Ryosuke Yorikane

Contribution of endogenous endothelin-1 to the progression of cardiopulmonary alterations in rats with monocrotaline-induced pulmonary hypertension.

Endothelin-1 (ET-1) is known to have potent contractile and proliferative effects on vascular smooth muscle cells and is known to induce myocardial cell hypertrophy. We studied the pathophysiological role of endogenous ET-1 in rats with monocrotaline-induced pulmonary hypertension. Four-week-old rats were given a single subcutaneous injection of 60 mg/kg monocrotaline (MCT rats) or saline (control rats) and were killed after 6, 10, 14, 18, and 25 days. In the MCT rats, right ventricular systolic pressure progressively increased and right ventricular hypertrophy developed in a parallel fashion. The venous plasma ET-1 concentration also progressively increased, and this increase preceded the development of pulmonary hypertension. The isolated pulmonary artery exhibited a significantly weaker response to ET-1 in the MCT rats on day 25 but not on days 6 and 14. In the MCT rats, the expression of prepro ET-1 mRNA as measured by Northern blot analysis significantly increased in the heart on days 18 and 25, whereas it gradually decreased in the lungs. The peptide level of ET-1 in the lungs also significantly decreased in the pulmonary hypertensive stage. The expression of prepro ET-1 mRNA had increased by day 6 only in the kidneys. Continuous infusion of BQ-123, a selective ETA receptor antagonist, by an osmotic minipump (14.3 mg per day per rat for 18 days) significantly inhibited the progression of both pulmonary hypertension (right ventricular systolic pressure, 77.8 +/- 4.2 [mean +/- SEM] mm Hg [n = 10] versus 52.3 +/- 2.4 mm Hg [n = 7]; P < .01) and right ventricular hypertrophy (right ventricle/[left ventricle +/- septum], 0.56 +/- 0.03 [n = 10] versus 0.41 +/- 0.02 [n = 7]; P < .01). Histological examination revealed that BQ-123 also effectively prevented pulmonary arterial medial thickening. The inhibition of right ventricular hypertrophy by BQ-123 may be partly ascribed to the blockade of excessive stimulation of the heart by ET-1, in addition to the prevention of pulmonary hypertension. The present findings suggest that endogenous ET-1 contributes to the progression of cardiopulmonary alterations in rats with MCT-induced pulmonary hypertension.

Increased production of endothelin-1 in the hypertrophied rat heart due to pressure overload.

Endothelin‐1 (ET‐1) has been demonstrated to induce hypertrophy in cultured cardiac myocytes. We investigated the production of ET‐1 in the heart of aorta‐banded rats in vivo. Seven days after the banding of the abdominal aorta, rats developed a significant left ventricular hypertrophy. The tissue content of mature ET‐1 and the level of expression of prepro ET‐1 mRNA were higher in the left ventricle of aorta‐banded rats than in those of sham‐operated rats. The expression of prepro ET‐1 mRNA in the right ventricle was not different between the two groups. These findings indicate that the production of ET‐1 increased in the hypertrophied left ventricle, thereby suggesting the possible involvement of endogenous ET‐1 in the development of cardiac hypertrophy due to pressure overload.

Altered expression of ETB-receptor mRNA in the lung of rats with pulmonary hypertension

To investigate the pathophysiologic role(s) of endothelin-1 (ET-1) in pulmonary hypertension, we studied the expression of ETB-receptor mRNA in the lung and venous plasma concentrations of ET-1 in rats with monocrotaline-induced pulmonary hypertension (PH). Three weeks after s.c. injection of monocrotaline (60 mg/kg), rats (PH rats, n = 6) were sacrificed. Vehicle-injected rats (n = 6) served as controls. The right ventricular systolic pressure of PH rats [58.0 +/- 4.7 mm Hg (mean +/- SEM)] was significantly higher than that in the vehicle-treated control rats (29.2 +/- 2.1; p < 0.01). Northern blot analysis showed that the expression of ETB-receptor mRNA decreased in the lung of PH rats. The venous plasma concentration of ET-1 measured by a sandwich-enzyme immunoassay was significantly higher in PH rats than in control rats (5.1 +/- 0.7 versus 1.3 +/- 0.2 pg/ml; p < 0.01). The present findings suggest that the expression of ETB-receptor mRNA decreases in the lung of PH rats, which might be closely related to the increase in plasma ET-1 concentration in these rats.

Electrophysiological effects of endothelin-1 on canine myocardial cells.

Endothelin-1 (ET-1) has been shown to induce severe ventricular arrhythmias associated with myocardial ischemia. However, ET-1 may have a direct arrhythmogenic action that is not related to myocardial ischemia. To examine this possibility, we studied the electrophysiological effects of ET-1 on cardiac tissues. The right bundle branch, false tendon, ventricular muscle, and atrial muscle were isolated from the dog, and transmembrane potentials were recorded by the conventional microelectrode technique. ET-1 prolonged the action potential duration (APD) in all of the tissues tested except in the atrial muscle, where the APD was shortened. Bay K 8644, a calcium channel agonist, prolonged the APD in all cardiac tissues. Spontaneous firing of the right bundle branch was suppressed by ET-1 but not Bay K 8644. The prolongation of the APD by ET-1 was far more marked in the right bundle branch than in other tissues, and it was followed by the development of early after depolarizations (EADs) only in the right bundle branch. The EADs induced by ET-1 or Bay K 8644 were abolished by nicardipine. These data suggest that L-type calcium current is involved in the genesis of EADs by ET-1, although other ionic mechanisms can not be ruled out. Since EADs underlie some types of arrhythmias, arrhythmias caused by ET-1 are at least partly attributable to the direct actions of the agent on myocardial cells.

Altered production of endothelin- 1 in the hypertrophied rat heart.

Summary: Although endothelin-1 (ET-1) has been shown to have potent hypertrophic effects in cultured cardiac myocytes, there is no evidence that the production of ET-1 is altered in the hypertrophied heart in vivo. We investigated mRNA and peptide levels of ET-1 in the rat heart hypertrophied due to pressure or volume overload. One week after surgery of abdominal aortic banding (AB) or aortic valve regurgitation (AR; the aortic valve was destroyed by a polyethylene catheter), the rats were sacrificed. Control sham-operated rats were also studied (AB-sham and AR-sham). The left ventricular (LV) weight to body weight ratio was significantly higher in AB rats than in AB-sham rats. It was also significantly higher in AR rats than in AR-sham rats. Therefore, AB rats and AR rats developed LV hypertrophy due to pressure and volume overload, respectively. The expression of ET-1 mRNA in the LV was markedly higher in AB rats than in AB-sham rats. The peptide level of ET-1 was also significantly higher in the LV of AB rats than in that of AB-sham rats. However, the expression of ET-1 mRNA in the LV of AR rats was similar to that of AR-sham rats. The present findings suggest that the production of ET-1 in the heart differs between pressure and volume overload-induced hypertrophy in rats.

Chronotropic effects of azelnidipine, a slow- and long-acting dihydropyridine-type calcium channel blocker, in anesthetized dogs: a comparison with amlodipine.

Azelnidipine, a dihydropyridine calcium channel blocker unlike other dihydropyridine calcium channel blockers, does not increase but slightly decreases heart rate (HR) in clinical settings. In the present study, the mechanism for the HR-lowering action characteristic of azelnidipine was investigated in anesthetized dogs. In the in situ perfused sinus node preparation, the negative chronotropic action of azelnidipine was almost 5 times more potent than that of amlodipine. When injected intravenously in intact anesthetized dogs, both drugs produced a long-lasting blood pressure reduction, but the action of azelnidipine developed more slowly than that of amlodipine. Azelnidipine hardly affected HR at lower doses and decreased HR at higher doses. On the other hand, amlodipine consistently induced slight tachycardia. The HR-lowering effect of azelnidipine in autonomically blocked dogs was not much different from that in intact dogs, whereas tachycardia induced by amlodipine was eliminated in autonomically blocked dogs. However, neither drug affected the HR gain of baroreceptor reflex induced by changing carotid sinus pressure. These results suggest that compared with amlodipine, azelnidipine has a greater intrinsic negative chronotropic action and induces a lower level of reflex tachycardia most probably due to a slower development of blood pressure-lowering effects and that these characteristics of azelnidipine underlies its HR-lowering effects when administered systemically.

Possible involvement of altered Na+ -Ca2+ exchange in negative inotropic effects of class I antiarrhythmic drugs on rabbit and rat ventricles.

We investigated the way in which Na+ channel blocking class I antiarrhythmic drugs, lidocaine (30 mu M), flecainide (30 mu M), and RS-2135 (100 mu M) affected contractions elicited by several protocols in rat and rabbit ventricular strips. Rabbit ventricles showed a positive force-frequency relation, and antiarrhythmic drugs inhibited the contraction, flattening the force-frequency curve. In contrast, rat ventricles showed a negative force-frequency relation, and the drugs shifted the force-frequency curve downward. Rapid-cooling contracture (RCC) also showed a positive or negative dependence on the frequency of preceding stimulation in rabbit or rat ventricles, respectively. All drugs inhibited the RCC, suggesting that they reduced the Ca2+ content in the sarcoplasmic reticulum. Ryanodine (1 mu M) abolished the RCC in both muscles and the contraction in rat muscles, but partially decreased contractions at high frequencies in rabbit ventricles. Antiarrhythmic drugs caused a further inhibition of contractions in the presence of ryanodine in rabbit ventricles. These results indicate that inhibition of Na+ channels by antiarrhythmic drugs alters Na+ -Ca2+ exchange, resulting in a decrease in the Ca2+ content in the sarcoplasmic reticulum (SR) and the Ca2+ entry through the exchanger.

Synthesis and collateral dilator activity of nitroxyalkylamides having direct or latent sulfhydryl moieties.

To develop an orally active, long-acting nitrate that does not induce tolerance, nitroxyalkyl compounds were prepared and their activities evaluated by the use of carotid collaterals in anesthetized dogs. A compound having a favorable pharmacological profile, that is, long-lasting collateral vasodilatation and little hypotension, and lack of nitrate tolerance, was chosen for further evaluation.

Effects of Rs-2135, a Novel Class I Antiarrhythmic Agent, on Sustained Ventricular Tachycardia After Coronary Embolization in Conscious Dogs

To assess the ability of RS-2135, a novel class I antiarrhythmic agent to suppress ischemia-induced ventricular arrhythmias, we produced myocardial infarction (MI) by introducing a glass bead into the coronary artery of the dog (bead model). Ventricular arrhythmias after coronary embolization were as severe and long-lasting as those that occur after two-stage coronary artery ligation as described by Harris. RS-2135 (1.25 and 2.5 mg/kg intravenously, i.v.) suppressed sustained ventricular tachycardia (SVT) 24 h after coronary embolization in the bead model. The antiarrhythmic effects of i.v. administration of RS-2135 were more potent and more long-lasting than those of lidocaine (5 and 10 mg/kg i.v.), mexiletine (5 and 10 mg/kg i.v.), disopyramide (2.5 and 5 mg/kg i.v.), and flecainide (2.5 and 5 mg/kg i.v.). The antiarrhythmic effects of oral (p.o.) administration of RS-2135 were evaluated 48 h after coronary embolization. RS-2135 (10 mg/kg p.o.) was equipotent to flecainide (10 mg/kg p.o.) and twice as potent as disopyramide (20 mg/kg p.o.) and mexiletine (20 mg/kg p.o.). Onset of antiarrhythmic effects after p.o. RS-2135 was slower than that of other drugs. These data suggest that the bead model is as useful as the Harris model for evaluation of the antiarrhythmic potential of chemicals and that RS-2135, either i.v. or p.o., is effective against SVT after acute MI.

Effects of Rs-2135, a New Antiarrhythmic Agent, on the Kinetics of Use-dependent Decrease in Maximum Upstroke Velocity in Guinea Pig Ventricular Myocardium

The use-dependent decrease in maximum upstroke velocity (Vmax) caused by RS-2135, a new antiarrhythmic compound was analyzed in isolated papillary muscles of guinea pigs. RS-2135 3 and 10 μM decreased Vmax of action potential (AP) in a concentration-related manner without affecting resting membrane potential (RMP). Vmax decay in the presence of RS-2135 was exponential at stimulation rates ≥ 0.5 Hz. This use-dependent block of Vmax was enhanced at higher stimulation frequencies. The time constants and onset rates per action potential of the use-dependent block were 10.7 26.9 s and 0.021–0.041 AP-1, respectively. The time constants of recovery from use-dependent block were 57.9 and 63.6 s, respectively, in the presence of 3 and 10 μM RS-2135. The predicted half-time of the recovery process calculated by physicochemical parameters of RS-2135 agreed well with the observed values. These results suggest that RS-2135 is a sodium channel blocking agent with slow kinetics and that the physicochemical properties underlie these characteristics.