Author + information
- Received August 3, 1998
- Revision received September 3, 1998
- Accepted September 4, 1998
- Published online January 1, 1999.
- Masafumi Kitakaze, MD, FACCa,* (, )
- Koichi Node, MDa,
- Tetsuo Minamino, MDa,
- Hiroshi Asanuma, MDa,
- Tsunehiko Kuzuya, MDa and
- Masatsugu Hori, MD, FACCa
- ↵*Address for correspondence: Dr. Masafumi Kitakaze, The First Department of Medicine, Osaka University School of Medicine, 2-2 Yamadaoka, Suita City, Osaka Pref. 565-0871, Japan
Objectives. This study was undertaken to examine whether a dihydropyridine Ca channel blocker, benidipine, increases cardiac NO levels, and thus coronary blood flow (CBF) in ischemic hearts.
Background. Benidipine protects endothelial cells against ischemia and reperfusion injury in hearts.
Methods and Results. In open chest dogs, coronary perfusion pressure (CPP) of the left anterior descending coronary artery was reduced so that CBF decreased to one-third of the control CBF, and thereafter CPP was maintained constant (103 ± 8 to 42 ± 1 mmHg). Both fractional shortening (FS: 6.1 ± 1.0%) and lactate extraction ratio (LER: −41 ± 4%) decreased. Ten minutes after the onset of an intracoronary infusion of benidipine (100 ng/kg/min), CBF increased from 32 ± 1 to 48 ± 4 ml/100g/min during 20 min without changing CPP (42 ± 2 mmHg). Both FS (10.7 ± 1.2%) and LER (−16 ± 4%) also increased. Benidipine increased cardiac NO levels (11 ± 2 to 17 ± 3 nmol/ml). The increases in CBF, FS, LER and cardiac NO levels due to benidipine were blunted by L-NAME. Benidipine increased cyclic GMP contents of the coronary artery of ischemic myocardium (139 ± 13 to 208 ± 15 fmol/mg protein), which was blunted by L-NAME.
Conclusion. Thus, we conclude that benidipine mediates coronary vasodilation and improves myocardial ischemia through NO-cyclic GMP-dependent mechanisms.
Calcium (Ca) channel blockers are widely used for the treatment of ischemic heart diseases because of the capability of effective coronary vasodilation (1,2). Ca channel blockers are believed to increase coronary blood flow (CBF) due to inhibition of Ca2+entry into the smooth muscle cells (3). Recently, benidipine, a dihydropyridine Ca channel blocker, can protect endothelial cellular function of renal resistance arteries of hypertensive rats (4), and mesenteric arteries of the circulatory shock rats (5). Endothelial function is important for the preservation of the organ function against ischemic or hypertensive stress (6–8), and indeed, benidipine is reported to be cardioprotective against myocardial ischemia and reperfusion injury (9). These observations suggest that benidipine may be protective against endothelial dysfunction during ischemia and reperfusion in the heart. Since myocardial ischemia deteriorates endothelial functions via the activation of platelets and leukocytes (10,11), benidipine may attenuate the endothelial functions and increase the production of nitric oxide in the ischemic hearts.
Therefore, we examined the idea that benidipine increases CBF via nitric oxide (NO)-dependent mechanisms in the canine hearts. Since dihydropyridine Ca channel blockers are reported to inhibit adenosine uptake (12,13), we also tested whether the effects of benidipine on coronary vasodilation in the ischemic hearts are attributable to adenosine.
Material and methods
Mongrel dogs weighing 14–26 kg were anesthetized with sodium pentobarbital (30 mg/kg, IV). The trachea was intubated and the dog was ventilated with room air mixed with oxygen. The chest was opened through the left fifth intercostal space, and the heart was suspended in a pericardial cradle. A proximal portion of the left anterior descending coronary artery (LAD) was cannulated and perfused with blood via the left carotid artery through an extracorporeal bypass tube. Coronary perfusion pressure (CPP) was monitored at the tip of the coronary arterial cannula, and CBF in the perfused area was measured with an electromagnetic flow probe attached at the bypass tube.
In 37 dogs (Protocols I–III), a small, short collecting tube (diam: 1 mm, length: 7 cm) was inserted into a small coronary vein near the center of the perfused area to sample coronary venous blood. The drained venous blood was collected in a reservoir placed at the level of the left atrium and was returned via the jugular vein. Left ventricular pressure was measured using a micromanometer (Konigsberg P-5, Pasadena, California) placed through the apex into the left ventricular cavity. Two paired ultrasonic crystals were placed in the inner one-third of the myocardium approximately one centimeter apart to measure the myocardial segment length with an ultrasonic dimension gauge (Schuessler, 5MHz). End-diastolic length was determined at the R wave of the electrocardiogram, and end-systolic length was determined at the minimal dP/dt (14). Fractional shortening (FS) was calculated by ([end-diastolic length]-[end-systolic length])/(end-diastolic length) and served as an index for myocardial contractility of the perfused area.
At the end of Protocols I–III, Evans blue was infused into the coronary artery via the bypass tube to identify the perfused area. We cut and weighed the perfused myocardium to normalize CBF.
Protocol I: effects of an intracoronary administration of benidipine on coronary hemodynamic and metabolic parameters in the non-ischemic myocardium
Five dogs were used in this protocol. Coronary arterial and venous blood were sampled for blood gas analysis and determination of adenosine and end products of NO (nitrate + nitrite) concentrations. Hemodynamic functions, that is, left ventricular pressure, dP/dt, and segment length of the perfused area, were measured. Four doses (25, 50, 100, and 200 ng/kg/min) of benidipine were randomly administered into the LAD for 5 min (n = 5) each. It took 3 min to obtain stable coronary hemodynamic conditions. Between 4 and 5 min during each infusion of benidipine, coronary arterial and venous blood were sampled and systemic and coronary hemodynamic parameters were measured. In the same dogs, we infused benidipine with the same four doses each for 5 min under the treatment with either Lω-nitro arginine methyl ester (L-NAME, 10 μg/kg/min) or 8-sulfophenyltheophylline (8-SPT, 50 μg/kg/min). In a preliminary study, we confirmed that the dose of L-NAME attenuates the coronary vasodilatory action of bradykinin (20 ng/kg/min, ic) by 85 ± 6%, and 8-SPT attenuates the coronary vasodilatory action of adenosine (4 μg/kg/min) by 82 ± 5%, respectively. Either L-NAME or 8-SPT was administered into the LAD 5 min before the administration of benidipine. We measured the hemodynamic parameters and sampled the coronary arterial and venous blood four minutes after the onset of infusion of either L-NAME or 8-SPT.
Protocol II: effects of benidipine on coronary hemodynamic and metabolic parameters in the ischemic myocardium (constant low CPP)
Twenty-two dogs were used in this protocol. After hemodynamic stabilization, CPP was reduced so that CBF was decreased to 33% of the control CBF, using an occluder attached at the extracorporeal bypass tube. After a low level of CPP was obtained, the occluder was adjusted to keep CPP constant. All of the hemodynamic parameters were measured 10 min after the onset of hypoperfusion, and both coronary arterial and venous blood were sampled. In five dogs, we tested the stability of the severity of ischemia for another 20 min, because previous studies indicate that myocardial ischemia may be changed during coronary hypoperfusion (15,16). In other dogs, benidipine (100 ng/kg/min, n = 7) was infused into the LAD, and measurements of all hemodynamic and metabolic parameters were repeated in 10 min. Thereafter, the infusion of either benidipine or the solvent was discontinued, and hemodynamic and metabolic parameters were observed until stabilization. In the other dogs, we infused benidipine under the treatment with either L-NAME (10 μg/kg/min, n = 5) or 8-SPT (50 μg/kg/min, n = 5) in the ischemic myocardium. Time and order of the administrations of the drugs before the onset of coronary hypoperfusion was the same as in Protocol I.
Protocol III: effects of benidipine on coronary hemodynamic and metabolic parameters in the ischemic myocardium (constant low CBF)
Because increased CBF may increase share stress of coronary artery, which may secondarily increase cardiac NO levels in the ischemic hearts, we tested the effects of benidipine on cardiac NO levels in the constant low CPP condition. In 10 dogs, after hemodynamic stabilization, CPP was reduced so that CBF was decreased to 33% of the control CBF, using an occluder attached at the extracorporeal bypass tube. After a low level of CPP and CBF was obtained, the occluder was adjusted to keep CBF constant. All of the hemodynamic parameters were measured 10 min after the onset of hypoperfusion, and both coronary arterial and venous blood were sampled. After these measurements, benidipine (100 ng/kg/min, n = 5) was infused into the LAD, and measurements of all hemodynamic and metabolic parameters were repeated in 10 min. Thereafter, the infusion of benidipine was discontinued, and hemodynamic and metabolic parameters were observed until stabilization. In the other dogs, we infused benidipine under the treatment with L-NAME (10 μg/kg/min, n = 5) in the ischemic myocardium. Time and order of the administrations of the drugs before the onset of coronary hypoperfusion was the same as in Protocol II.
Protocol IV: effects of benidipine on cyclic guanosine monophosphate (GMP) content of epicardial coronary arteries in ischemic hearts
In 20 dogs, we investigated whether benidipine increases the cyclic guanosine monophosphate (GMP) content of the epicardial arteries in the ischemic myocardium. With an occluder attached at the extracorporeal bypass tube, coronary perfusion pressure was reduced so that CBF decreased to one-third of the control coronary blood flow. After a low CPP was established, the occluder was adjusted to keep CPP at a constant low level. After this low CPP was maintained for 10 min, we infused either benidipine (100 ng/kg/min, n = 10) or the solvent (polyethylene glycol 400, 0.0167 ml/kg/min, n = 10) into the left anterior descending coronary artery for 10 min with and without L-NAME. Next, we rapidly removed the epicardial left anterior descending (ischemic region) and left circumflex (nonischemic control region) coronary arteries with precooled scissors and a tongue and stored them in liquid nitrogen.
Lactate concentration was assessed by the enzymatic assay (17). Lactate extraction ratio (LER) was obtained by multiplying the coronary arteriovenous differences in the lactate concentrations by 100 and dividing it by the arterial lactate concentration (18).
The methods of the measurements of plasma NO (19,20), and adenosine levels (21)have been reported previously. The method of the measurements of cyclic GMP levels in tissues have been previously described (22,23).
Statistical analysis was performed among the groups and in the time courses of the hemodynamic and metabolic parameters using analysis of variance (24,25). When analysis of variance revealed significant differences, Bonferroni probabilities were examined (25). All values were expressed as mean ± SEM. The p value of <0.05 was considered significant.
Effects of benidipine on the coronary hemodynamic and metabolic functions in the nonischemic myocardium
Either L-NAME or 8-SPT did not change either systolic and diastolic blood pressure, heart rate or CPP in the nonischemic condition (Table 1). In the untreated condition, benidipine increased the levels of nitrate + nitrite in the coronary venous blood over the arterial blood (ΔVA(NO)) (3.9 ± 0.7 nmol/ml at control, 4.9 ± 0.6 (p < 0.05 vs. the control), 5.8 ± 0.6 (p < 0.05), 7.2 ± 0.6 (p < 0.05), and 7.1 ± 0.3 nmol/ml (p < 0.05) during an intracoronary infusion of benidipine of 25, 50, 100, and 200 ng/kg/min, respectively); however, it did not affect the levels of adenosine in the coronary venous blood over the arterial blood (ΔVA(Ado)) (5.5 ± 1.3 pmol/ml at control, 4.6 ± 1.0, 5.1 ± 0.6, 5.3 ± 0.6, and 4.7 ± 1.2 pmol/ml (p < 0.05) during an intracoronary infusion of four doses of benidipine, respectively). Benidipine increased CBF dose-dependency (Fig. 1), which was partially attenuated by L-NAME, but not by 8-SPT.
Effects of intracoronary administration of benidipine on the coronary hemodynamic and metabolic functions in the ischemic myocardium
Either heart rate, systolic and diastolic blood pressure were unchanged with and without pharmacological interventions (Table 1). Without benidipine infusion, after the reduction of CPP (104 ± 4 mmHg at control; 41 ± 2, 41 ± 2, 41 ± 2, and 41 ± 2 mmHg at 5, 10, 20, and 30 min after the onset of coronary hypoperfusion, respectively), CBF (93 ± 3 ml/100g/min at control; 31 ± 2, 32 ± 3, 30 ± 3, and 30 ± 4 ml/100g/min at 5, 10, 20, and 30 min, respectively), FS (26.7 ± 3.1 mmHg at control; 4.9 ± 1.2, 4.8 ± 1.5, 5.2 ± 1.4, and 4.6 ± 1.5% at 5, 10, 20, and 30 min, respectively) and LER (27.8 ± 3.9% at control, −53.2 ± 4.9, −46.7 ± 7.8, −52.7 ± 4.5, and −62.3 ± 6.5% at 5, 10, 20, and 30 min after the onset of coronary hypoperfusion, respectively) did not change significantly for 30 min of coronary hypoperfusion.
Ten minutes after the reduction in CPP, ΔVA(NO) increased (Fig. 2), and benidipine further increased ΔVA(NO). In accordance with the increases in ΔVA(NO), benidipine increased CBF despite the constant CPP (Fig. 3). Fractional shortening and LER, which was reduced after the onset of coronary hypoperfusion, increased due to benidipine administration (Fig. 4). These beneficial effects of benidipine on the ischemic heart were attenuated by L-NAME but not by 8-SPT.
In the ischemic hearts with the low constant CBF condition, benidipine increased ΔVA(NO) and was attenuated by L-NAME (Fig. 5). Benidipine decreased CPP because of the coronary vasodilatory capability, which was prevented by L-NAME (Fig. 6). Since CBF remained constant, neither FS nor LER was altered (Fig. 7).
We measured cyclic GMP levels in the epicardial coronary artery; without L-NAME (CPP: 103 ± 3 to 43 ± 2 mmHg, CBF: 91 ± 2 to 31 ± 2 ml/100g/min), benidipine increased the cyclic GMP levels of the epicardial coronary arteries and this effect was blunted by L-NAME (Fig. 8).
This is the first report that a dihydropyridine Ca channel blocker, benidipine, has the capability of increasing cardiac NO levels in ischemic hearts, which mediates coronary vasodilation and attenuates the severity of myocardial ischemia. Since it is reported that amlodipine also increases NO levels in canine coronary microvessels (26), the capability of enhancing the effects of NO may not be attributable to the specific nature of benidipine, but the general characteristics of a dihydropyridine Ca channel antagonist, although the potency of the capability to enhance the NO levels due to Ca channel antagonists may be different.
Benidipine-induced coronary vasodilation in nonischemic and ischemic hearts
Because Ca channel blockers can inhibit adenosine uptake into the cells (12,13)and adenosine is a very potent coronary vasodilator (27), we at first thought that adenosine-mediated mechanisms may be involved in benidipine-induced coronary vasodilation, although they were not. On the contrary, benidipine increased the cardiac NO levels and benidipine-induced coronary vasodilation was blunted by L-NAME, indicating that NO-dependent mechanisms may be involved in benidipine-induced coronary vasodilation. However, it is quite different with regard to the extent of the involvement of NO in benidipine-induced coronary vasodilation between the ischemic and nonischemic hearts (Fig. 1 and 3). There are three possibilities that explain this difference. First of all, if the open probability of Ca channels in coronary smooth muscles is high in the nonischemic heart compared with the ischemic hearts, the capability for Ca channel blockers to inhibit Ca channels may be high enough to blunt the NO-dependent coronary vasodilation due to benidipine in the nonischemic hearts. On the other hand, if myocardial ischemia already decreases the open probability of Ca channels, only a slight potency may be left for benidipine to cause further coronary vasodilation via antagonization of Ca channels, and the relative importance of NO-dependent coronary vasodilation may increase. Indeed, endothelial-dependent hyperpolarization factor is released in the ischemic myocardium (28,29), which may hyperpolarize the cellular membrane, and it may decrease the open probability of Ca channels of coronary smooth muscles in the ischemic hearts. Secondly, since ischemia per se activates NO synthase via endogenous neurohumoral substances such as catecholamine, bradykinin and adenosine in the ischemic hearts, benidipine can further increase the activity of NO synthase that has been partially activated. Considering that the dose and action curves are generally sigmoidal, the extent of the increases in NO synthase may be augmented in the ischemic hearts compared with nonischemic hearts during infusions of benidipine. Third, the capability to increase the cardiac NO levels due to benidipine may be higher in ionic and neurohumoral circumstances of ischemic hearts than in nonischemic hearts. We did not determine which factor is likely to explain the difference.
The mechanisms of benidipine-induced increases in NO levels in ischemic hearts
Since Ca channel blockers increase CBF via inhibition of Ca2+entry into smooth muscle cells (3)and increases in shear stress in the endothelial cells due to increases in CBF increase NO production, one may argue that increases in NO levels due to benidipine are secondary to the increases in shear stress during the increases in CBF. However, in the present study, even when CBF was kept constant at a low level during infusion of benidipine, the cardiac NO levels were increased. This result indicates that benidipine directly affects the metabolism of NO. There are several stimuli to facilitate the NO production. Most of all, benidipine is reported to activate kallikrein in the kidney (30), which may increase bradykinin production in the heart. Indeed, it is reported that NO-dependent vasodilation due to amlodipine is partially attributable to bradykinin (26).
Limitation, and pathophysiological and clinical relevance
Although we used anesthetized open chest dogs, there are substantial differences between the anesthetized and conscious animals (31–33). In the conscious dogs, inhibition of nitric oxide synthase distal to a coronary stenosis reduced myocardial perfusion (31,32). In contrast, CPP in the L-NAME group during ischemia (45.8 ± 3.9 mm Hg) were slightly higher than the control group (42.5 ± 1.4 mm Hg), but there was no significant difference (p = 0.299). This result suggests that the NO-generating system may be weak in our model compared with the conscious dog models. However, this may be attributable to the variation in the physiological condition of the dogs. When L-NAME was administered into the coronary artery 10 min after the onset of coronary hypoperfusion in each dog, we found that coronary blood was decreased by about 30% in the anesthetized animals (18), suggesting that NO still works in the regulation of coronary vascular tones in the ischemic hearts. Furthermore, there is a difference in recruitable coronary flow reserve during ischemia in anesthetized and conscious animals (33); conscious dogs may have less recruitable flow reserve during ischemia compared with anesthetized animals. Since coronary vascular tone in the anesthetized animals may be higher compared with that of the conscious animals, we need to be careful to extend the present results to the clinical settings.
In turn, nitric oxide is believed to attenuate the severity of myocardial ischemia; NO increases CBF, attenuates the myocardial anaerobic metabolism, inhibits platelet aggregation and leukocytes activation, and attenuates the activation of sympathetic activity in ischemic hearts. Since the present study revealed that NO-dependent mechanisms are involved in the effects of benidipine, benidipine may be beneficial for ischemic hearts compared with other Ca channel antagonists. Furthermore, NO is reported to mediate cardioprotection at a late phase of ischemic preconditioning: 24 or 48 h after a brief period of ischemia mediates the infarct size-limiting effects (34,35)or the attenuation of myocardial stunning (36), and these are blunted by L-NAME.
Taken together, benidipine, a dihydropyridine Ca channel blocker, can be considered as an agent to mediate or enhance the NO-dependent cardioprotection in the diseased heart.
☆ Supported by Grants-in-aid for scientific research (No. 10557068 and 10670649) from the Ministry of Education, Science, and Culture, Japan and, in part, by grants from the Smoking Research Foundation in Japan.
- coronary blood flow
- coronary perfusion pressure
- fractional shortening
- guanosine monophosphate
- left anterior descending coronary artery
- ΔVA(NO) or ΔVA(Ado)
- levels of nitrate + nitrite or adenosine in the coronary venous blood over the arterial blood, respectively
- Lω-nitro arginine methyl ester
- nitric oxide
- Received August 3, 1998.
- Revision received September 3, 1998.
- Accepted September 4, 1998.
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