Presynaptic receptor systems on cardiac sympathetic nerves

Effect of Clonidine on Cardiac Norepinephrine Spillover in Isolated Rat Heart

The purpose of this study is to determine the effect of clonidine on cardiac norepinephrine spillover utilizing an isolated rat heart preparation with attached cardiac sympathetic nerves. Following a 20-minute stabilization period, the sympathetic ganglion for each heart preparation was electrically stimulated with 10V and 2 Hz for 30 seconds (S1: 60 pulses). Heart rate, left ventricular developed pressure, and coronary perfusion pressure was allowed to return to baseline and the perfusate was randomly switched to Krebs buffer containing one of two treatments: placebo or clonidine (1 microM). After 10 minutes of treatment, the sympathetic ganglion was again electrically stimulated with 10V and 2 Hz for 30 seconds (S2: 60 pulses). The perfusate exiting the heart before, during, and after each electrical stimulation was collected for the determination of cardiac norepinephrine spillover. Clonidine administration significantly reduced cardiac norepinephrine spillover by approximately 50% (P < 0.05) and was associated with a 36% reduction in heart rate (P < 0.05). These findings provide evidence that clonidine can directly suppress NE spillover from cardiac sympathetic nerve terminals. Thus, suppression of cardiac NE by clonidine may be due to stimulation of presynaptic alpha2-adrenergic receptors or imidazoline subtype I receptors located on cardiac sympathetic nerve terminals. Results from our study demonstrate a reduction in cardiac NE spillover by clonidine and provide additional evidence that it can directly suppress peripheral sympathetic activity in that our results were obtained utilizing an isolated perfused heart preparation with attached cardiac sympathetic nerves devoid of any CNS input.

Presynaptic modulation of evoked NE release contributes to sympathetic activation after pressure overload

Activation of the sympathetic nervous system is well documented in heart failure. Our previous studies demonstrated an increase in evoked norepinephrine (NE) release from left ventricle (LV) slices at 10 days of pressure overload. The purpose of this study was to test the hypothesis that presynaptic modulation of NE release contributes to sympathetic activation after pressure overload. We examined the functional status of the presynaptic α2- and β2-receptors and ANG II subtype 1 (AT1) receptors in LV slices from 10-day aortic constricted (AC) and sham-operated (SO) rats. Evoked 3H overflow from LV slices preloaded with [3H]NE was increased in AC rats. The α2-agonist UK-14,304 decreased evoked 3H overflow with no differences between groups. The β2-agonist salbutamol increased evoked 3H overflow with greater sensitivity in slices from AC rats. The β-antagonist propranolol decreased evoked 3H overflow from LV slices of AC rats but not controls. ANG II increased evoked 3H overflow with greater sensitivity in slices from AC rats. These data support the hypothesis that aberrant presynaptic modulation of catecholamine release contributes to sympathetic activation after pressure overload.

Facilitation of noradrenaline release by isoprenaline in rat isolated atria does not involve angiotensin II formation

1. Rat isolated atria were incubated with 3H-noradrenaline and the intramural sympathetic nerves were stimulated at 2 Hz for 60 s. The stimulation-induced (SI) efflux of radioactivity was used as an index of release of transmitter noradrenaline. 2. Isoprenaline (0.1 mumol/L) alone did not increase noradrenaline release. Cocaine (30 mumol/L) produced a 73% increase in the stimulation-induced release of noradrenaline. In the presence of cocaine, isoprenaline enhanced noradrenaline release by 22%. 3. In the presence of cocaine, both angiotensin I (0.3 mumol/L) and angiotensin II (0.3 mumol/L) produced almost two-fold enhancements in the SI release of noradrenaline. 4. Captopril (5 mumol/L) blocked the facilitatory effect of angiotensin I on noradrenaline release but did not alter that of isoprenaline. 5. Saralasin (0.1 mumol/L) reduced the facilitatory effect of angiotensin II on noradrenaline release but did not alter that of isoprenaline. 6. The findings indicate that the facilitation of noradrenaline release by isoprenaline in rat atria is not mediated by local formation of angiotensin II.

Development of cardiac sympathetic and adrenal-medullary responses in borderline hypertensive rats

Borderline hypertensive (BHR) rats are the first generation offspring of a cross of spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) normotensive rats. In adulthood, BHRs have systolic blood pressures in the 140-160 mm Hg range. If subjected to chronic stress paradigms, however, BHRs develop sustained and permanent elevations in systolic blood pressure (180-200 mm Hg). In the present study, we examined the functional development of cardiac and adrenal medullary responses to reflex activation of the sympathetic nervous system in preweanling BHR and WKY rats. Pups of the two groups were injected with insulin or saline at 4, 8, 12, or 16 days of age and sacrificed 3 h later. Insulin produces an acute lowering of blood glucose which is attended by a centrally mediated increase in sympathetic activity. The induction of ornithine decarboxylase (ODC) activity in heart and the depletion of epinephrine from the adrenal medulla were biochemical indicators of functional sympathetic neurotransmission. WKY and BHR pups had similar levels of cardiac ODC activity under basal conditions and following administration of insulin. In contrast, BHRs had higher amounts of adrenal norepinephrine and epinephrine from 4 to 16 days of age and greater depletion of adrenal epinephrine following insulin administration at 8, 12 and 16 days of age. These findings indicate that BHRs have a greater capacity for catecholamine biosynthesis, storage and release in the adrenal medulla during the preweanling period compared to age-matched normotensive WKY controls. This alteration in the adrenal medulla during the preweanling period may contribute to the susceptibility of adult BHR rats to stress-induced hypertension.

Distribution of muscarinic receptors and acetylcholinesterase in the rat heart

Experiments were performed to determine the degree of overlap in the distribution of muscarinic receptors and cholinergic innervation of the rat heart. Localization of muscarinic receptors was determined by autoradiography with [3H]quinuclidinyl benzilate. Adjacent sections were stained for acetylcholinesterase to determine innervation. The distribution of muscarinic receptors and cholinergic innervation overlapped in cardiac parasympathetic ganglia, nodal tissue, His bundle-Purkinje system, vena cava and pulmonary veins. Cholinergic innervation to the right atrium was greater than to the left atrium while muscarinic receptor density was equal in the two atria. Innervation of the ventricles was confined primarily to the base of the right ventricle. A low density of muscarinic receptors was observed throughout the ventricles. Neither cholinergic innervation nor muscarinic receptors were detected in the pulmonary trunk, ascending aorta or cardiac valves. Muscarinic receptors and cholinergic innervation in the nodal regions, ventricular conduction system and myocardium probably mediate negative chronotropic, dromotropic and inotropic effects of vagal nerve stimulation. Muscarinic receptors at sites not containing cholinergic innervation may be associated with noradrenergic nerves of the myocardium.

Interaction between presynaptic facilitatory angiotensin II receptors and inhibitory muscarinic cholinoceptors on 3H-noradrenaline release in the rabbit heart

The existence of a functional interaction between presynaptic receptors modulating the release of noradrenaline was studied in the rabbit heart. Isolated right atria were prelabelled with 3H-noradrenaline and the overflow of tritium was induced by field stimulation (2 Hz, 0.1 ms duration, supramaximal voltage for a total of 180 pulses). In atria superfused with Krebs' solution containing 10 mumol/l cocaine and 30 mumol/l corticosterone, angiotensin II (10 nmol/l) increased the stimulation-evoked overflow of 3H-transmitter by 2.8-fold. The addition of atropine (0.3 mumol/l) to the perfusion medium, either in the presence or in the absence of uptake inhibitors, further enhanced the facilitatory effect of angiotensin II (3H-transmitter release increased by 3.5-fold). Exposure to 1 mumol/l carbachol decreased by 65% the stimulation-evoked release of 3H-transmitter while the facilitatory effect of angiotensin II determined in the presence of the muscarinic cholinoceptor agonist was enhanced (3H-transmitter release increased by 6.6-fold). Conversely, during sustained activation of presynaptic angiotensin receptors producing a 2.5-fold increase in the release of 3H-transmitter, the inhibitory effect of carbachol remained unchanged. These results suggest a functional interaction between presynaptic inhibitory muscarinic cholinoceptors and the presynaptic facilitatory angiotensin receptor which modulate the release of noradrenaline from cardiac noradrenergic nerves.

Effects of locally generated angiotensin II on noradrenergic transmission in guinea-pig isolated atria

The extent to which cardiac neuroeffector function may be modulated by angiotensin II (AII) generated locally from angiotensin I (AI) was investigated in guinea-pig spontaneously beating isolated atria radiolabelled with [3H]noradrenaline. AI and AII were equipotent in increasing the rate and force of atrial contractions with a threshold concentration between 0.1 and 1.0 nM. In contrast, AI was approximately twenty times less potent than AII in enhancing the stimulation-induced (S-I) efflux of radioactivity. The actions of AI and AII on rate and force of atrial contractions, as well as those on S-I efflux of radioactivity, were substantially blocked by the AII receptor antagonist saralasin. On the other hand, the converting enzyme inhibitors captopril and MK-422 selectively blocked the actions of AI without affecting those of AII. These results suggest that significant local conversion of AI to AII occurs in guinea-pig isolated atria. This locally generated AII can act on myocardial AII receptors to increase rate and force of atrial contractions, and on AII receptors located on sympathetic nerve terminals to facilitate sympathetic neurotransmission. The lower potency of AI compared to AII in modulating transmitter release, in contrast to the equal potency of the peptides in mediating chronotropic and inotropic responses, may be due to conversion of AI to AII at sites remote from the neuroeffector junction.

Dopamine receptors in the guinea-pig heart. A binding study

The binding of dopaminergic agonists and antagonists to guinea-pig myocardial membrane preparations was studied using 3H-dopamine and 3H-spiperone as radioligand. 3H-Dopamine bound specifically to heart membranes while 3H-spiperone did not. A Scatchard analysis of 3H-dopamine binding showed a curvilinear plot indicating the presence of two dopamine receptor populations that we have termed high- (Kd = 1.2 nM, Bmx = 52.9 fmol/mg prot.) and low- (Kd = 11.8 nM, Bmx = 267.3 fmol/mg prot.) affinity binding sites, respectively. The characterisation of the high-affinity component of 3H-dopamine binding indicated that the binding is rapid, saturable, stereospecific, pH- and temperature-dependent, and displaced by dopaminergic agonists and antagonists known to act similarly in vivo. The finding that pretreatment with dibenamine (which has been described as an alpha-adrenoceptor irreversible blocker) did not affect the binding of dopamine to cardiac membrane preparations suggests that alpha-adrenoceptors and dopamine receptors have separate recognition sites in the heart. We conclude that 3H-dopamine binds to specific dopamine receptors in the heart of guinea-pigs.

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.

Effect of epinephrine on norepinephrine release from rat kidney during sympathetic nerve stimulation

Experiments were performed to study presynaptic beta-adrenoceptor facilitation of sympathetic neurotransmitter release in the isolated perfused rat kidney and evaluate the effect of epinephrine on norepinephrine release during sympathetic nerve stimulation. The right kidney was isolated and perfused with Krebs-Ringer solution. Norepinephrine storage sites were labelled with [3H]norepinephrine. Increasing concentrations of isoproterenol and salbutamol when perfused through the kidney, caused an enhancement of the stimulus-induced release of [3H]norepinephrine at 0.5 and 2 Hz, with the maximum facilitatory effect being observed at 0.5 Hz. The effect of salbutamol on [3H]norepinephrine release was concentration-dependent and more pronounced than that of isoproterenol. While propranolol (10(-9)-10(-5)M) by itself did not cause any significant changes in the stimulus-induced release of [3H]norepinephrine, it antagonized the facilitatory action of salbutamol on [3H]norepinephrine release during periarterial nerve stimulation. When epinephrine (10(-10)-10(-7)M), was perfused through the kidney in the presence of cocaine, it caused a concentration-dependent inhibition of the stimulus-induced release of [3H]norepinephrine release elicited during periarterial nerve stimulation. However, when epinephrine was perfused in the presence of cocaine, phentolamine and corticosterone it caused a slight but significant increase in the stimulus-induced release of [3H]norepinephrine; the highest concentration (10(-7)M) still caused a decrease in the [3H]norepinephrine release. These results, while providing evidence for the existence of presynaptic facilitatory beta-adrenoceptors on renal sympathetic nerves, fail to support the hypothesis that these receptors have a physiological role in the regulation of sympathetic neurotransmitter release.

Antagonist properties of phentolamine at both presynaptic alpha 2-adrenoceptors and presynaptic dopamine receptors using field stimulated right cat atria

Phentolamine, which is considered a specific alpha-adrenoceptor antagonist, was tested for antagonist properties at presynaptic dopamine receptors and presynaptic alpha 2-adrenoceptors present on sympathetic nerve terminals in isolated right cat atria. Field stimulation induced a transient tachycardia which was inhibited by stimulation of presynaptic dopamine receptors using apomorphine or by stimulation of presynaptic alpha 2-adrenoceptors using clonidine. The presynaptic inhibitory effects of apomorphine were competitively antagonized by sulpiride, which is considered a specific dopamine receptor antagonist, and by phentolamine and yohimbine which are considered alpha-adrenoceptor antagonists. The presynaptic inhibitory effects of clonidine were competitively antagonized by phentolamine and yohimbine but not by sulpiride. pA2 values for phentolamine against apomorphine and phentolamine against clonidine suggest that phentolamine may be an antagonist at both presynaptic dopamine receptors and presynaptic alpha 2-adrenoceptors.

Inhibition of cardiac sympathetic neurotransmission by histamine in the dog is mediated by H1-receptors

1 The role of histamine H1- and H2-receptors in mediating prejunctional inhibition of cardiac sympathetic neurotransmission and histamine-induced coronary vasodilatation were investigated in perfused dog hearts in situ. 2 Intra-arterial injections of histamine into the right coronary artery during the resting state caused slightly positive chronotropic responses in doses larger than 1 microgram. 3 Histamine in doses of 0.1 to 10 micrograms into the right coronary artery reduced the tachycardia resulting from electrical stimulation of the cardiac sympathetic nerves. 4 Intra-coronary infusions of chlorpheniramine (300 micrograms/min) significantly reduced the histamine-induced depression of cardiac nerve stimulation. The effects of cimetidine (300 micrograms/min) and metiamide (300 micrograms/min) were less pronounced. 5 Histamine (1 to 10 micrograms) further increased heart rate resulting from the continuous intra-coronary infusion of noradrenaline (1 or 3 micrograms/min). 6 Intra-arterial injections of histamine (0.1 to 10 micrograms) caused an increase in coronary blood flow in a dose-dependent manner. This was partially inhibited by intra-coronary infusion of chlorpheniramine (10 to 300 micrograms/min) and by cimetidine (10 to 300 micrograms/min). The combination of both drugs (10 to 100 micrograms/min of each) caused a larger inhibition. 7 The present results suggest that the histamine-induced depression of heart rate during cardiac sympathetic nerve stimulation is due to a prejunctional effect mediated mainly by H1-receptors. Histamine-induced coronary vasodilatation in the dog is mediated both by H1- and H2-receptors.

Increased norepinephrine release during sympathetic nerve stimulation and its inhibition by adenosine in the isolated perfused kidney of spontaneously hypertensive rats

The present study was performed to measure norepinephrine release during sympathetic nerve stimulation and determine the inhibitory action of adenosine on stimulus-induced release of norepinephrine in the isolated perfused kidney of WKY and SHR. Norepinephrine release during periarterial nerve stimulation was measured as total 3H-overflow since greater than 75% of total 3H-overflow was 3H-norepinephrine in both the WKY and SHR. A significantly greater increase in 3H-norepinephrine overflow was observed during periarterial nerve stimulation in SHR in comparison with WKY. Adenosine (0.3, 1.0, 3.0 and 10.0 micrograms/ml) produced dose-dependent inhibition of 3H-norepinephrine overflow elicited by periarterial nerve stimulation. However, the effect of adenosine on transmitter release was more pronounced in the SHR in that the threshold dose required to cause inhibition of stimulus-induced release of 3H-norepinephrine was smaller in the SHR. These results demonstrate that while norepinephrine release during sympathetic nerve stimulation is greater in the SHR, this finding can not be explained on the basis of a decrease in the presynaptic inhibitory action of adenosine. Therefore, the mechanism responsible for the increased release of norepinephrine in the SHR remains to be determined.

Cardiovascular dopamine receptors: physiological, pharmacological and therapeutic implications

Dopamine receptor activation can lead to pronounced changes in cardiovascular function. The myriad of effects produced by dopamine receptor agonists results from the activation of dopamine receptors located at different anatomical sites in the cardiovascular system. Further basic research is required to better characterize these dopamine receptors so as to allow the development of more specific dopamine receptor agonists. Endogenous dopamine may be involved in the physiological control of fluid and electrolyte balance and continuing research efforts in this area should provide for a better understanding of the role of cardiovascular dopamine receptors in the maintenance of overall circulatory homeostasis. Cardiovascular dopamine receptor stimulation represents an important and promising approach for the development of novel therapeutic agents for the treatment of hypertension, angina pectoris, congestive heart failure and acute renal failure.

Dopamine accumulation after dopamine beta-hydroxylase inhibition in rat heart as an index of norepinephrine turnover

Dopamine concentration in rat heart is normally very low, only a few percent of the concentration of norepinephrine. After treatment of rats with a dopamine beta-hydroxylase inhibitor, 1-cyclohexyl-2-mercapto-imidazole (CHMI), there was a rapid increase in dopamine concentration even before norepinephrine concentration had decreased perceptibility. This accumulation of dopamine was readily measured by liquid chromatography with electrochemical detection. Since the percentage change in dopamine was much greater than the percentage change in norepinephrine, especially at early times, measurement of dopamine accumulation rather than norepinephrine decline was considered as a useful measure of norepinephrine turnover. Drugs that act on noradrenergic receptors and are known to alter norepinephrine turnover were found to alter the rate of dopamine accumulation. Clonidine and guanabenz decreased dopamine accumulation after CHMI, whereas piperoxan (but not prazosin) increased dopamine accumulation after CHMI. Pergolide, a dopamine agonist whose lowering of blood pressure and cardiac rate has been suggested to be due to suppression of neurogenic release or norepinephrine, also decreased dopamine accumulation after CHMI. The results suggest that measuring dopamine accumulation may have advantages over measuring norepinephrine disappearance after dopamine beta-hydroxylase inhibition as an indicator of norepinephrine turnover in heart.

Presynaptic dopamine receptors and alpha-adrenoceptors as mediators of the bradycardic action of N-n-propyl-N-n-butyl dopamine

The role of presynaptic receptors in the bradycardic action of N-n-butyl dopamine (PBDA) was investigated. A biphasic effect on blood pressure and a decrease in heart rate were seen upon intravenous administration of PBDA to pentobarbital-anesthetized dogs. The bradycardia produced by PBDA was unaffected by bilateral vagotomy; however, it was abolished by cardiac sympathetic denervation. When PBDA was readministered following restoration of the cardiac sympathetic nerve activity by electrical stimulation, at frequency of 1 Hz, decrease in heart rate was again observed. The cardioinhibitory action of PBDA was completely abolished by sulpiride, whereas phentolamine and yohimbine caused only partial attenuation, suggesting the involvement of both presynaptic alpha-adrenoceptors as well as dopamine receptors in the bradycardiac action of PBDA. Additional experiments were performed to study the influence of stimulus frequency on the cardioinhibition produced by PBDA. Administration of PBDA to animals with different levels of cardiac sympathetic nerve activity (0.25-2 Hz) resulted in decreases in heart rate. However, yohimbine antagonized this action of PBDA only at the two higher frequencies of cardiac nerve stimulation. Sulpiride completely abolished the bradycardia observed at all the different frequencies of cardiac nerve stimulation. These results demonstrate that activation of presynaptic receptors on cardiac sympathetic nerves can result in a decrease in heart rate. PBDA causes bradycardia via an action on presynaptic dopamine receptors when the cardiac sympathetic nerve activity is low, while both presynaptic dopamine receptors as well as alpha-adrenoceptors are involved in the decrease in heart rate produced by this compound at higher levels of sympathetic nerve activity.

Strain differences in the facilitatory action of angiotensin on noradrenergic transmission in the rat mesentery

The isolated perfused rat mesenteric vasculature preparation was used to assess the contributions of presynaptic as well as postsynaptic mechanisms in the adrenergic potentiating action of angiotensin II in different rat strains. Angiotensin II (3 ng/ml; 2.5 x 10(12) M) potentiated vasoconstrictor responses to periarterial nerve stimulation in mesentery obtained from Sprague-Dawley (SD) rats by 102.5 +/- 6.2% (p less than .05) but did not significantly affect responses in mesentery from Wistar (W) rats. Responses to nerve stimulation were increased in W rats by 22.6 +/- 12.9% (p less than .05) at higher concentrations of angiotensin II (10 ng/ml; 8.5 x 10(12) M). The facilitatory actions of angiotensin II on nerve stimulation responses were antagonized by saralasin. Angiotensin II (2.5 or 8.5 x 10(12) M) had little or no effect on responses to norepinephrine in mesentery from either SD or W rats. Blockade or neuronal and extraneuronal uptake potentiated the effects of angiotensin II on the responses to nerve stimulation in mesentery from W rats but not in mesentery from SD rats. The data suggest that angiotensin II increases norepinephrine release during nerve stimulation in mesentery obtained from SD and W rats. However, due to differences in the ability of the two strains of rats to compensate for the effect through increased neurotransmitter reuptake, the net effect on nerve stimulation responses is a large increase (100-110%) in SD rats as opposed to only a slight increase (15-20%) in W rats.

Dopaminergic activity of cis-trans isomers of benzhydro[f]quinoline analogs

A comparison of the biological activity of isomers with varying alkyl substitutions on the heterocyclic nitrogen of benzhydro[f]quinoline derivatives was made. The secondary amines did not inhibit adrenergic transmission. The trans-isomer of the secondary amine was 0.5 as active as norepinephrine when evaluated for positive chronotropic action in anesthetized cats. The trans-isomers of the N-alkyl derivatives produce inhibition of responses produced by stimulation of cardioaccelerator nerves with doses of 1--5 mumol/kg. Likewise potent emetic activity (dog) and rotational behavior (rat) was observed with the N-alkyl derivatives when administered subcutaneously. The cis-isomers were much less active. The neuronal inhibitions in cats were antagonized by haloperidol 100 micrograms/kg, except the cis-isomer of the N-ethyl derivate which required phentolamine, 2 mg/kg. In general, the transisomers were more potent in contracting the isolated rabbit aorta. This response appeared to be mediated through an alpha-adrenergic mechanism since phentolamine blocked these responses. The trans-isomer TL-305 was effective in relaxing methacholine contracted guinea-pig trachea through a beta-adrenergic mechanism since propranolol blocked this response.

Modulation by histamine of peripheral vagal transmission in anesthetized mongrel dogs

Intravenous infusion of histamine (6.0 microgram/kg/min) attenuated bradycardic responses to electrical stimulation of right vagus in anesthetized mongrel dogs. Similarily histamine also reduced reflexly mediated decreases in the heart rate to phenylephrine, while bradycardia induced by exogenous acetylcholine was not altered. This inhibitory action of histamine on responses to vagal stimulation could be antagonized by pretreatment with metiamide, an H2-receptor antagonist; but not by pyrilamine, and H1-receptor blocker. Metiamide significantly potentiated the negative chronotrophic effect of vagal stimulation. These results indicate that histamine could inhibit peripheral vagal transmission via a presynaptic action. The ability of histamine to alter vagal transmission may have physiological significance and play a role in histamine-induced tachycardia.

Field-stimulus response of guinea-pig atria as influenced by the peptides angiotensin, bradykinin, and substance P

Field stimulation produced in isolated guinea-pig left atria a positive inotropic response, which was highly sensitive to the inhibitory actions of lidocaine, tetrodotoxin, and of pindolol. The field-stimulus effect was enhanced by angiotensin and by bradykinin, but not by substance P. Only bradykinin exerted also a direct positive inotropic effect (in the absence of field stimulation). It is concluded that the intramural adrenergic neurons of the guinea-pig atrium have receptors for both angiotensin and bradykinin, but not for substance P.

Inhibition of cardiac sympathetic neurotransmission by adenosine

Experiments were performed to study the effect of adenosine on sympathetic neurotransmission to the myocardium. Adenosine administration to pentobarbital-anesthetized dogs resulted in decrease in blood pressure and significant impairment of cardioacceleration produced by stimulation of cardiac sympathetic nerves. The positive chronotropic effect of intravenous norepinephrine was not affected by adenosine. The blood pressure lowering as well as the inhibitory effect of adenosine on cardiac sympathetic nerve function could be significantly antagonized by theophylline. These results provide in vivo evidence in support of the hypothesis that adenosine caused inhibition of sympathetic neurotransmission via an action on purinergic receptors located on sympathetic nerve terminals.

Analysis of the effects of bromocriptine on blood pressure and sympathetic nerve function

Bromocriptine, a dopamine receptor agonist, lowered arterial blood pressure and caused significant impairment of the cardioacceleration observed during the stimulation of the right postganglionic cardiac sympathetic nerve fibers in pentobarbital-anethetized dogs. Prior administration of pimozide antagonized the hypotensive and the inhibitory action of bromocriptine on cardiac sympathetic function. These results suggest that while presynaptic dopamine receptors seem to be involved in mediating the inhibitory action of bromocriptine on cardiac sympathetic function, the role of presynaptic as well as dopamine receptors located at other sites in mediating the hypotensive action of bromocriptine needs to be further investigated.