Kouichi Ashikawa

Mechanical interactions between four heart chambers with and without the pericardium in canine hearts.

By using excised postmortem hearts obtained from 15 mongrel dogs with the pericardium intact, we investigated mechanical interactions between the four heart chambers from the standpoint of ventricular pressure-volume relationships. The interactions investigated were those between (1) the atrium and the ventricle, (2) the right ventricle and left ventricles, (3) the atrium and one ventricle vs. the other ventricle, and finally (4) the left and right atrium and the right ventricle vs. the left ventricle. For these purposes, we inserted compliant balloons into the four heart chambers without injuring the pericardium, i.e., we incised the base of the atria which was not covered with the pericardium. We obtained the right and/or left ventricular pressure-volume relationships under a constant pressure in three other heart chambers by changing the height of the reservoir connected to each balloon. As a result, both ventricular pressure-volume relationships were hardly affected by an increase in the atrial pressure ranging from 5 to 30 cm H2O with the pericardium removed, although the ventricle became less compliant due to an increase of the same magnitude of the opposite ventricular pressure. On the other hand, the effect of an increase in atrial pressure was distinct with the pericardium intact. Also, all mechanical interactions were enhanced dramatically with the intact pericardium. Thus, the pericardium plays an important role in these mechanical interactions, especially when the filling pressures of all heart chambers increase simultaneously. Clinically, these findings may be important to understanding ventricular functions as related to various heart disease--especially acute heart failure.

Microvascular sites and mechanisms responsible for reactive hyperemia in the coronary circulation of the beating canine heart.

Our aim was to elucidate the site and mechanism responsible for reactive hyperemia in coronary circulation. In in vivo beating canine hearts, microvessels of the left anterior descending coronary artery (LAD) were observed through a microscope equipped with a floating objective. Flow velocity of the LAD was measured with a suction-type Doppler probe. The LAD was occluded for 20 or 30 seconds and then released, and reactive hyperemia was observed before and after 8-phenyltheophylline (7.5 mg/kg i.v.) or glibenclamide (200 micrograms/kg into the LAD) infusion. During the occlusion, only arterial microvessels smaller than 100 microns in diameter dilated. Dilation of those vessels was partially attenuated by 8-phenyltheophylline and completely abolished with glibenclamide. In the early phase of reactive hyperemia, all arterial microvessels dilated, and the magnitude of peak dilation was greater in vessels smaller than 100 microns compared with those larger than 100 microns. Vasodilation during reactive hyperemia ceased within 60 seconds in vessels smaller than 100 microns but was sustained for more than 120 seconds in those larger than 100 microns. 8-Phenyltheophylline did not change peak dilation of arterial microvessels but reduced dilation after the peak. Glibenclamide remarkably attenuated dilation of all arterial microvessels in the whole phase of reactive hyperemia. These results indicate that all arterial microvessels are responsible for reactive hyperemia after coronary artery occlusions of 20-30 seconds, but there is greater participation of vessels smaller than 100 microns in the early phase of reactive hyperemia. Dilation of vessels larger than 100 microns assumes an important role in the later phase. ATP-sensitive K+ channels mediate dilation of arterial microvessels both in brief ischemia and reactive hyperemia.

Phasic blood flow velocity pattern in epimyocardial microvessels in the beating canine left ventricle.

We quantitated phasic epimyocardial microcirculatory coronary blood flow velocity patterns in the beating left ventricle. Using a newly developed floating objective and high-speed cinematography, red cell velocities in small arterioles, capillaries, and small venules and microvascular diameters in the superficial layer of the epimyocardium of beating left ventricle were determined throughout the entire cardiac cycle in open-chest anesthetized dogs. Heart rate was maintained at 140 beats/min by means of left atrial pacing. Peak red cell velocity was observed in midsystole in small arterioles and capillaries, and in late systole in small venules. Abrupt decline in red cell velocity and, in many cases, a momentary cessation or reverse of flow, was observed in these microvessels during the pre-ejection period. The internal diameter of small venule was increased in late systole, while that of small arteriole remained almost constant during the cardiac cycle. Furthermore, in these epimyocardial microvessels, a higher percentage of the total area under the velocity curve occurred during the ejection phase; 51 % in small arterioles, 43% in capillaries, and 40% in small venules. These findings indicate that the phasic blood flow pattern is markedly different in the subepimyocardial microvessels from that in the large epicardial artery and the septal artery. During vasodilation following dilazep (50 fig/kg, i.v.), an adenosine potentiator, red cell velocity increased throughout the entire cardiac cycle in epimyocardial microvessels with significant increases in the total area under the velocity curves accompanied by significant dilation of the arterioles. The present data will provide information useful in predicting or simulating transmural differences in the phasic blood flow pattern.

Neuropeptide Y modulates vasoconstriction in coronary microvessels in the beating canine heart.

The purpose of this study was to determine whether neuropeptide Y has a direct vasoconstrictor effect at low doses, mimicking the physiological plasma concentration on the specific site(s) of coronary arterial microvessels in in situ beating canine left ventricles. Coronary microvessels were directly observed by means of an intravital microscope and video system equipped with a floating objective. Epi-illuminated fluorescence coronary microangiography was performed in open-chest anesthetized dogs (n = 14) to examine the changes in internal diameter of epimyocardial arterial microvessels. Flow velocity of fluorescently labeled microshperes in capillaries was also measured (n = 6). To eliminate secondary effects of neuropeptide Y on coronary microvessels via autonomic nervous modulation, experiments were conducted under pharmacological blockade of the regional autonomic nervous system by intracoronary injection of propranolol, 50 micrograms/kg; phentolamine, 100 micrograms/kg; and atropine, 5 micrograms/kg. Aortic pressure and heart rate were kept constant during the experiments. Intracoronary infusion of three different doses of neuropeptide Y (1, 10, and 100 pmol/kg/min) for 5 minutes significantly constricted small microvessels (less than 100 microns in diameter) (-5.2 +/- 1.4%, -8.5 +/- 1.5%, and -14.0 +/- 1.7%; p less than 0.05 versus before neuropeptide Y at each dose), medium microvessels (100-200 microns in diameter) (-5.5 +/- 1.6%, -10.6 +/- 1.8%, and -16.8 +/- 2.1%, p less than 0.05 versus before neuropeptide Y at each dose), and large microvessels (greater than 200 microns in diameter) (-3.6 +/- 0.6%, -5.8 +/- 0.8%, and -10.0 +/- 1.1%; p less than 0.05 versus before neuropeptide Y at each dose) in a dose-dependent manner. Capillary flow velocity was reduced by 17.2 +/- 3.1% by an intracoronary dose of 100 pmol/kg/min of neuropeptide Y (p less than 0.05). The present study indicates that low doses of neuropeptide Y exert a homogeneous direct vasoconstrictor effect on various sizes of coronary arterial microvessels and reduce capillary flow velocity. These results suggest that neuropeptide Y may play a physiological role in modulating coronary microvascular tone.

A new microscope system for the continuous observation of the coronary microcirculation in the beating canine left ventricle

A microscope system was designed using a new type of objective lens which makes possible the direct and continuous observation of the coronary microcirculation throughout the entire cardiac cycle in the beating canine heart. The microscope system consists of a standard microscope and a floating objective system which is composed of a pair of convex lenses and transmits a real image of the coronary microcirculatory bed to a standard microscope without any change in magnification. The convex lens facing the heart is supported by a weight-adjusting coil spring and low-resistance ball bearings which allow the lens to move perpendicularly in unison with cardiac motion. To reduce excessive cardiac movement, two 24-gauge needles connected to the animal table by a needle holder are horizontally inserted through the midmyocardium of the left ventricle beneath the area of interest. The epimyocardium of the left ventricle is transilluminated by means of a light pipe and a xenon-arc lamp. The distance between the floating lens and the cardiac surface is kept constant using a spacing device connected to the light pipe holder to prevent the compression of the tissue in the microscopic field of view. This improvement in the microscope system combined with high-speed cinematography greatly facilitates the continuous analysis of the coronary microcirculation in the beating left ventricle throughout the entire cardiac cycle, and may provide a useful approach to the understanding of the regulation mechanism of the coronary circulation.

Pressure-length loop in the ischemic segment during left circumflex coronary artery stenosis and its modification by afterload reducing in excised perfused canine hearts

SummaryBy using excised perfused heart preparations, we investigated the regional myocardial functions in the presence of a flow-limiting coronary stenosis of the left circumflex coronary artery (LCX) (approximately a 50% flow reduction of pre-ischemic control), as well as global cardiac functions during afterload reducing, while keeping left ventricular end-diastolic pressure (LVEDP) and heart rate constant. After inducing the LCX stenosis, cardiac output (CO), peak left ventricular pressure (peak LVP) and stroke work (SW) decreased from pre-ischemic control values, i.e., 81.1±3.2%, p<0.005, 88.1±3.8%, p<0.02 and 72.2±5.7%, p<0.005, respectively (n=7), whereas pressure-length (P-L) loop areas changed as follows; ischemic control values of the left anterior descending coronary artery (LAD) and LCX regions were 96.6±6.0%, n.s. and 72.6±9.0% of pre-ischemic control, p<0.02, respectively.Following afterload reducing with LCX stenosis, CO increased gradually, while the ischemic regional function started to further aggravate, and the initial point of further ischemic aggravation obtained in this experiment occurred at 63.5±6.9 mm Hg of mean aortic pressure (AoP). These results suggested that the increase of total cardiac function such as CO following afterload reducing was probably induced at the expense of aggravated regional ischemia. Therefore it was concluded that the treatment of ischemic myocardium by reducing afterload pressure should be done very carefully.

Effects of afterload elevation on the ischemic myocardium in isolated, paced canine heart with partial coronary stenosis

The effect of afterload elevation on the ischemic myocardium was examined in an isolated, paced canine heart with a partial coronary stenosis. The coronary blood flow of the left circumflex coronary artery was reduced to approximately one-third of the values before stenosis. The left circumflex coronary stenosis produced a decrease in global ventricular function, a decrease in systolic shortening and deviation of the ST-segment of the epicardial electrocardiogram and an increase in myocardial carbon dioxide (CO2) tension of the ischemic region. Then, afterload elevation with constant preload decreased the myocardial CO2 tension and improved the ST-segment deviation of the ischemic myocardium. Mechanical function, estimated by the relation between mean aortic pressure and systolic shortening, also improved with elevation of mean aortic pressure. In contrast, afterload elevation combined with preload elevation did not improve ischemic injury, as estimated by myocardial CO2 tension, and did not improve ST-segment deviation or mechanical function despite an increase in left circumflex coronary flow. These results suggest that the elevation of afterload pressure under constant preload improves ischemia produced by a partial coronary stenosis due to increased coronary blood supply; however, the preload elevation counterbalances the beneficial effects of afterload elevation.

Effects of α- and β-Adrenergic Blockade on Coronary Microcirculation in the Beating Canine Left Ventricle

This sudy was carried out to clarify the effects of α- and β-adrenergic blockade on coronary arterial microvessels in beating hearts. For this purpose, we used anesthetized open chest dogs (n = 47). Coronary microcirculation of the epimyocardium was observed with a microscope equipped with a floating objective and a high-speed video system. Diameters of arterial microvessels were measured by fluorescence microangiography. Heart rate and aortic pressure were kept nearly constant throughout the experimental period. In the following experiments, drugs were administered into the left anterior descending coronary artery (LAD). To assess the effect of α-adrenergic blockade, phentolamine (100 μg/kg) was administered in the absence or presence of β-adrenergic blockade (propranolol 50 μg/kg). To assess the effect of β-adrenergic blockade, propranolol (50 μg/kg) or three doses of ICI-118551 a selective β2-antagonist (0.1, 0.5, and 1.0 μg/kg per min) were administered. Coronary arterial microvessels were divided into three groups according to control internal diameters (ID) (small, ID < 100 μm; medium, 100 ≤ ID < 200 μm; and large, ID ≥ 200 μm). In the absence of β-adrenergic blockade, phentolamine significantly dilated vessels in all groups (small, 19.6 ± 5.6%; medium, 5.8 ± 2.3%; and large, 5.3 ± 0.9%; P < 0.05 vs control). In the presence of β- adrenergic blockade, the vasodilator effect of phentolamine was completely abolished.

Direct Observation of the Coronary Microvasculature in a Beating Heart by the Floating Objective System

Direct and continuous visualization of the coronary microcirculation has been almost impossible in the beating mammalian heart. To achieve this purpose, we have developed a new microscopic system which we termed the “floating objective system”, and which consists of a pair of convex lenses vertically aligned on the same optical axis. The real image on the front focus of a lens facing the heart (floating lens) is transmitted to the back focus of another convex lens fixed to the stage of a standard microscope. When the floating lens moves in unison with cardiac motion, the transmitted real image is not affected by the change in the distance between these two convex lenses. Therefore, in a beating heart, it is possible to observe coronary microvasculature with a standard microscope by observing the transmitted real image. The advantage of this system is that, in a beating heart, continuous observation of coronary microcirculation is possible throughout a cardiac cycle and resolution is sufficient to measure red cell velocity. The red cell velocity in epimyocardial microvessels reached the peak at midsystole in small arterioles and capillaries, and at late systole in small venules. In all vessels, red cell velocity gradually declined during diastole. Momentary cessation or reverse flow was observed during the pre-ejection period in all microvascular channels in the epimyo-cardium. Cyclic variation of diameter was observed in small venules but not in small arterioles.

Characteristics of Velocity Waveform and Radius in Epimyocardial Microvessels in Beating Left Ventricle

The phasic red cell velocity pattern and the phasic change in diameters in the epimyocardial microvessels were investigated in the beating canine left ventricle. We also studied the effects of nitroglycerin and dilazep, an adenosine potentiator, on the red cell velocity curves in epimyocardial microvessels and on the diameters of arterioles. Using a floating objective system and high-speed cinematography, we measured red cell velocities in arterioles, capillaries, and venules, and also microvascular diameters in the epimyocardium of left ventricle in open-chest anesthetized dogs. In arterioles and capillaries, peak red cell velocity occurred in mid-systole, followed by a decrease during diastole. In venules, red cell velocity reached its peak in late systole, followed by a gradual decline during diastole. An abrupt decline in red cell velocity and a momentary cessation or reverse flow were observed in these microvessels during the pre-ejection period. The internal diameter of small venules was significantly increased in late systole, while there was no significant change in the diameter of arterioles throughout the entire cardiac cycle. There was no significant change in the red cell velocity pattern in epimyocardial micro-vessels after administration of nitroglycerin. Nitroglycerin also failed to dilate the epimyocardial small arterioles. After administration of dilazep, red cell velocity curves in the epimyocardial microvessels were shifted upward throughout the entire cardiac cycle, with a significant increase in the total area under the velocity curves accompanied by significant dilation of small arterioles. These results suggest that the characteristics of velocity waveform in coronary microvessels in epimyocardium are quite different to those of endomyocardium in the beating left ventricle. Furthermore, using an intravital microscope system, we directly demonstrated that nitroglycerin has little effect on small arterioles and capillary flow, and that dilazep has a potent dilative effect on small arterioles and markedly increases capillary flow in the epimyocardium.