Autonomic nervous control of gastric somatostatin secretion from the perfused rat stomach

The effect of parasympathetic and sympathetic nerve stimulation on the secretion of gastric somatostatin and gastrin has been studied in an isolated perfused rat stomach preparation. Stimulation of the vagus nerve inhibited somatostatin secretion and increased gastrin release. Splanchnic nerve stimulation increased somatostatin release during simultaneous atropine perfusion, but not in its absence, whereas gastrin secretion was unchanged. The secretory activity of the gastric D-cell was therefore reciprocally influenced by the sympathetic and parasympathetic nerves but sympathetic stimulation was only effective during muscarinic blockade.     

Angiotensin II potentiates responses of the rabbit basilar artery to adrenergic nerve stimulation

Angiotensin II has little contractile effect on the isolated rabbit basilar artery; however, it markedly potentiates contractile responses to adrenergic nerve stimulation. This is not a post-synaptic effect of angiotensin, as responses to exogenous norepinephrine are not altered. Angiotensin increases stimulation-evoked release of norepinephrine, and this effect probably accounts for the increased response to adrenergic nerve stimulation. Since sympathetic stimulation may protect the cerebral circulation from hypertensive damage, increased responsiveness to adrenergic nerve activity produced by angiotensin may have a beneficial effect.     

Vagal afferent activity and renal nerve release of dopamine

To investigate the involvement of vagal afferents in renal nerve release of catecholamines, we compared norepinephrine, dopamine, and epinephrine excretion from innervated and chronically denervated kidneys in the same rat. The difference between innervated and denervated kidney excretion rates was taken as a measure of neurotransmitter release from renal nerves. During saline expansion, norepinephrine excretion from the innervated kidney was not statistically greater than from denervated kidneys. Vagotomy increased norepinephrine release from renal nerves. Thus vagal afferents participated in the suppression of renal sympathetic nerve activity during saline expansion. No significant vagal control of dopamine release by renal nerves was detected under these conditions. Bilateral carotid ligation stimulated renal nerve release of both norepinephrine and dopamine in saline-expanded rats. The effects of carotid ligation and vagotomy were not additive with respect to norepinephrine release by renal nerves. However, the baroreflex-stimulated renal nerve release of dopamine was abolished by vagotomy. Electrical stimulation of the left cervical vagus with a square wave electrical pulse (0.5 ms duration, 10 V, 2 Hz) increased dopamine excretion exclusively from the innervated kidney of hydropenic rats. No significant change in norepinephrine excretion was observed during vagal stimulation. Increased dopamine excretion during vagal stimulation was associated with a larger natriuretic response from the innervated kidney than from its denervated mate (p less than 0.05). We conclude that under appropriate conditions vagal afferents stimulate renal release of dopamine and produce a neurogenically mediated natriuresis.     

Actions of angiotensin on adrenergic nerve endings.

In the perfused vascular bed, vasoconstrictor responses to adrenergic nerve stimulation are augmented to a greater degree by angiotensin II than are the responses to injected norepinephrine. Overflow of adrenergic transmitter is also greater during nerve stimulation in the presence of angiotensin than in its absence. The evidence indicates that facilitation of adrenergic transmitter release rather than uptake blockade accounts for these results. In addition, an increased responsiveness of isolated arterial strips to norepinephrine as well as other agonists appears to contribute to the adrenergic potentiating effect of angiotensin II as well as angiotensin III. This action, which appears to be a cell membrane effect, seems to participate in adrenergic potentiation mainly in the arterial segment of the intact vascular bed. Both of these effects of angiotensin, i.e., facilitation of release and increased smooth muscle responsiveness, appear to be mediated by angiotensin receptors.     

Blockade of the pre- and postjunctional effects of angiotensin in vivo with a non-peptide angiotensin receptor antagonist

Recent reports indicate that some imidazole-5-acetic acid derivatives are competitive antagonists of angiotensin II receptors. However, to our knowledge, there is no published information regarding: 1) what constant infusion rate of these non-peptide angiotensin receptor blockers is necessary to effectively antagonize angiotensin receptors in vivo, 2) whether imidazole-5-acetic acid derivatives antagonize both prejunctional and postjunctional angiotensin receptors, and 3) whether effective levels of these compounds exert non-specific actions and/or partial agonist activity. To address these issues, either vehicle, 2-butyl-4-chloro-1-(2-nitrobenzyl) imidazole-5-acetic acid (CV-2961; 30 and 100 micrograms/min) or a standard angiotensin receptor blocker, 1Sar8Ile-angiotensin II (100 ng/min), was infused intravenously into captopril-treated rats that were prepared for in situ perfusion of their mesenteric vascular beds. Infusion of CV-2961 for two and one-half hours did not alter arterial blood pressure, mesenteric perfusion pressure, plasma aldosterone level, or mesenteric vascular responses to sympathetic nerve stimulation or exogenous norepinephrine. The higher dose of CV-2961 (100 micrograms/min) completely blocked angiotensin II-induced enhancement of vascular responses to sympathetic nerve stimulation and shifted the angiotensin dose-response curve 10-fold to the right with respect to angiotensin II-induced increases in mesenteric perfusion pressure. The effects of the lower dose of CV-2961 (30 micrograms/min) on these actions of angiotensin II were not statistically significant. 1Sar8Ile-angiotensin II abolished both the prejunctional and postjunctional effects of angiotensin II. We conclude that in intact rats CV-2961, infused at 100 micrograms/min, antagonizes both prejunctional and postjunctional angiotensin II receptors, yet has a somewhat greater effect on the prejunctional actions of angiotensin II. CV-2961 is devoid of partial agonist activity, and no non-specific actions of CV-2961 are evident. Imidazole-5-acetic acid derivatives may find considerable utility as pharmacological probes and as therapeutic agents.     

Attenuation of noradrenergic neurotransmission by the thromboxane synthetase inhibitor,UK 38,485

The purpose of this study was to determine the effects of chronic administration of the thromboxane synthetase inhibitor, UK 38,485, on noradrenergic neurotransmission. Male Sprague Dawley rats (n=14) were treated once daily with either UK 38,485 (100 mg/kg; n=7) or the vehicle of UK 38,485 (olive oil; n=7) by gavage. The dose of UK 38,485 chosen was sufficient to inhibit $\text{ex vivo}$ platelet TXB₂ production by >90% for 24 hours. One week into the treatment animals were prepared for $\text{in situ}$ perfusion of their mesenteric vascular beds. Vasoconstrictor responses to both exogenous norepinephrine and periarterial nerve stimulation were determined both before and during an infusion of angiotensin II (9ng/min) into the superior mesenteric artery. UK 38,485 significantly (P<0.02) attenuated the vascular response to periarterial nerve stimulation without altering the vascular response to either norepinephrine or angiotensin II. UK 38,485 did not influence the baseline perfusion pressure, the mean arterial blood pressure or the potentiation of neurotransmission by angiotensin II. These data indicate that in the $\text{in situ}$ rat mesentery UK 38,485 attenuates the release of neurotransmitter from sympathetic nerve terminals.     

Endothelin-induced modulation of neuropeptide Y and norepinephrine release from the rat mesenteric bed

The effect of three endothelin (ET) agonists [ET-1, ET-3, and sarafotoxin (STX6C)] on the nerve stimulation-induced release of norepinephrine (NE) and neuropeptide Y-immunoreactive compounds (NPY-ir) from the perfused mesenteric arterial bed of the rat as well as the effect on perfusion pressure were examined. ET-1, ET-3, and STX6C all produced a significant, concentration-dependent decrease in the evoked release of NPY-ir but had no effect on the release of NE. In contrast, all three ETs potentiated the nerve stimulation-induced increase in perfusion pressure. The inhibition of nerve stimulation-induced NPY-ir release by ET-1 was significantly blocked by the ET(A)/ET(B) antagonist PD-142893 and the ET(B) antagonist RES-701-1 but not by the ET(A) antagonist BQ-123. The potentiation of the nerve stimulation-induced increase in perfusion pressure by ET-1 was significantly blocked by PD-142893 and BQ-123 and attenuated by RES-701-1. Prior exposure of the preparation to indomethacin or meclofenamate failed to alter the attenuation of the evoked release of NPY-ir or the potentiation of the increase in perfusion pressure produced by ET-1 or ET-3. These results are consistent with the idea that sympathetic cotransmitters can be preferentially modulated by paracrine mediators at the vascular neuroeffector junction.     

Modulation by beta-adrenoceptors and angiotensin II receptors of splanchnic nerve evoked catecholamine release from the adrenal medulla.

In the present study, we have evaluated the effect of both facilitatory beta 2-adrenoceptor and angiotensin II receptor on the release of adrenal catecholamines induced by electrical stimulation of the splanchnic nerve in anaesthetized and vagotomized dog. In these experiments, individual or combined treatments with the beta 2-adrenoceptor antagonist ICI 118551 (0.3 mg/kg i.v.), the converting enzyme inhibitor captopril (2 mg/kg i.v.), or the angiotensin II receptor antagonist saralasin (2 micrograms.kg-1.min-1 i.v.) were found to significantly decrease the release of adrenal catecholamines during splanchnic nerve stimulation (5-V pulses of 2 ms duration for 3 min at 1 Hz) whatever the order of administration of the drugs. On the other hand, the infusion of angiotensin II (20 ng.kg-1.min-1) was shown to potentiate the release of adrenal catecholamines in response to electrical stimulation, and this effect was totally blocked by treatment with saralasin (4 micrograms.kg-1.min-1 i.v.). This facilitating angiotensin mechanism differed from beta-adrenoceptor facilitating mechanism, since following beta-blockade with ICI 118551, angiotensin II infusion still significantly potentiated the release of catecholamines during splanchnic nerve stimulation. These observations thus suggest that both facilitating beta 2-adrenoceptors and angiotensin II receptors can independently modulate the release of adrenal catecholamines.     

Canine vagus nerve stores cholecystokinin-58 and -8 but releases only cholecystokinin-8 upon electrical vagal stimulation

Cholecystokinin-58 has been shown to be the major form of cholecystokinin (CCK) released to the circulation upon lumenal stimulation of the small intestine in humans and dogs. In anesthetized dogs, electrical vagal stimulation evokes pancreatic exocrine secretion that is in part mediated through the release of CCK. We studied the molecular form of CCK stored in canine vagus nerves and that released into circulation upon electrical vagal stimulation. Gel filtration and radioimmunoassay of the water and acid extracts of canine vagus nerves indicated CCK-8 (35%) and CCK-58 (65%) as the major molecular forms in the vagus nerve. Both forms of CCK isolated from the vagal extracts were equally bioactive as the standard CCK-8 and CCK-58, respectively, in stimulation of amylase release from isolated rat pancreatic acini. Analysis of plasma collected after electrical vagal stimulation indicated that CCK-8 is the only form released into the circulation. The release of CCK-8 upon electrical vagal stimulation was not affected by application of lidocaine to the upper small intestinal mucosa, suggesting that it was released from vagal nerve terminals.     

Iloprost preserves kidney function against anoxia

Tissue protective activity of iloprost against anoxia was studied in the isolated perfused rabbit kidney. Addition of iloprost to the perfusion medium at concentrations between 10(-9)-10(-7) M attenuated the release of noradrenaline due to periarterial stimulation and decreased urine outflow. Iloprost also caused a concentration-dependent decrease in perfusion pressure. The potentiation by angiotensin II of the vasoconstriction due to periarterial stimulation and increase in urine volume were also decreased by further addition of iloprost into the medium. Iloprost at concentrations below 10(-7) M did not alter the vasoconstrictor effect of exogenously applied noradrenaline. UK 38 485, a powerful thromboxane A2 synthetase inhibitor, significantly suppressed the vascular but greatly potentiated the diuretic effects of angiotensin II. In kidneys exposed to anoxia for 24 hours in Krebs medium, the vascular and diuretic effects of angiotensin II and the release of noradrenaline due to periarterial stimulation were significantly diminished. In addition, interation between UK 38 485 and angiotensin II in both perfusion pressure and urine volume was also reduced after anoxia for 24 hours. On the other hand, no significant loss was observed in all investigated parameters measured in this study, in kidneys exposed to anoxia for 48 hours in the presence of iloprost. From these results it was concluded that iloprost preserves kidneys functionally against anoxia and possible mechanisms of this effect are discussed.     

Phorbol Ester Enhancement of Neurotransmitter Release from Rat Brain Synaptosomes

Neurotransmitter release from rat brain synaptosomes was measured following pretreatment with various phorbol esters. Ca2+-dependent, evoked neurotransmitter release was increased by phorbol esters that were active in stimulating protein kinase C. Protein kinase C activation was demonstrated by increased incorporation of 32P into 87-kilodalton phosphoprotein, a specific substrate for that kinase. Inactive phorbol esters had no effect on neurotransmitter release or on the phosphorylation of 87-kilodalton phosphoprotein. The increased release was observed in either crude cortical synaptosomal fractions (P2) or purified cortical synaptosomal fractions. The enhancement was found for all neurotransmitters (norepinephrine, acetylcholine, gamma-aminobutyric acid, serotonin, dopamine, and aspartate), all brain regions (cerebral cortex, hippocampus, and corpus striatum), and all secretagogues (elevated extracellular K+ level, veratridine, or A23187) examined. It was also observed at all calcium concentrations present during stimulation of release. The phorbol ester enhancement of Ca2+-dependent release occurred whether or not calcium was present during pretreatment. These results indicate that stimulation of protein kinase C leads to an enhanced sensitivity of the stimulus-secretion coupling processes to calcium within the nerve terminal. The results support the possibility that presynaptic activation of protein kinase C modulates nerve terminal neurotransmitter release in the CNS.     

Effects of pneumogastric nerve stimulation and injection of a beta 2 stimulant (terbutaline) on the lipid composition of the alveolar lining fluid and on pulmonary compliance in the rat

The effects of vagal stimulation and of terbutaline injection on lipidic composition of alveolar fluid and pulmonary compliance were studied. Three groups of rats were used: control, after right vagus nerve stimulation, after 0.2 mg terbutaline injection. The lungs of the rats were isolated. We studied pulmonary pressure-volume curves with air and we measured pulmonary compliance. We realised an alveolar lavage to obtain alveolar lipids. We observed: Vagus nerve stimulation and beta 2 agonist significantly increased fatty acids of total lipids respectively by 52.5% and 25.5% and phospholipids, respectively by 43.6% and 25.7%. beta 2 agonist did not change fatty acid composition of total lipids and phospholipids. Right vagus nerve stimulation increased the percentage of palmitic acid in phospholipids and decreased the percentage of saturated fatty acids and of palmitic acid in total lipids. Terbutaline injection induced more significant changes in pressure-volume curves and in pulmonary compliance than right vagus nerve stimulation. Our results suggest that both vagal stimulation and beta 2 agonists increase lipid release in alveolar lining, but only vagal stimulation modifies the composition of these lipids. These modifications could be, at least in part, correlated with the changes observed in the pressure-volume curves.     

Adenosine modulation of hepatic arterial but not portal venous constriction induced by sympathetic nerves, norepinephrine, angiotensin, and vasopressin in the cat

Intrinsic regulation of hepatic arterial blood flow depends upon local concentrations of adenosine. The present data show that i.a. infusions of adenosine cause dilation of the hepatic artery and inhibition of arterial vasoconstriction induced by norepinephrine, vasopressin, angiotensin, and hepatic nerve stimulation. Vasoconstriction induced by submaximal nerve stimulation (2 Hz) and norepinephrine infusions (0.25 and 0.5 micrograms X kg-1 X min-1, i.p.v.) were equally inhibited by adenosine. Supramaximal nerve stimulation (8 Hz) was inhibited to a lesser extent. The data are consistent with the hypotheses that (a) adenosine causes nonselective inhibition of vasoconstrictor influences on the hepatic artery; and (b) adenosine antagonizes neurally induced vasoconstriction by a purely postsynaptic effect and does not decrease norepinephrine release. In contrast with the hepatic artery, the intrahepatic portal resistance vessels are not affected by even large doses of adenosine; neither responses in basal tone nor antagonism of vasoconstrictor effects of nerve stimulation, norepinephrine, or angiotensin could be demonstrated. The data are consistent with the hypothesis that the smooth muscle of the portal resistance vessels does not contain adenosine receptors, whereas adenosine receptors on the smooth muscle of the hepatic arterial resistance vessels are of major regulatory importance. Whether endogenous levels of adenosine can reach sufficient concentration to modulate endogenous constrictors remains to be determined.     

Vagal modulation of arginine- and glucagon-induced pancreatic somatostatin secretion

In order to elucidate the role of the vagus nerve in the regulation of pancreatic somatostatin secretion, the effect of electrical stimulation of the vagus on the isolated perfused rat pancreas was studied. Somatostatin release induced by 19 mM arginine in the presence of 11 mM glucose or 10(-6)M glucagon in the presence of 5.5 mM glucose was suppressed by vagal stimulation. This suppressive effect on somatostatin was eliminated in the presence of 10(-5)M atropine plus glucagon, while somatostatin release was significantly enhanced in the presence of atropine plus arginine. We conclude that pancreatic somatostatin secretion may be regulated not only by a cholinergic inhibitory neuron but also by a stimulatory non-cholinergic neuron.     

Vagal innervation of guinea pig bronchial smooth muscle

We isolated the guinea pig right bronchus with the vagus nerves intact and evaluated the changes in isometric tension of the smooth muscle in response to nerve stimulation. Brief (10-s) trains of electrical field stimulation or vagus nerve stimulation caused a biphasic contraction: the "first phase" sensitive to atropine and the "second phase" sensitive to capsaicin. The two phases could be dissociated by adjusting the stimulus intensity; greater stimulus intensities (pulse durations or voltage) were required to evoke the capsaicin-sensitive phase. When stimulated at 30-min intervals, the magnitude of both phases of the contractions declined over a 2-h period of repeated stimulation; however, this was prevented by indomethacin. Stimulation of the left vagus nerve resulted in a monophasic contraction of the right bronchus, with little evidence of a capsaicin-sensitive phase. Blocking neurotransmission through the bronchial ganglion, as monitored by intracellular recording techniques, abolished the first-phase contraction but had no effect on the capsaicin-sensitive phase. Selective blockade of muscarinic M1 receptors had no effect on vagus nerve-mediated contractions. The results demonstrate that the left and right vagus nerves carry preganglionic fibers to the right bronchial ganglion. The right but not the left vagus nerve also carries capsaicin-sensitive afferent fibers that, when stimulated, result in a persistent contraction of the right bronchus. Finally, we provide functional and electrophysiological evidence supporting the hypothesis that capsaicin-sensitive afferent neurons communicate with postganglionic motoneurons within the bronchus.     

Facilitatory role of NO in neural norepinephrine release in the rat kidney

We examined modulation by nitric oxide (NO) of sympathetic neurotransmitter release and vasoconstriction in the isolated pump-perfused rat kidney. Electrical renal nerve stimulation (RNS; 1 and 2 Hz) increased renal perfusion pressure and renal norepinephrine (NE) efflux. Nonselective NO synthase (NOS) inhibitors [N(omega)-nitro-L-arginine methyl ester (L-NAME) or N(omega)-nitro-L-arginine], but not a selective neuronal NO synthase inhibitor (7-nitroindazole sodium salt), suppressed the NE efflux response and enhanced the perfusion pressure response. Pretreatment with L-arginine prevented the effects of L-NAME on the RNS-induced responses. 2-(4-Carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (carboxy-PTIO), which eliminates NO by oxidizing it to NO(2), suppressed the NE efflux response, whereas the perfusion pressure response was less susceptible to carboxy-PTIO. 8-Bromoguanosine cGMP suppressed and a guanylate cyclase inhibitor [4H-8-bromo-1,2,4-oxadiazolo(3,4-d)benz(b)(1,4)oxazin-1-one] enhanced the RNS-induced perfusion pressure response, but neither of these drugs affected the NE efflux response. These results suggest that endogenous NO facilitates the NE release through cGMP-independent mechanisms, NO metabolites formed after NO(2) rather than NO itself counteract the vasoconstriction, and neuronal NOS does not contribute to these modulatory mechanisms in the sympathetic nervous system of the rat kidney.     

Potentiation of cholecystokinin-induced exocrine secretion by either electrical stimulation of the vagus nerve or exogenous VIP administration in the guinea pig pancreas

In the anaesthetized guinea pig, the secretory responses (pancreatic juice flow and protein output) induced by electrical stimulation of the vagus nerve were not blocked by atropine but by hexamethonium. Excitation of the left vagus nerve induced by electrical stimulation significantly potentiated the C-terminal octapeptide of cholecystokinin (CCK-OP)-induced secretory responses. In the isolated perfused pancreas of guinea pig, the secretory responses induced by CCK-OP at concentrations in the physiological range were markedly potentiated by simultaneous stimulation with vasoactive intestinal peptide (VIP). However, the secretory responses induced by CCK-OP at higher concentrations were not potentiated but inhibited by simultaneous stimulation of VIP. The secretory responses induced by carbachol (CCh) at any concentrations were not potentiated but inhibited by simultaneous stimulation of VIP. These results support the view that VIP released from the nerve endings can potentiate the hormonal action of CCK-OP.     

Effect of catecholamine on aldosterone release in isolated rat glomerulosa cell suspensions

Effect of adrenergic activity on the adrenal steroidogenesis and the modulation by catecholamines of aldosterone release were studied in isolated rat adrenal cell suspensions. Isoproterenol, norepinephrine and epinephrine, but not dopamine, caused statistically significant increase in aldosterone release. Both prazosin (alpha 1 antagonist) and yohimbine (alpha 2 antagonist) suppressed the norepinephrine-induced aldosterone release in a dose dependent manner, respectively. Both atenolol (beta 1 antagonist) and ICI 118-551 (beta 2 antagonist) also blocked (-)-isoproterenol-induced aldosterone release in a dose dependent manner, respectively. Neither (-)-isoproterenol nor (+/-)-norepinephrine at concentrations of 10(-6) M potentiated aldosterone release stimulated by angiotensin II or ACTH. These results suggest that catecholamines stimulate aldosteroidogenesis, but it appears unlikely that aldosterone release induced by ACTH or angiotensin-II is modulated by adrenergic stimulation.     

Monophasic and biphasic effects of angiotensin II and III on norepinephrine uptake and release in rat adrenal medulla.

Angiotensin II and III have hypertensive effects. They induce vascular smooth muscle constriction, increase sodium reabsorption by renal tubules, stimulate the anteroventral third ventricle area, increase vasopressin and aldosterone secretions, and modify catecholamine metabolism. In this work, angiotensin II and III effects on norepinephrine uptake and release in rat adrenal medulla were investigated. Both angiotensins decreased total and neuronal norepinephrine uptake. Angiotensin II showed a biphasic effect only on evoked neuronal norepinephrine release (an earlier decrease followed by a later increase), while increasing the spontaneous norepinephrine release only after 12 min. On the other hand, angiotensin III showed a biphasic effect on evoked and spontaneous neuronal norepinephrine release. Both angiotensins altered norepinephrine distribution into intracellular stores, concentrating the amine into the granular pool and decreasing the cytosolic store. The results suggest a physiological biphasic effect of angiotensin II as well as angiotensin III that may be involved in the modulation of sympathetic activity in the rat adrenal medulla.     

The effect of cold on adrenergic neurotransmission in canine saphenous arteries and veins

The effect of severe cold (5 to 10 degrees C) on adrenergic neurotransmission was compared in the isolated cutaneous (saphenous) artery and vein of the dog. The vein contracted to sympathetic nerve stimulation at temperatures as low as 10 degrees C; higher temperatures were needed for the artery to contract. Both blood vessels contracted to exogenous norepinephrine at temperatures as low as 5 degrees C. However, the contractile response to exogenous norepinephrine was less in the saphenous artery, and contractions to high K+ solution were depressed by cooling more in the artery than in the vein. During electrical stimulation of the sympathetic nerves in saphenous arteries and veins previously incubated with labeled norepinephrine, progressive cooling from 37 to 5 degrees C caused a sharp decline in overflow of [3H]norepinephrine and its metabolites. However, overflow of labeled norepinephrine in both blood vessels continued at very cold temperatures. Thus the inability of the saphenous artery to contract to sympathetic nerve stimulation at 10 degrees C can be explained by a greater sensitivity of the arterial smooth muscle to the direct depressant effect of cold, rather than to a differential release or metabolism or norepinephrine in the arterial wall or a loss of responsiveness to norepinephrine at very cold temperatures.