Isolated Dog Atrium Research Articles

Adenosine infusion inhibits adenosine-induced cardiac actions but enhances acetylcholine-induced actions in isolated dog atria.

Using isolated, blood-perfused canine atrial preparations, adenosine was continuously administered at various infusion rates into the cannulated sinus node artery. Adenosine induced negative chrono- and inotropic effects in an infusion-rate related manner. The inotropic responses to the adenosine infusion apparently faded but the chronotropic responses did not. Effects of a bolus injection of adenosine, acetylcholine (ACh), or norepinephrine and effects of intracardiac autonomic nerve stimulation (ICNS) were examined before and during the adenosine infusion. During the adenosine infusion, adenosine-induced effects were significantly reduced but ACh-induced ones were significantly potentiated, and the norepinephrine-induced effects were slightly depressed. ICNS readily induced negative and positive chrono- and inotropic responses which were blocked by atropine and propranolol. These negative responses were enhanced but positive ones were slightly but insignificantly depressed during the adenosine infusion. From these results, it is concluded that adenosine infusion (1) produces a stable continuous bradycardia but in the developed tension an initial decrease is followed by a gradual recovery response showing the fade phenomenon, (2) causes an acute desensitizing action on a subsequent bolus dose of adenosine, and (3) induces a weak antiadrenergic effect, while enhancing cholinergic effects in isolated canine atria.

Acute Desensitization of Adenosine by Adenosine Infusion in Isolated Dog Atria

ABSTRACT We investigated the effects of a continuous infusion of adenosine on responses of sinus rate and developed tension to bolus injections of adenosine and acetylcholine in isolated, blood-perfused canine right atria. Each drug was directly injected into the sinus node artery of the isolated atrium. During adenosine infusions, the negative chronotropic and inotropic responses to bolus injections of adenosine were significantly inhibited in a dose-related manner, while the negative responses to administered acetylcholine were consistently potentiated. These results indicate that adenosine infusion has an acute desensitizing action on subsequent bolus adenosine, although the cholinergic action is enhanced by a continuous infusion of adenosine.

Acute desensitization of adenosine by adenosine infusion in isolated dog atria.

We investigated the effects of a continuous infusion of adenosine on responses of sinus rate and developed tension to bolus injections of adenosine and acetylcholine in isolated, blood-perfused canine right atria. Each drug was directly injected into the sinus node artery of the isolated atrium. During adenosine infusions, the negative chronotropic and inotropic responses to bolus injections of adenosine were significantly inhibited in a dose-related manner, while the negative responses to administered acetylcholine were consistently potentiated. These results indicate that adenosine infusion has an acute desensitizing action on subsequent bolus adenosine, although the cholinergic action is enhanced by a continuous infusion of adenosine.

Comparison of inotropic and chronotropic effects of vasoactive intestinal peptide in isolated dog atria

The positive chronotropic and inotropic effects of vasoactive intestinal peptide, VIP, were studied in an isolated canine right atrial preparation. Atria were removed, maintained in a bath, and perfused with Tyrode's solution. Contractile force and atrial depolarization were measured. VIP (18.8–600 pmol) was injected into a cannulated sinoatrial nodal artery and dose response curves were obtained. The mean EC 50 was similar for the inotropic and the chronotropic responses (136 and 144 pmol, respectively). Time courses of the onset and of recovery from the responses were measured. Times for onset of VIP effects were similar but, once the effect was initiated, rate of development of the response and recovery time from the responses were dose dependent. The increases in atrial rate lasted two to four times longer than did the increases in contractile force. Recovery from the chronotropic and inotropic responses to VIP differ, suggesting that the intracellular responses are coupled differently to the receptors. The responses to VIP were compared to those of 100 pmol isoproterenol, another positive chronotropic and inotropic agent. Isoproterenol was a slightly more potent chronotropic and inotropic agent than VIP. Desensitization of the responses was determined. Repeated exposures to VIP decreased the chronotropic response but not the inotropic response to VIP. There was no significant decrease in responsiveness to isoproterenol.

Inhibition by bertosamil of cardiac responses to pinacidil or Bay k 8644 in isolated dog atria and ventricles

We investigated the effects of a novel bradycardic agent, bertosamil (3-isobutyl-7-isopropyl-9,9-pentamethylene-3,7-diazabicyclo[3.3.1]nonane sesquihydrogenfumarate), on the sinus rate and atrial contractile force and the left ventricular contractile force in isolated, blood-perfused dog hearts and the blocking effects of bertosamil on the chronotropic and inotropic responses to pinacidil and Bay k 8644 (methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-piridine-5-carboxylate). Bertosamil (0.1–100 nmol) caused transient positive, followed by continuous negative, chronotropic responses and positive inotropic responses in atria, and increased the left ventricular contractile force. Neither propranolol nor atropine affected the cardiac responses to bertosamil. Bertosamil (3–100 nmol) dose dependently attenuated the negative chronotropic and inotropic responses to pinacidil but not to acetylcholine. Bertosamil at a high dose attenuated the positive cardiac responses to Bay k 8644, norepinephrine and isoproterenol. These results suggest that bertosamil inhibits negative cardiac responses mediated by an ATP-sensitive K + channel but not an acetylcholine muscarinic receptor and, at a high dose, attenuates the L-type Ca 2+ channel-mediated positive cardiac responses in isolated dog hearts.

Negative Chronotropic and Inotropic Effects of U-92032, a Novel T-Type Ca2+ Channel Blocker, on the Isolated, Blood-Perfused Dog Atrium

We investigated the effects of U-92032 ((7-((bis-4-fluorophenyl)methyl)-1-piperazinyl)-2-2(2-hydroxyethylamino)-4-(1-methylethyl)-2,4,6-cycloheptatrien-1-one), a novel T-type Ca2+ channel blocker, on sinus rate and atrial contractile force in the isolated, blood-perfused atrium of the dog. U-92032 (1 to 300 nmol) induced negative chronotropic and inotropic responses in a dose-dependent manner, and the percentage decrease in sinus rate was less than that in atrial contractile force. Atropine did not affect the negative responses to U-92032. These results suggest that U-92032, a T-type Ca2+ channel blocker, simultaneously decreases the sinus rate and atrial force as do L-type Ca2+ channel blockers in the isolated dog atrium.

O-137 - Differential inhibition of positive chrono- and inotropic responses to cardiotonics by UL-FS 49, a specific bradycardic agent in the isolated dog atrium.

O-137 - Differential inhibition of positive chrono- and inotropic responses to cardiotonics by UL-FS 49, a specific bradycardic agent in the isolated dog atrium.

K + channel blocking and anti-muscarinic effects of a novel piperazine derivative, INO 2628, on the isolated dog atrium

The effects of a novel piperazine derivative, INO 2628, on the negative inotropic and chronotropic responses to intracardiac parasympathetic nerve stimulation and carbachol, to adenosine and to the K + channel openers, pinacidil and nicorandil, were investigated in isolated, blood-perfused dog heart preparations. INO 2628 (0.1–10 μmol) injected into the sinus node artery of the isolated atrium induced negative chronotropic and small positive inotropic responses in a dose-dependent manner. INO 2628 antagonized the negative chronotropic and inotropic responses to intracardiac vagus stimulation and carbachol in a dose-dependent manner, whereas INO 2628 did not antagonize the negative cardiac responses to adenosine. Pinacidil and nicorandil caused ose-dependent negative inotropic and small negative chronotropic responses in isolated atria and ventricles, suggesting that pinacidil-related K + channels are much sparser in SA nodal pacemaker cells than in cardiac muscle cells. INO 2628 dose dependently antagonized the negative inotropic responses to pinacidil and nicorandil, but it did not modify the nicardipine-, pentobarbital- or G-strophanthin-induced cardiac responses. These results suggest that INO 2628 inhibits the negative cardiac effects of acetylcholine at muscarinic receptors and directly inhibits K + channels in the isolated dog heart.

Muscarinic receptor subtypes mediating negative chrono‐ and inotropic responses in isolated, blood‐perfused dog right atria

1. Negative chrono- and inotropic responses to both carbachol (CCh) into the sinus node artery and electrical stimulation of vagal nerve fibres (ES) were studied in the isolated, blood-perfused canine right atrium, using four muscarinic receptor antagonists, atropine, 4-DAMP, AF-DX 116 and pirenzepine. 2. ES and CCh evoked negative chrono- and inotropic responses in a frequency-dependent manner and in a dose-related manner, respectively. 3. Each antagonist inhibited these negative chrono- and inotropic responses in a dose-dependent manner. The ranking order of blocking potency (ID50) was atropine greater than or equal to 4-DAMP greater than AF-DX 116 greater than pirenzepine. 4. The ID50 of atropine, 4-DAMP or AF-DX 116 against sinus rate decreases induced by CCh or ES was not significantly different from that against the atrial tension decreases. In contrast, the ID50 of pirenzepine against sinus rate decreases evoked by CCh (17 nmol) or ES (20 nmol) was significantly smaller than that for the atrial tension decrease (CCh, 200 and ES, 53 nmol, respectively). 5. These results suggest that, in the isolated dog atrium, M2-receptor-mediated-responses are predominant. However, M1-receptor activation may also be involved in sinus rate response.

Differential vagal inhibition of the positive chronotropic and inotropic responses to cardiotonics in the isolated dog atrium

The effects of vagal nerve stimulation on the chronotropic and inotropic responses to norepinephrine (NE), dobutamine, forskolin, 3-isobutyl-1-methylxanthine (IBMX) and Bay k 8644 were investigated in the isolated, blood-perfused right atrium of the dog. Electrical stimulation of intramural vagal nerves evoked decreases in the sinus rate and atrial contractile force, which were maintained at almost constant levels during stimulation. Vagal stimulation consistently attenuated both the positive chronotropic and inotropic responses to NE, dobutamine, forskolin and IBMX. The vagal inhibition of the chronotropic response to each substance was greater than that of the inotropic one except that to Bay k 8644. Vagal stimulation did not depress the positive chronotropic and inotropic responses to Bay k 8644. These results, therefore, suggest that, under parasympathetic tonic conditions, NE, dobutamine, forskolin and IBMX induce a positive chronotropic effect much less than a positive inotropic effect in the isolated dog atrium. Our results also suggest that the vagal inhibition of the chronotropic response to a β-adrenoceptor agonist is induced at intracellular sites in the cyclic AMP cascade proximal to the Ca channel activation and also at a site proximal to the catalytic unit of adenylate cyclase.

Negative chronotropic and positive inotropic effects of a unique cardioactive agent, N,N'-dicyclopropyl methyl piperazine(bis) hydrochloride(INO 2628), in isolated, blood-perfused dog atria.

The effects of a novel piperazine derivative (INO 2628) on sinus node pacemaker activity and atrial contractile force were investigated in isolated, blood-perfused dog atrium. Injections of INO 2628 (0.03-100 mumol) into the sinus node artery of the isolated atrium induced a dose-dependent decrease in sinus rate and an increase in contractile force. The positive inotropic effects at more than 10 mumol were accompanied by a transient negative inotropism. Propranolol did not affect the positive inotropic response to INO 2628, but it significantly suppressed positive chronotropic and inotropic responses to norepinephrine. Atropine at a dose which completely blocked negative chronotropic and inotropic responses to carbachol, produced a slight but significant depression of INO 2628-induced negative chronotropic responses; inotropic responses were unaffected. These results suggest that INO 2628 induces noncholinergic negative chronotropic and nonadrenergic positive inotropic responses in isolated dog atria.

Effects of Ketanserin on the Pacemaker Activity and Contractility in the Isolated, Blood-Perfused Dog Atrium

Effects of ketanserin on the pacemaker activity and atrial contractility and on 5-hydroxytryptamine (5-HT)-induced cardiac responses were investigated in the isolated, blood-perfused dog atrium. Ketanserin (1-300 micrograms) injected into the sinus node artery evoked a transient positive followed by a negative chronotropic effect, and a dose above 30 micrograms of ketanserin produced a dose-dependent negative inotropic effect with a little positive one. Ketanserin-induced negative chronotropic and inotropic effects were not affected by atropine in a dose which blocked ACh-induced responses. Propranolol inhibited positive inotropic responses to ketanserin and norepinephrine and significantly augmented the negative chronotropic and inotropic responses to ketanserin. Imipramine did not affect the transient positive followed by negative chronotropic and the negative inotropic responses to ketanserin but it induced the positive cardiac responses following the negative ones. Tetrodotoxin, phentolamine, and diphenhydramine did not modify the effects of ketanserin. From these results, it is concluded that ketanserin might induce the direct negative chronotropic and inotropic effects and the indirect effects by catecholamine release. Ketanserin-induced catecholamine release might not be due to tyramine-like or nicotine-like action. Ketanserin significantly inhibited a low dose (3 micrograms) of 5-HT-induced negative chronotropic and inotropic effects, suggesting the possibility of 5-HT2 receptors in the isolated dog atrium.

Attenuation of the responses to repeated cholinergic interventions in the isolated dog atrium.

In the isolated, blood-perfused canine right atrium, which was pretreated with propranolol, negative chronotropic and inotropic responses were evoked by stimulation of the intramural parasympathetic nerve fibers or by intra-arterial infusion of acetylcholine (ACh). Successive cholinergic interventions were applied; first, a conditioning intervention for 2 min was given, then this was followed by a test intervention for 4 min. The two interventions were separated by a rest period that varied from 15 to 240 s. The cardiac responses to the conditioning parasympathetic nerve stimulation quickly reached maximum levels, and then they "faded" or progressively diminished back toward the control level. The inotropic responses to the conditioning infusion of ACh (1 microgram/min) faded slightly but the chronotropic response did not. After the rest period, the test nerve stimulation evoked responses that also gradually faded with time. The maximal amplitude of the responses to the test stimuli were less than those to the conditioning stimuli. This reduction in the maximal amplitude of the cardiac responses to the test stimuli was more pronounced with high frequency stimulation (30 Hz) than with low frequency stimulation (5 Hz). The decrement was also more pronounced the shorter the rest period, and it was greater at earlier times after beginning the stimulation. Conversely, the maximal cardiac responses to test infusions of ACh were not appreciably less than the responses to the conditioning infusions. We conclude, therefore, that the diminution of the cardiac responses to the second test stimulation of the parasympathetic nerve fibers was mainly ascribable to a prejunctional rather than to a postjunctional mechanism.

Effects of 4-Aminopyridine on the Fade Response to Vagal Nerve Stimulation in the Isolated Dog Atrium

Intramural parasympathetic nerve stimulation (5 Hz, 2 min) after propranolol treatment evoked negative chronotropic and inotropic responses in the isolated dog atrium. The cardiac responses were not maintained during stimulation; they reached maximum values and then faded back toward their control values (“fade” response). 4-Aminopyridme increased the maximum values of both responses, and it increased the fade of the chronotropic response but not that of the inotropic response. These results suggest that the fade response to parasympathetic nerve stimulation is modulated at the parasympathetic nerve terminals of the isolated dog atrium.

Potentiating effects of 4-aminopyridine on responses to intramural vagal stimulation in the isolated dog atrium

The effects of 4-aminopyridine (4-AP) on the chronotropic and inotropic responses to intramural parasympathetic nerve stimulation and to injection of acetylcholine (ACh) into the sinus node artery were investigated after treatment with propranolol in the isolated, blood-perfused dog atrium. 4-AP (3–100 μg) induced positive chronotropic and inotropic effects and 300 μg of 4-AP induced biphasic, negative and positive effects. The negative responses to 4-AP were completely blocked by 10 μg of atropine. 4-AP (more than 30 μg) potentiated significantly the negative chronotropic and inotropic responses to intramural parasympathetic nerve stimulation at a frequency of 3–30 Hz. The potentiation of the responses to stimulation was greater at a low than at a high frequency of stimulation. Potentiation of the chronotropic effect was greater than that of the inotropic one. 4-AP in the doses used did not change the negative chronotropic and inotropic responses to injection of ACh into the sinus node artery. These results suggest that the potentiation by 4-AP of the cardiac responses to parasympathetic nerve stimulation was due to an increase in release of ACh from nerve terminals but not to an increase in responsiveness at the effector site.

Suppression of Strychnine on the Chronotropic and Inotropic Effects in the Isolated, Blood-Perfused Canine Atrium

In the isolated, blood-perfused dog atrium, strychnine evoked dose-dependent decreases in atrial rate and contractile force, and the negative cardiac effects were not influenced by atropine. Strychnine suppressed dose-dependently the negative chronotropic and inotropic responses to intramural parasympathetic nerve stimulation, but not the response to acetylcholine. These results suggest that strychnine has direct cardiac depressant properties and inhibits the release of acetylcholine from parasympathetic nerve terminals in isolated dog atrium.

Effects of dopamine on sinoatrial conduction in isolated, blood-perfused dog atria.

Dopamine, administered at a constant infusion rate of 1-2 micrograms/min into the cannulated sinus node artery of the isolated dog atrium, decreased sinus cycle length (SCL) from 630 +/- 19 to 501 +/- 22 msec (mean +/- SEM, 38 trials in 12 atria). However, on sinoatrial conduction time (SACT) estimated by a constant atrial pacing method, dopamine produced 2 types of response (shortening and lengthening) with sinus tachycardia. In 24 trials in 11 atria, the drug decreased SACT from 86 +/- 8 to 56 +/- 4 msec, and in 14 trials in 6 atria it increased SACT from 67 +/- 7 to 101 +/- 9 msec. In general, the effects of dopamine on SACT were dependent on the control levels of SCL: dopamine caused a reduction of SACT at small levels of SCL and a prolongation at large levels. At a control sinus rate of 120 beats/min, dopamine usually shortened SACT. Dopamine-induced shortening of SACT was blocked by a beta-adrenoceptor blocker, propranolol, and an uptake blocker, imipramine, but not by a dopaminergic inhibitor, sulpiride. Furthermore, dopamine-induced lengthening of SACT tended to be suppressed by propranolol, but not by sulpiride. It is concluded that the dopamine-induced changes in SACT are mediated via beta-adrenergic mechanism and partially due to a tyramine-like action.

Double-peaked positive chronotropic responses of isolated and cross-perfused dog atria to ATP.

The effects of a large amount of adenosine and ATP (100 to 3000 micrograms) were investigated on sinus rate and developed tension, using the isolated dog atrium which was perfused with arterial blood from a heparinized donor dog. Each of the substances used for study was administered into the cannulated sinus node artery of the isolated atrium. Adenosine caused monophasic negative chronotropic and inotropic effects in a dose-related manner. However, ATP induced two-peaked positive chronotropic phases during a long-lasting negative chronotropic phase, i.e., initially, brief positive (t-1) effects and secondarily, relatively longer positive chronotropic (t-2) effects. These responses were repetitively induced during the experiment. The t-1 and t-2 were not influenced by treatment with propranolol which significantly blocked the positive chronotropic effect of norepinephrine. Aminophylline treatment significantly suppressed t-2 but not t-1. Quinidine (100-1000 micrograms) did not affect either the t-1 or t-2. It is suggested that ATP-induced tachycardia in the dog is partially due to activation of the P1-purinoceptors named by Burnstock.

DOUBLE-PEAKED POSITIVE CHRONOTROPIC RESPONSES OF ISOLATED AND CROSS-PERFUSED DOG ATRIA TO ATP

The effects of a large amount of adenosine and ATP (100 to 3000 μg) were investigated on sinus rate and developed tension, using the isolated dog atrium which was perfused with arterial blood from a heparinized donor dog. Each of the substances used for study was administered into the cannulated sinus node artery of the isolated atrium. Adenosine caused monophasic negative chronotropic and inotropic effects in a dose-related manner. However, ATP induced two-peaked positive chronotropic phases during a long-lasting negative chronotropic phase, i.e., initially, brief positive (t-1) effects and secondarily, relatively longer positive chronotropic (t-2) effects. These responses were repetitively induced during the experiment. The t-1 and t-2 were not influenced by treatment with propranolol which significantly blocked the positive chronotropic effect of norepinephrine. Aminophylline treatment significantly suppressed t-2 but not t-1. Quinidine (100-1000 μg) did not affect either the t-1 or t-2. It is suggested that ATP-induced tachycardia in the dog is partially due to activation of the P1-purinoceptors named by Burnstock.

CARDIOVASCULAR EFFECTS OF DOBUTAMINE, DOPAMINE AND ISOPROTERENOL ON THE WHOLE ANIMAL AND ISOLATED CROSS-PERFUSED ATRIUM IN DOGS

The right atrium of the dog was isolated and cross-perfused with arterial blood of another (donor) dog anesthetized with pentobarbital. When dobutamine, dopamine or isoproterenol was administered intravenously to the donor dog, the following changes were elicited in them: Dobutamine caused slight biphasic changes in heart rate. Dopamine induced biphasic changes in blood pressure and heart rate. Isoproterenol oroduced a decrease in blood pressure and a marked increase in heart rate. All these 3 catecholamines, however, induced positive chronotropic and inotropic effects in a similar fashion on the isolated atrium cross-perfused with blood of the donor dog. When injected into the cannulated sinus node artery of the isolated dog atrium, dobutamine, dopamine or isoproterenol induced monophasic positive chronotropic and inotropic effects in a dose-related manner, although the potency of each of the drugs was different. Dobutamine-induced effects were abolished by propranolol, but not modified by imicramine which suppressed significantly tyramine- and dopamine-induced effects. The difference in dobutamine-induced chronotropic effect between the whole animal and the isolated atrium may be due mainly to modification by extracardiac factors in the whole animal.