Inhibition of delayed rectifier K+ current by dofetilide and E-4031 differentially affects electrical cardiac responses to vagus stimulation in anesthetized dogs.

Vagal activation influences various cardiac functions as well as occurrence of arrhythmias. Inhibition of a rapid type of delayed rectifier K+ current (I[Kr]) has been reported to be effective for the treatment of both ventricular and supraventricular arrhythmias. However, it is unknown how I[Kr] inhibition modulates the cardiac responses to vagal activation in situ. We analyzed the effects of I[Kr] inhibitors, dofetilide and E-4031, and a class I antiarrhythmic agent, disopyramide, on electrical cardiac responses to vagus stimulation in anesthetized dogs. Dofetilide (0.003-0.3 micromol/kg, i.v.), E-4031 (0.01-1 micromol/kg, i.v.) and disopyramide (2.9-29 micromol/kg, i.v.) prolonged sinus cycle length (SCL), right atrial effective refractory period (AERP) and ventricular effective refractory period (VERP) dose-dependently. During cervical vagus stimulation-induced prolongation of SCL, atrio-His (AH) interval and VERP and shortening of AERP, dofetilide and E-4031 inhibited the prolongation of SCL but potentiated the shortening of AERP. Dofetilide and E-4031 did not affect prolongations of AH interval and VERP. On the other hand, disopyramide inhibited all electrical cardiac responses to vagus stimulation. These results suggest that I(Kr) inhibition differentially modulate cardiac responses to vagus activation probably due to a different role of I(Kr) in each cardiac function in the heart in situ.

Inhibition by E-4031 of the prolongation of the first returning cycle length after overdrive in anesthetized dog hearts.

Prolongation of the functional recovery of the sinoatrial (SA) nodal pacemaker activity after overdrive, "overdrive suppression," is determined by intrinsic pacemaker activity, pacemaker site, and SA conduction. We investigated the effects of E-4031, a blocker of a rapid type of the delayed rectifier K+ current (Ikr) and stimulation of the intracardiac parasympathetic nerves to the SA nodal region (SAP) on the prolongation of the first returning cycle length (1st RCL) after overdrive in autonomically decentralized hearts of open-chest anesthetized dogs. Second and third RCLs also were measured. We determined SA node recovery time (SNRT) and corrected SNRT (CSNRT) after atrial pacing at rates of 120, 150, and 200% of the control rate for 1 min and also determined SA conduction time (SACT) by the constant-atrial-pacing method. E-4031 (0.1-3 micromol/kg i.v.) increased the sinus cycle length (SCL) and SNRT dose dependently. However, E-4031 decreased CSNRT when the pacing rate was low or the number of pacing stimuli was small, although the agent did not induce a significant change in CSNRT when sufficient pacing stimuli were applied. E-4031 decreased SACT dose dependently. After E-4031 treatment. we observed changes in atrial electrical configurations, suggesting the possibility of pacemaker shift. When SAP stimulation increased SCL, SNRT, CSNRT, and SACT, E-4031 selectively inhibited the prolongation of SCL by SAP stimulation but did not affect the prolongation of CSNRT or SACT. These results suggest that functional recovery of the SA nodal pacemaker activity after overdrive is regulated by Ikr at least in part and that Ikr inhibition attenuates prolongation of the SCL but not the 1st RCL induced by parasympathetic nerve activation in the heart in situ.

Effects of low temperature on the chronotropic and inotropic responses to zatebradine, E-4031 and verapamil in isolated perfused dog atria.

We investigated the effects of hypothermia (25 degrees C) on the chronotropic and inotropic effects of zatebradine (a blocker of hyperpolarization-activated inward current, I(f)), E-4031 (a blocker of the rapid type of the delayed rectifier K+ current, I(Kr)) and verapamil, and on the positive cardiac responses to isoproterenol after treatment with zatebradine and E-4031 in isolated, blood-perfused dog atria. Hypothermia shifted the dose-response curves to the right for the negative chronotropic and inotropic effects of verapamil and for the negative chronotropic and positive inotropic effects of zatebradine, but not for the negative chronotropic and positive inotropic effects of E-4031. Hypothermia attenuated the positive chronotropic response to isoproterenol or Bay k 8644 (an L type Ca2+ channel agonist) and was attenuated more than the inotropic one. Zatebradine selectively inhibited the positive chronotropic response to isoproterenol at a normal temperature, but in hypothermia, it inhibited neither the chronotropic nor inotropic responses. E-4031 did not affect the positive responses to isoproterenol. These results suggest that verapamil and zatebradine but not E-4031 influence the atrial rate and contractile force much less in hypothermia than in normothermia and that the I(f) and inward Ca2+ current are sensitive to hypothermia in the heart.

Parasympathetic inhibition of sympathetic effects on atrioventricular conduction in anesthetized dogs.

To investigate the selective parasympathetic control of atrioventricular (AV) conduction during sympathetic activation, we studied the effects of cervical vagus nerve stimulation on the positive dromotropic responses to sympathetic interventions before and after surgical dissection of dual fatty tissues at the junction of the inferior vena cava and inferior left atrium and at the right atrial side of the atrial junctions of the right pulmonary veins in open-chest anesthetized dogs. In atrial-paced hearts, vagus stimulation at low frequencies prolonged atrio-His (A-H) interval and at high frequencies induced second- and third-degree AV blocks. Vagus stimulation additively prolonged A-H interval shortened by stimulation of the ansae subclaviae or isoproterenol infusion. After dissection of dual fatty tissues, vagus stimulation prolonged A-H interval by only 7%. However, during sympathetic stimulation but not during isoproterenol infusion, vagus stimulation prolonged the shortened A-H interval. Atropine abolished the responses to vagus stimulation. These results suggest that even during sympathetic activation, regional vagus inputs selectively control A-H interval, and even after denervation of the regional parasympathetic nerves, presynaptic parasympathetic inhibition of the positive cardiac responses to sympathetic activation works in the heart in situ.

Effects of PACAP-38 on the SA nodal pacemaker activity in autonomically decentralized hearts of anesthetized dogs.

Pituitary adenylate cyclase-activating polypeptide (PACAP) receptors exist, but the physiologic role of PACAP is unclear in the heart in situ. We investigated effects of PACAP-38 on sinus rate and on the negative chronotropic response to acetylcholine (ACh) or stimulation of the intracardiac parasympathetic nerve fibers to the sinoatrial nodal region in the automatically decentralized heart of the open chest, anesthetized dog. PACAP-38 (0.1-1 nmol) injected directly into the sinus node artery caused transient positive followed by negative chronotropic responses. Both pretreatment with atropine and tetrodotoxin inhibited the negative chronotropic responses to PACAP-38. However, hexamethonium did not block the negative responses to PACAP-38. After treatment with PACAP-38 (0.1-1 nmol), ACh induced atrial fibrillation significantly (p < 0.01). On the other hand, the negative chronotropic responses to intracardiac parasympathetic stimulation were not changed. These results suggest that (a) PACAP-38 induces negative chronotropic responses and liberates ACh from intracardiac postganglionic parasympathetic nerves, and that (b) PACAP-38 reduces ACh-induced atrial fibrillation threshold in the dog heart in situ.

Atrioventricular junctional rhythm induced by sympathetic stimulation in E-4031-treated dog hearts.

To investigate the role of delayed rectifier potassium current (IK) on the sympathetic control of the heart, we studied the effects of E-4031, a blocker of the rapidly activating type of IK (IKr), on the chronotropic, dromotropic, and inotropic responses to sympathetic nerve stimulation in the autonomically decentralized hearts of open-chest anesthetized dogs, E-4031 (0.01-3 mumol/kg intravenously, i.v.) decreased the heart rate (HR) dose dependently without affecting other cardiac functions. After E-4031 treatment, cardiac sympathetic nerve stimulation changed the sinus rhythm to the atrioventricular (AV) junctional rhythm in 6 of 11 anesthetized dogs (55%). In three of six junctional rhythm hearts, sinus rhythm supervened during sympathetic stimulation for 2 min. The number of pacemaker shifts to junctional rhythm increased as the dose of E-4031 was increased. However, E-4031 attenuated neither the positive chronotropic, dromotropic, nor right atrial and ventricular inotropic responses to sympathetic nerve stimulation. These results suggest that IKr inhibition may induce the AV junctional rhythm due to the combination of the different participation of IKr, the different resting potentials, and the different sensitivity to sympathetic activation among cardiac pacemaker cells.

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 bradycardiac 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 Ca2+ channel-mediated positive cardiac responses in isolated dog hearts.

Parasympathetic inhibition of sympathetic effects on sinus rate in anesthetized dogs.

The intracardiac parasympathetic neural elements that control sinus rate are found in the fatty tissue overlying the atrial junctions of the right pulmonary veins of mammalian hearts. We refer to these nerves as the sinus rate-related parasympathetic nerves (SRRPN). Thus, to elucidate the role of SRRPN, we studied the effects of cervical vagus stimulation on the positive chronotropic responses to cardiac sympathetic nerve stimulation and isoproterenol infusion before and after the SRRPN were removed in the open-chest anesthetized dog heart. Before SRRPN denervation, cervical vagus stimulation suppressed the sinus rate and the positive chronotropic response to sympathetic nerve stimulation or isoproterenol infusion. After SRRPN denervation, cervical vagus stimulation hardly decreased the sinus rate. On the other hand, even after SRRPN denervation, cervical vagus stimulation suppressed the rate increased by sympathetic stimulation. Cervical vagus stimulation also attenuated the sinus rate increased by isoproterenol. The inhibition by vagus stimulation of the chronotropic response to sympathetic stimulation was greater than that of the response to isoproterenol. The attenuation by cervical vagus stimulation was abolished by atropine. These results suggest that 1) a small number of vagus nerves to the sinoatrial nodal area different from the SRRPN decrease the sinus rate increased by adrenergic interventions and 2) the same activation that causes relatively small effects on sinus rate is capable of causing much larger changes in sinus rate during increased sympathetic tone or in the case of beta-adrenoceptor agonist treatment in the heart in situ.

Selective inhibition by a class III antiarrhythmic agent, E-4031, of the negative chronotropic response to parasympathetic stimulation in anesthetized dogs.

To investigate the influence of a class III antiarrhythmic agent, E-4031, on the vagus control of the heart, we studied the effects of E-4031 on the chronotropic, dromotropic and inotropic responses to parasympathetic stimulation in the autonomically decentralized hearts of the open-chest, anesthetized dogs. E-4031 (0.01-3 mumol/kg i.v.) decreased heart rate dose-dependently without affecting atrioventricular (AV) conduction time, first derivative of "a" wave component of the right atrial pressure (RA dP/dt), maximum rate of the right ventricular pressure development (RV +dP/dt) and arterial blood pressure. When cervical vagus stimulation decreased the heart rate, RA dP/dt and RV +dP/dt and prolonged the AV conduction time, E-4031 antagonized the negative chronotropic response in a dose-dependent manner but affected neither dromotropic nor atrial inotropic responses. E-4031 at a high dose of 3 mumol/kg i.v. attenuated the ventricular inotropic response. ID50 for the chronotropism was 0.20 +/- 0.05 mumol/kg. Stimulation of the selective intracardiac parasympathetic nerves to the sinoatrial nodal area decreased the heart rate and RA dP/dt without a dromotropic response. E-4031 antagonized the negative chronotropic response to the stimulation but not the inotropic response. Stimulation of the selective intracardiac parasympathetic nerves to the AV nodal area prolonged the AV conduction time without a chronotropic response. E-4031 at a high dose of 3 mumol/kg i.v. attenuated the negative dromotropic response to the stimulation by 35 +/- 10%. These results suggest that E-4031 preferentially blocks the negative chronotropic response to vagus stimulation without significantly affecting other cardiac responses at a site distal to the muscarinic receptor in the heart in situ.

Acute presynaptic inhibition by doxorubicin of negative chrono- and inotropic responses to parasympathetic nerve stimulation in isolated, blood-perfused dog atrium.

The clinical use of doxorubicin, an anthracycline antineoplastic agent, is limited by its cardiotoxicity. Although several previous reports have shown neurotoxic effects of doxorubicin, there is little information about the acute effects of doxorubicin on the autonomic nerve functions in the heart. Accordingly, to evaluate the effects of doxorubicin on the cardiac responses to autonomic nerve activation, we studied the effects of doxorubicin on the negative chrono- and inotropic responses to intracardiac parasympathetic nerve stimulation and acetylcholine (ACh), and the positive chrono- and inotropic responses to norepinephrine (NE) in the isolated, blood-perfused dog atrium. Doxorubicin (0.01-3 mumol), injected into the sinus node artery of the isolated atrium, induced negative inotropic effects dose dependently and weak negative chronotropic effects. Doxorubicin inhibited the negative chrono- and inotropic responses to parasympathetic nerve stimulation dose dependently. However, doxorubicin affected neither the negative chrono- and inotropic responses to ACh nor the positive chrono- and inotropic responses to NE. These results indicate that doxorubicin interacts with neither muscarinic receptors nor beta-adrenoceptors and suggest that doxorubicin inhibits the negative cardiac responses to parasympathetic nerve activation due to the inhibition of ACh release from nerve varicosities in the heart.

Effect of thromboxane A2 synthetase inhibitor, OKY-046, on sputum in chronic bronchitis and diffuse panbronchiolitis.

The mechanisms of excessive sputum production are only partially understood. We speculated that a selective thromboxane (Tx) A2 synthetase inhibitor, OKY-046, now used in the treatment of asthma in Japan, could decrease excess sputum production in patients with chronic airways disease. To test this hypothesis, we carried out a double-blind, placebo-controlled study of the effects of OKY-046, administered orally at 400 mg.day-1, on the sputum of patients with chronic bronchitis and patients with diffuse panbronchiolitis. Patients treated with OKY-046 showed a significant decrease (22%) in sputum volume after 1 month, and a 39% decrease after 3 months. Although the rheological properties of the sputum and the concentrations of fucose and immunoglobulin (Ig) A in the sputum remained unchanged, significant decreases were observed in the concentrations of total protein, albumin, sialic acid and phospholipid. Since albumin and fucose are chemical markers of plasma exudation and mucus secretion, respectively, whilst sialic acid and phospholipid are derived both from serum and mucus, our results indicate that this TxA2 synthetase inhibitor reduced sputum volume by inhibiting microvascular leakage in the airway. OKY-046 may, therefore, be of value in the treatment of chronic bronchitis and diffuse panbronchiolitis.

Selective inhibition by zatebradine and discrete parasympathetic stimulation of the positive chronotropic response to sympathetic stimulation in anesthetized dogs.

To investigate how to reduce the positive chronotropic response to sympathetic nerve activation selectively without affecting other cardiac actions, we studied the effects of zatebradine, an inhibitor of the hyperpolarization-activated current (I(f)), verapamil and parasympathetic nerve stimulation on the positive chronotropic, dromotropic and inotropic responses to sympathetic nerve stimulation in the autonomically decentralized heart of the open-chest anesthetized dog. Parasympathetic input was activated by stimulation of the cervical vagus (CV) or parasympathetic nerves to the sinoatrial (SA) nodal region (SAP). Zatebradine (0.1-3 mumol/kg i.v.) decreased the heart rate but not other cardiac responses to sympathetic nerve stimulation, i.e., a wave component of the right atrial pressure (RAP), the first derivative of the RAP (dRAP/dt), atrioventricular (AV) conduction time (AVCT), right ventricular pressure (RVP) and its first derivative (dRVP/dt). Zatebradine (1 mumol/kg) inhibited basal heart rate by 28% but inhibited the chronotropic response to sympathetic stimulation by 85%. Verapamil (0.06-0.6 mumol/kg i.v.) attenuated the increases in heart rate, RVP and dRVP/dt elicited by sympathetic stimulation but potentiated shortening of the AVCT from the prolonged basal AVCT. The SAP stimulation attenuated the heart rate and dRAP/dt responses to sympathetic stimulation without affecting other cardiac responses, whereas CV stimulation decreased the positive chronotropic and atrial and ventricular inotropic responses. Cervical vagus stimulation did not change the positive dromotropic response. These results demonstrate that in contrast to CV nerve activation or verapamil, zatebradine and SAP stimulation cause bradycardia but preserve the myocardial contractile force and AVCT in response to sympathetic nerve activation or sympathomimetic drugs in the heart in situ.