cell with a velocity of approximately 10 microm/s. High K+ (80 mmol/L) induced a rapid rise in [Ca2+]i, the peak level being consistently reached approximately 10 s after stimulation. In contrast, the time to peak level of [Ca2+]i induced by ET-1 (0.3 micromol/L) varied widely between 13 and 26 s among individual cells, an indication that the extent of nonuniform coordination of increases in [Ca2+]i in individual cells may be partly responsible for the different time courses of tension development of vascular smooth muscle in response to the vasoactive stimulants. The increase in [Ca2+]i induced by ET-1 was transient but a pronounced and sustained contraction developed further in response to ET-1. Thus ET-1 has a biological property as a potential candidate to elicit cerebral vasospasm. Confocal laser microscopy could be a useful tool to measure the changes in [Ca2+]i in individual smooth muscle cells of cerebral artery.

Contribution of alpha-adrenoceptors to depolarization and contraction evoked by continuous asynchronous sympathetic nerve activity in rat tail artery

1. The effects of continuous but asynchronous nerve activity induced by ciguatoxin (CTX-1) on the membrane potential and contraction of smooth muscle cells have been investigated in rat proximal tail arteries isolated in vitro. These effects have been compared with those produced by the continuous application of phenylephrine (PE). 2. CTX-1 (0.4 nM) and PE (10 microM) produced a maintained depolarization of the arterial smooth muscle that was almost completely blocked by alpha-adrenoceptor blockade. In both cases, the depolarization was more sensitive to the selective alpha-adrenoceptor antagonist, idazoxan (0.1 microM), than to the selective alpha 1-adrenoceptor antagonist, prazosin (0.01 microM). 3. In contrast, the maintained contraction of the tail artery induced by CTX-1 (0.2 nM) and PE (2 and 10 microM) was more sensitive to prazosin (0.01) microM, than to idazoxan (0.01 microM). In combination, these antagonists almost completely inhibited contraction to both agents. 4. Application of the calcium channel antagonist, nifedipine (1 microM), had no effect on the depolarization induced by either CTX-1 or PE but maximally reduced the force of the maintained contraction to both agents by about 50%. 5. We conclude that the constriction of the tail artery induced by CTX-1, which mimics the natural discharge of postganglionic perivascular axons, is due almost entirely to alpha-adrenoceptor activation. The results indicate that neuronally released noradrenaline activates more than one alpha-adrenoceptor subtype. The depolarization is dependent primarily on alpha 2-adrenoceptor activation whereas the contraction is dependent primarily on alpha 1-adrenoceptor activation. The links between alpha-adrenoceptor activation and the voltage-dependent and voltage-independent mechanisms that deliver Ca2+ to the contractile apparatus appear to be complex.

Electrical activity in rat tail artery during asynchronous activation of postganglionic nerve terminals by ciguatoxin-1

1. The effects of ciguatoxin-1 (CTX-1) on the membrane potential of smooth muscle cells have been examined in rat proximal tail arteries isolated in vitro. 2. CTX-1 (> or = 10 pM) increased the frequency of spontaneous excitatory junction potentials (s.e.j.ps). At 100-400 pM, there was also a marked and maintained depolarization (19.7 +/- 1.4 mV, n = 14, at 400 pM). 3. In 20-400 pM CTX-1, perivascular stimuli evoked excitatory junction potentials (e.j.ps) which were prolonged in time course relative to control. 4. Although threshold and latency of the e.j.p. were not affected by CTX-1 (< or = 400 pM), propagated impulses were blocked at > or = 100 pM. 5. The spontaneous activity and the depolarization produced by CTX-1 were reduced in the presence of Ca2+ (0.1 mM)/Mg2+ (25 mM), omega-conotoxin (0.1 microM) or Cd2+ (50-100 microM). 6. All effects of CTX-1 were abolished by tetrodotoxin (0.3 microM). 7. Raised Ca2+ (6 mM) reduced the depolarization and spontaneous activity produced by CTX-1. 8. In 400 pM CTX-1, the membrane repolarized (17 +/- 3.2 mV, n = 4) following the addition of phentolamine (1 microM). S.e.j.ps and e.j.ps were selectively abolished by suramin (1 mM), and the membrane repolarized by 1.3 +/- 1.6 mV (n = 4). 9. We conclude that CTX-1 releases noradrenaline and ATP by initiating asynchronous discharge of postganglionic perivascular axons. In 100-400 pM CTX-1, the smooth muscle was depolarized to levels resembling those recorded in this artery during ongoing vasoconstrictor discharge in vivo.

The effects of suramin on purinergic and noradrenergic neurotransmission in the rat isolated tail artery

Intracellular microelectrode recording was used to examine the effects of suramin, a P2-purinoceptor antagonist, on the electrical responses evoked by sympathetic nerve stimulation in the rat isolated tail artery. Field stimulation (10 or 20 pulses at 0.5, 1 and 2 Hz) evoked a biphasic electrical response, consisting of fast, transient excitatory junctional potentials (e.j.p.s) and a slow, prolonged depolarisation. Suramin (100 microM) abolished the e.j.p.s and significantly increased the amplitude of the slow depolarisation at all frequencies. In contrast, phentolamine (2 microM) abolished the slow depolarisation, but had no effect on the magnitude of e.j.p.s. Neither drug altered the resting membrane potential of cells. The ability of suramin to inhibit e.j.p.s in rat tail artery is consistent with the proposal that it is a P2X-purinoceptor antagonist and supports a role for ATP as an excitatory cotransmitter from the sympathetic nerves innervating this tissue. Suramin is also able to increase the alpha-adrenoceptor-mediated slow depolarisation by an unknown mechanism.

The role of alpha 2-adrenoceptors in the vasculature of the rat tail

1. The effects of alpha 2-adrenoceptor agonists and antagonists on rat tail skin temperature (tts), an indicator of local cutaneous blood flow, were studied in conscious and anaesthetized rats and in the isolated, Krebs perfused, vascular bed of the rat tail. 2. In conscious rats, at an ambient temperature of 18.5-20 degrees C, tts was 21.0 +/- 0.2 degrees C and core (rectal) temperature (tc) was 38.2 +/- 0.04 degrees C (n = 126). The alpha 2-adrenoceptor antagonist, delequamine (RS-15385-197; 1 mg kg-1, s.c., n = 6), produced a rapid elevation in tts to 29.1 +/- 0.7 degrees C (P < 0.001 vs. saline-treated control group), attained 10 min after injection. tc fell slightly, by 1.0 +/- 0.1 degrees C. The tts response was dose-related over the dose-range tested (0.01-1 mg kg-1, s.c.), with an ED50 of 17 micrograms kg-1, s.c. (n = 6 per dose). 3. The maximum increases in tts in response to a dose of 1 mg kg-1, s.c. of alpha 2-adrenoceptor antagonists were as follows (n = 6 per drug): delequamine (+9.6 +/- 0.8 degrees C) > yohimbine (+9.0 +/- 1.0 degrees C) > WY-26703 (+7.9 +/- 1.3 degrees C) > piperoxan (+5.6 +/- 1.7 degrees C) > idazoxan (+4.6 +/- 1.3 degrees C) > imiloxan (+4.1 +/- 1.3 degrees C) > SKF 104078 (+2.0 +/- 1.9 degrees C) > BDF-6143 (+1.3 +/- 0.8 degrees C). 4. Prazosin (0.3 mg kg-1, s.c.), hydralazine (10 mg kg-1, s.c.) and nifedipine (3 mg kg-1, s.c.) did not increase tts, whereas propranolol (10 mg kg-1, s.c.) evoked a small increase in tts (+2.9 +/- 1.0 degrees C). Pentolinium (2-10 mg kg-1, s.c.) elicited a dose-related increase in tts, which was elevated by 4.4 +/- 1.3 degrees C after a dose of 10 mg kg-1; tc was reduced in a dose-related manner. Drug vehicles (1 ml kg-1, s.c.) had no effect on tts or tc. 5. In anaesthetized rats, idazoxan (300 microg, i.v.) produced a rapid increase in tts which was detectable 2 min after beginning the injection, reaching a peak after 7 min. When the same dose was administered i.c.v., tts also rose, but more slowly. The peak response (+ 3.6 +/- 0.70C, n = 5) was significantly smaller than when idazoxan was administered intravenously (+ 6.3 +/- 1.2 C, n = 5), which suggests that the increase in tts following systemic administration of M2-adrenoceptor antagonists is not due to a central effect. The change in tts was not secondary to changes in blood pressure.6. In the isolated, Krebs perfused, tail vascular bed of the rat, at an ambient temperature of 20-21C,under constant flow conditions (3.5-4.0 ml min-1; n = 4), baseline perfusion pressure was 57 +/- 4 mmHg.5-Hydroxytryptamine (5-HT; 70-150 nM) increased perfusion pressure by 56+/- 9 mmHg. The alpha2-adrenoceptor agonist, UK-14,304 (10 nmol), elicited a further increase in perfusion pressure by27.5 +/- 15 mmHg but had no effect in the absence of 5-HT; this response to UK-14,304 was abolished by rauwolscine (1 microM).7. Under constant pressure conditions (-100 mmHg; n = 9), baseline mean perfusion flow was 2.1 +/- 0.2 ml min-1, and mean tail skin temperature was 31.6 +/- 0.6C. 5-HT (119 +/- 28 nM) decreased tts.by 3.3 +/- 2.0 C and reduced flow by 1.2 +/- 0.3 ml min-1. UK-14,304 (10 nmol) further reduced tts by 3.0 +/- 0.3 C without significant effect on flow; this effect was also abolished by 1 microM rauwolscine.8. We conclude that post-junctional M2-adrenoceptors in the vasculature of the rat tail have a major vasoconstrictor role, controlling both the flow and distribution of blood within the tail and thereby thermoregulatory heat loss from its surface.

Kinetics of ATP- and noradrenaline-mediated sympathetic neuromuscular transmission in rat tail artery

Electrophysiological, electrochemical and mechanical recordings were employed to study the kinetics of the release and clearance of adenosine 5'-triphosphate (ATP) and noradrenaline (NA) as sympathetic co-transmitters and of the neurogenic and non-neurogenic contractions in rat isolated tail artery. The life-time of ATP and NA released by a single pulse or 10 pulses at 50 Hz was brief (< 100 ms, or < 3 s, respectively); the neurogenic contractile responses occurred largely after the transmitters had been removed from the extracellular space. The ATP-induced neurogenic contractile responses to a single pulse or 10 pulses at 50 Hz were similar in time-course to the responses to direct muscle stimulation at low voltage; both seemed to be caused by activation of nifedipine-sensitive voltage-gated L-type Ca2+ channels. The alpha 1- and alpha 2-adrenoceptor-mediated components of the NA-induced neurogenic contractile response to 10 pulses at 50 Hz were more delayed and prolonged and determined by properties of the post-receptor mechanisms. The per pulse release of both ATP and NA faded rapidly during long high-frequency trains. So did the ATP level at the receptors and the ATP-induced neurogenic contraction. The NA levels and the contractile responses induced via alpha 1- and alpha 2-adrenoceptors were much better maintained during ongoing stimulation at 20 Hz but relaxed rapidly afterwards, suggesting that nerve activity suppressed, and cessation of nerve activity reactivated NA clearance.

Frequency- and train length-dependent variation in the roles of postjunctional alpha 1- and alpha 2-adrenoceptors for the field stimulation-induced neurogenic contraction of rat tail artery

The present paper examines the roles of postjunctional alpha 1- and alpha 2-adrenoceptors for the noradrenaline (NA)-induced neurogenic contractile response to field stimulation mainly with 1-100 pulses at 2 or 20 Hz, in the tail artery of adult normotensive rats. Pharmacological tools were employed to isolate and characterize the alpha 1- and alpha 2-adrenoceptor-mediated components of this response. The degree to which the drugs influenced NA release or reuptake was assessed by their effects on the electrochemically determined, stimulation-induced rise in the NA concentration at the innervated outer surface of the media. This response was unaffected by alpha,beta-methylene ATP (10 microM) or suramin (500 microM), added to desensitize or block P2-purinoceptors, respectively prazosin (0.1 microM) or SK&F 104078 (6-chloro-9-[(3-methyl-2-butenyl)oxyl]-3-methyl- 1H-2,3,4,5-tetrohydro-3-benzazepine, 0.1 microM), used to block postjunctional alpha 1- and alpha 2-adrenoceptors respectively, nifedipine (10 microM), blocker of Ca2+ influx through L-type channels, and ryanodine (10 microM), which blocks mobilization of Ca2+ from intracellular stores; it was moderately enhanced by yohimbine (0.1 microM), blocker of pre- and postjunctional alpha 2-adrenoceptors, and strongly enhanced by cocaine (3 microM) or desipramine (1 microM), blockers of NA reuptake. Judging from their inhibitory effects on the contractile responses to the alpha 1- and alpha 2-adrenoceptor agonists, phenylephrine and xylazine, prazosin (0.1 microM) and SK&F 104078 (0.1 microM) could be used to selectively block alpha 1- and alpha 2-adrenoceptors respectively, while yohimbine (0.1 microM) was less selective, strongly depressing alpha 2- and slightly depressing alpha 1-adrenoceptor-mediated responses. The alpha 1-adrenoceptor-mediated component of the contractile response to short trains at 20 Hz was fast in onset, brief in duration and abolished by ryanodine; that mediated by alpha 2-adrenoceptors was more delayed, prolonged and insensitive to ryanodine. Both components were dose-dependently depressed by nifedipine (0.1-10 microM). The small contractile responses to single pulses, or up to 50 pulses at 2 Hz, or short train (< 4 pulses) at 20 Hz, were more markedly depressed by 0.1 microM yohimbine or SK&F 104078 than by 0.1 microM prazosin and, hence, mediated mainly by alpha 2-adrenoceptors. The reverse was true of the much larger response to longer trains at 20 Hz, which thus probably was mediated mainly by alpha 1-adrenoceptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of alpha 1-adrenoceptors and functional consequences in the bisected rat vas deferens following chronic inhibition of neuronal noradrenaline uptake

1. The adaptational changes induced after chronic inhibition of neuronal noradrenaline uptake on both functional responsiveness of alpha 1-adrenoceptor activation and [3H]-prazosin binding were investigated in prostatic and epididymal portions of the rat vas deferens. 2. Contractile concentration-response curves to phenylephrine and saturation isotherms of [3H]-prazosin binding to homogenates of each of the portions of the bisected rat vas deferens were determined 48 h after the last injection of desipramine, nomifensine or nisoxetine (10 mg kg-1; i.p. for 14 days). 3. Treatment with both nomifensine and nisoxetine decreased the potency (pD2) of phenylephrine by about 10 and 8 fold respectively in the epididymal portion. However, administration of desipramine only reduced the potency of the alpha 1-adrenoceptor agonist by about 1.8 fold. None of the treatments modified the maximal effect (Emax) elicited by phenylephrine in this portion of the vas deferens. In the prostatic portion only the treatment with nomifensine (1.4 fold) and nisoxetine (1.8 fold) decreased the potency of phenylephrine; the maximal contraction elicited by the agonist after the treatments was also reduced. 4. Chronic treatment with either nomifensine or nisoxetine did not change the KD for [3H]-prazosin binding in either epididymal or prostatic membranes. However, these two treatments resulted in a significant decrease in the [3H]-prazosin Bmax in membranes in both portions of rat vas deferens. The reduction in density of alpha 1-adrenoceptors was higher in the epididymal than the prostatic half. Desipramine reduced the Bmax only in the epididymal portion. 5. These results indicate that differential regulation of ax-adrenoceptors in either portion of the rat vas deferens could result from a greater degree of activation of these receptors in the epididymal half after chronic inhibition of neuronal noradrenaline uptake. The different functional consequences of the loss of alpha l-adrenoceptors in each portion seems to be explained on the basis of a different relationship between the occupancy of the receptor and the response elicited.

Electrophysiological and electrochemical analysis of the secretion of ATP and noradrenaline from the sympathetic nerves in rat tail artery: effects of ?2-adrenoceptor agonists and antagonists and noradrenaline reuptake blockers

The aim of this study was to investigate whether or not nerve impulses release ATP and noradrenaline in parallel from the sympathetic nerve terminals of the rat tail artery. The extracellularly recorded excitatory junction current (EJC) was used to study, pulse by pulse, the release of ATP. An electrochemical method was used to study online the nerve stimulation-induced rise in the extracellular concentration of endogenous noradrenaline at the probe, a carbon fibre electrode (CF). This parameter, which does not directly represent noradrenaline release, but reflects release minus clearance, has been termed delta[NA]CF. The effects of a number of pharmacological agents on the EJCs were examined both at 0.1 and 2 Hz, and the effects on the EJC response to 100 pulses at 2 Hz compared with that on the delta[NA]CF response. Clonidine and xylazine were used as alpha 2-agonists, yohimbine and idazoxan as alpha 2-antagonists and desipramine and cocaine as blockers of noradrenaline reuptake. Most of these agents had unwanted side effects, especially at higher concentrations. However, clonidine and xylazine depressed at lower concentrations the EJC and delta[NA]CF responses to about the same extent; these effects were partially or completely reversed by yohimbine. Yohimbine or idazoxan did not affect the EJCs at 0.1 Hz but enhanced the EJC and delta[NA]CF responses to 100 pulses at 2 Hz to the same extent. All effects of desipramine (1 microM) seemed explainable as a result of block of noradrenaline reuptake, while cocaine (10 microM) in addition exerted an 'unspecific' depressant (probably local anesthetic) effect. Under control conditions, both agents depressed the EJC but dramatically enhanced the delta[NA]CF response to 100 pulses at 2 Hz. Addition of yohimbine prevented the depressant effect of desipramine on the EJCs completely and reduced that of cocaine, but increased their effects on the delta[NA]CF response. These results are compatible with the view that ATP and noradrenaline are released in parallel from the sympathetic nerve terminals of this tissue. The different, and under some conditions even opposite, effects of desipramine or cocaine on the EJC and delta[NA]CF responses are explainable in terms of the known post-secretory effects of these agents.

Electrophysiological responses in the rat tail artery during reinnervation following lesions of the sympathetic supply

1. Responses to perivascular stimuli have been recorded with intracellular microelectrodes from the smooth muscle of isolated segments of the main caudal artery of rats at various times between 7 and 128 days after all four collector nerve trunks had been lesioned near the base of the tail at 21 days of age. 2. In proximal segments (< 40 mm distal to the lesions), excitatory junction potentials (EJPs) and neurogenic alpha-depolarizations (NADs) evoked by stimuli presented via a proximally located suction electrode were similar to those in the same segments of unoperated control animals of the same age. Supramaximal EJPs in these segments decreased in amplitude with age. 3. Stimuli just supramaximal for EJPs in innervated preparations failed to evoke responses in segments farther than 30-40 mm distal to the lesions at any time after the nerves had been cut and 1 cm excised. Higher voltages evoked slow depolarizing potentials (SDPs) which were of longer time course than EJPs. Similar responses occurred in segments over 60 mm distal to the lesions at 20-50 days after the nerves had been frozen, and in all segments sampled over 100 mm distal to nerve lesions. 4. Spontaneous transient depolarizations (STDs) were recorded at all depths of the media in denervated segments. These occurred at frequencies similar to those of spontaneous events (including attenuated spontaneous EJPs) in innervated segments. 5. The earliest signs of reinnervation (24-42 days after freeze lesions) consisted of very small amplitude EJPs of normal time course which facilitated markedly during a short train of stimuli (5-10 Hz); these were followed by NADs which were large relative to the amplitudes of the EJPs. Less commonly, small focal EJPs of brief time course (resembling spontaneous EJPs in superficial cells of innervated arteries) were evoked in very restricted regions of the vessel wall. 6. At later times (57-128 days postoperative), six of eight segments located 40-70 mm distal to freeze lesions showed EJPs of nearly control amplitude, but NADs that were larger than in equivalent segments from control animals. In the remaining two cases, reinnervation at this level was similar to that seen at the earliest postoperative times. High stimulus voltages prolonged the decay of EJPs in both control and reinnervated arteries. 7. Sensitivity to exogenous noradrenaline, assessed in terms of membrane depolarization, was increased in both denervated and reinnervated segments. 8. Catecholamine fluorescence disappeared from the arteries at a distance greater than 30-40 mm distal to the site of the nerve lesions.(ABSTRACT TRUNCATED AT 400 WORDS)

An electrophysiological study of responses evoked in isolated segments of rat tail artery during growth and maturation

1. Intracellular recordings from the smooth muscle of isolated segments of the main caudal artery of rats at various ages between 45 and 150 days postnatal were made in order to relate the spontaneous depolarizations and responses to perivascular stimulation at different levels along the artery to the differences in vessel structure and innervation density during growth of the animals. 2. In the outermost smooth muscle cells close to the neuromuscular junctions, spontaneous depolarizations with fast time courses (spontaneous excitatory junction potentials or SEJPs) were recorded. In cells lying deeper in the media, spontaneous depolarizations had a wide range of time courses and amplitudes, but only a few of those could be attributed to electrotonic attenuation of SEJPs. 3. In arterial segments taken from animals of all ages, stimuli which evoked maximal amplitude excitatory junction potentials (EJPs) 1-2 mm caudal to a suction electrode also evoked neurogenic alpha-depolarizations (NADs) with time to peak of 15 s and duration nearly 1 min. Both responses decreased progressively in amplitude along the length of the artery. NADs were blocked by phentolamine (10(-6) M) or idazoxan (10(-7) M) which were without effects on EJPs. 4. During short trains of stimuli (5 at 1 or 10 Hz), EJPs facilitated but to a lesser extent with distance along the tail. Such trains evoked NADs of greater amplitude than those following a single stimulus; these were often preceded by contractions of the artery which were restricted to the region close to the stimulating electrode. 5. Increasing stimulus voltage led to progressive prolongation of the decay phase of the EJP. After the addition of tetrodotoxin (10(-7) M), or in the presence of reduced Ca2+ and raised Mg2+ concentration, slow depolarizing potentials (SDPs) (with time to peak of 150-300 ms and decay lasting > 2 s) were recorded which were graded in amplitude with stimulus voltage. SDPs were attenuated by increasing Ca2+ concentration to 5 mM. These responses often added to the EJP at supramaximal stimulus voltages. 6. The mean amplitudes of the EJP and NAD declined significantly with age, the former to a greater degree than the latter. These changes may be explained by changes in the electrical properties of the media related to hypertrophy of smooth muscle cells as the animals grew, and emphasize the need to allow for such growth effects in studies of young rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of neuronal uptake on pre- and postjunctional effects of alpha-adrenoceptor agonists in tissues with noradrenaline--ATP cotransmission

This study was undertaken in an attempt to explain why in some of the tissues in which noradrenaline and ATP act as co-transmitters the noradrenergic component predominates, while in others the predominant component is purinergic. Four different tissues were used: the epididymal portion of the rat vas deferens and the rabbit ear artery, tissues where the noradrenergic component predominates, and the prostatic portion of the rat vas deferens and the rabbit jejunal artery, where the purinergic component predominates. The noradrenaline content as well as the electrically-evoked release of noradrenaline were determined in all tissues. To determine the evoked release, the tissues were pretreated with pargyline (1 mmol.l-1) and then exposed to 3H-noradrenaline, washed out and transmurally stimulated (1 Hz). In addition, the influence of inhibition of neuronal uptake by desipramine (40 nmol.l-1) on pre- and postjunctional effects of adrenaline and alpha-methylnoradrenaline (and/or noradrenaline) was compared. The noradrenaline content of the tissues averaged: 17.4, 23.2, 3.1, and 4.8 micrograms.g-1 for the epididymal and the prostatic portions of the rat vas deferens and for the ear and the jejunal arteries of the rabbit, respectively. The fractional electrically-evoked release of 3H-noradrenaline was 2.02 and 2.04 x 10(-5) for the epididymal and the prostatic portions of the rat vas deferens, respectively, and 3.33 and 3.26 x 10(-5) for the ear and the jejunal arteries of the rabbit, respectively. Desipramine enhanced much more the postjunctional effect of noradrenaline, adrenaline, and alpha-methylnoradrenaline in the epididymal than in the prostatic portion of the rat vas deferens.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of neurotransmission with nerve trunk and transmural field stimulation in guinea-pig mesenteric artery

1. Intracellular electrical and contractile responses to sympathetic nerve trunk stimulation (NTS) and transmural electrical field stimulation (TMS) were compared in the guinea-pig mesenteric artery in vitro. 2. Step increases in voltage with NTS gave rise to excitatory junctional potentials (EJPs) which reached a plateau amplitude of 5-10 mV, whereas with TMS larger amplitude EJPs and sometimes action potentials were obtained. 3. EJPs of equal amplitude (1-7 mV) elicited with TMS and NTS had the same rise time, duration and decay half-time. 4. Slow depolarization obtained with repetitive stimulation was significantly greater in amplitude with TMS than with NTS. 5. Equal amplitude EJPs were obtained throughout the preparation with NTS. With TMS, the amplitude of responses declined substantially with distance from the stimulating electrodes. 6. Tetrodotoxin (TTX) completely blocked EJPs, slow depolarization and contraction with NTS; however, with TMS a TTX-resistant component was observed. The TTX-resistant response to TMS was abolished in the presence of a low-Ca2+ superfusion solution but was not affected by endothelium removal. 7. Phentolamine or prazosin abolished slow depolarization but not EJPs with NTS or TMS. Prazosin abolished contraction with NTS and reduced but did not abolish contraction with TMS. 8. alpha, beta-Methylene ATP abolished EJPs with NTS, whereas with TMS only EJPs obtained with low stimulus intensities were abolished. alpha, beta-Methylene ATP did not block contraction with either NTS or TMS. 9. Combined TTX, prazosin and alpha, beta-methylene ATP abolished EJPs initiated with TMS at all but the highest stimulus intensities (12-20 V, 0.3 ms duration). 10. It is concluded that responses obtained with NTS can be reliably attributed to the release of transmitter by the conduction of action potentials in paravascular nerves, whereas activation by TMS is a more complex phenomenon dependent upon stimulus strength and probably involving multiple forms of activation.

Neurotransmitters and pre- and post-junctional receptors involved in the vasoconstrictor response to sympathetic nerve stimulation in rat tail artery

The study was prompted by the need to re-evaluate, in view of the complexity of the evidence in the literature, the relative roles of different sympathetic transmitters and receptors in the contractile response of the tail artery of adult normotensive rats to electrical field stimulation. By the pharmacological approach employed, noradrenaline and adenosine 5'-triphosphate appeared to contribute to this response; the possible roles of other putative transmitters such as neuropeptide Y could not be examined due to lack of specific antagonists. Noradrenaline, clearly the main mediator, exerted both excitatory and inhibitory effects, acting in part via different receptors depending on the stimulus parameters. Thus, yohimbine and prazosin (alpha 2- and alpha 1-adrenoceptor antagonists, respectively) were about equally effective as inhibitors of the noradrenaline-mediated contractile response to stimulation with short trains and/or at low frequency, but the response caused by long trains and/or high-frequency stimulation was much more strongly inhibited by prazosin than by yohimbine. As expected, yohimbine enhanced the [3H]noradrenaline overflow response to long but not to short stimulus trains, presumably because in the latter case the noradrenaline concentration in the relevant biophase was too low to activate the pre-junctional alpha 2-adrenoceptors. Finally, propranolol, an unselective beta-adrenoceptor antagonist, enhanced the neurogenic contraction, indicating that noradrenaline restricts this response by effects via post-junctional beta-adrenoceptors. Adenosine triphosphate appeared to exert dual, excitatory as well as inhibitory, post-junctional effects. Thus, the P2x-purinoceptor desensitizing agent, alpha, beta-methylene adenosine triphosphate, abolished the initial phase, but enhanced the amplitude of the neurogenic contraction, without affecting the nerve stimulation-induced overflow of [3H]noradrenaline. The results indicate that noradrenaline and adenosine triphosphate, the main mediators of the neurogenic contraction of this preparation, act in a more complex fashion than earlier thought; they argue against a significant direct contribution by other putative transmitters but do not exclude that such agents may act indirectly as modulators of this response.

Presynaptic dopamine DA2-receptors in rabbit jejunal arteries. An electrophysiological study

Excitatory junction potentials (e.j.ps) evoked by nerve stimulation with 15 pulses at 1 Hz were recorded from muscle cells of rabbit isolated jejunal arteries. LY 171555 1 mumol/l, SKF 38393 10 mumol/l, dopamine 10 mumol/l and clonidine 0.1 mumol/l depressed all e.j.ps in the train. The percentage inhibition was inversely related to the number of pulses. S- and R-sulpiride, 10 mumol/l, domperidone 1 mumol/l, SCH 23390 1 mumol/l and rauwolscine 1 mumol/l did not change, or even depressed the first e.j.ps. Of these compounds only S- and R-sulpiride, 10 mumol/l and rauwolscine 1 mumol/l facilitated the late e.j.ps. The percentage facilitation increased with the number of pulses until a maximum was reached; rauwolscine 1 mumol/l had the largest effect. S- and R-sulpiride, 10 mumol/l, as well as domperidone 1 mumol/l antagonized the action of LY 171555 1 mumol/l. S-Sulpiride was more potent than its R-isomer. SCH 23390 1 mumol/l and rauwolscine 1 mumol/l blunted the effect of SKF 38393 10 mumol/l. Rauwolscine 1 mumol/l slightly reduced the inhibition by dopamine 10 mumol/l; S-sulpiride 10 mumol/l was antagonistic only in the presence of rauwolscine 1 mumol/l. When rauwolscine 1 mumol/l, prazosin 0.1 mumol/l, propranolol 1 mumol/l and cocaine 10 mumol/l was added to the medium, dopamine 10 mumol/l continued to produce the same depression of e.j.ps, as in the absence of these compounds. Under such conditions S-sulpiride 10 mumol/l also counteracted dopamine 10 mumol/l. Rauwolscine 1 mumol/l prevented the effect of clonidine 0.1 mumol/l. The antagonists were not absolutely selective against only one type of agonist. We suggest that both presynaptic DA2- and postsynaptic DA1-receptors are present in rabbit jejunal arteries. The activation of either receptor-type may depress the e.j.ps. Dopamine interferes with neuroeffector transmission due to alpha 2-adrenoceptor agonist properties; its DA2-effect is unmasked only after alpha 2-adrenoceptor blockade. There was no evidence for a co-transmitter function of dopamine.

Selective inhibition by nifedipine of the purinergic component of neurogenic vasoconstriction in the dog mesenteric artery

The neurogenic contractions evoked by perivascular sympathetic nerve stimulation of dog mesenteric artery consist of purinergic and adrenergic components, and these components were selectively inhibited by alpha, beta-methylene ATP and prazosin, respectively. We examined the effects of Ca antagonists on both these components in dog mesenteric arteries. Nifedipine (10(-8)-10(-6) M) inhibited the purinergic and adrenergic contractions evoked by transmural electrical stimulation, and this inhibition was more evident for the purinergic component of the response. Nifedipine was also more potent to inhibit the contractile response to alpha, beta-methylene ATP than it was to inhibit the responses to noradrenaline. Verapamil and diltiazem also inhibited the purinergic and adrenergic responses induced by transmural electrical stimulation, alpha, beta-methylene ATP or noradrenaline, but the extend of the inhibition was less than that seen with nifedipine. These three Ca antagonists had little effect on the 3H efflux evoked by electrical transmural stimulation of arteries that had been preincubated with [3H]noradrenaline. These results show that nifedipine is a selective inhibitor of the purinergic component of contractions evoked by sympathetic nerve stimulation of blood vessels.

The effect of temperature on neuromuscular transmission in the main caudal artery of the rat

1. Excitatory junction potentials (EJPs) recorded in isolated segments of the proximal main ventral artery of the rat tail were reduced in amplitude and prolonged in time course as temperature was lowered from 35 to 15 degrees C. 2. The slow depolarization that followed the EJPs after supramaximal or repetitive perivascular stimulation was markedly slowed in time course, but little affected in amplitude, as temperature was lowered. 3. The time constant (tau EJP) of the exponential decay phase of the EJP recorded from cells deep in the media was similar to the membrane time constant, so that the increase in tau EJP at low temperatures is consistent with a decrease in membrane conductance. 4. The value of tau EJP was also prolonged if the EJP was evoked at the time of the peak of the slow depolarization; this effect was blocked by idazoxan (10(-7) M) but not by prazosin (10(-6) M). 5. During repeated short bursts of high-frequency stimulation, action potential initiation was facilitated by both the prolongation of EJPs and summation of slow depolarizations; these effects were greater at 25 than at 35 degrees C. 6. The interactions between EJPs and alpha-adrenoreceptor-mediated membrane conductance changes are considered with respect to the electrical events occurring during sympathetic neuromuscular transmission at the natural temperatures of the rat tail.

Regulation of coronary artery tone in relation to the activation of signal transductors that regulate calcium homeostasis

Muscle tone of coronary arteries is regulated by free calcium concentration in the myoplasm. Various agonists, autacoids and putative peptides modify the calcium concentration directly or through actions of second messengers (signal transductors), such as cyclic guanosine monophosphate (GMP), cyclic adenosine monophosphate (AMP), inositol 1,4,5-trisphosphate, diacylglycerol or calmodulin. For example, acetylcholine (in the presence of intact endothelium cells), alpha-human natriuretic peptide or nitrate compounds increase the amount of cyclic GMP and isoproterenol, prostacyclin (prostaglandin I2) or vasoactive intestinal polypeptide increases cyclic AMP. Both cyclic nucleotides reduce free calcium concentration. On the other hand, acetylcholine (in the presence or absence of endothelium cells), norepinephrine or thromboxane A2 increases inositol 1,4,5-trisphosphate and diacylglycerol, thus causing the increase in the free calcium concentration, whereas vasoactive intestinal peptide and alpha-human natriuretic peptide reduce them. Calmodulin acts as an internal calcium receptor for regulation of the contractile machinary. Regulation of calcium homeostasis in relation to the muscle tone in the coronary arteries including other vascular tissues is discussed together with the role of second messengers.

Autonomic innervation of the canine penile artery and vein in relation to neural mechanisms involved in erection

Neural control of the penile artery and vein of dogs was investigated using isometric tension recording and microelectrode methods. Field stimulation evoked twitch-like contractions of these two vessels and these contractions were blocked by guanethidine. In the artery, twitch-like contractions were more effectively blocked by yohimbine than by prazosin. During high tone of arterial and venous tissues evoked by noradrenaline (NA) in the presence of guanethidine, field stimulation evoked muscle relaxation that was not affected by atropine but was abolished by tetrodotoxin. In parallel to the mechanical responses, field stimulation evoked excitatory junction potentials (EJPs) in both arterial and venous smooth muscle cells. In the case of the artery, an action potential was superimposed on the EJP. The NA-induced contraction was suppressed by vasoactive intestinal polypeptide (VIP), dose-dependently. On the other hand, alpha, beta-methylene-ATP did not affect the muscle relaxation induced by field stimulation. These results indicate that the penile artery and vein of the dog are innervated by adrenergic excitatory nerves and non-adrenergic, non-cholinergic inhibitory nerves. The transmitter possibly involved in the latter is discussed.

Comparison of the frequency dependence of venous and arterial responses to sympathetic nerve stimulation in guinea-pigs

1. Intracellular potentials and measurements of contractions were recorded in adjacent veins and arteries in the colonic mesentery of the guinea-pig in vitro during stimulation of post-ganglionic nerve trunks. 2. Repetitive stimulation (0.5-5 Hz) of lumbar colonic nerve trunks produced frequency-dependent slow depolarizations in all venous and in 92% of arterial smooth muscle cells. Excitatory junction potentials were observed for each nerve shock in arteries, but not in veins. 3. Low-frequency stimulations produced slow depolarizations of greater amplitude and longer duration in veins than in arteries. The frequencies at which half-maximal depolarizations and contractions occurred were always lower for veins than for arteries. 4. The alpha 1-adrenergic antagonist prazosin (5 X 10(-7) M) reduced the mean arterial slow depolarizations by 82% and reduced mean venous slow depolarizations by 58% for 5 Hz stimulations. Arterial contractions were completely inhibited by prazosin but venous contractions were incompletely reduced in a frequency-dependent manner. 5. These findings suggest that functional differences in activation between mesenteric veins and arteries during sympathetic stimulation are a result of differences in neuromuscular transmission.

Lack of involvement of alpha-adrenoceptors in sympathetic neural vasoconstriction in the hindquarters of the rabbit

The hypothesis that sympathetic nerves in arterial blood vessels activate excitatory receptors distinct from alpha-adrenoceptors was investigated in vivo in the rabbit. In anaesthetized, ganglion-blocked rabbits, graded stimulation of the lumbar sympathetic nerve chains caused graded hind limb vasoconstriction. The responses to single pulses and short trains of stimuli were unaffected by benextramine (10 mg kg-1) and the longer trains were enhanced. Phenoxybenzamine (5 mg kg-1) slightly reduced the responses to short trains of stimuli and did not affect the responses to long trains. The dose-response curve to intra-arterial noradrenaline (after beta-adrenoceptor blockade) was shifted rightwards about ten fold by benextramine (10 mg kg-1) and by phenoxybenzamine (5 mg kg-1). In conscious rabbits the vasoconstriction caused by the nasopharyngeal reflex initiated by smoke inhalation was unaffected by benextramine (10 mg kg-1). Small mesenteric arteries (less than 250 microns) taken from untreated rabbits responded to noradrenaline with a threshold concentration of about 1 microM. Similar tissues from benextramine (10 mg kg-1)-treated rabbits were unresponsive to noradrenaline at concentrations up to 300 microM. However, these tissues were able to respond to potassium and angiotensin II. Aortic ring segments taken from the same rabbits were only about ten fold less sensitive to noradrenaline than segments from control rabbits. These results are in accord with the hypothesis that sympathetic nerves activate non-alpha-receptors in the vasculature of the rabbit.

Vascular responsiveness of simian digital artery to various vasoactive substances

The stainless steel cannula inserting method was used to observe vascular effects of 7 vasoactive substances on the isolated and cold-stored digital artery of the monkey. The arterial preparation was suspended in a bath at 37 degrees C and perfused with Ringer's solution under constant flow rate. Drugs were administered in the endothelial side of the artery through a cannulated tubing, and the response was obtained as changes in perfusion pressure. All used agonists increased perfusion pressure in a dose-related manner. The order of potency for inducing vasoconstriction was norepinephrine greater than serotonin = phenylephrine = clonidine greater than xylazine greater than tyramine greater than potassium chloride. Inhibiting effect of prazosin (0.001-0.01 microgram) was greater on phenylephrine (0.03 microgram)-induced contractile responses than on clonidine (0.03 microgram)-induced responses, and the reverse was true in the case of yohimbine (0.01-1.0 microgram). Thus, it was suspected that the simian digital artery would be a good model for in vitro investigation of vascular responsiveness as a conduit artery which mainly supplies blood flow to the skin, and that this artery might be contracted not only via postjunctional alpha 1- but also alpha 2-adrenoceptors.

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.

Actions of ATP and alpha, beta-methylene ATP on neuromuscular transmission and smooth muscle membrane of the rabbit and guinea-pig mesenteric arteries

In the rabbit mesenteric artery, adenosine triphosphate (ATP), showed two actions on the membrane potential of muscle cells: low concentrations (1-10 microM) hyperpolarized and high concentrations (greater than or equal to 50 microM) depolarized the membrane. Both changes in the potential were accompanied by increases in ionic conductance. In the rabbit mesenteric artery, alpha, beta-methylene ATP (MeATP), (greater than or equal to 30 nM) depolarized the muscle membrane at a lower concentration than ATP (greater than or equal to 50 microM), and increased the ionic conductance of the membrane. The depolarization induced by ATP was prevented by low concentrations of MeATP, but the hyperpolarization was retained. Furthermore, the hyperpolarization was not affected by theophylline (10 microM). In the guinea-pig mesenteric artery, ATP and MeATP depolarized and increased the ionic conductance of muscle membrane, but to depolarize the membrane, higher concentrations of both agents were required, compared to those in the rabbit mesenteric artery. In the mesenteric arteries from both species, perivascular nerve stimulation evoked excitatory junction potentials (e.j.ps). In both tissues, MeATP inhibited the amplitude of e.j.ps at lower concentrations than did ATP, and both agents had more potent inhibitory actions on rabbit than on guinea-pig. The inhibition of e.j.p. induced by low concentrations of these agents showed no relationship to depolarization, but the inhibition induced by high concentrations was paralleled by depolarization and increase in ionic conductance of the membrane. In the rabbit mesenteric artery, overflow of noradrenaline (NA) and its metabolite (3,4-dihydroxyphenylglycol; DOPEG) produced by perivascular nerve stimulation was examined. ATP (0.1 mM) but not MeATP (0.1 microM) reduced the overflow of NA, whereas both agents had no effect on the overflow of DOPEG. Exogenously applied high concentrations of NA (greater than or equal to 3 microM) depolarized the muscle membrane in both species. These NA-induced depolarizations were not affected by treatment with ATP or MeATP. It is concluded that, in the rabbit mesenteric artery, ATP is more likely to be involved in generation of e.j.ps than is NA. A similar interpretation in the guinea-pig mesenteric artery is complicated by the depolarization produced by high concentrations of ATP or MeATP.

Comparison of the antagonistic effects of phentolamine on vasoconstrictor responses to exogenous and neurally released noradrenaline in vivo

The antagonistic effects of the alpha-adrenoceptor blocking agent phentolamine on vasoconstrictor responses to intraluminal noradrenaline and lumbar sympathetic nerve stimulation were compared in the hindlimb of the anaesthetized dog. Sympathetic stimulation with 1 pulse or trains of 4-10 pulses at 0.4-40 Hz produced graded vasoconstrictor responses that were matched in amplitude by intra-arterial injections of 10(-8) - 10(-6) g noradrenaline. Phentolamine (0.5 mg kg-1 i.v.) attenuated amplitude-matched responses to both types of stimuli to quite similar extents. The extent of the effect of phentolamine on neurogenic responses was greater with 1 pulse stimulation than with trains, and greater with 4 pulse than with 10 pulse trains. The effect was maximal within 2 min of phentolamine administration and wore off in parallel with that on responses to injected noradrenaline. The results are consistent with the view that transmitter released from noradrenergic vasoconstrictor nerves acts primarily on subjunctional alpha-adrenoceptors.

Actions of nipradilol (K-351), a new alpha- and beta-adrenoceptor blocker, on the rabbit portal vein

Nipradilol but not desnitro nipradilol [N-) nipradilol) inhibited the norepinephrine (NE)-induced depolarization and contraction of the rabbit portal vein. The NE-induced contraction and depolarization were also blocked by prazosin, but not blocked by yohimbine. Therefore, nipradilol possesses an alpha 1-blocking action. The order of potency was prazosin greater than nipradilol greater than yohimbine greater than (N-)-nipradilol = 0. With applications of field stimulations to muscle tissues, the smooth muscle membrane was depolarized with a latency of several seconds, and the action potential was generated. These phenomena were blocked by tetrodotoxin (TTX), prazosin or nipradilol, but not by yohimbine. Isoproterenol (Isop) inhibited the 30 mM K-induced contraction, and this inhibitory action was blocked by (N-), nipradilol, nipradilol or propranolol, dose-dependently. The potency of beta-blocking actions of nipradilol was much the same as that observed by propranolol and (N-) nipradilol. When nipradilol (10(-5) M) was applied to the tissue, the amplitude of the 30 mM K contraction was slightly reduced. Such inhibitory action was not observed by application of (N-) nipradilol. The Ki values of nipradilol for blocking actions on the NE-induced contraction and Isop-induced relaxation were of the same order of 10(-7) M. Therefore, the potencies of alpha 1-blocking and beta-blocking actions of nipradilol may be the same in the rabbit portal vein. These findings suggest that the vasodilating action of nipradilol on the rabbit portal vein is mainly due to the alpha 1-blocking action and that the nitrate action of this agent may be weak.

Mechanisms of the ergonovine-induced vasoconstriction in the rabbit main coronary artery

The mean membrane potential of smooth muscle cells of the rabbit main coronary artery was -60.3 mV and an evoked action potential could be recorded in response to acetylcholine (ACh). Ergonovine or 5-hydroxytryptamine (5-HT) slightly depolarized the membrane and methysergide, a relatively selective antagonist for the 5-HT receptor, had a slight inhibitory action on these depolarizations. 5-HT produced larger contractions than ergonovine, and the concentration-effect relationships obtained for both agents shifted to higher concentrations following pre-equilibration with methysergide. ACh (10(-11)M) slightly hyperpolarized the membrane and relaxed the tissue, and high concentrations of ACh (greater than 10(-8)M) depolarized the membrane, increased the membrane resistance and produced a contraction. ACh but not ergonovine or 5-HT, produced a contraction in Ca-free EGTA-containing solution. Following a 60 min pre-equilibration with indomethacin, the ergonovine-induced contraction was markedly enhanced but the 5-HT- or ACh-induced contractions were not. Removal of the endothelium by rubbing the vascular lumen enhanced the ergonovine- or ACh-induced contractions, but not those to 5-HT. The results obtained can be summarized as follows: ergonovine probably accelerates Ca influx and thereby produces contraction in the rabbit main coronary artery. This contraction is due to activation of the 5-HT receptor as an agonist, but the ergonovine-induced contraction is attenuated due to activation of the endothelium from which inhibitory prostanoid substances may be released. Ergonovine, therefore, may produce greater contractions in coronary arteries with damaged endothelium than in intact tissues.

The presynaptic regulation of noradrenaline release differs in mesenteric arteries of the rabbit and guinea-pig

Prejunctional control mechanisms of neuromuscular transmission in mesenteric arteries of the guinea-pig and rabbit were compared by examining excitatory junction potentials (e.j.p.s). The various agents used did not modify the membrane potential or resistance of smooth muscle cells of either tissue at the concentrations used in the present experiments. In the rabbit mesenteric artery, 10(-7) M-phentolamine and yohimbine had almost no effect on the amplitude of e.j.p.s or on facilitation. In the guinea-pig mesenteric artery, phentolamine enlarged and yohimbine reduced the amplitude of the first e.j.p. but both agents markedly enlarged the amplitude of e.j.p.s evoked by repetitive perivascular nerve stimulation at frequencies over 0.1 Hz. In the rabbit mesenteric artery, isoprenaline and dibutyryl cyclic AMP inhibited the e.j.p.s and the facilitation evoked by repetitive stimulation, while in the guniea-pig mesenteric artery, isoprenaline enhanced the amplitude of e.j.p.s and the facilitation process. In the rabbit mesenteric artery, ATP inhibited the amplitude of e.j.p.s with no change in facilitation and adenosine inhibited mainly the facilitation process evoked by repetitive perivascular nerve stimulation. In the guinea-pig mesenteric artery, these agents had no effect on e.j.p.s or facilitation. In both the rabbit and guinea-pig, indomethacin enlarged the amplitude and prolonged the duration of e.j.p.s. Prostaglandin E2 and F2 alpha inhibited the amplitude of e.j.p.s. Enlarged amplitudes of the e.j.p.s following pre-treatment with indomethacin were inhibited by treatment with PGE2 or ATP. In the rabbit mesenteric artery, 8-phenyltheophylline (an antagonist of P1 subtype of purinergic receptor) had no effect on the e.j.p.s, but these agents did reverse the inhibitory action of 2-chloroadenosine (an agonist of P1 subtype of purinergic receptor) or adenosine on facilitation of e.j.p.s. Theophylline did not have any direct effect on e.j.p.s nor did it reverse the effects of adenosine or 2-chloroadenosine. In the rabbit mesenteric artery, ATP and adenyl-imidophosphate showed much the same potency in inhibiting the amplitude of e.j.p.s. Therefore, the ATP action on nerve terminals is not due to a high energy requiring mechanism. From these results we conclude that the control mechanisms related to noradrenaline (NA) release in nerve terminals differ in the guinea-pig and rabbit mesenteric arteries.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of a new alpha 1-adrenoceptor blocking agent, 3-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-(1H,3H)-quinazoline-2, 4-dione monohydrochloride (SGB1534), on electrical and mechanical properties of guinea-pig mesenteric artery

Effects of a newly synthesized alpha-adrenoceptor blocking agent, 3-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-1H, 3H)-quinazoline-2, 4-dione monochloride (SGB1534), on the electrical and mechanical properties of smooth muscles of guinea-pig mesenteric artery and on the electrical properties of smooth muscles of rat tail artery were investigated. SGB1534 (10(-10) M-10(-5) M) did not modify the membrane potential and ionic conductance of the membrane in guinea-pig mesenteric artery, but this agent did inhibit depolarizations induced by noradrenaline, phenylephrine, or histamine with similar potencies. This agent inhibited serotonin-induced depolarization, but with a weak potency. In rat tail artery, the noradrenaline-induced depolarization (10(-5) M) was inhibited by yohimbine (5 X 10(-7) M), but not by SGB1534 (10(-6) M). SGB1534 (10(-6) M) did not modify the amplitude of excitatory junction potentials (e.j.ps), the facilitation process or spike potential evoked by perivascular nerve stimulation in the mesenteric artery. Noradrenaline, phenylephrine or histamine evoked the contraction in guinea-pig mesenteric artery, and this contraction was inhibited by SGB1534 (over 10(-10) M). The serotonin-induced contraction was inhibited by higher concentrations of SGB1534 (over 10(-6) M). The concentration of SGB1534 required to inhibit the contractions evoked by these amines was much higher than that required to inhibit the depolarizations. SGB1534 (less than 10(-6) M) had no effect on the excess K-induced depolarization and contraction. These results indicate that SGB1534 possesses the property of an alpha 1-but not alpha 2-adrenoceptor blocking agent.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of alpha-adrenoceptor antagonists on electrical and mechanical responses of the isolated dog mesenteric vein to perivascular nerve stimulation and exogenous noradrenaline

The effects of four different alpha-adrenoceptor antagonists (prazosin, phentolamine, yohimbine, and nipradilol) on the electrical and mechanical responses of smooth muscle cells of the dog isolated mesenteric vein to perivascular nerve stimulation and exogenous noradrenaline were investigated. Perivascular nerve stimulation generated an excitatory junction potential (e.j.p.), a spike potential and a slow depolarization. The latter component was blocked by yohimbine or phentolamine at doses over 10(-7) M, while the former two components were suppressed by 10(-6)-10(-5) M yohimbine, but not by prazosin, nipradilol or phentolamine (up to 10(-5) M). Nerve-mediated muscle contractions were suppressed by these alpha-adrenoceptor antagonists in a concentration-dependent manner, at doses over 10(-7) M. The order of potency was yohimbine greater than nipradilol = phentolamine greater than prazosin. Exogenously applied noradrenaline (10(-6) M) depolarized the smooth muscle membrane and generated slow waves. The slow waves were blocked by all of these alpha-adrenoceptor antagonists (10(-5) M), while the depolarizations were inhibited by yohimbine (greater than 10(-7) M) or phentolamine (10(-5) M), but not by nipradilol or prazosin (up to 10(-5) M). Contractions produced by exogenously applied noradrenaline (10(-6) M) were inhibited by the alpha-adrenoceptor antagonists; yohimbine or phentolamine (10(-6)-10(-5) M) showed complete inhibition and prazosin or nipradilol (up to 10(-5) M) partial inhibition. Contractions produced by high-potassium or current-stimulation were suppressed by high-concentrations (10(-6)-10(-5) M) of these alpha-adrenoceptor antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic transmission in the dog mesenteric vein and its modulation by alpha-adrenoceptor antagonists

Adrenergic transmission was investigated in the dog mesenteric vein by recording electrical responses of single smooth muscle cells to perivascular nerve stimulation. Perivascular nerve stimulation generated an excitatory junction potential (e.j.p.) and a slow depolarization of the membrane. The amplitude of the e.j.p. was increased by increasing the stimulus intensity, and at high intensity, a spike potential was generated. Repetitive stimulation of the nerves showed facilitation of e.j.ps and enhanced the amplitude of slow depolarization. A linear relationship was observed between the amplitude of the e.j.p. and of slow depolarization. The slow depolarization was inhibited by application of yohimbine or phentolamine, but not by prazosin. The amplitude of e.j.p. was increased by prazosin and was decreased by yohimbine. Both e.j.p. and slow depolarization were inhibited by guanethidine or tetrodotoxin. Exogenously applied noradrenaline depolarized the muscle membrane and, in high concentrations (greater than 10(-7)M), generated slow waves. These effects of noradrenaline were blocked by yohimbine. High concentrations of prazosin (greater than 10(-6)M) showed weak inhibitory effects on the noradrenaline actions. The amplitude of e.j.p. was decreased by exogenously applied noradrenaline in a dose-dependent manner. The inhibitory effect of noradrenaline on the e.j.p. was suppressed by yohimbine, but not by prazosin or phentolamine. Phentolamine, but not prazosin, enhanced the facilitation process of e.j.ps. This effect was not suppressed by exogenously applied noradrenaline. Application of neostigmine but not of atropine, reduced the e.j.p. amplitude without affecting the slow depolarization. It was concluded that, in the dog mesenteric vein, perivascular nerve stimulation produced three types of electrical responses of the smooth muscle membrane, i.e., e.j.p., slow depolarization and spike potential. The slow depolarization was generated by activation of alpha 2-adrenoceptors. Exogenously applied noradrenaline reduced the e.j.p. amplitude through activation of prejunctional alpha 2-adrenoceptors, but the reduction may not involve alpha-autoinhibitory mechanisms.