Eric O. Feigl

An unexpected role for brain-type sodium channels in coupling of cell surface depolarization to contraction in the heart

Voltage-gated sodium channels composed of pore-forming α and auxiliary β subunits are responsible for the rising phase of the action potential in cardiac muscle, but the functional roles of distinct sodium channel subtypes have not been clearly defined. Immunocytochemical studies show that the principal cardiac pore-forming α subunit isoform Nav1.5 is preferentially localized in intercalated disks, whereas the brain α subunit isoforms Nav1.1, Nav1.3, and Nav1.6 are localized in the transverse tubules. Sodium currents due to the highly tetrodotoxin (TTX)-sensitive brain isoforms in the transverse tubules are small and are detectable only after activation with β scorpion toxin. Nevertheless, they play an important role in coupling depolarization of the cell surface membrane to contraction, because low TTX concentrations reduce left ventricular function. Our results suggest that the principal cardiac isoform in the intercalated disks is primarily responsible for action potential conduction between cells and reveal an unexpected role for brain sodium channel isoforms in the transverse tubules in coupling electrical excitation to contraction in cardiac muscle.

Parasympathetic Control of Coronary Blood Flow in Dogs

The role of the vagus in the control of coronary blood flow was studied in chloralose-anesthetized dogs with open chests. Propranolol, 1.0 mg/kg iv, was used for beta-receptor blockade. Heart rate was maintained at a constant level with atrial and ventricular pacing. Left circumflex coronary artery blood flow was measured with an electromagnetic flowmeter. The experimental design accounted for the four major determinants of coronary blood flow: (1) aortic pressure, (2) myocardial systolic compression, (3) alterations in myocardial oxygen tension and metabolism secondary to changes in contractile force, (4) neural control. Efferent stimulation of the cut cervical vagi (30 Hz, 2 msec, 8-10 v) resulted in decreased aortic blood pressure and increased coronary blood flow. Late diastolic coronary artery resistance fell to 58% of the control value after 5 seconds of vagal stimulation. Atropine, 0.5 mg/kg iv, blocked these effects. It is concluded that direct parasympathetic coronary vasodilation results from vagal stimulation, which is independent of vagal chronotropic and inotropic effects.

Control of Coronary Blood Flow during Exercise

Under normal physiological conditions, coronary blood flow is closely matched with the rate of myocardial oxygen consumption. This matching of flow and metabolism is physiologically Important due to the limited oxygen extraction reserve of the heart. Thus, when myocardial oxygen consumption is increased, as during exercise, coronary vasodilation and increased oxygen delivery are critical to preventing myocardial underperfusion and Ischemia. Exercise coronary vasodilation is thought to be mediated primarily by the production of local metabolic vasodilators released from cardiomyocytes secondary to an increase in myocardial oxygen consumption. However, despite various investigations into this mechanism, the medlator(s) of metabolic coronary vasodilation remain unknown. As will be seen in this review, the adenosine, K+ATP channel and nitric oxide hypotheses have been found to be inadequate, either alone or in combination as multiple redundant compensatory mechanisms. Prostaglandins and potassium are also not important in steady-state coronary flow regulation. Other factors such as ATP and endothelium-derived hyperpolarizing factors have been proposed as potential local metabolic factors, but have not been examined during exercise coronary vasodilation. In contrast, norepinephrine released from sympathetic nerve endings mediates a feed-forward ß-adrenoceptor coronary vasodilation that accounts for -25% of coronary vasodilation observed during exercise. There is also a feed-forward α-adrenoceptor-mediated vasoconstriction that helps maintain blood flow to the vulnerable subendocardium when heart rate, myocardial contractility, and oxygen consumption are elevated during exercise. Control of coronary blood flow during pathophysiological conditions such as hypertension, diabetes mellitus, and heart failure is also addressed.

Adrenergic coronary vasoconstriction helps maintain uniform transmural blood flow distribution during exercise.

The hypothesis that α-adrenergic coronary vasoconstriction helps maintain a uniform transmural distribution of myocardial blood flow during exercise was tested in dogs. Carotid artery loops were surgically constructed and a splenectomy performed three weeks prior to study. On the day of study, the dog was anesthetized briefly (fentanyl and nitrous oxide) for percutaneous catheterization, and α-receptors in one myocardial region were blocked with phenoxybenzamine (0.25 mg/kg) infused selectively into the left circumflex coronary artery. Recirculation of phenoxybenzamine was minimized by drainage of coronary sinus outflow during the infusion. After the dog recovered from the anesthesia, regional blood flow was measured at rest and during graded treadmill exercise with the microsphere technique calibrated by reference blood samples. Average transmural flow was limited by α-vasoconstriction and was less in the region where α-receptors were intact than in the region where they were blocked, as has been described by others. The ratio of inner layer myocardial blood flow to outer layer flow was better maintained in the region with α-receptors intact than in the region with α-receptors blocked when myocardial oxygen consumption was 150 μl/min/g or greater (p <0.001). Even though average transmural flow was limited by α-receptor activation, inner layer myocardial blood flow was greater in the region with α-receptors intact than in the region with α-receptors blocked when myocardial oxygen consumption was 500 μl/min/g or more (p <0.05). In conclusion, adrenergic coronary vasoconstriction mediated by α-receptors helps to maintain a uniform transmural distribution of myocardial blood flow during exercise in spite of limiting average transmural flow.

Control of myocardial oxygen tension by sympathetic coronary vasoconstriction in the dog.

The effect of sympathetic alpha-receptor coronary vasoconstriction on myocardial oxygen tension was studied in open- and closed-chest, chloralose-anesthetized dogs. Blood oxygen tension in the coronary sinus and blood flow in the circumflex coronary artery were continuously measured in a three-part experiment. With stimulation of the left stellate ganglion (15 Hz, 3 msec, 4–7 v, 90-second train) under vagotomy control conditions (part 1), heart rate, blood pressure, and coronary blood flow increased, but coronary sinus oxygen tension decreased from 19 mm Hg to 15 mm Hg. In part 2, beta-receptor blockade with propranolol (2.0 mg/kg, iv) in the same does blunted the Dositive inotroDic and chronotropic effects of sympathetic stimulation; coronary alpha-receptor vasoconstriction was unmasked, and coronary sinus blood oxygen tension fell from 17 mm Hg to 11 mm Hg. Since increases in oxygen metabolism were blunted, it appeared that the decrease in coronary sinus oxygen tension was caused by alpha-receptor coronary artery vasoconstriction. This hypothesis was tested in part 3 by the addition of alpha-receptor blockade with Dibozane (3.0 mg/kg, iv). Sympathetic stimulation no longer resulted in a change in either coronary vascular resistance or coronary sinus oxygen tension. These results indicate that the fall in oxygen tension during beta-receptor blockade in part 2 was due to alpha-receptor coronary vasoconstriction. Thus, myocardial oxygen tension may be regulated by coronary sympathetic vasomotion as well as by myocardial oxygen metabolism and local vascular control mechanisms.

Sympathetic Control of Cerebral Blood Flow in Dogs

The effect of sympathetic stimulation on cerebral blood flow was investigated in dogs anesthetized with chloralose. A preparation has been developed for the moment-to-moment measurement of cerebral venous outflow with an electromagnetic flow transducer. The brains arterial supply was left undisturbed. The sympathetic innervation of the cerebral vessels was stimulated at the stellate ganglion (3−9 v, 3 msec, and 1, 3, 6, 10, and 15 Hz for 60 or 90 seconds). Stimulation at 15 Hz resulted in an average decrease in cerebral blood flow of 79.7%. During stimulation the arterial oxygen tension decreased from 93.2 to 84.9 mm Hg, the arterial carbon dioxide tension increased from 32.9 to 34.6 mm Hg, and arterial pH fell from 7.392 to 7.378. These changes in blood gas variables all opposed the observed vasoconstriction. Interactions between intracranial pressure and sympathetic cerebral vasoconstriction were evaluated by measuring cerebrospinal fluid pressure and cerebral venous outflow pressure. Stimulation of the left sympathetic stellate ganglion produced a 64% decrease in cerebral blood flow and an 8 mm Hg increase in intracranial pressure. Infusion of saline into the cisterna magna, raising intracranial pressure to 47 mm Hg, produced a 3% decrease in cerebral blood flow. Opening the cerebrospinal fluid space and thus fixing intracranial pressure at zero (atmospheric pressure) did not alter the cerebral blood flow response to sympathetic stimulation. It was concluded that stimulation of the sympathetic stellate ganglion resulted in cerebral vasoconstriction which was independent of changes in arterial PCO2, Po2, and pH and was also independent of changes in cerebrospinal fluid pressure.

Neural Control of Coronary Blood Flow

Parasympathetic control of coronary blood flow has been extensively studied in dogs, and a clear vasodilator effect not dependent on changes in myocardial metabolism was observed. Parasympathetic vasodilatation is mediated via nitric oxide (EDRF) and is activated during carotid baroreceptor and chemoreceptor reflexes. Intracoronary infusions of acetylcholine in humans results in increased coronary blood flow and epicardial coronary artery dilatation except in atherosclerotic epicardial coronary vessels, which show a paradoxical vasoconstriction. Sympathetic α-adrenoceptor-mediated coronary vasoconstriction has been repeatedly demonstrated whenever there is adrenergic activation of the heart, as during exercise or a carotid sinus baroreceptor reflex in dogs or during a cold pressor reflex in humans. Recent evidence indicates that there is a beneficial effect of this paradoxical vasoconstrictor influence in that it helps preserve flow to the vulnerable inner layer of the left ventricle, but only when both heart rate and coronary flow are high. β-Adrenoceptor-mediated coronary vasodilatation also occurs during adrenergic activation of the heart. The dominant site for β-vasodilatation is in small arterioles, while the dominant site for α-vasoconstriction is in microvessels larger than ∼100 µm diameter. The β-adrenoceptor coronary vasodilatation is an example of feedforward open-loop control that complements the closed-loop negative feedback control by local metabolic factors. The combined feedback and feedforward control mechanism has the advantage of an excellent match between coronary blood flow and myocardial oxygen consumption with a rapid response time but without the instability inherent in high gain feedback systems.

Feedforward control of coronary blood flow via coronary beta-receptor stimulation.

It is usually assumed that the increase in coronary blood flow observed with norepinephrine occurs through local metabolic vasodilation secondary to cardiac beta-receptor activation. However, direct feedforward beta-receptor-mediated coronary vasodilation is also a possibility. In dogs with alpha-receptor blockade, the left circumflex artery was perfused at constant pressure. The vasodilator effect of intracoronary norepinephrine injections was determined during prolonged diastoles to avoid the chronotropic and intropic effects of norepinephrine. Norepinephrine caused a dose-dependent increase in coronary blood flow that was attenuated by both the selective beta 1-antagonist practolol and the selective beta 2-antagonist ICI 118,551. These data indicate that norepinephrine activates beta 1- and beta 2-receptors in coronary resistance vessels to cause vasodilation independent of inotropic and chronotropic effects. The physiological significance of coronary beta-receptor-mediated vasodilation was investigated in the beating heart. The coronary blood flow response and coronary venous oxygen tension response were compared when myocardial oxygen consumption was increased over the same range by one of three positive inotropic interventions: (1) norepinephrine, (2) paired-pulse stimulation, or (3) norepinephrine after alpha-blockade. During norepinephrine infusion (intervention 1), coronary venous oxygen tension decreased, indicating that the match between myocardial oxygen consumption and oxygen delivery is not maintained when coronary blood flow is controlled by alpha- and beta-receptors in addition to local metabolic factors. Paired-pulse stimulation (intervention 2) also resulted in a decrease in coronary venous oxygen tension, demonstrating that the balance between oxygen consumption and delivery is not maintained when blood flow is controlled by local metabolic factors alone. However, when coronary beta-receptor-mediated vasodilation was unmasked by alpha-blockade, norepinephrine infusion (intervention 3) produced no change in coronary venous oxygen tension. Therefore, coronary beta-receptor vasodilation helps maintain the balance between flow and metabolism in a feedforward manner in the beating heart.

Quantitative Relation Between Interstitial Adenosine Concentration and Coronary Blood Flow

The effect of exogenous and endogenous adenosine in controlling coronary flow was determined using an axially distributed mathematical model of the myocardium to estimate interstitial adenosine concentration from coronary arterial and venous adenosine values. The left main coronary artery was perfused at constant pressure in closed-chest, anesthetized dogs, and exogenous adenosine was infused intracoronary to increase coronary flow. Basal interstitial adenosine was 92 nmol/L, just at the threshold for increasing coronary flow. An increase in interstitial adenosine concentration of only 62% was sufficient to increase coronary flow from 5% to 50% of maximal flow. The possible contribution of an endothelial dilator secondary to activation of adenosine receptors on endothelial cells was tested by comparing the response to exogenous intracoronary adenosine infusion with increases in endogenous adenosine produced by inhibition of adenosine kinase and adenosine deaminase. If adenosine increases coronary flow by an endothelial mechanism, then the interstitial ED50 of exogenous adenosine would be lower than that for endogenous adenosine due to the postulated additional endothelial dilator. The interstitial ED50 for exogenous adenosine was 156 nmol/L, not different from the endogenous ED50 of 150 nmol/L. In conclusion, basal interstitial adenosine concentration is at the threshold of a remarkably steep dose-response curve for increasing coronary blood flow. No evidence was found for an endothelium-mediated vasodilator mechanism secondary to adenosine receptor activation of endothelial cells in vivo. The steep adenosine dose-response curve indicates that measurements of adenosine concentration should be interpreted with caution, because small changes in adenosine concentration cause large changes in coronary flow.

Adrenergic blockade blunts adenosine concentration and coronary vasodilation during hypoxia.

Myocardial hypoxia is thought to be an important stimulus for increasing interstitial adenosine concentration. The adenosine hypothesis of coronary control was investigated during steady-state hypoxia by making measurements of coronary venous and epicardial well adenosine concentrations in adrenergically intact dogs and in animals with alpha- and beta-receptor blockade. In the adrenergically intact group, hypoxia sufficient to lower coronary venous oxygen tension to 8 mm Hg increased coronary blood flow 243% from normoxic values. Both coronary venous and epicardial well adenosine concentrations were increased throughout the hypoxic period. In the adrenergically blocked group, hypoxia to a similar level of coronary venous oxygen tension produced an increase in coronary blood flow of only 75%, which was significantly less than in the adrenergically intact group (p less than 0.01). Coronary venous adenosine was only transiently elevated, and epicardial well adenosine was unchanged from control levels. In a separate group of alpha- and beta-receptor-blocked animals that received an infusion of L-homocysteine thiolactone during hypoxia, there was no difference in tissue S-adenosylhomocysteine levels compared with those of normoxic controls. It is concluded that much of the coronary vasodilation associated with systemic hypoxia is dependent on adrenergic activation and that adenosine may only play a role in sustained hypoxic vasodilation when adrenergic receptors are intact.