Positive chronotropic and inotropic responses to guanosine in the isolated dog atrium.

The effects of guanosine on chronotropism and inotropism in isolated dog atria were studied in spontaneously beating preparations which were suspended in a bath perfused with arterial blood from a carotid artery of a heparinized support dog. Guanosine administered into the cannulated sinus node artery in a dose range of 30 microgram to 3 mg produced a dose-related positive inotropic and chronotropic effect. The positive responses to guanosine were not inhibited by treatment with propranolol or a non-depressant beta-blocker, carteolol, in doses which blocked responses to norepinephrine. From these results, it is concluded that guanosine has a direct effect on atrial rate and contractility.     

Inhibition of the negative chronotropic action of adenosine by caffeine in the dog

The sinus node artery was perfused in situ in 15 dogs. Injection of adenosine into the sinus node artery induced a negative chronotropic effect at doses of 1, 3 and 10 μg. The negative chronotropic response to adenosine was blocked by 300 μg to of caffeine given into the same artery. On the contrarym, the negative chronotropic response to ACh was not inhibited but rather enhanced by caffeine, so that the atrial fibrillation threshold to ACh was lowered. Neither norepinephrine nor calcium when injected into the sinus node artery inhibited the deceleration response to adenosine.     

A Na+/Ca2+ exchanger inhibitor, KB-R7943, caused negative inotropic responses and negative followed by positive chronotropic responses in isolated, blood-perfused dog heart preparations

Effects of a Na+/Ca2+ exchanger inhibitor, KB-R7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl] isothiourea methanesulfonate), on the sinoatrial nodal pacemaker activity, atrial contractility and ventricular contractility were investigated in the isolated and blood-perfused right atrium and left ventricle of the dog. KB-R7943 (0.03- 3 micromol) induced negative inotropic effects and negative followed by positive chronotropic effects in the right atrium and negative inotropic effects in the left ventricle. Neither atropine nor hexamethonium affected the cardiac responses to KB-R7943. Propranolol attenuated the positive chronotropic response to KB-R7943 but imipramine did not. Tetrodotoxin potentiated the positive chronotropic response to KB-R7943 in 6 of 11 isolated atria. When NaCl infusion increased atrial contractile force and atrial rate, KB-R7943-induced negative inotropic and chronotropic responses were attenuated in a dose-dependent manner. CaCl2 infusion potentiated the negative chronotropic response to KB-R7943 but did not affect the inotropic response significantly. On the other hand, ouabain (17 nmol) attenuated the negative inotropic response, but not chronotropic response, to KB-R7943. These results suggest that KB-R7943-induced cardiac effects relate to the Na+ activity, probably mediated through the Na+/Ca2+ exchanger, and the Na+/Ca2+ exchanger modifies the pacemaker activity and myocardial contractility in the dog heart.     

The mode of action of 4-aminopyridine on the chronotropic and inotropic responses in the isolated, blood-perfused dog heart

The effects of 4-aminopyridine (4-AP, 0.3 to 300 micrograms) on atrial rate and atrial muscle contractile force were investigated in the isolated, blood-perfused dog atrial preparation. Low doses (less than 30 micrograms) of 4-AP injected into the sinus node artery caused a dose-dependent positive inotropic response and a negligible positive chronotropic response. A higher dose (300 micrograms) of 4-AP induced a transient positive chronotropic response followed by a negative response and positive and negative inotropic responses. The negative cardiac responses to 4-AP were completely inhibited by an adequate dose of atropine and were potentiated by physostigmine. TTX or C6 suppressed partly, but significantly the negative responses induced by 4-AP. On the other hand, the negative responses to electrical stimulation (ES) were inhibited by atropine, C6 and TTX and those to ACh were blocked by atropine but not by C6 or TTX. The positive responses to 4-AP were not affected by propranolol but were potentiated by atropine. The positive inotropic response to 4-AP was significantly suppressed by a calcium channel blocker, diltiazem. From these results, it is concluded that the cardiac responses to 4-AP are composed of direct positive responses and negative responses which are related to activation of parasympathetic ganglionic neurotransmission.     

Analysis of the atypical characteristics of adenosine receptors mediating negative inotropic and chronotropic responses of guinea-pig isolated atria and papillary muscles.

1. Adenosine receptor(s) mediating negative inotropy of paced left atria, isoprenaline-stimulated paced left atria and papillary muscles, and negative chronotropy of spontaneously beating right atria were characterized. 2. Isometric tension of guinea-pig isolated paced left atria and left ventricular papillary muscles and rate of contraction of spontaneously beating right atria were recorded. Papillary muscles were pre-stimulated with isoprenaline (1x10-8 M). Concentration-response curves (CRCs) for tension or rate reduction by N6-cyclopentyladenosine (CPA), the stereoisomers of N6-(2-phenylisopropyl)adenosine ((+)-PIA and (-)-PIA), 5'-(N-carboxamido)adenosine (NECA), N6-2-(4-aminophenyl)ethyladenosine (APNEA) and N6-(3-iodobenzyl)adenosine-5'-N-methyuromide (IB-MECA) revealed a potency order of CPA=(-)-PIA>NECA in right atria and papillary muscles, which is consistent with involvement of A1-receptors. The potency order in left atria was CPA=NECA>(-)-PIA>(+)-PIA>APNEA, which is not typical of A1 adenosine receptors. Weak activity of APNEA and IB-MECA discounts involvement of A3 receptors. 3. pA2 values for the antagonism of CPA by 8(p-sulfophenyl)theophylline (8-SPT) were calculated from Schild plots (log concentration-ratio against log 8-SPT concentration), the unity slopes of which indicated competitive antagonism. The pA2 value in the papillary muscles was significantly higher than for atrial preparations, indicating a possible difference in receptor characteristics between atrial and papillary muscle responses. 4.In left and right atria there was a limit to the displacement of the CPA CRCs at higher concentrations of 8-SPT. The 8-SPT-resistant component of the response is suggested to arise from duality of coupling of a common A1 receptor through either different G proteins or G protein subunits to independent transduction pathways. The results with papillary muscles can be explained by a typical A1 receptor coupled to a single transduction pathway.     

Direct chronotropic and inotropic responses to guanosine and five different guanine nucleotides in isolated perfused dog atria.

The effects of guanosine and of five guanine nucleotides on sinus rate and contractile force were investigated using the isolated, blood-perfused dog atrium preparation. Guanosine consistently induced a positive chronotropic and inotropic effect which was not suppressed by treatment with a potent beta-adrenoceptor blocking agent, carteolol. GMP, GDP and CTP induced a biphasic chronotropic and inotropic effect. The potencies for inducing the positive effects of GMP, GDP and GTP were almost the same. However, their rank order of potency for inducing negative effects was GTP greater than or equal to GDP greater than GMP. The negative responses to GTP were not inhibited by atropine. Cyclic GMP and dibutyryl cyclic GMP produced only slight negative chronotropic and inotropic effects but not consistently even at larger doses. The positive chronotropic and inotropic responses to norepinephrine were not modified by treatment with dibutyryl cyclic GMP.     

Positive chronotropic and inotropic effects of angiotensin II in the dog heart

The effects of angiotensin II on sinus rate and atrial contractility were investigated in 17 isolated canine atria and 5 isolated paced ventricular preparations perfused with arterial blood conducted from a heparinized donor dog. When angiotensin II was injected into the cannulated sinus node artery, positive chronotropic responses were dose-dependently produced starting from the 0.01 microgram dose although inotropic responses to angiotensin II were not consistently induced. Angiotensin II produced a similar inotropic response pattern in the paced ventricular preparation. Moreover, when angiotensin II was given into the jugular vein of the donor dog, similar positive chronotropic and inotropic responses were also shown in the isolated atrium. Angiotensin II-induced positive chronotropic and slight inotropic effects were not influenced by treatment with the beta-adrenoceptor blocking agents, propranolol and carteolol, but significantly suppressed by saralasin which has been reported to be a competitive antagonist of angiotensin II. From these results, it is suggested that angiotensin II induced a positive chronotropic and slight positive inotropic effect via angiotensin II receptors in the dog heart.     

Different antagonism of the positive chronotropic and inotropic responses of the isolated, blood-perfused dog atrium to Bay k 8644 by nicardipine and verapamil

The effects of Bay k 8644, a dihydropyridine calcium agonist, on sinoatrial (SA) node pacemaker activity and atrial contractility and its interaction with the calcium antagonists, nicardipine and verapamil, were investigated in the isolated, blood-perfused dog atrium. Bay k 8644 (0.03-30 micrograms) increased sinus rate and atrial contractility dose dependently. Norepinephrine (NE) and CaCl2 dose dependently increased the sinus rate and contractility but the positive chronotropic effect of CaCl2 was much less than that of the other drugs. The positive chronotropic effect of Bay K 8644 (0.3-1 microgram) was dose dependently depressed by nicardipine at doses of 1-10 micrograms but the inotropic effect was depressed only by a large dose of 10 micrograms. After sinus arrest induced by nicardipine (10-30 micrograms), SA node pacemaker activity was readily restored by Bay k 8644 or NE. Verapamil (1-3 micrograms) also depressed the positive chronotropic effect of Bay k 8644 more effectively than the inotropic effect although it did not attenuate the chronotropic effect of NE. The inotropic interaction could not be evaluated at higher doses of antagonists because of sinus arrest. Propranolol (3 micrograms) suppressed both positive chronotropic and inotropic effects of NE but did not depress the Bay k 8644-induced responses. These results show that the antagonism between Bay k 8644 and calcium antagonists is predominant on SA node pacemaker activity in cardiac tissues.     

Effects of isoprenaline and aminophylline on the chronotropic responses of the isolated guinea-pig heart to vagal stimulation and acetylcholine

1. In isolated perfused guinea-pig hearts, isoprenaline (2 × 10^-8 g/ml) reduced the negative chronotropic responses to vagal stimulation and to exogenous acetylcholine.

2. The β-adrenoceptor antagonist LB46 (2 × 10^-8 g/ml) abolished the effect of isoprenaline in reducing vagal bradycardia. LB46 itself did not alter the responses to vagal stimulation.

3. Increase in ⁸⁶Rb efflux induced by acetylcholine was not affected by isoprenaline.

4. Aminophylline (2·3 × 10^-4 g/ml) almost abolished the negative chronotropic effect of vagal stimulation.

5. The `antivagal' effects of isoprenaline and aminophylline may be at a common site beyond the level of the cardiac β-adrenoceptors, perhaps related to cyclic-3′,5′-AMP and/or sodium transport.

     

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.     

The effects of imipramine, mianserin and trazodone on the chronotropic, inotropic and coronary vascular responses in the isolated perfused rat heart

1. The cardiovascular effects of the tricyclic antidepressant imipramine and two second generation antidepressants mianserin, a tetracyclic, and trazodone, a triazolopyridine derivative, were investigated in the isolated perfused rat heart. 2. Imipramine caused cardiac slowing and a negative inotropic effect at 2.5 microM after 30 min of perfusion. Conversely mianserin and trazodone had no effect on heart rate at 5 microM with inotropic state remaining above control values after 30 min of perfusion. 3. Varying effects on coronary flow, which appear to correlate well with the documented receptor actions of each drug, were demonstrated. Imipramine caused a decrease in coronary flow at 1.25 and 2.5 microM, followed by an increase at 10 microM. Mianserin decreased coronary flow at all concentrations between 1 and 20 microM. Trazodone elicited a marked elevation in coronary flow over the dose range of 2.5 to 250 microM. 4. The results in this model suggest that although the second generation agents appear to cause less cardiodepression all three agents elicit quantitatively different coronary vascular responses.     

Effects of potassium channel blockers on the negative inotropic responses induced by cromakalim and pinacidil in guinea pig atrium.

The K+ channel openers cromakalim and pinacidil induced a concentration-dependent reduction in atrial contraction force with EC50 values of 25 +/- 2 and 37 +/- 2 mumol/l, respectively. This depressant effect was antagonised by 50 mumol/l tacrine which displaced the concentration-response curves of cromakalim and pinacidil to the right. The respective DR50 values were 3.8 and 2.3. Increasing the tacrine concentration (100 and 500 mumol/l) produced no additional effect on the concentration-response relationships. Addition of 1 mumol/l atropine enhanced the antagonism due to tacrine by increasing the DR50 value from 3.8 to 6.5 for cromakalim and from 2.3 to 5.2 for pinacidil. Glibenclamide, an ATP-sensitive K+ channel blocker, competitively inhibited the negative inotropic effects of cromakalim and pinacidil. The respective dissociation constants for glibenclamide against cromakalim and pinacidil were 0.57 and 0.35 mumol/l. Neither apamin nor variation in external Ca2+ concentration affected the negative inotropic effects of the K+ channel openers. It was suggested that the mechanical effects of cromakalim and pinacidil are mediated through the ATP-sensitive K+ channels in the heart.     

Positive inotropic and negative chronotropic effects of (-)-cis-diltiazem in rat isolated atria.

1. The cardiovascular effects of (-)-cis-diltiazem, an optical isomer of diltiazem, were studied in the isolated atrium and aortic strip. (-)-cis-Diltiazem (30 microM or more) increased the developed tension of the rat left atrium, while (+)-cis-diltiazem (1 microM or more) decreased it. 2. (-)-cis-Diltiazem (1 to 100 microM) decreased the rate of spontaneous beating in the right atrium as did (+)-cis-diltiazem. 3. The potency of the positive inotropic action of (-)-cis-diltiazem was almost the same as that of ouabain in the rat left atrium, but in the guinea-pig left atrium it was considerably weaker than that of ouabain. 4. In both endothelium-intact and endothelium-denuded aortic strips, (-)-cis-diltiazem relaxed the Ca(2+)-induced contraction. In the endothelium-intact rat aortic strip depolarized by 15 mM KCl, Bay K 8644, a calcium channel agonist, increased the contractile force, whereas (-)-cis-diltiazem did not. 5. These results indicate that (-)-cis-diltiazem has a positive inotropic action in isolated atria in rats and guinea-pigs, but the mode of positive inotropic action of (-)-cis-diltiazem is different from that of ouabain or Bay K 8644.     

Selective reserpine-induced supersensitivity of the positive inotropic and chronotropic responses to isoprenaline and salbutamol in guinea-pig isolated atria.

1 Dose-response curves for the positive inotropic and chronotropic responses to isoprenaline were obtained in atria from untreated guinea-pigs and those receiving various reserpine pretreatments. 2 Tension responses were unaffected, whereas rate responses were depressed by the lowest dose of reserpine (0.05 mg/kg i.p. at 24 hours). 3 With larger 24 h doses and a 3 day pretreatment, the rate and tension dose-response curves were progressively displaced to the left, indicating supersensitivity which was greater for tension at each pretreatment. 4 No supersensitivity to histamine or Ca2+ could be detected, leading to the conclusion that it was selective for the beta-adrenoceptor agonists possibly at the receptor level. 5 As an indication of the adrenergic neurone depleting effectiveness of each reserpine dosage, preparations were exposed to test doses of beta-phenylethylamine. 6 Salbutamol was a partial agonist in untreated atria, the maximum rate (63.3%) and tension (10.0%) responses being less than those for isoprenaline. In atria from reserpine pretreated animals the supersensitivity was revealed as an increase of this maximum compared with isoprenaline. 7 The significance of this observation in relation to the possible mechanism of the supersensitivity is discussed.     

Positive inotropic and negative chronotropic effects of proton pump inhibitors in isolated rat atrium

The effects of three specific H+/K+-ATPase inhibitors (omeprazole, lansoprazole and SCH 28080 (2-methyl-8-(phenylmethoxy)-imidazo[1,2-a] pyridine-3-acetonitrile)) were investigated on the mechanical and electrophysiological properties of rat atrium, in vitro. Omeprazole (100-300 microM), lansoprazole (100-300 microM) and SCH 28080 (10-100 microM) increased the amplitude of contractions and decreased the beating rate. These effects are reversible, reproducible and correlated with their order of potency as gastric H+/K+-ATPase inhibitors; SCH 28080 > omeprazole = lansoprazole. Cardiac effects of proton pump inhibitors were not inhibited with phentolamine (5 microM), propranolol (15 microM), atropine (1 microM), ouabain (2 microM), theophylline (300 microM) and milrinone (100 microM). Ouabain-induced increase in beating rate and contracture development were antagonized by H+/K+-ATPase inhibitors. Ouabain increased the positive inotropic effect of H+/K+-ATPase inhibitors. Lansoprazole (300 microM) significantly prolonged the duration of action potentials in rat atrial cells. H+/K+-ATPase may play a crucial role in the mechanical and electrophysiological properties of rat atrial myocardium.     

A negative inotropic effect of omega-conotoxin GVIA in isolated, blood-perfused dog right atria.

Using an isolated, blood-perfused canine atrial preparation, we investigated the effects of omega-conotoxin GVIA (omega-CgTX), a blocker of N-type voltage-operated calcium channels, on the atrial contractile force and sinus rate. Omega-CgTX in a dose range of 0.3-3 nmol gradually attenuated the atrial contractile force in a dose-dependent manner, but did not affect the sinus rate, indicating that it can depress contractility but not pacemaker activity.