Yuji Hoyano

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.

Effects of Ritobegron (KUC-7483), a Novel Selective β3-Adrenoceptor Agonist, on Bladder Function in Cynomolgus Monkey

We evaluated the pharmacological profile of ritobegron [KUC-7483; (−)-ethyl 2-[4-(2-{[(1S,2R)-2-hydroxy-2-(4-hydroxyphenyl)-1-methylethyl]amino}ethyl)-2,5-dimethylphenyloxy]acetate monohydrochloride] and its effects on the bladder in cynomolgus monkeys by in vitro and in vivo experiments. In vitro, ritobegron decreased the resting tension of the isolated bladder in a concentration-dependent manner (EC50 8.2 ± 2.3 × 10−7 M; maximal relaxation 88.7 ± 3.7%). The β3-adrenoceptor (AR) antagonist 3-(2-allylphenoxy)-1-[(1S)-1,2,3,4-tetrahydronaphth-1-ylamino]-(2S)-2-propanol hydrochloride (SR58894A) produced a rightward shift of this concentration-response curve without altering the maximal response (pKB value 6.56 ± 0.35). In isolated atria, ritobegron increased the atrial rate only at high concentrations (EC50 6.5 ± 1.2 × 10−5 M). Ritobegron had no effect on tracheal contraction at concentrations from 10−9 to 10−4 M, and even at the highest concentration tested, 10−3 M, the maximal relaxation it induced was only 26.7 ± 8.1%. Tests of the selectivity of ritobegron for the bladder gave values of 79.3- and 1200-fold higher versus atria and trachea, respectively. In the in vivo study ritobegron significantly decreased intravesical pressure (ED50 1.44 mg/kg) without affecting either mean blood pressure or heart rate. In conclusion, ritobegron displayed potent and selective β3-AR agonistic activity and relaxed the monkey isolated bladder, and in vivo it decreased intravesical pressure without affecting cardiovascular parameters. These results suggest that ritobegron may be a promising potential agent for the treatment of overactive bladder.

Effects of silodosin and tamsulosin on the urethra and cardiovascular system in young and old dogs with benign prostatic hyperplasia

We examined whether the effects (efficacy on the urethra and hypotension) of silodosin (alpha(1A)-adrenoceptor antagonist) and tamsulosin (alpha(1A+1D)-adrenoceptor antagonist) in dogs with benign prostatic hyperplasia altered with age. We used young and old dogs, diagnosed as having benign prostatic hyperplasia by veterinarians palpation. Under anesthesia, the increase in intraurethral pressure evoked by hypogastric nerve stimulation was measured, together with the level of systemic mean blood pressure. Each drug was administered intravenously in progressively increasing doses. At the end of the experiment, the prostate was isolated from each dog, then weighed and investigated pathologically to confirm benign prostatic hyperplasia. The wet weight of the prostate was greater in old dogs with benign prostatic hyperplasia than in young dogs with benign prostatic hyperplasia. By light microscopy, hyperplasia in the prostatic epithelium was confirmed in both groups. Silodosin (0.3-300 microg/kg) dose-dependently inhibited the hypogastric nerve stimulation-induced increase in intraurethral pressure (without significant hypotensive effects) in both young and old dogs with benign prostatic hyperplasia. Tamsulosin (0.3-300 microg/kg) also dose-dependently inhibited the intraurethral pressure increase in both groups, but it had a hypotensive effect that was significantly greater in old than in young dogs with benign prostatic hyperplasia. In conclusion, as regards the effect of silodosin on intraurethral pressure, potency was similar between young and old dogs with benign prostatic hyperplasia, and it was without significant hypotensive effects. We therefore suggest that silodosin might be a good medication for lower urinary tract symptoms in patients with benign prostatic hyperplasia in all age groups.

Comparison of the Effects of Four α1-Adrenoceptor Antagonists on Ejaculatory Function in Rats

OBJECTIVE To compare the effects of four α(1)-adrenoceptor (AR) subtype-selective antagonists on ejaculatory function in rats to investigate whether the differences in their modes of action-based on their selectivities for the α(1A)-AR subtype-would be related to the prevalence of ejaculation disorder (EjD). METHODS The effects of α(1)-AR antagonists on noradrenaline-induced contractions were studied in rat isolated seminal vesicles, vas deferens, bladder trigone, and prostate. Male rats were given α(1)-AR antagonists orally and, 1 hour after the drug administration they were cohoused in pairs for 1 hour with untreated female rats certified to be in estrus. The number of copulatory plugs (NP) present after mating was measured as a marker of EjD. Drug effects on ejaculatory function (ie, on NP) were compared with those on the prostatic urethra (ie, phenylephrine-induced increase in intraurethral pressure [IUP]). RESULTS All α(1)-AR antagonists concentration-dependently inhibited noradrenaline-induced contraction in all 4 tissues, and there were no differences in the rank order of potencies (tamsulosin > silodosin > alfuzosin > naftopidil) among the tissues. All α(1)-AR antagonists dose-dependently decreased NP and inhibited the phenylephrine-induced increase in IUP. There was little difference in the dose ratio ID(50) value (dose required to produce 50% inhibition) for NP/ID(50) value for IUP response among the four drugs. Drug potencies associated NP and IUP correlated closely with affinities for the human α(1A)-AR. CONCLUSION α(1)-AR antagonists cause EjD as a class effect that depends on affinity for α(1A)-AR. Differences in α(1A)-AR selectivity would be unlikely to be related to the incidence of EjD.

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.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.

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.

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.

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.