Influence of age on control of norepinephrine release from the rat tail artery.

Stimulation-evoked norepinephrine release from the rat tail artery increases with age; therefore, the sensitivity of prejunctional alpha-2 adrenergic receptors to antagonists and agonists was compared in perfused tail arteries from Fischer-344 rats, aged 6 and 20 months. The increase in endogenous fractional norepinephrine release produced by blockade of alpha-2 adrenergic receptors with submaximal concentrations of either yohimbine or idazoxan was significantly greater in 6-month-old animals as compared to 20 months; however, the effect of a maximal concentration of idazoxan was not significantly different. Inhibition of norepinephrine release by the alpha-2 receptor agonist UK14304 was reduced in 20-month-old animals compared to 6 months. In contrast, there were no age-related differences in inhibition of contractile responses to nerve stimulation by the prejunctionally acting dopamine D2 agonist, N-0923. These data suggest that age-related changes in the sensitivity of prejunctional alpha-2 receptors to agonists and antagonists may not reflect any fundamental alteration in the function of this receptor system, but may be related to competition between alpha-2 agonists or antagonists and increased biophase concentrations of norepinephrine. This conclusion is supported by lack of an age-related change in function of prejunctional dopamine D2 receptors. Persistence of age-related increases in norepinephrine release when alpha-2 adrenergic receptors are fully blocked may reflect an alteration in other fundamental mechanisms that control norepinephrine release.     

Complex prejunctional actions of the D2 dopamine agonists N-0923 and N-0924 in the rat tail artery.

In rat tail arteries preloaded with [3H]norepinephrine, the D2 dopamine receptor agonist N-0923 [(S)-(-)-2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin++ +], (S)-(-)-enantiomer of N-0437 [2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin], inhibited tritium efflux and contractile responses evoked by nerve stimulation, indicating a prejunctional site of action. At higher concentrations (greater than or equal to 10(-7) M), N-0923 had additional effects which were blocked by the alpha-2 receptor antagonist yohimbine. In the presence of yohimbine, inhibition by N-0923 (10(-9) to 10(-6) M) of stimulation-evoked contractile responses correlated well with inhibition of tritium efflux, effects which were antagonized by sulpiride. The (R)-(+) enantiomer, N-0924 [(R)-(+)-2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin++ +], also inhibited stimulation-evoked contractile responses (EC50 = 5.0 x 10(-7) M) and tritium efflux (EC50 = 6.0 x 10(-7) M) in the presence of yohimbine, but with reduced potency compared to N-0923 (EC50 = 4.0 x 10(-9) and 4.2 x 10(-9) M, respectively). At high concentrations (10(-6) M), both enantiomers also increased basal tritium efflux, an effect which coincided with contraction in the case of N-0923. The effect of N-0923 at the D2 receptor, measured as inhibition of stimulation-evoked contractile responses, was greatest when the intensity of stimulation was low (low frequency or short train lengths). When extracellular calcium was lowered to 1.0 mM, the inhibitory effect of N-0923 was increased whereas elevated calcium (5.0 mM) attenuated the action of N-0923.(ABSTRACT TRUNCATED AT 250 WORDS)     

Serotonergic prejunctional inhibition of canine coronary adrenergic nerves

The actions of 5-hydroxytryptamine (5-HT) on the response of isolated canine coronary arteries to adrenergic nerve stimulation and norepinephrine were studied. 5-HT inhibited the beta adrenergic relaxation of left circumflex coronary rings in response to transmural electrical stimulation. The sensitivity to exogenously added norepinephrine was unaffected, suggesting that the effect on the response to electrical stimulation is prejunctional. Inhibition of norepinephrine release by 5-HT was confirmed in strips of coronary artery preincubated in [3H]norepinephrine. Serotonergic antagonists were tested for their ability to block the prejunctional inhibition by 5-HT, as well as their effects on the response of the coronary artery to electrical stimulation and norepinephrine. The nonselective serotonergic antagonists, methiothepin and metergoline, but not the selective 5-HT2 antagonists, ketanserin and LY 53857, prevented the inhibition by 5-HT of the response to electrical stimulation and of the stimulated overflow of [3H]norepinephrine. All of the serotonergic antagonists studied had additional effects on the response of the coronary artery to electrical stimulation or to norepinephrine. However, the alpha adrenergic antagonist, phentolamine, had additional effects similar to the serotonergic antagonists, but did not antagonize prejunctional inhibition caused by 5-HT. Furthermore, methiothepin did not block prejunctional inhibition caused by acetylcholine, suggesting the specificity of the nonselective serotonergic antagonists. Because the prejunctional inhibition by 5-HT was unaffected by neuronal uptake blockade with cocaine, these results suggest specific, non-5-HT2 serotonergic receptors on coronary adrenergic nerves which, when activated, inhibit the stimulated release of norepinephrine.     

Prejunctional alpha adrenoceptor and angiotensin receptor function in isolated human, monkey and dog arteries

Transmural electrical stimulation (2-20 Hz) produced a frequency-dependent contraction in dog mesenteric, monkey mesenteric and human gastroepiploic arterial strips, which was abolished by tetrodotoxin and suppressed by phentolamine. Treatment with yohimbine (10(-9) and 10(-8) M) potentiated the response to nerve stimulation dose-dependently in the dog arteries, but rather attenuated the response of the primate arteries. Yohimbine (10(-8) M) attenuated the contraction caused by exogenous norepinephrine in the dog and monkey arteries. Angiotensin (ANG) II (2 X 10(-10) M) and ANG I (10(-9) M) potentiated the response to transmural stimulation in the dog and monkey arteries, whereas the response to norepinephrine was unaffected. The ANG I-induced potentiation was suppressed by treatment with ANG converting enzyme inhibitors. 3H-Overflow evoked by transmural stimulation in tissues prelabeled with [3H] norepinephrine was increased by yohimbine in the superfused dog arteries but was not increased significantly in the monkey arteries. The overflow was increased significantly by ANG II in the dog and monkey arteries. It may be concluded that prejunctional alpha-2 adrenoceptors mediating the inhibition of transmitter release do not function significantly in human gastroepiploic and monkey mesenteric arteries, and postjunctional alpha-2 receptors are involved partly in contractions of the monkey arteries due to adrenergic nerve stimulation. ANG II appears to be synthesized from ANG I via ANG converting enzyme in the primate arteries; the octapeptide potentiates the contraction caused by adrenergic nerve stimulation, possibly due to prejunctional ANG receptor activation and increased norepinephrine release.     

Neuropeptide Y preferentially potentiates responses to adrenergic nerve stimulation by increasing rate of contraction

Neuropeptide Y (NPY) is colocalized with norepinephrine and coreleased from adrenergic nerves. NPY reportedly can influence vascular neuroeffector transmission in both negative and positive directions. Mechanisms by which NPY effects are exerted and their relative contribution may vary between different blood vessels. Therefore, we investigated effects of NPY on adrenergic neurotransmission in ring segments of the rat tail artery by measuring isometric force development and [3H]norepinephrine release. NPY (1-100 nM) potentiated responses to transmural nerve stimulation (3 Hz, six pulses) by more than 300% but produced no direct contractile effects. NPY potentiated responses to longer transmural nerve stimulation trains (30-100 pulses) or to exogenous norepinephrine by only 30%. NPY had no effect on [3H]norepinephrine overflow with either short (six pulses) or long (30 pulses) stimulation trains. Contractile responses to transmural nerve stimulation were completely blocked by prazosin (10(-7) M) both in the absence and presence of NPY. Since the impact of an increased rate of contraction on the contractile response achieved would be greater with short stimulation trains, the effect of NPY on contraction rate was analyzed. NPY increased the rate of contraction to both norepinephrine and transmural nerve stimulation. It is hypothesized that NPY increases the early (phasic) component of the contractile response by preferentially influencing release of intracellular calcium stores. These findings suggest that the effects of NPY will be more profound on phasic rather than sustained patterns of nerve traffic.     

Interaction of the prejunctional inhibitory action of 5-hydroxytryptamine on noradrenergic transmission with neuronal amine uptake in rabbit isolated ear artery

The interaction of the prejunctional inhibitory action of 5-hydroxytryptamine (5-HT) on noradrenergic transmission with the neuronal amine uptake mechanism has been studied in rabbit isolated ear artery preparations. Release of norepinephrine in response to stimulation of periarterial sympathetic nerves (30 pulses, 1 Hz) was deduced from the efflux of radioactivity which had been incorporated into the noradrenergic transmitter pool as [3H]norepinephrine. 5-HT (100 nM), applied alone, had no effect on the stimulation-induced efflux of radioactivity. However, in the presence of cocaine (1 microM), 5-HT reduced stimulation-induced efflux. The inhibitory effect of 5-HT, in the presence of cocaine, on stimulation-induced efflux was abolished by the nonselective 5-HT1/5-HT2 receptor antagonist, methiothepin (30 nM), but not by the selective 5-HT2 receptor antagonist, ketanserin (6 nM), or by the alpha adrenoceptor antagonist, phentolamine (30 nM). These findings indicate that the uptake of 5-HT into periarterial sympathetic nerves may limit its prejunctional "5-HT1-like" receptor-mediated inhibitory effect on noradrenergic transmission. In arteries which were incubated with 5-HT (1 microM) and the monoamine oxidase inhibitor, pargyline (10 microM), before loading the transmitter stores with [3H]norepinephrine, methiothepin (30 nM) enhanced stimulation-induced efflux markedly. The enhancing effect of methiothepin was not observed in arteries which were preincubated with cocaine (10 microM) together with 5-HT and pargyline. It is suggested that, following its uptake into periarterial sympathetic nerves, 5-HT may be coreleased with norepinephrine to activate prejunctional 5-HT1-like receptors and thereby mediate an autoinhibitory effect on transmitter release.     

Effects of YM-12617, an alpha adrenoceptor blocking agent, on electrical and mechanical properties of the guinea-pig mesenteric and pulmonary arteries

The effects of YM-12617 on the electrophysiological properties of smooth muscle membranes and prejunctional nerve terminals and contractions evoked by different procedures were studied using guinea-pig mesenteric and pulmonary arteries. In concentrations over 1 nM, YM-12617 inhibited the depolarization induced by norepinephrine in both muscle tissues. Yohimbine had no effect whereas prazosin inhibited the norepinephrine-induced depolarization to a lesser extent than YM-12617. When YM-12617, in concentrations over 1 microM, was applied to the mesenteric artery the amplitude of the first excitatory junction potential evoked by a train stimulation of perivascular nerves was inhibited, but the facilitation of excitatory junction potentials evoked by frequencies over 0.1 Hz was enhanced. As a consequence, the amplitude of the excitatory junction potentials after completion of the facilitation exceeded the control, as was expected to occur with a typical alpha-2 adrenoceptor blocker. YM-12617 inhibited the contraction evoked by exogenously applied norepinephrine or perivascular nerve stimulation, with a higher potency than seen with prazosin, but this agent had no effect on the contraction evoked by excess concentrations of K+ or by direct muscle stimulation. These results indicate that YM-12617 possesses a more potent alpha-1 adrenoceptor blocking action than does prazosin, and is more selective for alpha-1 than for alpha-2 adrenoceptors.     

Effects of Losartan, a nonpeptide angiotensin II receptor antagonist, on norepinephrine overflow and antidiuresis induced by stimulation of renal nerves in anesthetized dogs.

We examined the role of endogenous angiotensin II on renal noradrenergic neurotransmission in anesthetized dogs, using Losartan, a nonpeptide angiotensin II receptor (angiotensin subtype 1) antagonist. The renal nerve stimulation caused a frequency-dependent increase in renal norepinephrine secretion rate. The low frequency renal nerve stimulation (0.5-2.0 Hz) significantly decreased urine flow and urinary excretion of sodium, without affecting renal hemodynamics. The high frequency renal nerve stimulation (2.5-5.0 Hz) produced a more potent antidiuresis and a renal vasoconstriction that resulted in reductions of renal blood flow and glomerular filtration rate. Intrarenal arterial infusion of Losartan (10 and 100 micrograms/kg/min) did not affect the basal levels of norepinephrine secretion rate, although increased urine formation with some renal vasodilation were observed during infusion of the drug. The administration of Losartan had an inhibitory action on the decreased urine formation, renal vasoconstriction and enhanced norepinephrine secretion rate, in response to renal nerve stimulation. Based on these findings, we suggest that endogenous angiotensin II seems to regulate renal noradrenergic neurotransmission by facilitating norepinephrine release, through the prejunctional angiotensin subtype 1 receptor.     

Effects of alpha,beta-methylene ATP on the prejunctional purinoceptors of the sympathetic nerves of the rat caudal artery

The effects of alpha,beta-methylene ATP, an agent known to stimulate and then to desensitize P2-purinoceptors, on the release of endogenous norepinephrine from the electrically stimulated rat caudal artery were determined. Norepinephrine was quantified by high-performance liquid chromatography-electrochemical detection techniques. alpha,beta-Methylene ATP over the concentration range of 1 to 100 microM did not affect the release of norepinephrine evoked by stimulation for 3 min at 1 Hz. In contrast, 2-chloroadenosine, a P1 receptor agonist, and beta,gamma-methylene ATP, a P2 receptor agonist, produced a concentration-related inhibition of the release of norepinephrine presumably by activating prejunctional purinoceptors. The failure of alpha,beta-methylene ATP to inhibit transmitter release was apparently not related to the length of pretreatment with this agent because pretreatments of 0.5 to 15 min yielded similar results. These findings indicate that the ability of alpha,beta-methylene ATP to decrease excitatory junction potentials and vasoconstriction of the caudal artery, as reported by others, is not due to a decrease in the release of transmitter. In spite of not possessing agonistic properties, alpha,beta-methylene ATP does interact with prejunctional purinoceptors as judged by the finding that the inhibitory effects of 2-chloroadenosine and beta,gamma-methylene ATP, but not those of the alpha-2 agonist clonidine, were antagonized by alpha,beta-methylene ATP. alpha,beta-Methylene ATP thus appears to be an antagonist at prejunctional purinoceptors (classified by us previously as P3-purinoceptors). alpha,beta-Methylene ATP also appears to act as an antagonist against endogenously released adenine nucleosides and nucleotides.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dopamine regulation of [3H]acetylcholine release from guinea-pig stomach

The involvement of dopamine receptors in cholinergic transmission of guinea-pig stomach was investigated by analyzing the effects of dopamine receptor agonists and antagonists on acetylcholine (ACh) release from this organ. Electrical stimulation (1-20 Hz) of strips of guinea-pig stomach preloaded with [3H] choline induced a [3H]ACh release that was calcium dependent and tetrodotoxin sensitive. Dopamine inhibited this transmural stimulation-induced [3H]ACh release in a concentration-dependent manner (10(-8)-10(-4) M). This effect of dopamine was not altered by 10(-5) M hexamethonium, thereby suggesting that the major dopamine receptors are located on the postganglionic cholinergic neurons. Concentration-response curves for dopamine on [3H]ACh release were inhibited by haloperidol, sulpiride and domperidone but not by prazosin, yohimbine, propranolol and ketanserin. LY 171555, an agonist for the D2 dopamine receptor, but not SKF 38-393, an agonist for the D1 dopamine receptor, to some extent decreased the release of [3H]ACh induced by transmural stimulation. In view of the results, the release of ACh from postganglionic cholinergic neurons is probably required through dopamine receptors antagonized by D2 antagonists but not by adrenergic or serotonin receptor antagonists.     

Alpha-2 adrenoceptors modulate [3H]dopamine release from rabbit retina

In the rabbit retina, preloaded in vitro with [3H]dopamine, calcium-dependent release of radioactivity was elicited by a 1-min period of field stimulation at 3 Hz (20 mA, 2 msec). In the presence of the catecholamine uptake inhibitor nomifensine (30 microM), unlabeled catecholamines (0.01-3 microM), namely, dopamine, norepinephrine and epinephrine, inhibited in a concentration-dependent manner the field stimulation-evoked release of [3H]dopamine from the retina. The concentrations of dopamine, norepinephrine or epinephrine which inhibited by 50% the release of [3H]dopamine (IC50) were 0.30, 0.25 and 0.25 microM, respectively. In the presence of 30 microM nomifensine, S-sulpiride (1 microM) significantly increased the calcium-dependent release of [3H]dopamine, suggesting that this dopamine antagonist blocks a receptor tonically activated by endogenous dopamine in the rabbit retina. In contrast, the alpha receptor antagonist phentolamine (1 microM) alone did not affect the release of [3H]dopamine from the retina. The inhibitory effect of norepinephrine and epinephrine on [3H]dopamine overflow was not modified by S-sulpiride which, on the contrary, selectively antagonized the effect of exogenous dopamine. Phentolamine (1 microM) competitively antagonized the inhibitory effect of norepinephrine and epinephrine on [3H]dopamine release, suggesting that these catecholamines activate alpha adrenoceptors in retina. In the absence of nomifensine, the selective alpha-2 agonist clonidine (IC50 = 0.056 microM) inhibited the stimulation-evoked release of [3H]dopamine from retina, whereas the alpha agonist methoxamine was without effect. The inhibitory effect of clonidine was antagonized by yohimbine (1 microM), but not prazosin, suggesting that the release modulating alpha receptors of the retina are of the alpha-2 subtype.(ABSTRACT TRUNCATED AT 250 WORDS)     

SK&F 89124, A POTENT AND SELECTIVE AGONIST AT PREJUNCTIONAL DOPAMINE RECEPTORS

SK&F 89124 (4-[2-(N,N-di-n-propylamino)ethyl]-7-hydroxy-2(3H) indolone) can be considered as a derivative of N,N-di-n-propyldopamine (DPDA) in which the meta-hydroxyl is replaced by a cyclic amide function. SK&F 89124 is at least one order of magnitude more potent than DPDA as an agonist at peripheral inhibitory prejunctional dopamine receptors (DA2 receptors) in the isolated perfused rabbit ear artery. A potent agonist action of SK&F 89124 at the DA2 receptor can also be demonstrated by inhibition of radioactive overflow from prelabelled canine coronary artery or saphenous vein, and in the anesthetized dog as an inhibition of the tachycardia induced by cardioaccelerator nerve stimulation or the increase in hind-limb perfusion pressure induced by stimulation of the lumbar sympathetic chain. SK&F 89124 is a potent inhibitor of the binding of [3H]spiroperidol to D2 receptors in bovine pituitary homogenates. High concentrations of SK&F 89124 do not activate the adenylate cyclase D1 receptor in rat caudate homogenates, nor produce activation of alpha 2-adrenoceptors or H2-histamine receptors in the guinea pig atrium. Although some alpha 1-adrenoceptor mediated vasoconstriction is produced in the rabbit ear artery and rabbit aorta, the concentrations required are several orders of magnitude higher than those active at the DA2 receptor. From these data it is evident that this structural modification can increase both the potency and selectivity of DPDA as a DA2 receptor agonist. The potency and selectivity of SK&F 89124 make this agent a useful tool for determination of the functional role of the DA2 receptor.     

Influence of the DA2-receptor agonist quinpirole on the guinea pig ileum

The influence of the dopamine-receptor agonist quinpirole was studied in segments of the guinea pig ileum, mounted to measure longitudinal muscle activity. In concentrations selective for dopamine receptors (10(-9)-10(-7) M), quinpirole did not influence cholinergic twitch responses induced by transmural stimulation at supramaximal voltage, but it concentration-dependently facilitated them in the concentration range 10(-6) to 10(-4) M; the potency of quinpirole was similar to that of metoclopramide (ED50 values, 6.0 x 10(-6) M and 2.2 x 10(-6) M, respectively). Pretreatment of the tissues with 5-hydroxytryptamine or with the 5-hydroxytryptamine receptor antagonists methysergide, ketanserin and R53434 did not influence the facilitatory effect of quinpirole. It was also not influenced by drugs blocking adrenoceptors, dopamine receptors, opioid receptors, muscarinic M1 receptors, GABAA receptors and inhibiting the synthesis of prostaglandins. It was antagonized by the nicotinic receptor antagonist hexamethonium but not by pentolinium. Quinpirole did not enhance contractile responses to acetylcholine and carbachol, but in high concentrations induced contractions in unstimulated preparations. It is concluded that quinpirole facilitates twitch responses in the guinea pig ileum via a prejunctional site of action. This effect is not related to activation of dopamine receptors, but the exact mechanism remains unclear.     

Influence of stimulation train length on the opioid-induced inhibition of norepinephrine release in the rabbit ear artery

In perfused segments of rabbit ear artery the effects of opioid agonists on vasoconstrictor responses to adrenergic nerve stimulation and stimulation-evoked [3H]norepinephrine overflow were compared using trains of 8, 40 or 120 monophasic pulses at 8 Hz with 1 msec duration and 40 V amplitude. Both delta selective peptides, [D-Ala2, D-Leu5]-enkephalin, met-enkephalin and leu-enkephalin, and preferential kappa ligands, dynorphin1-13 and ethylketocyclazocine, significantly reduced the vasoconstriction evoked by 8 pulses at 8 Hz. The magnitude of this effect was inversely related to the stimulus train length. In most cases, opioid agonists were highly effective with short stimulus trains, but failed to decrease vasoconstrictor responses by more than 10 to 15% with trains of 120 pulses at 8 Hz. In good correlation with these data, in tissue preincubated with [3H]norepinephrine the inhibitory effect of opioids on the tritium overflow evoked by 8 or 120 pulses at 8 Hz was inversely proportional to the length of stimulation. Our findings confirm that opioid agonists interact with a heterogeneous prejunctional receptor population to modulate the action potential-evoked release of norepinephrine in the isolated ear artery of the rabbit. This modulation is more pronounced at the beginning of a stimulus train when the negative feedback inhibition of norepinephrine release is still not fully operative. It is suggested that the physiological role of opioid peptides could be more significant during a short rather than a prolonged vasoconstriction.     

Dopamine selectively inhibits angiotensin II-induced aldosterone secretion by interacting with D-2 receptors

Aldosterone secretion is subject to both stimulatory and inhibitory controls. Angiotensin II (AII) is the primary stimulator of aldosterone production and an inhibitory role of dopamine (DA) has been suggested recently. In this study we investigated the interactions between DA and AII in the intracellular events leading to aldosterone secretion. By measuring aldosterone secretion and cyclic AMP (cAMP) formation in intact adrenal glomerulosa cells we show that AII induced a sustained stimulation of aldosterone secretion (EC50, 0.41 nM) and a rapid and transient increase in intracellular cAMP content (EC50, 4 nM). DA inhibited both aldosterone secretion (IC50, 300 nM) and cAMP formation (IC50, 100 nM) elicited by submaximal concentrations of AII; in contrast, DA did not attenuate either basal or adrenocorticotropic hormone-stimulated cell activity. The pharmacological characterization of DA effects with dopaminergic agonists and antagonists strongly indicated an involvement of D-2 receptors. Indeed, selective D-2 agonists were more effective than DA in inhibiting the glomerulosa cell responses to AII; in addition, the effects of DA on both aldosterone secretion and cAMP formation were prevented by D-2 antagonists, such as (-)-sulpiride and domperidone, but not by the selective D-1 antagonist SCH 23390. These data suggest a specific functional interaction between D-2 receptors apparently associated with inhibition of cAMP formation and AII in the regulation of aldosterone production.     

Nonadrenergic nature of prazosin-resistant, sympathetic contraction in the dog mesenteric artery

Electrical transmural stimulation of the isolated dog mesenteric artery produced a contractile response which was abolished by guanethidine and 6-hydroxydopamine but not by prazosin. Approximately 60% of the response seen with a frequency of 3 Hz remained after the treatment with prazosin. The prazosin-resistant contraction induced by electrical transmural stimulation was potentiated by other alpha adrenoceptor antagonists (phentolamine, phenoxybenzamine, tolazoline and DG-5128). Alpha-2 adrenoceptor agonists (including norepinephrine) attenuated the prazosin-resistant contraction and this attenuation was antagonized by the alpha antagonists mentioned above. Cocaine slightly inhibited the prazosin-resistant contraction, whereas this drug markedly augmented the contractile response to electrical stimulation before treatment with prazosin. In reserpine-treated mesenteric arteries also, electrical transmural stimulation produced a contraction and this was neither suppressed nor potentiated by prazosin and other alpha antagonists but was attenuated by alpha-2 agonists. Guanethidine and 6-hydroxydopamine abolished the prazosin-resistant contraction in reserpine-treated arteries. Nicotine, but not tyramine, also produced such prazosin-resistant contraction in reserpine-treated and untreated arteries. Exogenous norepinephrine produced a concentration-dependent contraction in reserpine-treated and untreated arteries and the responses were competitively antagonized by prazosin. These results indicate that the prazosin-resistant contractions of the dog mesenteric artery induced by electrical transmural stimulation and nicotine are sympathetic in origin but not adrenergic in nature. Such prazosin- resistant contraction was observed in the dog mesenteric vein but not in carotid and femoral arteries, thereby suggesting that the nonadrenergic component may play an important role in the regulation of visceral blood flow.     

Motor innervation of the smooth muscle of the rat seminal vesicle.

Frequency-related isovolumetric contractions of the rat seminal vesicle elicited with transmural electrical stimulation were blocked by tetrodotoxin but unaffected by hexamethonium. The postganglionic motor innervation of the rat seminal vesicle is purely excitatory and contains both an adrenergic and a cholinergic component which are excited simultaneously during transmural stimulation. Contractions elicited by adrenergic nerve stimulation were mediated by norepinephrine acting via alpha adrenoceptors, i.e., 1) responses of untreated vesicles to transmural stimulation and to exogenous norepinephrine were antagonized by phentolamine and potentiated by cocaine, 2) pretreatment of animals with reserpine or 6-hydroxydopamine produced a marked depletion of tissue norepinephrine concentration and reduced the responses to transmural stimulation to a level which resembled that of untreated organs in the presence of phentolamine, 3) the residual responses of vesicles from pretreated rats were not modified by phentolamine or cocaine, and 4) responses to tyramine in untreated organs were antagonized by phentolamine but not by cocaine and were observed in organs from reserpine-pretreated rats only after repletion with exogenous norepinephrine. Responses elicited by cholinergic nerve stimulation were mediated by acetylcholine through muscarinic receptors, i.e., 1) responses of untreated vesicles to transmural stimulation and to exogenous acetylcholine were antagonized by atropine, 2) the residual responses to transmural stimulation of vesicles from animals pretreated with reserpine of 6-hydroxydopamine were nearly abolished by atropine and 3) physostigmine potentiated and prolonged the responses of organs from untreated and reserpine-pretreatd animals to transmural stimulation; these effects of physostigmine were abolished by atropine.     

McN-5652: a highly potent inhibitor of serotonin uptake

McN-5652 is one of a series of substituted pyrrolo-isoquinolines that, as a group, potently inhibit the uptake of one or more of the monoamines, norepinephrine, serotonin and dopamine. McN-5652 is characterized by exceptionally high potency as an inhibitor of the uptake of serotonin by rat brain synaptosomes in vitro (Ki approximately 0.6 nM) and ex vivo (ED50 approximately 2 mg/kg p.o.). The high potency of McN-5652 as a serotonin uptake inhibitor in vivo is indicated further by the low doses required to potentiate L-5-hydroxytryptophan-induced head twitches in mice (ED50 = 0.4 mg/kg 2 hr after p.o. dosing) and the serotonin syndrome in rats (ED50 = 1.5 mg/kg 2 hr after p.o. dosing). McN-5652 also potently inhibited the synaptosomal uptake of norepinephrine (Ki approximately 3 nM) and was a moderately potent inhibitor of the synaptosomal uptake of dopamine (Ki approximately 40 nM). McN-5652 inhibited tetrabenazine-induced ptosis in rats and mice but was much less effective in blocking the sedation caused by tetrabenazine. In rats, McN-5652 did not induce the stereotyped behavior often caused by dopamine agonists and inhibitors of dopamine uptake. Receptor binding experiments indicated that McN-5652 has a weak affinity for serotonin 5-HT2 and alpha-1 adrenergic receptors (apparent Ki approximately 200 nM) and a very low affinity for dopamine D1 and D2 receptors, serotonin 5-HT1, alpha-2 adrenergic, muscarinic and gamma-aminobutyric acid-A receptors. Experiments using the guinea pig ileum indicate that McN-5652 is a weak, noncompetitive antagonist of histamine.     

Subtypes of muscarinic receptors on adrenergic nerves and vascular smooth muscle of the canine saphenous vein

Experiments were designed to characterize pre- and postjunctional muscarinic receptors in the canine saphenous vein. In rings contracted by 2-Hz electrical stimulation, acetylcholine produced concentration-dependent relaxations that result from prejunctional inhibition of norepinephrine release from adrenergic nerve endings (prejunctional effect). In quiescent preparations, acetylcholine caused concentration-dependent contractions (postjunctional effect). Both responses to acetylcholine were inhibited in a competitive manner by atropine, pirenzepine and gallamine. The affinity of atropine for pre- and postjunctional muscarinic receptors was similar (pKB = 8.7 and 9.2, respectively). However, gallamine displayed higher affinity for the prejunctional than for the postjunctional receptor (pKB = 6.1 and 4.7, respectively), whereas the converse was true for pirenzepine (pKB = 6.5 and 8.1). The data support the presence in the canine saphenous vein of two muscarinic receptor subtypes, M1 postjunctionally and M2 prejunctionally, which are distinguishable by selective antagonists.     

The effect of cold stress on the modulation of vascular adrenergic transmission by acetylcholine

Acetylcholine acts via presynaptic receptors to inhibit adrenergic neurotransmission in vascular tissue. To test the possibility that this modulation might be altered by hyperactivity of the sympathetic nervous system, rats were exposed to cold stress for 5 days. Rat caudal (tail) arteries were excised, cannulated and perfused at constant flow. Responses to transmural nerve stimulation and/or acetylcholine were measured. In arteries from nonstressed rats, acetylcholine produced a dose-dependent inhibition of responses to nerve stimulation of 10 Hz. Likewise, acetylcholine (32 ng/ml) produced a frequency-dependent inhibition to nerve stimulation. Cold stress elevated sympathetic nerve activity in the tail artery as indicated by increased tyrosine hydroxylase activity. Responses to exogenous norepinephrine alone were not different between arteries from cold-stressed and nonstressed rats. In arteries from cold-stressed rats, acetylcholine inhibited the response to nerve stimulation in a dose-dependent manner and at each dose of acetylcholine the inhibition was greater than in arteries from nonstressed rats. Likewise, the inhibition of the frequency-dependent responses to nerve stimulation by acetylcholine was greater in arteries from cold-stressed than from nonstressed rats. These data show that after chronic elevation of vasomotor tone, acetylcholine is a more effective modulator of neurogenic tone which indicates the development of the functional equivalent of supersensitivity of presynaptic receptor-mediated events.