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Alpha Adrenergic Vasoconstriction and Nitroglycerin Vasodilation of Large Coronary Arteries in the Conscious Dog

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Alpha Adrenergic Vasoconstriction and Nitroglycerin Vasodilation of Large Coronary Arteries in the Conscious Dog Alpha Adrenergic Vasoconstriction and Nitroglycerin Vasodilation of Large Coronary Arteries in the Conscious Dog Stephen F. Vatner, … , W. Thomas Manders, Ares D. Pasipoularides J Clin Invest. 1980;65(1):5-14. https://doi.org/10.1172/JCI109659. Research Article The effects of methoxamine and nitroglycerin on measurements of large vessel (left circumflex) coronary dimensions were examined in eight conscious dogs using an ultrasonic dimension gauge, and total coronary resistance was calculated from measurements of arterial pressure and coronary blood flow. Methoxamine (50 μg/kg per min), after transiently increasing left circumflex coronary dimensions, induced sustained reductions in left circumflex diameter (9±2%) and external (18±4%) and internal (27±5%) cross-sectional areas, at a time when mean arterial pressure rose by 65±5%, left ventricular dP/dt had decreased only slightly, and heart rate and mean coronary blood flow remained at control levels. Calculated large vessel and total coronary resistances rose similarly, i.e., by 108±29 and 92±14%, respectively. Methoxamine reduced coronary arterial wall stiffness from control at comparable stress levels, although at any common radius, wall stiffness was augmented substantially. Nitroglycerin (25 μg/kg) induced an initial decrease in coronary dimensions along with the fall in arterial pressure. However, left circumflex coronary dimensions then rose, reaching a maximum 5 min later at a time when left circumflex coronary blood flow was reduced and heart rate and left ventricular dP/dt were at control levels. At this time, significantly different effects were observed on large vessel coronary resistance, which fell by 18±2%, and on total coronary resistance, which rose by 11±4%. Thus, in the conscious dog, large coronary […] Find the latest version: https://jci.me/109659/pdf Alpha Adrenergic Vasoconstriction and Nitroglycerin Vasodilation of Large Coronary Arteries in the Conscious Dog STEPHEN F. VATNER, MASSIMO PAGANI, W. THOMAS MANDERS, and ARES D. PASIPOULARIDES, Departments of Medicine, Harvard Medical School and Peter Bent Brigham Hospital, Boston, Massachusetts 02115; New England Regional Primate Research Center, Southboro, Massachusetts 01772 A B S T RA C T The effects of methoxamine and nitro- active changes. Alpha adrenergic stimulation is suf- glycerin on measurements of large vessel (left circum- ficiently powerful to reduce cross-sectional area, flex) coronary dimensions were examined in eight con- despite the opposing elevation of distending pressure. scious dogs using an ultrasonic dimension gauge, and total coronary resistance was calculated from measure- INTRODUCTION ments of arterial pressure and coronary blood flow. The concept of coronary artery vasoconstriction is at- Methoxamine (50 ,ug/kg per min), after transiently in- creasing left circumflex coronary dimensions, induced tracting increasing clinical interest as a possible mech- sustained reductions in left circumflex diameter (9±2%) anism, not only for Prinzmetal's variant angina (1-3), and external but also for typical angina pectoris and myocardial in- (18±4%) and internal (27±5%) cross-sec- farction (2, 3). In support of this concept are cinean- tional areas, at a time when mean arterial pressure giographic studies in man and studies in rose by 65±+5%, left ventricular dPIdt had decreased experimental and animals (2, 3). Moreover, an important neural com- only slightly, heart rate and mean coronary blood ponent of the control of the coronary circulation has flow remained at control levels. Calculated large vessel been demonstrated and total coronary resistances rose similarly, i.e., by that particularly involves alpha 108±29 and adrenergic control of the coronary bed (2-11). How- 92±14%, respectively. Methoxamine ever, in these studies, changes in coronary vascular reduced coronary arterial wall stiffness from control cross-sectional area have been assessed at comparable stress levels, although at any common indirectly by radius, wall stiffness calculation of resistance from measurements of blood was augmented substantially. flow and pressure (4-11). There are relatively few stud- Nitroglycerin (25 ,g/kg) induced an initial decrease in ies on coronary dimensions with the fall in sympathetic regulation of coronary vascular along arterial pres- caliber, in which these measurements have been made sure. However, left circumflex coronary dimensions and then rose, reaching a maximum 5 min later at a time directly continuously. when left circumflex coronary blood flow was reduced The primary goal of this investigation was to deter- and heart rate and left ventricular dPIdt were at con- mine the extent to which alpha adrenergic activation trol levels. At this time, significantly different effects with methoxamine altered coronary vascular caliber, were observed on large vessel coronary resistance, using an ultrasonic dimension gauge to measure the which fell by 18±2%, and on total coronary resistance, diameter of a coronary artery directly and continuously which rose by 11±4%. Thus, in the conscious dog, in conscious dogs. This intervention was compared large with the effects of nitroglycerin, a potent coronary coronary vessels not only react passively to vasodilator. In both instances, calculations of large changes in aortic pressure but also undergo substantial vessel resistance, determined from the measurement of left circumflex coronary diameter, and total coronary Dr. Vatner was an Established Investigator, American Heart resistance, from Association. Dr. Pagani's current address is the University of determined measurements of aortic Milan, Milan, Italy. pressure and left circumflex coronary blood flow, were Received for publication 25 January 1979 and in revised compared. In addition, an analysis was undertaken to form 6 August 1979. assess the effects of augmented alpha adrenergic stimu- J. Clin. Invest. X) The American Society for Clinical Investigation, Inc. * 0021-9738/80/01/0005/10 $1.00 5 Volume 65 January 1980 5-14 lation on the elastic properties of smooth muscle in ing at the sonic velocity of -1.5 x 106 mm/s between the 7 the coronary arterial wall. MHz piezoelectric crystals, thus giving a record of instan- taneous external coronary arterial diameter. To measure these relatively small dimensions accurately, the instrument used METHODS in this study was further modified to minimize the acoustic Eight mongrel dogs weighing 28-38 kg were anesthetized disturbance generated by the electrical excitation ofthe trans- with pentobarbital Na, 30 mg/kg i.v. The transducers were im- mitting crystal. This was accomplished by placing 1,000-ohm planted through a thoracotomy in the fifth left intercostal rheostats in parallel with the crystals at the exciter output space. One pair of miniature 7 MHz ultrasonic transducers and receiver input. These rheostats were adjusted to minimize (2 x 1 mm, 12 mg) was implanted on opposing surfaces of the ringing observed in the receiving crystal, without sub- the left circumflex coronary artery 3-6 cm from its origin. stantially affecting the amplitude ofthe received echo. In addi- The ultrasonic transducers were covered with Insl-X (Insl- tion, the basic 1 MHz repetition rate of the dimension gauge X Products Corp., Yonkers, N. Y.), and attached to a Dacron was changed to 2 MHz. This doubled the amplitude ofthe out- (Du Pont, de Nemours & Co., Inc., Wilmington, Del.) back- put voltage and also permitted precise calibrations in 0.5- ing. The Dacron was sutured to the outer adventitia of the ,us steps. The frequency response of the dimension gauge is coronary artery using Ethicon 6-0 suture (Ethicon, Inc., flat to 100 Hz. The drift of the instrument is minimal and in Somerville, N. J.). In the five dogs in which it was feasible, these experiments never exceeded 0.01 mm in 6 h. To further a Zepeda electromagnetic flow transducer (Zepeda Instru- ensure data reliability, repeated calibration references were ments, Seattle, Wash.) and hydraulic occluder were implanted obtained regularly throughout the experiment, and the re- 1-3 cm distally on the same vessel. In all dogs, a miniature ceived ultrasonic signal was monitored continuously on an pressure gauge (Konigsberg Instruments, Inc., Pasadena, oscilloscope. Any major change in alignment of the crystals Calif.) was implanted in the left ventricle, and heparin-filled would be detected in the received signal and invalidate the Tygon (Norton Co., Plastics and Synthetics Div., Tallmadge, experiment. Ohio) catheters were implanted in the left atrium and de- The experiments were conducted with the conscious dogs scending thoracic aorta. In these eight dogs, a hydraulic lying quietly 1 wk after operation. Measurements of left occluder was implanted around the aortic root, and in two circumflex coronary arterial diameter, aortic root pressure, left additional dogs the occluder was implanted around the de- ventricular pressure, dP/dt, intrapleural pressure, right and scending thoracic aorta. 1 wk later, at the time of the experi- left atrial pressures, left circumflex coronary blood flow, and ment, using local anesthesia with 2% lidocaine (Astra Pharma- heart rate were continuously recorded. Aortic pressure was ceutical Products, Inc., Framingham, Mass.), catheters (Intra- raised mechanically by inflating the aortic hydraulic occluder. cath 3184, Deseret Pharmaceutical Co., Sandy, Utah) were Nitroglycerin was administered in a bolus, 25 Ag/kg i.v. At advanced into the chest cavity to measure intrapleural pres- least 1-2 h recovery
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