Plasma expansion effect on cardiac capillary and adrenergic exchange in intact dogs

The effect of plasma volume expansion on transcapillary exchange and norepinephrine release in the heart was examined in pentobarbital sodium-anesthetized dogs by use of the multiple indicator-dilution technique. Animals were studied under basal conditions and following infusion of the plasma expander, dextran. Catheters were placed in coronary artery and coronary sinus in a closed-chest preparation. Labeled albumin, sucrose, and norepinephrine were injected into the coronary artery and outflow-dilution curves were secured. Analysis of these provided parameters reflecting coronary flow and permeability-surface product, and a norepinephrine tracer kinetic-bulk model provided simultaneous estimates of the rate of norepinephrine release into the myocardial interstitial space. The infusion of dextran resulted in a large increase in coronary flow without significant changes in myocardial norepinephrine release; at the same time the permeability-surface product values increased, amplifying the capacity of the higher flow to deliver substrates to sarcolemmal cells. The findings indicate that plasma volume expansion increases transcapillary exchange in the heart without activating the cardiac sympathetic system.     

Regulation of chemoreceptor sensitivity in the carotid body: the role of presynaptic sensory nerves

Several neural and vascular mechanisms regulate the sensitivity of carotid body chemoreceptors to hypoxia, hypercapnia, and acidosis. Factors that control blood flow and oxygen delivery in the carotid body along with those that augment or diminish catecholamine release from glomus cells can have major effects on chemoreceptor function. In addition, the sensory nerves themselves may participate in the regulation of chemoreceptor sensitivity. A portion of the carotid body's sensory nerves are presynaptic to glomus cells. In response to stimulation, the sensory nerve terminals exhibit ultrastructural changes that resemble changes associated with increased release of transmitter from motor nerves: 1) the number of small (synaptic) vesicles decreases; and 2) coated vesicles and coated regions of cisternal membrane increase in number during stimulation. If sensory nerves of the carotid body release a neurotransmitters, sensory nerve activity could influence glomus cell secretion of catecholamines or other substances tha modify chemoreceptor sensitivity. Such an effect could be produced in the carotid body by hypoxia and other conditions that stimulate the sensory nerves or it could result from antidromic activity evoked in the sensory nerves by primary afferent depolarization of their terminals in the CNS.     

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.     

Quantitative analysis of feedforward sympathetic coronary vasodilation in exercising dogs.

Recent experiments demonstrate that feedforward sympathetic beta-adrenoceptor coronary vasodilation occurs during exercise. The present study quantitatively examined the contributions of epinephrine and norepinephrine to exercise coronary hyperemia and tested the hypothesis that circulating epinephrine causes feedforward beta-receptor-mediated coronary dilation. Dogs (n = 10) were chronically instrumented with a circumflex coronary artery flow transducer and catheters in the aorta and coronary sinus. During strenuous treadmill exercise, myocardial oxygen consumption increased by approximately 3.9-fold, coronary blood flow increased by approximately 3.6-fold, and arterial plasma epinephrine concentration increased by approximately 2.4-fold over resting levels. At arterial concentrations matching those during strenuous exercise, epinephrine infused at rest (n = 6) produced modest increases (18%) in flow and myocardial oxygen consumption but no evidence of direct beta-adrenoceptor-mediated coronary vasodilation. Arterial norepinephrine concentration increased by approximately 5. 4-fold during exercise, and coronary venous norepinephrine was always higher than arterial, indicating norepinephrine release from cardiac sympathetic nerves. With the use of a mathematical model of cardiac capillary norepinephrine transport, these norepinephrine concentrations predict an average interstitial norepinephrine concentration of approximately 12 nM during strenuous exercise. Published dose-response data indicate that this norepinephrine concentration increases isolated coronary arteriolar conductance by approximately 67%, which can account for approximately 25% of the increase in flow observed during exercise. It is concluded that a significant portion of coronary exercise hyperemia ( approximately 25%) can be accounted for by direct feedforward beta-adrenoceptor coronary vascular effects of norepinephrine, with little effect from circulating epinephrine.     

Opioid Peptides in the Rabbit Carotid Body: Identification and Evidence for Co-Utilization and Interactions with Dopamine

Abstract: The rabbit carotid body is a catecholaminergic organ that contains dopamine and norepinephrine in a proportion of nearly 5:1. Chronic (15 days) carotid sinus nerve denervation or superior cervical ganglionectomy did not modify the carotid body dopamine content (5–6 nmol/mg of protein, equivalent to 250 pmol per carotid body), but sympathectomy reduced by ～ 50% the norepinephrine content. The carotid body has also a very high content of opioid activity (250 equivalent pmol of Leu-enkephalin/mg of protein) as measured by a radioreceptor assay that detects preferentially δ-opioid activity. In the carotid body the degree of opioid posttranslational processing to low-molecular-weight peptides (mostly Leu- and Met-enkephalin) is nearly 80%. HPLC identification of opioid peptides revealed that the sequences of Met- and Leu-enkephalin were in a proportion of nearly 6:1, indicating that the main opioid precursor in the carotid body is proenkephalin A. Chronic denervations of the carotid body did not modify the levels or the degree of opioid precursor processing. Acute hypoxic exposure of the animals (8% 0₂ in N₂; 3 h) resulted in a parallel decrease of dopamine and opioid activity, without any change in the degree of opioid processing. Norepinephrine levels were not affected by hypoxia. These findings suggest corelease of dopamine and opioids during natural hypoxic stimulation. In agreement with the analytical data. [d -Ala², d -Leu⁵]enkephalin, but not [d -Ala²,N-Me-Phe⁴ Gly⁵-ol]-enkephalin, reduced the in vitro release of dopamine induced by low Po₂, a high external K⁺ concentration, and dinitrophenol. Naloxone augmented the release response elicited by low Po₂ stimulation. These findings indicate that the previously described inhibitory actions of opioids are mediated, at least in part, by receptors located in chemoreceptor cells. Additional targets for opioid peptides, e.g., sensory nerve endings or blood vessels, and additional actions of opioids on chemoreceptor cells, e.g., long-term trophic actions, are not excluded.     

Chronic CO inhalation and carotid body catecholamines: testing of hypotheses

The purpose of this study was twofold: one concerns carotid blood flow and tissue PO2 and the other the effect of chronic hypoxic hypoxia on enhanced catecholamine content. The rationale was that chronic CO inhalation would not mimic the effect of hypoxia on the carotid body if its tissue blood flow is sufficiently high to counteract the effect of CO on O2 delivery and, hence, on tissue PO2. The differential effects of CO on the carotid body and erythropoietin-producing tissue would also indicate that the effect of hypoxic hypoxia on the carotid body is the result of a direct action of a local low O2 stimulus rather than secondary to a systemic effect initiated by other O2-sensing tissues. To test these alternatives we studied the effects of chronic CO inhalation on carotid body catecholamine content and hematocrit in the rats, which were exposed to an inspired PCO of 0.4-0.5 Torr at an inspired PO2 of approximately 150 Torr for 22 days. The hematocrit of CO-exposed rats was 75 +/- 1.1% compared with 48 +/- 0.7% in controls. Dopamine and norepinephrine content of the carotid bodies (per pair) was 5.88 +/- 0.91 and 3.02 +/- 0.19 ng, respectively, in the CO-exposed rats compared with 6.20 +/- 1.0 and 3.29 +/- 0.6 ng, respectively, in the controls. Protein content of the carotid bodies (per pair) was 18.4 +/- 1.6 and 20.5 +/- 0.9 micrograms, respectively. Thus, despite a vigorous erythropoietic response, the CO-exposed rats failed to show any significant stimulation of carotid body in terms of the content of either catecholamine or protein. The results suggest that carotid body tissue PO2 is not compromised by moderate carboxyhemoglobinemia because of its high tissue blood flow and that the chronic effect of hypoxic hypoxia on carotid body is direct.     

Prostaglandins modulate baroreflex-induced changes in blood pressure and myocardial function in anesthetized dogs

The purpose of our study was to investigate the role of prostaglandins in the changes in myocardial function and peripheral and coronary vascular resistance which accompany a generalized increase in sympathetic tone caused by carotid baroreflex unloading in the anesthetized dog. Bilateral carotid artery occlusion (BCO) with heart rate held constant by electrical pacing (150 beats/min) resulted in increases in systolic, (33%) diastolic (40%), and mean (35%) arterial pressures, LV systolic pressure (33%) and left ventricular (LV) dP/dt (37%). After blockade of prostaglandin synthesis with indomethacin (N = 11) or meclofenamate (N = 6) the increases in systolic (41%), diastolic (45%), and mean (41%) arterial pressures, LV systolic pressure (39%), LV dP/dt (52%), and cardiac work caused by BCO were significantly greater, in spite of the initially higher baseline values (11-18%) following the administration of the drugs. In contrast, the changes in circumflex coronary blood flow and coronary vascular resistance to BCO were essentially the same before and after inhibition of prostaglandin synthesis. Systemic prostaglandin synthesis may, therefore, play a significant role in the control of systemic arterial pressure and myocardial function, most probably by modulating the release of norepinephrine from adrenergic nerve terminals, without adversely affecting coronary blood flow regulation.     

Sympathetic nervous system effects on rat brain metabolism.

Brain tissue lactate and pyruvate were measured in rats under normoxic and normocapnic conditions. A significant increase in brain lactate was observed following (i) 15 minutes of unilateral carotid ligation, (ii) 30 minutes of norepinephrine infusion and (iii) 30 minutes of electrical stimulation of the superior cervical ganglion. Lactate values were not significantly altered after (i) a 30 minute epinephrine infusion, and (ii) a partial chemical sympathectomy obtained through an injection of 6-Hydroxydopamine. No alterations in brain tissue pyruvate concentration were obtained. These findings suggest that sympathetic stimulation causing the release of norepinephrine in the cerebral vessels results in increased anaerobic metabolism.     

The ratio of systemic and coronary fractions of left ventricular output in reflex pressor reactions

In acute experiments the participation of coronary and systemic fractions was studied during suppression of the carotid sinus baroreceptors by the occlusion of the carotid arteries and stimulation of the tibial nerve afferent fibers. In most tests systemic fraction was reduced in carotid arteries occlusion and increased in tibial nerve stimulation. The coronary fraction was always increased. The cardiac output (the sum of systemic and coronary fractions) was steady with blood pressure increase by 20 to 70%. The role of coronary fraction in the mechanism of homeometric regulation of the heart is discussed.     

Contribution of adenosine to changes in coronary flow in metabolically stimulated rat heart

The extent to which endogenous, extracellular adenosine mediates increased coronary flow in crystalloid-perfused, isovolumic rat hearts stimulated with either norepinephrine or isoproterenol was examined. When infused into the coronary circulation, norepinephrine (1 x 10(-7) M) rapidly increased left ventricular developed pressure (LVDP) from 81 +/- 6 to 235 +/- 13 mmHg (1 mmHg = 133.3 Pa) and coronary flow from 12.7 +/- 0.8 to 18.4 +/- 0.7 mL.min-1.g-1. The presence of either adenosine deaminase (2 U.mL-1) or the adenosine receptor antagonist, 8-phenyltheophylline (5 x 10(-6) M) in the perfusate of norepinephrine-stimulated hearts augmented the increase in LVDP and +/- dP/dtmax by 10-20% but reduced the increase in coronary flow by 34%. Doubling the rate of adenosine deaminase infusion, or infusing the enzyme and 8-phenyltheophylline together did not alter their inhibitory effectiveness. Similar results were observed with hearts stimulated with isoproterenol (5 x 10(-8) M). These data show that about a third of the vasodilation that results from the metabolic stimulation of rat heart by catecholamines is due to the receptor-mediated action of extracellular adenosine.     

Minireview. Presynaptic receptors and alterations in norepinephrine release in spontaneously hypertensive rats

The ability of blood vessels to constrict to a given stimulus is significantly increased in spontaneously hypertensive rats (SHR). Such an increase in the vasoconstrictor responsiveness contributes to the elevated peripheral vascular resistance noted in SHR. The present review discusses evidence in support of the concept that an increased release of norepinephrine during sympathetic nerve stimulation may contribute to the increase in vasoconstrictor responsiveness and, subsequently, to an increase in vascular resistance in the SHR. Several studies suggest that the exocytotic release of norepinephrine from sympathetic nerves may be altered by endogenously occurring neurohumoral substances which produce their effects by interacting with presynaptic receptors located on postganglionic sympathetic nerves. Therefore, it is postulated that alterations in presynaptic regulation of norepinephrine release, resulting from changes in the functioning of one or more of these presynaptic receptors, may lead to a greater release of norepinephrine in the SHR. This review summarizes the results of studies evaluating presynaptic receptor mechanisms and norepinephrine release in the SHR. These studies suggest that norepinephrine release during sympathetic nerve stimulation is greater in the SHR and that alterations in some of the presynaptic receptor mechanisms may be responsible for this phenomenon.     

Parasympathetic control of coronary blood flow

Active parasympathetic coronary vasodilation in excess of any changes in myocardial metabolism has been observed in a number of circumstances. Electrical stimulation of the cardiac end of the cut vagus nerve produces a cholinergic coronary vasodilation that is blocked by atropine. Activation of carotid body chemoreceptors, carotid sinus baroreceptors, or left ventricular receptors elicits reflex parasympathetic coronary vasodilation. The coronary vasodilation produced by these reflexes can be prevented by vagotomy or atropine. The relative importance of parasympathetic coronary control in relation to sympathetic and local metabolic coronary control awaits further research.     

Flow stimulates nitric oxide release in guinea pig heart: role of stretch-activated ion channels.

Blood flow regulates vessel tone triggering the release of nitric oxide; however, the mechanism involved in this phenomenon is unknown. We investigated whether coronary flow induces nitric oxide release in the isolated perfused guinea pig heart and the role of the stretch-activated ion channels in the effect of flow. We used gadolinium (3 microM) in order to block these channels, and estimated nitric oxide release by an oxyhemoglobin method. The results have shown a flow-dependent stimulation of nitric oxide release (fivefold increase at perfusion flow of 25 ml/min). Gadolinium inhibited this effect in a dose-dependent fashion. Acetylcholine was able to stimulate nitric oxide release in presence of gadolinium. We concluded that coronary flow stimulates nitric oxide release in the guinea pig heart. Stretch-activated ion channels mediate this effect. Acetylcholine and flow stimulate nitric oxide release by different mechanisms of action.     

The alpha adrenoceptors on endothelial cells

Endothelial cells release a powerful factor (endothelium-derived relaxing factor [EDRF]) that relaxes smooth muscle cells in response to some vasodilating agents such as acetylcholine. Contraction curves to norepinephrine (NE) in greyhound, mongrel dog, and pig coronary artery rings were studied in vitro in the presence of propranolol. Removal of endothelium increased the sensitivity and maximum contraction in response to NE. In other experiments pig coronary rings were precontracted with a thromboxane mimetic U 46619 in the presence of propranolol. NE relaxed these arteries only if endothelium was present. Methoxamine was without effect but the relaxation response to NE was antagonized by phentolamine, idazoxan, and yohimbine, which suggests that there are alpha 2 adrenoceptors on endothelial cells that mediate the release of EDRF. Greyhound and mongrel dog large coronary arteries relaxed to NE only if prazosin was present, which suggests that alpha 1-adrenoceptor stimulation on the vascular smooth muscle can override the relaxation response to EDRF. Comparison of NE responses in carotid, mesenteric, renal, and femoral large arteries of the pig, greyhound, and mongrel dog indicate the nonuniformity of distribution of alpha 2 adrenoceptors on endothelium and alpha 1 and alpha 2 adrenoceptors on vascular smooth muscle. The integrity of the endothelium must now be considered in interpreting the vascular responses to alpha-adrenoceptor agonists.     

Evidence for alpha-1 adrenergic receptor regulation of atriopeptin release from the isolated rat heart

Atrial myocardium is the source of a recently described peptide hormone termed atriopeptin. Atriopeptin is thought to have a role in the regulation of systemic arterial pressure, fluid balance and plasma electrolyte homeostasis. Isolated rat hearts release atriopeptin into the coronary effluent, and we have found that this release is stimulated by the administration of norepinephrine, a compound with alpha and beta adrenergic properties. Infusion of the pure beta-receptor agonist, isoproterenol, failed to stimulate the release; however, the alpha-1 receptor agonist phenylephrine induced the release in a dose-dependent manner. The stimulation of atriopeptin release by norepinephrine and phenylephrine was inhibited by alpha-blockade with phentolamine. Administration of BHT-920, a selective alpha-2 agonist, had no effect on atriopeptin release. We conclude that atriopeptin secretion by the atrial myocyte is stimulated by activation of the alpha-1 adrenergic receptor. This finding suggests an involvement of the sympathetic nervous system in the physiologic regulation of the secretion of this hormone.     

Acetylcholine release from the carotid body by hypoxia: evidence for the involvement of autoinhibitory receptors.

The purpose of the present study was to investigate whether hypoxia influences acetylcholine (ACh) release from the rabbit carotid body and, if so, to determine the mechanism(s) associated with this response. ACh is expressed in the rabbit carotid body (5.6 +/- 1.3 pmol/carotid body) as evidenced by electrochemical analysis. Immunocytochemical analysis of the primary cultures of the carotid body with antibody specific to ACh further showed that ACh-like immunoreactivity is localized to many glomus cells. The effect of hypoxia on ACh release was examined in ex vivo carotid bodies harvested from anesthetized rabbits. The basal release of ACh during normoxia ( approximately 150 Torr) averaged 5.9 +/- 0.5 fmol.min-1.carotid body-1. Lowering the Po2 to 90 and 20 Torr progressively decreased ACh release by approximately 15 and approximately 68%, respectively. ACh release returned to the basal value on reoxygenation. Simultaneous monitoring of dopamine showed a sixfold increase in dopamine release during hypoxia. Hypercapnia (21% O2 + 10% CO2) as well as high K+ (100 mM) facilitated ACh release from the carotid body, suggesting that hypoxia-induced inhibition of ACh release is not due to deterioration of the carotid body. Hypoxia had no significant effect on acetylcholinesterase activity in the medium, implying that increased hydrolysis of ACh does not account for hypoxia-induced inhibition of ACh release. In the presence of either atropine (10 microM) or domperidone (10 microM), hypoxia stimulated ACh release. These results demonstrate that glomus cells of the rabbit carotid body express ACh and that hypoxia overall inhibits ACh release via activation of muscarinic and dopaminergic autoinhibitory receptors in the carotid body.     

Potential usefulness of renal vasodilators in hypertension and renal disease: SK&F 82526

SK&F 82526 and its enantiomers have been shown to increase renal blood flow and decrease renal vascular resistance in the anesthetized dog. The effect of the racemate on lowering systemic blood pressure in the anesthetized dog and the spontaneously hypertensive rat has been shown to be caused by the R-enantiomer with the S-enantiomer being devoid of significant activity on blood pressure. The mechanism by which the R-enantiomer decreases blood pressure is not systemic vasodilatation or prejunctional inhibition of norepinephrine release but appears to result from a unique stimulation of the postjunctional dopamine receptor. Racemic SK&F 82526 also has been shown to increase renal blood flow in an ischemic model of acute renal failure.     

Guanine nucleotide stimulation of norepinephrine secretion from permeabilized PC12 cells: effects of Mg2+, other nucleotide triphosphates and N-ethylmaleimide.

The addition of either Ca2+ or guanosine 5'-O-3-(thiotriphosphate), GTP gamma S, to digitonin-permeabilized rat pheochromocytoma PC12 cells stimulates norepinephrine release. Unlike Ca(2+)-stimulated release, there is a delay between the time of addition of GTP gamma S to digitonin-permeabilized PC12 cells and stimulation of norepinephrine release. Preincubation of the permeabilized cells in the absence of Mg2+ eliminates this lag and increases the initial rate of GTP-gamma S-stimulated norepinephrine secretion. This suggests that the rate of GDP dissociation from the GTP-binding protein responsible for this stimulation is faster in the absence of Mg2+ than in its presence. While an equimolar concentration of GTP gives 50% inhibition of GTP gamma S-stimulated release, 100-fold excesses of ITP, ATP, UTP and CTP gave no inhibition of GTP gamma S-stimulated release. Both the inability of ITP to inhibit GTP gamma S-stimulated secretion and the increase in GTP gamma S-stimulated secretion caused by preincubation in the absence of Mg2+ indicate that some of the properties of the GTP-binding protein responsible for this stimulation are more like those of the low molecular weight GTP-binding proteins rap1 and ras than those of a heterotrimeric G-protein. Low concentrations of N-ethylmaleimide gave more inhibition of GTP gamma S-stimulated release than Ca(2+)-stimulated release which suggests that the mechanisms by which Ca2+ and GTP gamma S stimulate norepinephrine release are at least in part distinct.     

Ca2+ Mobilized by Caffeine from the Inositol 1,4,5-Trisphosphate-Insensitive Pool of Ca2+ in Somatic Regions of Sympathetic Neurons Does Not Evoke [3H]Norepinephrine Release

The effects of electrical stimulation, muscarinic and serotonergic agonists, and caffeine on [3H]inositol 1,4,5-trisphosphate ([3H]Ins(1,4,5)P3) content, intracellular free Ca2+ concentration ([Ca2+]i), and release of [3H]norepinephrine ([3H]NE) were studied in cultured sympathetic neurons. Neuronal cell body [Ca2+]i was unaffected by muscarinic or serotonergic receptor stimulation, which significantly increased [3H]Ins(1,4,5)P3 content. Stimulation at 2 Hz and caffeine had no effect on [3H]Ins(1,4,5)P3, but caused greater than two-fold increase in [Ca2+]i. Only 2-Hz stimulation released [3H]NE. Caffeine had no effect on the release. When [Ca2+]i was measured in growth cones, only electrical stimulation produced an increase in [Ca2+]i. The other agents had no effect on Ca2+ at the terminal regions of the neurons. We conclude that Ins(1,4,5)P3-insensitive, but caffeine-sensitive Ca2+ stores in sympathetic neurons are located only in the cell body and are not coupled to [3H]NE release.     

Availability of glucose given orally during exercise

The present study was designed to determine whether daily exercise alters adrenergic and muscarinic neural control of coronary blood flow during resting and exercising conditions in the conscious dog. Mean left circumflex artery blood flow (CBF), mean coronary blood pressure, and heart rate were measured during resting conditions (55 +/- 9 ml/min, 108 +/- 6 mmHg, and 93 +/- 2 beats/min, respectively) and during submaximal exercise (85 +/- 9 ml/min, 108 +/- 7 mmHg, and 210 +/- 15 beats/min). Injection of phentolamine into the left circumflex coronary artery during treadmill exercise resulted in a 10 +/- 1% increase in CBF before training (untrained, UT) and a 21 +/- 6% increase after 4-5 wk of daily exercise (partially trained, PT) (P less than 0.02 UT vs. PT). Intracoronary atenolol or propranolol caused a 15 +/- 6% reduction in CBF during exercise in dogs before and after PT. While the dogs were lying quietly at rest intracoronary injections of norepinephrine initially increased CBF 85%, followed by a prolonged 19 +/- 9% decrease in CBF. CBF decreased 16 +/- 3% after intracoronary injection of phenylephrine. After PT the coronary vasoconstriction following norepinephrine and phenylephrine injections was significantly potentiated (31 +/- 6 and 35 +/- 4%, respectively). These data suggest that exercise training caused significant changes in the coronary vascular response to alpha-receptor stimulation so that an alteration in the neural control of the coronary circulation occurred.