Refractory period fluid stimulation of right atria: a method for studying presynaptic receptors in cardiac autonomic transmission

Contrasting cardiovascular properties of the µ-opioid agonists morphine and methadone in the rat

Morphine and methadone share the property of μ-opioid receptor agonism yet have markedly different cardiovascular actions suggesting additional properties are at play. We investigated the i.v. dose-response relationships of the opioids on cardiovascular metameters in anaesthetised rats in the absence or presence of H1- and H2-receptor antagonism and the μ-opioid antagonist naloxone. In vitro tissue assays were employed to define more clearly cardiac and vascular mechanisms of action. Morphine (9, 30, 90mg/kg i.v.) decreased heart rate (HR) and mean arterial pressure (MAP) - responses that were blocked by naloxone pretreatment (10mg/kg i.v.). In contrast, methadone (3, 10, 30mg/kg i.v.) caused dramatic short-lived (1-3min) bradycardia, hypotension and lengthening of the QT interval before stabilising 5min after i.v. dosing. Only the steady-state responses of HR and MAP were blocked by naloxone. Mepyramine (10mg/kg i.v.) and cimetidine (100mg/kg i.v.) also blocked the naloxone-sensitive components. In isolated small mesenteric arteries precontracted by K(+) 62mM or endothelin-1, methadone (1-30μM) relaxed vessels while morphine (1-100μM) had no effect. Pretreatment with naloxone (10μM), indomethacin (30μM) or nitro-l-arginine (100μM) did not affect the relaxation to methadone. In rat isolated left atria, morphine and methadone inhibited inotropic responses at high concentrations (100μM). In rat papillary muscle and right atria, methadone was more than 30 times more potent at lengthening the refractory period and slowing the atrial rate than morphine. We conclude that methadone is a potent vasodilator agent, possibly through blocking L-type calcium channels.

Synthesis of six mexiletine derivatives with isoindolines attached as potential antioxidants and their evaluation as cardioprotective agents

Some mexiletine derivatives with isoindoline based antioxidants attached have been shown to have significant cardioprotective properties.

Cardiac contractility modulation by non-excitatory currents: Studies in isolated cardiac muscle

BACKGROUND

Myocardial contractility can be altered using voltage clamp techniques by modulating amplitude and duration of the action potential resulting in enhanced calcium entry in the cell of isolated muscle strips (Non-Excitatory Currents; NEC). Extracellular electrical stimuli delivered during the absolute refractory period (Cardiac Contractility Modulation; CCM) have recently been shown to produce inotropic effects in-vivo.

AIM

Understanding the cellular mechanism, underlying the CCM effect, is essential for evaluating its clinical potential. We tested the hypothesis that NEC and CCM modulate contractility via similar cellular mechanisms.

METHODS

Square wave electric currents were applied in the organ bath to isometrically contracting rabbit RV papillary muscle and human failing trabecular muscle during the absolute refractory period (ARP).

RESULTS

These currents, which did not initiate new action potentials or contractions, modulated action potential duration (shortened or lengthened) and contractility (enhanced or depressed) in a manner that depended upon their amplitude, duration and delay from the pacing stimulus. The contractility modulation effect in the rabbit RV papillary muscle was markedly blunted after exposure to ryanodine, indicating that the sarcoplasmic reticulum plays an important role in the contractility modulation.

CONCLUSION

Like voltage clamping, extracellular currents applied during the ARP can similarly modulate action potential duration in-vitro and modulate myocardial contractility by similar intracellular mechanisms. This concept provides the potential of a therapeutic strategy in patients with heart failure to enhance contractility.

Central endogenous histamine modulates sympathetic outflow through H3 receptors in the conscious rabbit

1. This study examined the role of histamine H(3) receptors in vagal and sympathetic autonomic reflexes in the conscious rabbit, and in rabbit and guinea-pig isolated right atria. 2. The baroreceptor-heart rate reflex (baroreflex), Bezold-Jarisch-like and nasopharyngeal reflexes were assessed after these treatments (i.v.; with H(1) and H(2) receptor block): (i) vehicle (saline; n=11); (ii) H(3) receptor agonist, (R)-alpha-methylhistamine (R-alpha-MH) 100 micro g kg(-1)+100 micro g kg(-1) h(-1) (n=9); (iii) H(3) receptor antagonist, thioperamide 1 mg kg(-1)+1 mg kg(-1) h(-1) (n=11); (iv) R-alpha-MH and thioperamide (n=6); and (v) H(2) and H(3) antagonist, burimamide 6.3 mg kg(-1)+6.3 mg kg(-1) h(-1) (n=4). 3. R-alpha-MH caused a thioperamide-sensitive fall in mean arterial pressure (MAP) of 8+/-1 mmHg and tachycardia of 18+/-2 bpm (P<0.0005). Burimamide was without effect, however thioperamide elicited an increase in MAP of 4+/-1 mmHg (P<0.01), but no change in heart rate (HR). 4. R-alpha-MH caused a 44% decrease in the average gain of the baroreflex (P=0.0001); this effect was antagonised by thioperamide. Thioperamide caused a parallel rightward shift in the barocurve with an increase in MAP of 5 mmHg (P<0.05). Burimamide had no effect on the baroreflex. The vagally mediated bradycardia elicited by the Bezold-Jarisch and nasopharyngeal reflexes was unaffected by H(3) receptor ligand administration. 5. R-alpha-MH (<or=10 micro M) caused a thioperamide-sensitive depression of both sympathetic and vagal responses in guinea-pig atria, but had no effect in rabbit atria. 6. As H(3) receptor activation caused a significant decrease in baroreflex gain without affecting HR range, the former is unlikely to be simply due to peripheral sympatholysis (supported by the lack of effect in isolated atria). Central H(3) receptors may have a tonic role in the baroreflex as thioperamide caused a rightward resetting of the barocurve. In contrast, the peripherally acting H(3) antagonist burimamide was without effect. These findings suggest a role for central histamine H(3) receptors in cardiovascular homeostasis in the rabbit.

Pharmacological characterisation of cannabinoid CB(1) receptors in the rat and mouse

The role of cannabinoid CB(1) receptors in sympathetic neurotransmission was characterised in nerve-mediated responses of isolated right atria, vasa deferentia and small mesenteric resistance arteries using the cannabinoid CB(1) receptor agonists Delta(9)-tetrahydrocannabinol, CP 55,940 and anandamide and the cannabinoid CB(1)-selective antagonist SR 141716A. In the mouse vas deferens, the twitch response was completely inhibited by each of the putative cannabinoid receptor agonists with pIC(50) values of CP 55,940, 9.2+/-0.1; Delta(9)-tetrahydrocannabinol, 8.4+/-0.1; anandamide, 7.1+/-0.1. SR 141716A 10-100 nM was a competitive antagonist of all three agonists with a pK(B) value of 8.4-8.6, consistent with an interaction at the cannabinoid CB(1) receptor. In the rat vas deferens CP 55,940 (0.01-10 microM) inhibited the contractions to a significant extent (88.5+/-0.5% at 10 microM; pIC(50) of 7.1+/-0.1) while Delta(9)-tetrahydrocannabinol and anandamide (both up to 10 microM) were inactive. CP 55,940 exhibited low potency in rat compared with mouse vas deferens and the rat concentration-response curve was not competitively antagonised by SR 141716A (100 nM) or SR 144528 (10 nM-10 microM), suggesting an interaction at a receptor(s) distinct from cannabinoid CB(1) or CB(2). Sympathetic nerve-induced tachycardia in rat and mouse atria, and rat mesenteric artery smooth muscle contractile responses to perivascular nerve stimulation, were not inhibited by Delta(9)-tetrahydrocannabinol, CP 55,940 or anandamide up to 1 microM. These data indicate that cannabinoid CB(1) receptor activation inhibits sympathetic neurotransmission only in the mouse vas deferens and thus point to species and regional differences in cannabinoid CB(1) receptor involvement in pre-synaptic inhibition of sympathetic neurotransmission and CP 55,940 may have inhibitory actions in rat vas deferens unrelated to cannabinoid receptor activity.

Role of N-type calcium channels in autonomic neurotransmission in guinea-pig isolated left atria

1. Calcium entry via neuronal calcium channels is essential for the process of neurotransmission. We investigated the calcium channel subtypes involved in the operation of cardiac autonomic neurotransmission by examining the effects of selective calcium channel blockers on the inotropic responses to electrical field stimulation (EFS) of driven (4 Hz) guinea-pig isolated left atria. In this tissue, a previous report (Hong & Chang, 1995) found no evidence for N-type channels involved in the vagal negative inotropic response and only weak involvement in sympathetic responses. 2. The effects of cumulative concentrations of the selective N-type calcium channel blocker, omega-conotoxin GVIA (GVIA; 0.1-10 nM) and the non-selective N-, P/Q-type calcium channel blocker, omega-conotoxin MVIIC (MVIIC; 0.01-10 nM) were examined on the positive (with atropine, 1 microM present) and negative (with propranolol, 1 microM and clonidine, 1 microM present) inotropic responses to EFS (eight trains, each train four pulses per punctate stimulus). 3. GVIA caused complete inhibition of both cardiac vagal and sympathetic inotropic responses to EFS. GVIA was equipotent at inhibiting positive (pIC50 9.29+/-0.08) and negative (pIC50 9.13+/-0.17) inotropic responses. MVIIC also mediated complete inhibition of inotropic responses to EFS and was 160 and 85 fold less potent than GVIA at inhibiting positive (pIC50 7.08+/-0.10) and negative (pIC50 7.20+/-0.14) inotropic responses, respectively. MVIIC was also equipotent at inhibiting both sympathetic and vagal responses. 4. Our data demonstrates that N-type calcium channels account for all the calcium current required for cardiac autonomic neurotransmission in the guinea-pig isolated left atrium.

Neuropeptide Y is a prejunctional inhibitor of vagal but not sympathetic inotropic responses in guinea-pig isolated left atria

1. The effects of NPY and related peptides were examined on basal contractile force and nerve-mediated inotropic responses to electrical field stimulation of the guinea-pig isolated left atrium. 2. Electrical field stimulus (EFS)-inotropic response curves were constructed by applying 1-64 trains of four field pulses (200 Hz, 0.1 ms duration, 100 V) across isolated left atria (paced at 4 Hz, 2 ms, 1-4 V) within the atrial refractory period. Curves were constructed in presence of vehicle, propranolol (1 microM) or atropine (1 microM) to determine appropriate stimulus conditions. 3. The effects of PYY (1-10,000 nM), NPY (0.01-10 microM), N-Ac-[Leu28,31]NPY(24-36) (N-A[L]NPY(24-36); 0.01-10 microM) and clonidine (0.1-1000 nM) were examined on the positive and negative inotropic responses to EFS (eight trains, four pulses per refractory period). 4. NPY-related peptides had no effect on basal force of contraction nor on the inotropic concentration-response curves to bethanechol or isoprenaline. All three peptides inhibited vagally-mediated negative inotropic responses; rank order of potency PYY>NPY> or =N-A[L]NPY(24-36) was consistent with an action at prejunctional Y2-receptors. Clonidine concentration-dependently inhibited sympathetic inotropic responses. However, PYY, NPY and N-A[L]NPY(24-36) failed to mediate any significant inhibition of the positive inotropic response to EFS. 5. These data demonstrate that NPY is an effective inhibitor of vagal but not sympathetically-mediated inotropic responses in the guinea-pig isolated left atria. This may suggest that endogenously co-released NPY is important in mediating cross talk between efferent components of the autonomic nervous system modulating cardiac contractility, acting overall to sustain positive inotropic responses.

Heterogeneity of prejunctional NPY receptor-mediated inhibition of cardiac neurotransmission

Neuropeptide Y (NPY) has been proposed as the candidate inhibitory peptide mediating interactions between sympathetic and vagal neurotransmission in several species, including man. Here, we have defined the NPY receptors involved in modulation of cardiac autonomic neurotransmission using receptor-selective agonists and antagonists in the rabbit and guinea-pig isolated right atria. In isolated atrial preparations, sympathetically-mediated tachycardia (ST; with atropine 1 microM) or vagally-mediated bradycardia (VB; with propranolol 0.1-1 microM) in response to electrical field stimulation (EFS, 1-4 pulses) were tested 0-30 min after incubation with single concentrations of vehicle, NPY (0.01-10 microM), the Y2 receptor agonist N-Acetyl-[Leu28,31]NPY(24-36) (termed N-A[L]NPY(24-36)) or the Y1 receptor agonist [Leu31,Pro34]NPY (LP). The effect of NPY on the concentration-chronotropic response curves to isoprenaline and bethanechol were also assessed. Guinea-pig atria: NPY and N-A[L]NPY(24-36) caused concentration-dependent inhibition of VB and ST to EFS. Both peptides caused maximal inhibition of VB and ST within 10 min incubation and this remained constant. LP caused a concentration-dependent, transient inhibition of ST which was antagonized by the Y1-receptor antagonist GR231118 (0.3 microM), with apparent competitive kinetics. Rabbit atria: NPY (1 or 10 microM) had no effect on VB at any time point, but both NPY and LP caused a transient (approximately 10 min) inhibition of sympathetic tachycardia. This inhibition could be prevented by 0.3 microM GR231118. N-A[L]NPY(24-36) had no effect on ST. NPY had no effect on the response to beta-adrenoceptor stimulation by isoprenaline nor muscarinic-receptor stimulation by bethanechol in either species. Thus, in the guinea-pig, NPY causes a stable inhibition of both VB and ST to EFS via Y2 receptors and transient inhibition of ST via Y1 receptors. In contrast in the rabbit, NPY has no effect on the cardiac vagus and prejunctional inhibition of ST is transient and mediated by a Y1-like receptor (rather than Y2). Therefore it would be surprising if NPY plays a functional role in modulation of cardiac neurotransmission in the rabbit.

Effects of N-, P- and Q-type neuronal calcium channel antagonists on mammalian peripheral neurotransmission

1. The effects of N-, P- and Q-type neuronal voltage-operated calcium (Ca2+) channel antagonists on neurotransmission were determined in a range of cardiovascular and urogenital tissues, as well as the diaphragm, isolated from rat or mouse. 2. The pharmacological tools chosen were omega-conotoxin GVIA (CTX GVIA), a selective N-type Ca2+ channel antagonist, the P-type channel blocker (< or = 100 nM) omega-agatoxin IVA (AGA IVA) and omega-conotoxin MVIIC (CTX MVIIC), a non-selective antagonist of N-, P- and Q-type channels. The effects of these antagonists on nerve-mediated responses were assessed in right atria, vasa deferentia, phrenic nerve-hemidiaphragms and small mesenteric arteries. 3. Rat mesenteric artery contractile responses to perivascular nerve stimulation were concentration-dependently inhibited by CTX GVIA (1-10 nM); inhibition was 92% with 10 nM. CTX MVIIC was > 100 fold less potent and only caused an inhibition of 46% at the highest concentration (1000 nM). AGA IVA (100 nM) had no effect. 4. In rat vas deferens stimulated at 0.05 Hz, CTX GVIA (10 nM) completely inhibited the twitch response and CTX MVIIC, about 100 fold less potent, caused total inhibition at 1000 nM. AGA IVA did not affect the twitch. In rat preparations stimulated at 20 Hz, a CTX GVIA-resistant (< or = 1000 nM) twitch response of 25% was apparent which could be blocked by 1000 nM AGA IVA or CTX MVIIC. In mouse vas deferens (20 Hz stimulation), CTX GVIA 10 nM caused an 87% inhibition of the twitch, the remainder being resistant to CTX GVIA, 100 nM. CTX MVIIC was only 10 fold less potent than CTX GVIA and completely inhibited the response at 1000 nM. AGA IVA (100 nM) inhibited the twitch by 55%. 5. The twitch response of the mouse phrenic nerve-hemidiaphragm was concentration-dependently inhibited by AGA IVA (1-100 nM); inhibition was 92% at 100 nM. CTX MVIIC was about 10 fold less potent than AGA IVA with an inhibition of 80% at 1000 nM. CTX GVIA was without effect. In the rat diaphragm preparation, AGA IVA (< or = 100 nM) and CTX GVIA (< or = 1000 nM) had little effect on the twitch response. CTX MVIIC (1000 nM) inhibited the twitch by 57%. 6. In rat and mouse right atria, sympathetic responses were concentration-dependently inhibited by CTX GVIA with almost complete block at 10-100 nM. CTX MVIIC was 100 fold less potent and caused complete inhibition at 1000 nM in the mouse preparation. AGA IVA did not affect atrial sympathetic responses. 7. These data suggest that N-type Ca2+ channels predominate in the control of sympathetic transmission in the mesenteric artery, vas deferens and right atrium. In the mouse vas deferens (and rat tissue at high stimulus frequency), P- and Q-type channels also mediate Ca2+ influx. P- and Q-type Ca2+ channels control neurosecretion at the motor endplate, with no role for N type channels.

Histamine H3-receptors inhibit sympathetic neurotransmission in guinea pig myocardium

The histamine H3 agonist, (R)-alpha-methylhistamine (alpha-MeHA, 10(-10) to 10(-5) M), caused a concentration-dependent inhibition of the sympathetic contractile response to electrical field stimulation of guinea pig isolated atria, but alpha-MeHA did not alter the basal tension or the contraction induced by exogenously applied norepinephrine. Blockade of H1 and H2 histamine receptors, and alpha- and beta-adrenoceptors failed to prevent the inhibitory effect of alpha-MeHA, whereas the specific H3 receptor antagonist, thioperamide, concentration dependently reversed the inhibitory effect of alpha-MeHA. At the concentration of 10(-7) M, which was effective for antagonizing the action of alpha-MeHA, thioperamide did not modify the sympathetic responses facilitated by the beta 2-adrenoceptor agonist, clenbuterol, or attenuated by the alpha 2-adrenoceptor agonist, clonidine. Our results suggest that H3 receptors exist on the cardiac sympathetic terminals, which may modulate adrenergic neurotransmission in guinea pig myocardium.

Different effects of prostaglandins on adrenergic neurotransmission in atrial and ventricular preparations

1. The effects of prostaglandin E2 (PGE2) and iloprost on the cardiac response to adrenergic nerve stimulation in guinea-pig atrial and ventricular preparations have been studied. 2. In guinea-pig isolated atria both PGE2 (0.1-10 nM) and iloprost (0.1-3 microM) concentration-dependently reduced the cardiac response to adrenergic nerve stimulation. 3. The inhibition of cyclo-oxygenase by indomethacin and acetylsalicylic acid potentiated the response to nerve stimulation in the atrial preparations. 4. Arachidonic acid (1-10 microM) reduced the response to nerve stimulation in atria. This effect was prevented by indomethacin and acetylsalicylic acid. 5. In guinea-pig ventricles PGE2 and iloprost were found to be effective at higher concentrations than in atrial preparations: arachidonic acid, indomethacin or acetylsalicylic acid did not modify the cardiac response to adrenergic nerve stimulation. 6. These results suggest a different modulator role for endogenous prostaglandins in atrial and ventricular tissue.

Comparative assay of neuronal uptake and autoinhibitory feedback in guinea-pig and rat atria

1. The roles of neuronal uptake and autoinhibitory feedback were studied in guinea-pig and rat isolated right atria. Tachycardia was used as a measure of noradrenaline concentration at the post-junctional beta-adrenoreceptors in response to electrical field stimulation, 1-16 pulses at 1 Hz, or to exogenous noradrenaline. 2. The EC50 values for noradrenaline concentration-response curves were similar (7.3, -log M) in the two species when neuronal uptake was inhibited by desipramine (DMI, 1 microM). In the absence of DMI, however, the EC50 values were different for the rat (7.0) and guinea-pig (6.3). 3. DMI (0.01-1 microM) caused a substantial increase in half-response time (t 1/2), an integrated measure of tachycardia, in the rat during field stimulation but only caused modest increases in the guinea-pig atria. Following pretreatment with the irreversible alpha 2-adrenoceptor antagonist benextramine, the t 1/2 values were substantially raised in guinea-pig atria with no further change to t 1/2 in the rat atria. 4. The existence of presynaptic inhibitory alpha 2-adrenoceptors in the rat was established using clonidine which caused parallel rightward shifts of field stimulation-tachycardia curves. 5. These results suggest that in the guinea-pig atria blockade of both autoinhibitory feedback and neuronal uptake cause a very large increase in tachycardia compared with blockade of either system alone. In rat atria the most important modulation is from neuronal uptake, which suggests that in this species autoinhibitory feedback is of little consequence.

Amiloride analogues cause endothelium-dependent relaxation in the canine coronary artery in vitro: possible role of Na+/Ca2+ exchange

1. A number of amiloride analogues were used to test the proposal that Na+/Ca2+ exchange may play a role in the secretion of endothelium-derived relaxing factor (EDRF). The analogues used were those substituted on either the 5-amino group or the terminal guanidino nitrogen atom. The former block both Na+/Ca2+ and Na+/H+ exchange whilst the latter block the Na+ channel and the Na+/Ca2+ exchange. 2. Both series of compounds caused relaxation in isolated rings of dog coronary artery (EC50 values, 1-10 microM) presumably due to release of EDRF since removal of endothelium greatly attenuated the response. 3. Amiloride (1-100 microM) had little effect on either endothelium-intact or denuded arteries. 4. The guanidino substituted analogues also appeared to block selectively the relaxation response to acetylcholine in the coronary artery, independently of their EDRF-releasing activity. 5. It is proposed that endothelial cells have an active Na+/Ca2+ exchange operating in the forward mode to extrude Ca2+. This mechanism may be important in the control of EDRF release.

No effect of atrial natriuretic factor on cardiac rate, force and transmitter release

The effects of atrial natriuretic factor (ANF, 1-300 nmol/l) on cardiac rate, force and neurotransmitter release were examined in guinea-pig isolated heart preparations. Synthetic ANF had little effect on the contractile force of electrically driven papillary muscle regardless of the presence or absence of isoprenaline. The pulse period of spontaneously beating right atria was not affected by ANF. Neither the positive nor negative chronotropic effect of isoprenaline or bethanechol respectively were changed in the presence of ANF. ANF did not affect the release of neurotransmitter from the nerve endings in the isolated atrium. ANF was confirmed to relax isolated aortic rings precontracted with either noradrenaline (ANF,IC50 = 3 nmol/l,s.e.m. = 0.2, n = 5) or potassium (ANF,IC50 = 24 nmol/l, s.e.m. = 0.2, n = 5). These results demonstrate that ANF within doses effective for vasorelaxation has no appreciable effect on myocardial function or neutrotransmitter release in the heart.

Cardiodepressant effects of ethanol on guinea-pig atria: presynaptic and postsynaptic components

Ethanol, at concentrations ranging from 0.5 to 1.5%, depressed myocardial contractility of electrically-stimulated guinea-pig atria. This effect was evident in preparations bathed with a low calcium concentration, but was progressively reduced by increasing the extracellular calcium. The same concentrations of ethanol produced a dose-dependent inhibition of the cardiac response to field stimulation of the adrenergic nerve terminals. This effect was again calcium-dependent. These results support the hypothesis that the pre- and postsynaptic components of the cardiodepressant effects of ethanol are due to a reduction in calcium availability both at the nerve endings and in the contractile cells.

Inhibition of the cardiac response to sympathetic nerve stimulation by opioid peptides and its potentiation by morphine and methadone

[D-Ala2,D-Leu5]enkephalin (1-10 microM) and [Met5]enkephalin-Arg-Phe (1-10 microM) produced concentration-dependent inhibition of the cardiac response to field stimulation of the adrenergic nerve terminals in preparations pretreated with peptidase inhibitors (captopril 10 microM, bestatin 10 microM, thiorphan 0.3 microM and L-leucyl-L-leucine 2 mM). The inhibitory response to the opioid agonists was evident in preparations superfused with solutions containing 1.8 mM calcium, but not in those containing 3.6 mM calcium. Moreover the inhibition was antagonized by naloxone 10 microM. [D-Ala2,Met5]enkephalinamide (1-3 microM) and beta-endorphin (1-3 microM) did not significantly affect the sympathetic response. The cardiac response to sympathetic stimulation was not inhibited but, on the contrary, was potentiated by morphine (3-10 microM) and methadone (3-10 microM). It is suggested that the depressant effect of the opioid peptides was due to stimulation of presynaptic inhibitory opiate receptors on adrenergic nerve terminals of the heart, and that the potentiation of the sympathetic response by morphine and methadone was probably attributable to an unspecific inhibitory effect on the neuronal uptake of noradrenaline.

Differences between the prejunctional effects of phenylephrine and clonidine in guinea-pig isolated atria

The prejunctional effects of clonidine and phenylephrine were studied in guinea-pig isolated atria by means of field stimulation of the sympathetic nerve terminals during the cardiac refractory period, in the presence of 1 microM atropine. Clonidine (10-100 nM) produced a dose-dependent decrease in the stimulus-inotropic response curve; the IC50 for clonidine was increased about 70 times by the pretreatment of the preparations with 1 microM yohimbine. The effect of clonidine was not modified by 0.5 microM prazosin. Unlike clonidine, phenylephrine (1-10 microM) induced a statistically insignificant increase in the contractile force of preparations stimulated at 4 Hz. The inhibitory effect of phenylephrine (1-10 microM) was partially prevented by either 1 microM yohimbine or 0.5 microM prazosin. However, it was antagonized, to about the same degree as that observed with clonidine, by the pretreatment of the preparations with both 1 microM yohimbine and 0.5 microM prazosin. The results seem to indicate that one component of the prejunctional effects of phenylephrine may be mediated by presynaptic alpha-adrenoceptors belonging to the alpha 1-subtype.

Phentolamine--an unexpected agonist in the rabbit

Phentolamine (0.1-10 microM) caused an anomalous rightward shift of the relationship between the number of electrical field pulses and tachycardia in the rabbit isolated right atrium. Phentolamine was apparently acting as a presynaptic agonist on sympathetic nerve endings to inhibit transmitter release. The effect was prevented by benextramine treatment and antagonized 10 fold by yohimbine (1 microM) but not by prazosin (0.1 microM). In ganglion-blocked (mecamylamine) conscious or anaesthetized rabbits, phentolamine (3-1000 micrograms kg-1) caused a dose-related rise in blood pressure that was antagonized by yohimbine (1 mg kg-1). These pressor and inhibitory cardiac sympathetic nerve effects of phentolamine are not found in similar preparations from the guinea-pig or rat. Therefore, these rabbit-specific agonist effects of phentolamine at sites similar to alpha 2-adrenoceptors make this drug unsuitable as an alpha-adrenoceptor antagonist in rabbits.

Reversible inhibition of neuronal uptake by benextramine, an irreversible presynaptic alpha-adrenoceptor antagonist

Benextramine, a covalently binding alpha-adrenoceptor blocking agent, potentiated the action of noradrenaline but not isoprenaline in guinea pig isolated right atria. This potentiation was probably caused by inhibition of neuronal uptake. When the benextramine was washed from the tissues for 60 min, no potentiation of the action of noradrenaline was observed. This easily reversed inhibition of neuronal uptake by benextramine contrasts with the effects of desipramine and phenoxybenzamine because the potentiating effect of these drugs was unaffected by 60 min of washing. The presence of benextramine also caused a small tachycardia in both rabbit and guinea pig right atria which was probably due to the release of endogenous noradrenaline. Clonidine a presynaptic alpha 2-adrenoceptor agonist, inhibited the responses to electrical field stimulation. Pretreatment with benextramine greatly diminished the effect of clonidine. This alpha 2-adrenoceptor antagonism was not reversed by washing the benextramine from the tissue for 240 min. We conclude that benextramine is a readily reversible inhibitor of neuronal uptake and an irreversible antagonist of presynaptic alpha 2-adrenoceptors.

Role of auto-inhibitory feed-back in cardiac sympathetic transmission assessed by simultaneous measurements of changes in 3H-efflux and atrial rate in guinea-pig atrium

Guinea-pig right atria were labelled with [3H]-noradrenaline or [3H]-dopamine before superfusion in a flow-cell. Choice of label did not significantly alter either the relationship between 3H-efflux and number of electrical field pulses or the inhomogeneity of labelling. The relationship between 3H-efflux and frequency of 4 field pulses (0.125-2 Hz) was hyperbolic and similar to the tachycardia-frequency relationship measured simultaneously. No evidence was found for a U shaped 3H-efflux-frequency relationship (Story, McCulloch, Rand & Standford-Starr, 1981). Phentolamine (1 microM) did not alter the 3H-efflux or atrial rate responses to 4 field pulses at stimulus levels that gave 50-60% of the maximum rate response. In the presence of neuronal uptake inhibition (desipramine, DMI 0.1 microM), rate and 3H-efflux responses to 4 field pulses were enhanced at all frequencies and were further increased by phentolamine. In the absence of DMI, prolonged trains of field pulses (8 and 12 pulses) at low frequency (0.25 Hz) were not sufficient to activate auto-inhibitory feed-back. At 2 Hz phentolamine enhanced both 3H-efflux and rate responses at 12 field pulses. We conclude that in guinea-pig right atrium auto-inhibitory feed-back plays little role in the modulation of transmitter release at levels of stimulation that cause 50-60% of maximum tissue response. This is because neuronal uptake normally prevents synaptic concentrations of noradrenaline from activating prejunctional alpha 2-adrenoceptors. Stimulation sufficient to induce a near-maximal response or the presence of neuronal uptake inhibition are necessary to evoke autoinhibitory feed-back.

Role of autoinhibitory feedback in cardiac sympathetic transmission

The relationship between two indices of transmitter release measured simultaneously and the frequency of 4 field pulses (0.125-2 Hz) were obtained from superfused guinea pig right atria after labelling with 3H-noradrenaline. The relationships between 3H-efflux or rate responses and frequency were hyperbolic. Autoinhibitory feedback did not play a role since phentolamine (1 microM) did not alter the 3H-efflux or rate responses to 4 field pulses that gave 50-60% of the maximum rate response. In the presence of neuronal uptake block (desipramine (0.1 microM) phentolamine enhanced 3H-efflux and rate responses to 4 field pulses at all frequencies. In the absence of desipramine prolonged trains of field pulses (8-12 pulses) at low frequency (0.25 Hz) were not sufficient to activate autoinhibitory feedback. At 2 Hz phentolamine enhanced both responses at 12 field pulses. We conclude that in the right atrium autoinhibitory feedback plays little role in the modulation of transmitter release at levels of stimulation that cause 50-60% of maximum tissue response. The presence of neuronal uptake inhibition or high stimulus strengths are necessary to evoke autoinhibitory feedback.

Disadvantages of cocaine as a neuronal uptake blocking agent: comparison with desipramine in guinea-pig right atrium

Cocaine and desipramine (DMI) are widely used as neuronal uptake blocking agents in studies of cardiac sympathetic transmission in isolated tissue preparations. It is generally assumed that these pharmacological tools do not alter transmitter release or postjunctional effector response. To test this assumption, we have compared the effects of cocaine and DMI on rate responses to sympathetic nerve stimulation and exogenous noradrenaline in guinea-pig isolated right atria. Right atria were equilibrated with the irreversible alpha-adrenoreceptor antagonist benextramine to prevent any effect of presynaptic alpha-adrenoreceptors. Cumulative (-) noradrenaline concentration-response curves were shifted to the left by DMI (0.01-1 microM) without significant change in the resting or maximum rates. Cocaine (1-100 microM) also caused sensitisation to noradrenaline but caused a biphasic change in resting atrial rate. In addition there was a small but significant depression of the maximum rate at cocaine 10 and 100 microM. Sympathetic nerve stimulation was achieved by applying trains of 1, 2 and 4 electrical field pulses delivered during one atrial refractory period. DMI caused a concentration dependent potentiation of responses to field stimulation. Cocaine (1 microM) caused significant enhancement of peak responses to field stimulation but no further enhancement and indeed depressed peak responses were observed at cocaine 10 and 100 microM respectively. The time for atrial period to return halfway to baseline after field stimulation (t 1/2) was enhanced by cocaine in a concentration dependent manner as was observed with DMI. We conclude that cocaine (but not DMI) decreases the maximum response to exogenous noradrenaline (postjunctional depression). The reduction of the peak response to sympathetic nerve stimulation in the presence of cocaine to below control responses suggests that cocaine also depresses the release of transmitter. These additional depressant properties of cocaine, which occur in a concentration range of neuronal uptake block, are important disadvantages and should discourage its use in experiments on sympathetic transmission.

A simple technique to study the effect of drugs on the negative inotropic response elicited by postganglionic cholinergic nerve stimulation in guinea pig atria

A simple technique is described for evaluating the effects of drugs on postganglionic cholinergic nerve stimulation in guinea pig atrial tissue. Raising the voltage of stimulation tenfold (high voltage stimulation, HVS) in the left atrium produced a positive inotropic response. Propranolol abolished this effect but failed to reveal a negative inotropic response. However, in atria obtained from 6-hydroxydopamine (6-OHDA) pretreated animals, HVS produced a negative inotropic effect. The magnitude of the response was frequency-dependent and was potentiated by the anticholinesterase dyflos. Tetrodotoxin abolished the response but mecamylamine had no significant effect. Drugs which inhibit cardiac muscarinic receptors caused a parallel shift of the frequency-response relationship. In the right atrium, HVS caused a biphasic inotropic response. Propranolol or 6-OHDA pretreatment inhibited the positive inotropic effect, thereby enhancing the negative inotropic response. The effect did not show as great a dependence on the frequency of stimulation as in the left atrium and was less useful for comparing the effectiveness of muscarinic receptor antagonists.

Possible presynaptic inhibitory effect of etorphine on sympathetic nerve terminals of guinea-pig heart

Etorphine (1-4 microM) dose dependently reduced the sympathetic response induced by trains of field pulses in guinea-pig isolated atria stimulated at 4 Hz; this effect was antagonized by 10 microM naloxone. Since etorphine did not modify the dose-inotropic effect curve of exogenous noradrenaline in the same preparation, it is suggested that the depressant effect of the opioid agonist was due to stimulation of presynaptic inhibitory opiate receptors on adrenergic nerve terminals of the heart.

Effects of alinidine (ST 567) on baroreceptor-heart rate reflexes and its interactions with clonidine on the baroreflex and on the sympathetic terminals of the isolated atrium

Alinidine (ST 567), an N-allyl derivative of clonidine, slowed the heart rate of conscious rabbits by 41 +/- 2.3 (S.E.D.) b/min and reduced mean arterial pressure (MAP) by 6.4 +/- 1.4 mmHg (P less than 0.001). The cardiac slowing was considered to be a direct effect in agreement with previous findings by others, since it was present in rabbits without functioning autonomic nerves, but the fall in blood pressure did not occur in these animals. Alinidine produced no significant changes in the reflex tachycardia response evoked by infusing nitroprusside, or in the pressure-related parameters of the MAP-heart period (HP) curve of the baroreceptor-heart rate reflex (i.e. HP range, gain, or median blood pressure BP50). Intravenous (i.v.) clonidine produced characteristic rises in baroreflex HP range and gain, which were due to vagal facilitation, and also produced falls in BP50 and resting MAP. I.v. alinidine suppressed the clonidine-induced vagal facilitation, but had no effect on the blood pressure changes. Intracisternal alinidine could be given in only relatively low dose, but reduced the clonidine-induced rise in vagal component of HP range. The main site of antagonism between i.v. alinidine and clonidine was probably in the CNS. We studied the nature of the antagonism at the sympathetic nerve terminal of the isolated left guinea pig atrium. Clonidine depressed the inotropic response to field stimulation of the sympathetic nerves and this was competitively antagonised by phentolamine greater than yohimbine greater than alinidine at potencies of about 1200:80:1. Alinidine was considered to be a weak but specific alpha 2-antagonist; it has no alpha 1-antagonist properties since it was without effect on the contractile response to noradrenaline of the guinea pig aorta. The alpha 2-antagonist property explains the suppression by alinidine of the clonidine-induced facilitation of the vagal component of the baroreceptor-heart rate reflex.

Chronotropic effects of angiotensin I, angiotensin II, bradykinin and vasopressin in guinea pig atria

Chronotropic responses to angiotensin I and angiotensin II, vasopressin and bradykinin were measured in guinea pig isolated right atria. Angiotensin II (100-30,000 pg/ml) was slightly more potent than angiotensin I and caused a maximum tachycardia of 30-40 b/min; only 20% of the maximum response to (--)-noradrenaline. Propranolol (1 micro M) or reserpine pretreatment (1 mg/kg i.p., 24 h) did not alter the response to angiotensin II or bradykinin. Converting enzyme inhibition by captopril (10 micrograms/ml) did not affect resting rate nor the response to angiotensin II but shifted the location of the angiotensin I curve by 40 fold to the right. Bradykinin (5-500 ng/ml) caused small increases in rate while vasopressin 1-100 ng/ml was completely without effect. These results suggest that angiotensin II has a small positive chronotropic effect that is not dependent on tissue noradrenaline release or beta-adrenoceptors and that tissue converting enzyme is active in right atria. Relatively high concentrations of angiotensin and bradykinin were required to directly stimulate the sino-atrial node compared with plasma levels measured during physiological stimuli. Therefore these effects on atria are probably of little physiological significance for peptide concentrations in plasma but may be important in relation to local tissue generation of angiotensin II.

Comparison of angiotensin converting enzyme inhibitors captopril and MK421-diacid in guinea pig atria

Angiotensin I (AI) and angiotensin II (AII) caused concentration-dependent increases in atrial rate in guinea pig isolated right atria. Converting enzyme inhibitors captopril and MK421-diacid did not alter the responses to AII but displaced the curves to AI to the right. The atrial response to generated AII from AI was used as a bioassay to estimate the dissociation constants of converting enzyme inhibitors (Kb) and test for kinetics of simple competition. MK421-diacid was 12-40 times more potent than captopril. However, estimations of Kb for captopril and MK421-diacid were unsatisfactory because at high concentrations of inhibitors the curves to AI were not displaced according to simple competition. We conclude that AI in high concentration can stimulate AII receptors accounting for the stationary displacement of curves to AI in the presence of converting enzyme inhibitors. MK421-diacid also potentiates responses to bradykinin in this assay.