Effects of adrenoceptor agonists and antagonists on smooth muscle cells and neuromuscular transmission in the guinea-pig renal artery and vein

The purinergic neurotransmitter revisited: a single substance or multiple players?

The past half century has witnessed tremendous advances in our understanding of extracellular purinergic signaling pathways. Purinergic neurotransmission, in particular, has emerged as a key contributor in the efficient control mechanisms in the nervous system. The identity of the purine neurotransmitter, however, remains controversial. Identifying it is difficult because purines are present in all cell types, have a large variety of cell sources, and are released via numerous pathways. Moreover, studies on purinergic neurotransmission have relied heavily on indirect measurements of integrated postjunctional responses that do not provide direct information for neurotransmitter identity. This paper discusses experimental support for adenosine 5'-triphosphate (ATP) as a neurotransmitter and recent evidence for possible contribution of other purines, in addition to or instead of ATP, in chemical neurotransmission in the peripheral, enteric and central nervous systems. Sites of release and action of purines in model systems such as vas deferens, blood vessels, urinary bladder and chromaffin cells are discussed. This is preceded by a brief discussion of studies demonstrating storage of purines in synaptic vesicles. We examine recent evidence for cell type targets (e.g., smooth muscle cells, interstitial cells, neurons and glia) for purine neurotransmitters in different systems. This is followed by brief discussion of mechanisms of terminating the action of purine neurotransmitters, including extracellular nucleotide hydrolysis and possible salvage and reuptake in the cell. The significance of direct neurotransmitter release measurements is highlighted. Possibilities for involvement of multiple purines (e.g., ATP, ADP, NAD(+), ADP-ribose, adenosine, and diadenosine polyphosphates) in neurotransmission are considered throughout.

Functions of large conductance Ca2+-activated (BKCa), delayed rectifier (KV) and background K+ channels in the control of membrane potential in rabbit renal arcuate artery

1. The types of K+ channel which determine the membrane potential of arcuate artery smooth muscle cells were investigated by patch-clamp recording from isolated cells and lumenal diameter measurements from intact pressurized renal arcuate arteries. 2. Single cells had a mean resting potential of -38 mV and were depolarized by 130 mM K+ but not by the Cl- channel blocker 4,4'-diisothiocyanatostilbene-2, 2'-disulphonic acid (DIDS). 3. Iberiotoxin did not affect the resting potential but inhibited spontaneous transient hyperpolarizations. Iberiotoxin or 1 mM tetraethylammonium (TEA+) constricted intact arteries. 3,4-Diaminopyridine (3,4-DAP)-sensitive delayed rectifier K+ (KV) channel current was elicited by depolarization but 3,4-DAP did not affect the resting potential or induce constriction in the intact artery. 4. A voltage-independent K+ current was inhibited by >= 0.1 mM barium (Ba2+) and unaffected by iberiotoxin, glibenclamide, apamin, 3,4-DAP and ouabain. In six out of ten cells, 1 mM Ba2+ depolarized the resting potential, while in the other cells the potential was resistant to all of the K+ channel blockers and ouabain. Ba2+ (0.1-1 mM) constricted the intact artery, but 10 microM Ba2+, 1 microM glibenclamide or 100 nM apamin had no effect. 5. The data suggest that resting potential is determined by background K+ channels, one type being Ba2+ sensitive and voltage independent, and another type being poorly defined due to its resistance to any inhibitor. Large conductance Ca2+-activated K+ (BKCa) and KV channels do not determine the resting potential but have separate functions to underlie transient Ca2+-induced hyperpolarizations and to protect against depolarization past about -30 mV.

Properties of sympathetic neuromuscular transmission and smooth muscle cell membranes in vascular beds

In vascular smooth muscle tissues, the cycle of contraction-relaxation is mainly regulated by the cytosolic Ca, and many other factors, such as substances released from endothelial cells and perivascular nerve terminals (mainly sympathetic nerves). In this article, we introduce regional differences in specific features of ionic channels in vascular smooth muscle membranes (mainly on features of Ca, Na and K channels) in relation to mobilization of the cytosolic Ca. In many vascular tissues, neurotransmitters released from sympathetic nerve terminals activate post-junctional receptors, and subsequently modify ion channels (receptor-activated cation channel and voltage-dependent Ca channel), whereas in some tissues, ionic channels are not modified by receptor activations (pharmaco-mechanical coupling). However, activation of receptors, with or without modulation of ionic channels, regulates the cytosolic Ca through synthesis of second messengers. In addition, receptors distributed on prejunctional nerve terminals positively or negatively regulate the release of transmitters. Roles of neurotransmitters (mainly ATP and noradrenaline) are also discussed in relation to the generation of excitatory junction potentials.

Isolated bovine cerebral arteries from rostral and caudal regions: distinct responses to adrenoceptor agonists

Responsiveness to norepinephrine and related agents was compared in isolated bovine anterior cerebral (ACA). middle cerebral (MCA), intracranial internal carotid (ICA), posterior communicating (PCOM), posterior cerebral (PCA) and basilar arteries (BA). Norepinephrine contracted the strips from ACA, MCA and ICA, but relaxed those from PCOM, PCA and BA. In the presence of propranolol, the amine-induced contractions tended to be potentiated in ACA, MCA and ICA, and the relaxations in PCOM, PCA and BA were reversed to contractions. The maximum contractions induced by norepinephrine in ICA and ACA treated with propranolol were significantly greater than those in PCA and BA, but the EC50 values did not differ among arteries. In ACA and MCA, the contractions induced by phenylephrine were greater than those induced by clonidine. The contractions induced by norepinephrine and phenylephrine were attenuated by prazosin but not influenced by yohimbine in ACA and MCA treated with propranolol. These findings indicate that the responses to norepinephrine evidently differ in bovine cerebral arteries of rostral (ACA, MCA and ICA) and caudal regions (PCOM, PCA and BA), possibly due to different functioning of alpha/beta receptors. The amine-induced contraction predominantly seen in the rostral arteries appears to be associated with activation of the alpha 1 adrenoceptor subtype.

Phentolamine lacks alpha 2-adrenoceptor agonist activity in anaesthetized dogs

1. This study was performed in order to determine whether the blockade of sympathetic vasoconstriction in anaesthetized dogs by phentolamine is due to the antagonist action of the drug at postjunctional adrenoceptors, or is due to depression of neurotransmitter release by an agonist action at prejunctional adrenoceptors. 2. In dogs made areflexic by ganglion blockade with hexamethonium, phentolamine (0.5 mg i.a. or 0.5 mg kg-1 i.v.) elevated or did not affect femoral blood flow. By contrast, clonidine (0.5-2.5 nmol, i.a.) produced femoral vasoconstriction, which was attenuated by prior administration of phentolamine. 3. Prior blockade of prejunctional alpha 2-adrenoceptors with yohimbine (30 micrograms kg-1, i.v.) did not reduce the blocking effect of phentolamine (0.5 mg kg-1, i.v.) on neurogenic vasoconstriction. 4. The results indicate that, in anaesthetized dogs, phentolamine lacks appreciable agonist activity at either prejunctional or postjunctional alpha 2-adrenoceptors. The blockade of neurogenic responses by phentolamine is therefore likely to be due to postjunctional adrenoceptor blockade.

Postjunctional alpha 2-adrenoceptors mediate venoconstriction in the hindquarters circulation of anaesthetized cats

1 A study was made of the subtypes of postjunctional alpha-adrenoceptors which mediate arterial and venous constriction in the hindquarters circulation of anaesthetized cats, as measured by changes in perfusion pressure and vena cava blood flow, respectively. 2 It was found that, while noradrenaline caused constriction in both the arterial and venous compartments, methoxamine caused only arterial constriction. Clonidine and B-HT 920 also caused arterial and venous constriction although autodesensitization to both drugs occurred. 3 The ability of either prazosin or yohimbine to antagonize the constrictor effects of noradrenaline was also examined. It was found that the combination of both antagonist drugs abolished both the arterial and venous constrictor effects of noradrenaline. However, there was a greater prazosin-resistant response to noradrenaline in the venous compartment as compared with the arterial effects of noradrenaline. Yohimbine caused approximately equal reductions in the effect of noradrenaline in both arteries and veins, which was greater than that observed with prazosin. 4 These results suggest that, in the cat hindquarters, both alpha 1- and alpha 2-adrenoceptors are present in the arterial circulation, whereas there are mainly alpha 2-adrenoceptors in the venous circulation.

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.

Mechanism of vasodilation induced by alpha-human atrial natriuretic polypeptide in rabbit and guinea-pig renal arteries

Effects of alpha-human atrial natriuretic polypeptide (alpha-HANP) on electrical and mechanical properties of smooth muscle cells of the guinea-pig and rabbit renal arteries and of the guinea-pig mesenteric artery were investigated. alpha-HANP (up to 10 nM) modified neither the membrane potential nor resistance of smooth muscle cells of the guinea-pig and rabbit renal arteries. In the guinea-pig mesenteric and renal arteries, alpha-HANP (up to 10 nM) had no effect on the amplitude and facilitation (mesenteric artery) or depression (renal artery) of excitatory junction potentials nor on action potentials. In the guinea-pig renal artery, alpha-HANP (up to 10 nM) had no effect on the depolarization induced by noradrenaline (NA) (up to 10 microM) but markedly inhibited NA-induced contraction. alpha-HANP (10 nM) slightly inhibited the K-induced contraction. In the rabbit renal artery, alpha-HANP (10 nM) inhibited the NA-induced contraction and to a lesser extent the K-induced contraction. In the rabbit renal artery, the effects of alpha-HANP on the release of Ca from the cellular storage by two applications of NA, and its re-storage, were investigated in Ca-free solution containing 2 mM-EGTA. When 5 nM-alpha-HANP was applied before and during the first application of 0.5 microM-NA, the contraction was markedly inhibited but the contraction to a second application of 10 microM-NA was potentiated. If the first dose of NA was 10 microM the effect was very small. Under the same experimental procedures, nitroglycerine (10 microM) showed almost the same effects as alpha-HANP on the NA-induced contractions. When both the first (3 mM) and second (10 mM) contractions were evoked by caffeine in Ca-free solution, alpha-HANP (5 nM) and nitroglycerine (10 microM) inhibited both contractions to the same extent. In the rabbit renal artery, applications of alpha-HANP or nitroglycerine increased the amount of guanosine 3',5'-phosphate (cyclic GMP) in a dose-dependent manner. However, a much higher concentration of nitroglycerine was required (2 X 10(3) times). In the rabbit renal artery, hydrolysis of phosphatidyl inositol 4,5-bisphosphate (PI-P2) activated by 0.5 microM-NA was inhibited by alpha-HANP, in a dose-dependent manner, but activation by 10 microM-NA was not inhibited by alpha-HANP (up to 100 nM).(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Effects of amosulalol on the electrical responses of guinea-pig vascular smooth muscle to adrenoceptor activation

The effects of amosulalol, a newly synthesized sulphonamide-substituted phenylethylamine derivative, on electrical responses of smooth muscle cells of the guinea-pig vascular tissues to noradrenaline, isoprenaline and perivascular nerve stimulation were investigated. Amosulalol (10(-10) -10(-5)M) did not alter the resting membrane potential of smooth muscle cells of the mesenteric artery, the mesenteric vein, the main pulmonary artery and the portal vein. In the mesenteric artery, main pulmonary artery and portal vein, but not in the mesenteric vein, membrane depolarizations produced by noradrenaline were antagonized by amosulalol. In the portal vein, membrane hyperpolarizations produced by isoprenaline were antagonized by amosulalol. In the mesenteric artery, amosulalol (over 10(-6)M) enhanced the amplitude of excitatory junction potentials (e.j.ps) produced by perivascular nerve stimulation. Amosulalol antagonized the noradrenaline-induced decrease in the e.j.p. amplitude; this effect was much weaker than that of phentolamine. Amosulalol also antagonized the isoprenaline-induced enhancement of the e.j.p. amplitude. In the mesenteric vein, the slow depolarizations produced by perivascular nerve stimulation were depressed by amosulalol (over 10(-6)M), but the effect was much weaker than that of prazosin, yohimbine or phentolamine. Actions of amosulalol on electrical properties of vascular tissues can be summarized as follows: amosulalol blocks alpha 1- and beta-adrenoceptors. It also blocks alpha 2-adrenoceptors, though weakly.

An electrophysiological study of alpha-adrenoceptor mediated excitation-contraction coupling in the smooth muscle cells of the rat saphenous vein

The effects of perivascular nerve stimulation and application of exogenous alpha-adrenoceptor agonists on the rat saphenous vein were studied by simultaneous recordings of electrical and mechanical activities. The resting membrane potential of the saphenous vein averaged -65 mV. Perivascular nerve stimulation elicited excitatory junction potentials (e.j.ps) and slow depolarizations. Contraction was observed when either the e.j.p. or the slow depolarization reached a critical threshold of about -55 mV. Exogenously applied noradrenaline, B-HT 920 and clonidine induced depolarization and contraction similar to the slow depolarization. The responses to these agonists and the slow depolarizations were antagonized by yohimbine, but not by prazosin. The selective alpha 1-adrenoceptor agonists phenylephrine and methoxamine had very little effect on the electrical and mechanical activities of the saphenous vein. It was concluded that in the rat saphenous vein, only alpha 2-adrenoceptors are present and that these receptors mediate the slow depolarization and contraction induced by nerve stimulation.

Amine oxidase activities in brown adipose tissue of the rat: identification of semicarbazide-sensitive (clorgyline-resistant) activity at the fat cell membrane

Amine oxidase activity, previously described in homogenates of brown adipose tissue of the rat, has now been investigated in preparations of isolated fat cells. It was found that the specific activities of both monoamine oxidase A (MAO) and of the semicarbazide-sensitive clorgyline-resistant amine oxidase (SSAO) were higher in isolated fat cells than in the original whole tissue. Brown adipocytes therefore represent a major source of both these enzymes. In plasma membranes prepared from these isolated brown fat cells by borate extraction there was a similar enrichment of activity of SSAO and of the plasma membrane marker enzyme, phosphodiesterase I. However in preparations of cell membranes made by binding the cells to polycation-coated beads, enrichment of phosphodiesterase I activity was much greater than that of SSAO. It is suggested that the disposition of the enzyme within the cell membrane may account for the discrepancy in these results, i.e. the sidedness of the membrane may be important. Histochemical visualization of enzyme activity in whole tissue at the ultrastructural level was undertaken. Positive staining of mitochondria was achieved in the presence of the MAO substrate, tryptamine. Staining around the edges of the brown fat cells was observed with the SSAO substrates, tyramine and benzylamine. Staining was largely absent when substrate was omitted or after pretreatment with the irreversible SSAO inhibitor, hydralazine and the slowly reversible inhibitor, semicarbazide. It is not definitely proven that this staining represents sites of enzyme activity but the results are consistent with evidence from other studies indicating that SSAO in brown adipose tissue of the rat may be found predominantly at the fat cell surface.(ABSTRACT TRUNCATED AT 250 WORDS)

Solubilization and some properties of a semicarbazide-sensitive amine oxidase in brown adipose tissue of the rat

A semicarbazide-sensitive clorgyline-resistant amine oxidase (SSAO) was solubilized from membrane fractions of rat brown adipose tissue by the non-ionic detergent, Triton X-100. Alteration of ionic strength or addition of chelating agents alone failed to release the enzyme from its membrane. Lipid-depletion led to loss of enzyme activity and alteration of substrate affinity. Over 80% of the activity of the solubilized enzyme was found in gel filtration fractions corresponding to an Mr of between 160 000 and 180 000. The glycoprotein nature of SSAO was established from affinity chromatography with either immobilized concanavalin A or Lens culinaris lectin. Elution of over 50% SSAO activity from the lentil lectin was achieved with 0.25M-alpha-methyl D-mannoside to give 80-90-fold purification of the enzyme. Irradiation inactivation gave a value for Mr of around 183 000 for both soluble and membrane-bound SSAO. Substrate affinity and inhibitor sensitivity of the enzyme were unaltered by solubilization. The copper-chelating agent, diethyldithiocarbamate, did not affect the enzyme, shedding doubt on the suggestion that SSAO is a copper-requiring enzyme. The significance of these findings in relation to the nature of SSAO and to its disposition within the cell membrane is discussed.