Youichi Yabuuchi

Characterization of a novel aquaretic agent, OPC‐31260, as an orally effective, nonpeptide vasopressin V2 receptor antagonist

1 OPC‐31260, a benzazepine derivative, has been studied for its ability to antagonize the binding of arginine vasopressin (AVP) to receptors in rat liver (V1) and kidney (V2) plasma membranes, for antagonism of the antidiuretic action of AVP in alcohol‐anaesthetized rats and for diuretic action in conscious normal rats. 2 OPC‐31260 caused a competitive displacement of [3H]‐AVP binding to both V1 and V2 receptors with IC50 values of 1.2 ± 0.2 × 10−6 m and 1.4 ± 0.2 × 10−8 m, respectively. 3 OPC‐31260 at doses of 10 to 100 μg kg−1, i.v., inhibited the antidiuretic action of exogenously administered AVP in water‐loaded, alcohol‐anaesthetized rats in a dose‐dependent manner. OPC‐31260 did not exert an antidiuretic activity suggesting that it is not a partial V2 receptor agonist. 4 After oral administration at doses of 1 to 30 mg kg−1 in normal conscious rats, OPC‐31260 dose‐dependently increased urine flow and decreased urine osmolality. The diuretic action of OPC‐31260 was characterized as aquaresis, the mode of diuretic action being different from previously known diuretic agents such as furosemide, hydrochlorothiazide and spironolactone. 5 The results indicate that OPC‐31260 is a selective V2 receptor antagonist and behaves as an aquaretic agent. OPC‐31260 will be a useful tool in studying the physiological role of AVP and in the treatment of various conditions characterized by water retention.

OPC-21268, a vasopressin V1 antagonist, produces hypotension in spontaneously hypertensive rats.

We studied the hypotensive effects of OPC-21268, an orally effective nonpeptide vasopressin V1 receptor antagonist, in spontaneously hypertensive rats (SHR) and stroke-prone SHR (SHRSP). OPC-21268 was given intravenously to conscious, freely moving SHR and SHRSP. We used young and aged animals to examine the contribution of vasopressin to the development and maintenance of hypertension in both types of rats. In SHR, hypertension was fully established at 38 weeks of age, and intravenous injection of OPC-21268 produced slight hypotensive effects at either 38 or 70 weeks of age. In SHRSP, hypertension developed at 25 weeks of age, and blood pressure was sustained at a high level (approximately 250 mm Hg systolic blood pressure) thereafter. Intravenous administration of OPC-21268 did not cause hypotensive effects in young rats at 15 weeks, but at 25 weeks a significant decrease in blood pressure was observed. Furthermore, in the malignant state of SHRSP (35 to 41 weeks), OPC-21268 significantly decreased mean blood pressure by 32.4 +/- 7.9 mm Hg (mean +/- SEM) at 3 mg/kg IV, and the decrease was dose dependent (0.3 to 3.0 mg/kg). Plasma vasopressin concentrations were increased in a more malignant phase of SHRSP at 45 weeks of age, whereas at other ages of SHRSP or in SHR, plasma vasopressin levels were not increased. These results suggest that vasopressin plays an important role through V1 receptors in the maintenance of hypertension, at least in the malignant phase of SHRSP, and OPC-21268 may be therapeutically useful in the treatment of some types of hypertension.

Cardiovascular actions of OPC-18790: A novel positive inotropic agent with little chronotropic action

SummaryOPC-18790 [(±)-6-[3-(3,4-dimethoxy-benzylamino)- 2 - hydroxypropoxy] - 2(1H) - quinolinone], a novel positive inotropic agent, was investigated in several in vitro and in vivo experiments to elucidate its cardiovascular effects and its mechanism of action. In isolated blood-perfused dog heart preparations, OPC-18790 increased contractile force at 10 to 1,000 nmol i.a.; increased coronary arterial blood flow at 30 to 1,000 nmol; and deceased sinus rate slightly at 1,000 nmol. Atrio-ventricular nodal conduction was slightly facilitated with OPC-18790 (10 to 1,000 nmol), whereas ventricular automaticity tended to decrease. OPC-18790 (10−6 to 10−4 M) increased contractile force in isolated ventricular muscles of dogs, cats, rabbits and guinea pigs but not rats. OPC-18790 increased left ventricular contractile force dosedependently in anesthetized open-chest dogs and in conscious dogs with slight or no changes in heart rate and blood pressure. The positive inotropic effect of OPC-18790 was not affected by β-blockade. OPC-18790 (10−5 to 10−4 M) prolonged the duration of action potential in guinea pig papillary muscles. Na+, K+-ATPase was not inhibited, but peak-III phosphodiesterase (low Km cyclic AMP specific fraction, inhibited by cyclic GMP) was inhibited by OPC-18790 (IC50 = 0.41 × 10−6 M) in dog myocardium. However, such an inhibitory action of phosphodiesterase can hardly be reconciled with the lack of a positive chronotropic effect shown by OPC-18790. Thus, these results suggest that OPC-18790 may have an additional mechanism. The cardiovascular effects revealed by this study suggest that OPC-18790 may exert a beneficial effect in the treatment of congestive heart failure.

Antihypertensive activity of OPC-13340, a new potent and long-acting dihydropyridine calcium antagonist, in rats

The antihypertensive action of OPC-13340, a new dihydropyridine, was studied in rats and compared with the action of nicardipine and other dihydropyridines. OPC-13340 showed more potent and longer hypotensive action than nicardipine when administered either intraveneously (i.v.) or orally in normotensive and hypertensive rats. Among 6 compounds tested, (OPC-13340, nifedipine, nitrendipine, nisoldipine, nicardipine and diltiazem), OPC-13340 was the most potent and long-acting when administered orally to spontaneously hypertensive rats (SHR). Tachycardia after administration of OPC-13340 was less or diminished earlier than that of nicardipine. Oral administration of OPC-13340 (3 mg/kg) once daily for 13 days did not cause any rebound phenomena in SHR. The compound inhibited Ca- or K-induced contractions in isolated rat aorta and shortened action potential duration in guinea pig papillary muscle, suggesting Ca channel blocking action. OPC-13340 might be useful as a drug for once-daily therapy of essential hypertension.

Actions of procaterol (OPC-2009), a new β2-adrenoceptor stimulant, on pulmonary resistance, contractions of the soleus muscle, and cardiovascular system of the anaesthetized cat

The β‐adrenoceptor stimulant actions of procaterol hydrochloride [5‐(1‐hydroxy‐2‐iso‐propylaminobutyl)‐8‐hydroxycarbostyril hydrochloride hemihydrate] were compared with those of isoprenaline, orciprenaline and salbutamol on pulmonary resistance, contractions of the soleus muscle, heart rate and diastolic blood pressure in the anaesthetized cat. All four drugs reduced the 5‐HT‐induced increase in pulmonary resistance, decreased the tension of incomplete tetanic contractions of the soleus muscle and the diastolic blood pressure, and increased the heart rate in a dose‐related manner. The duration of the bronchodilator action of procaterol was far longer than that of orciprenaline or salbutamol while isoprenaline had the shortest duration. Procaterol was about 1·5 times more potent, and orciprenaline and salbutamol about 80 and 10 times less potent than isoprenaline in reducing the 5‐HT‐induced increase in pulmonary resistance and in decreasing the tension of incomplete tetanic contractions of the soleus muscle. Procaterol, orciprenaline and salbutamol were 3·5, 91·9 and 43·9 times less potent than isoprenaline in increasing the heart rate. Procaterol, orciprenaline and salbutamol were 3·4, 130·2 and 12·9 times less potent than isoprenaline in decreasing the diastolic blood pressure. Calculation of selectivity for bronchial vs cardiac β‐adrenoceptors indicates that procaterol and salbutamol have a similar degree of selectivity for β2‐adrenoceptors mediating the bronchodilation, and that orciprenaline was an essentially unselective stimulant. Furthermore, the results support the suggestion that at present it is not possible to separate the bronchodilating and tremor‐enhancing properties of β‐adrenoceptor stimulants.

Cilostazol inhibits modified low-density lipoprotein uptake and foam cell formation in mouse peritoneal macrophages

Internalization of modified low-density lipoprotein (LDL) via macrophage scavenger receptors (e.g. scavenger receptor A and CD36) is thought to play a crucial role in the development of atherosclerotic lesions. Cilostazol, an antiplatelet agent with selective phosphodiesterase 3 inhibitory action, has been reported to ameliorate atherosclerosis in mouse models. However, the effect of cilostazol on modified LDL uptake in macrophages is not known. Thus, we investigated the effect of cilostazol on LDL uptake in mouse peritoneal macrophages (MPM). Cilostazol significantly inhibited oxidized and acetylated LDL uptake in MPM, while cyclic AMP (cAMP)-elevating agents, db-cAMP and other phosphodiesterase 3 or 4 inhibitors, did not inhibit the uptake. Cilostazol did not change cytosolic cAMP levels in MPM, and a protein kinase A (PKA) inhibitor did not influence the inhibitory effects of cilostazol. Cilostazol decreased scavenger receptor A but not CD36 expression. Moreover, cilostazol significantly inhibited foam cell formation, which was represented by an increase in esterified cholesterol content. In conclusion, cilostazol significantly inhibits the uptake of modified LDL and foam cell formation in mouse peritoneal macrophages, and the inhibitory effect of cilostazol can be induced in a cAMP- and PKA-independent manner.

Effect of cilostazol on delayed cerebral vasospasm after subarachnoid hemorrhage in rats: evaluation using black blood magnetic resonance imaging.

The purpose of this study was to use black blood magnetic resonance imaging (BB-MRI) to assess delayed cerebral vasospasm (DCV) after subarachnoid hemorrhage (SAH) in rats, and evaluate whether delayed treatment with the anti-platelet agent cilostazol was effective on DCV. BA vasospasm was sequentially assessed at 1, 2, and 3 h, and 1-6 days after SAH by BB-MRI. BB-MRI clearly visualized biphasic vasospasm; early vasospasm at 1 h later and the maximal DCV at day 2. Cilostazol was perorally administered twice at day 1 after having confirmed significant DCV using BB-MRI. The effect of cilostazol on DCV was evaluated at day 2. Cilostazol significantly attenuated DCV and suppressed the levels of malondialdehide and 8-isoprostane in CSF after SAH. This study shows that BB-MRI is a useful and less invasive method for the evaluation of DCV, and cilostazol may be effective on DCV.

Effects of OPC-18790, a new positive inotropic agent, on energetics in the ischaemic canine heart : a 31P-MRS study

OBJECTIVE Effects of OPC-18790, a novel positive inotropic agent, on cardiohaemodynamics and cardiac energetics were assessed simultaneously in dogs with cardiac ischaemia using phosphorus-31 magnetic resonance spectroscopy (31P-MRS) and compared with those of amrinone, a pure cGMP-inhibited PDE inhibitor. METHODS Cardiac ischaemia was produced by partial stenosis of the coronary artery. Dogs with cardiac ischaemia were instrumented for the determination of regional coronary blood flow (non-radioactive coloured microsphere method), regional contractile function (sonomicrometry), and haemodynamics. Myocardial phosphate compounds were measured simultaneously by 31P-MRS. RESULTS Coronary stenosis produced regional dyskinesis, a slight decrease in cardiac output (CO), intracellular acidosis, an increase in the inorganic phosphate (Pi)/creatine phosphate (PCr) ratio concomitantly with a decrease in regional coronary blood flow (CBF) in the ischaemic region. OPC-18790 dose-dependently produced an increase in contractility (measured by peak LVdP/dt) and CO, with only slight changes in heart rate (HR) and mean blood pressure (mBP). OPC-18790 did not change regional dyskinesis, but improved the Pi/PCr ratio at the high dose compared with ischaemic values (before drug administration). Amrinone produced an increase in CO comparable to that of OPC-18790; however, the increase in peak LVdP/dt was smaller while the increase in HR and decrease in mBP were larger than those seen with OPC-18790. Amrinone worsened the Pi/PCr ratio and intracellular acidosis only at the high dose. CONCLUSION These observed differences in energy metabolism between OPC-18790 and amrinone at the high dose may be due to the ability of OPC-18790 to increase CBF in the ischaemic region and which may attributed to its differing effect on overall haemodynamics. Thus, OPC-18790 may be useful in the management of ischaemic heart failure.

Differential effects of OPC‐18790, amrinone and dobutamine on cardiac function and energy metabolism in the guinea‐pig isolated ischaemic heart

1 The effects of OPC‐18790, a novel positive inotropic agent, on cardiac function and myocardial energy metabolism in the guinea‐pig isolated heart with ischaemia were studied by 31P‐magnetic resonance spectroscopy (MRS) and compared with those of amrinone and dobutamine. 2 Cardiac ischaemia was induced by intracoronary infusion of 15 μm microspheres to reduce coronary perfusion flow (CPF) by 50%. Microsphere embolisation caused a 40% decrease in left ventricular systolic pressure (LVSP), cardiac contractility measured by peak of ventricular pressure development (LVdP/dt) and slightly reduced heart rate. There was also a decrease in ATP and creatine phosphate (PCr) by 20%, an increase in inorganic phosphate (Pi) by 25% and an acidic shift of intracellular pH in the ischaemic heart. 3 In the ischaemic heart, OPC‐18790, amrinone and dobutamine were applied at concentrations which increased LVdP/dt by about 60%. These compounds increased LVP by 15% to 30% and increased CPF by about 10%. Amrinone and dobutamine but not OPC‐18790 increased heart rate. When these drugs produced the haemodynamic changes described above, amrinone and dobutamine reduced ATP and PCr, increased Pi and produced further intracellular acidosis, whereas, OPC‐18790 did not change these parameters. 4 Cardiac pacing at 285 beats min−1 produced decreases in LVP, LVdP/dt and CPF by about 30%, 20%, 5%, respectively and an increase in Pi, decreases in PCr and ATP, and intracellular acidosis. 5 These results suggest that degradation of high energy phosphate compounds closely relates to increase in heart rate in the ischaemic heart. Positive inotropic agents without chronotropic action seem to be beneficial in support of the ischaemic heart.

Blockade of ATP-sensitive K+ channels attenuates preconditioning effect on myocardial metabolism in swine:: Myocardial metabolism and ATP-sensitive K+ channels

OBJECTIVE We investigated if blockade of ATP-sensitive K+ channels (KATP) abolishes the protective effect of ischemic preconditioning (IP) on myocardial metabolism and ischemia-induced reactive hyperemia (RH) in pigs. METHODS IP was elicited by a single cycle of 5 min occlusion and 5 min reperfusion of coronary artery, followed by 15 min of test ischemia and 120 min of reperfusion. Vehicle or the ATP-sensitive K+ channels (KATP) blocker, glibenclamide (3 or 6 mg/kg; G3 or G6) was administered before IP (groups; IP, G3+IP, G6+IP). As respective controls, the same treatment was performed in groups without IP (groups; C, G3, G6). Tissue levels of ATP, creatine phosphate (CP) and intracellular pH (pHi) in the area at risk were measured by 31P-nuclear magnetic resonance spectroscopy. RH after 5 min of preconditioning ischemia was assessed by regional myocardial blood flow. RESULTS ATP and pHi were preserved after 15 min of ischemia in the IP group [C/IP; ATP=57+/-4/76+/-10% of baseline, pHi=6.18+/-0.08/6.66+/-0.03, P<0.05, C vs. IP]. Both doses of glibenclamide completely abolished the ATP sparing effect of IP. The high dose completely abolished pHi preservation (G6+IP=6.33+/-0.06), while the low dose showed only a partial effect (G3+IP=6.48+/-0.03). Glibenclamide did not adversely affect myocardial metabolism in groups without IP. Glibenclamide attenuated RH after 5 min of ischemia by 30% in both subendocardium and subepicardium. CONCLUSIONS Blockade of KATP abolished the preconditioning effect on myocardial metabolism, and partially attenuated post-ischemic reactive hyperemia in pigs. These results indicate that KATP activation might be involved in the mechanisms of these phenomena, reactive hyperemia is not sufficient to induce IP protection.