C. K. Chapler

Circulatory Adjustments to Anemic Hypoxia

Oxygen transfer in the circulation can be reduced in a number of ways, as Barcroft described many years ago (Barcroft, 1920). Following his nomenclature, we will describe some of the circulatory responses to hypoxic hypoxia (Barcroft’s anoxic type), which is a condition of lowered arterial PO2, and to anemic hypoxia, which is a condition of lowered arterial O2 concentration. Most attention will be directed toward the latter type because, in the case of experimental hemodilution particularly, there are some fascinating differences from hypoxic hypoxia that occur for reasons that are not immediately obvious.

Receptor-mediated vascular and metabolic actions of endothelin-1 in canine small intestine

The effects of endothelin-1 (ET-1) infusion on blood flow (Q˙G) and O2 uptake (V˙o 2G) were examined in the small intestine of anesthetized dogs ( n = 10). Arterial and venous flows of a gut segment were isolated, and the segment was perfused at constant pressure. Arterial and gut venous blood samples were taken, gut perfusion pressure andQ˙G were measured, and O2 extraction ratio (OERG) andV˙o 2Gwere calculated. ET-1 was infused (0.118 μg ⋅ kg-1 ⋅ min-1ia) throughout the experiment. In group 1 ( n = 5), ETA receptors were blocked using BQ-123 (0.143 mg ⋅ kg-1 ⋅ min-1ia) followed by blockade of ETBreceptors with BQ-788 (0.145 mg ⋅ kg-1 ⋅ min-1ia). The order of ETA and ETB receptor blockade was reversed in group 2( n = 5). In group 1, the decrease inQ˙G observed with ET-1 infusion was partially reversed with BQ-123; no further change occurred after BQ-788 administration. In group 2, addition of BQ-788 to the infusate further decreasedQ˙G, whereas addition of BQ-123 returnedQ˙G to a value not different from that with ET-1 infusion alone. These data indicated that ET-1-induced vasoconstriction in the gut was mediated via ETA receptors and that this constriction was buffered by activation of ETB receptors.V˙o 2Gdecreased in proportion to the decrease inQ˙G with ET-1, decreased further with ET-1 plus ETB receptor blockade ( group 2), and increased in proportion to the increases in Q˙Gwith ETA receptor blockade (both groups). No changes in OERGoccurred during ETA and ETB receptor antagonism in either group. This study is the first to demonstrate that a flow-limited decrease in gutV˙o 2Goccurred with infusion of ET-1 in gut vasculature. An intriguing and novel finding was that, during O2limitation, OERG was only 50% of that normally associated with ischemia in this tissue.The effects of endothelin-1 (ET-1) infusion on blood flow (QG) and O2 uptake (VO2G) were examined in the small intestine of anesthetized dogs (n = 10). Arterial and venous flows of a gut segment were isolated, and the segment was perfused at constant pressure. Arterial and gut venous blood samples were taken, gut perfusion pressure and QG were measured, and O2 extraction ratio (OERG) and VO2G were calculated. ET-1 was infused (0.118 microgram. kg-1. min-1 ia) throughout the experiment. In group 1 (n = 5), ETA receptors were blocked using BQ-123 (0.143 mg. kg-1. min-1 ia) followed by blockade of ETB receptors with BQ-788 (0.145 mg. kg-1. min-1 ia). The order of ETA and ETB receptor blockade was reversed in group 2 (n = 5). In group 1, the decrease in QG observed with ET-1 infusion was partially reversed with BQ-123; no further change occurred after BQ-788 administration. In group 2, addition of BQ-788 to the infusate further decreased QG, whereas addition of BQ-123 returned QG to a value not different from that with ET-1 infusion alone. These data indicated that ET-1-induced vasoconstriction in the gut was mediated via ETA receptors and that this constriction was buffered by activation of ETB receptors. VO2G decreased in proportion to the decrease in QG with ET-1, decreased further with ET-1 plus ETB receptor blockade (group 2), and increased in proportion to the increases in QG with ETA receptor blockade (both groups). No changes in OERG occurred during ETA and ETB receptor antagonism in either group. This study is the first to demonstrate that a flow-limited decrease in gut VO2G occurred with infusion of ET-1 in gut vasculature. An intriguing and novel finding was that, during O2 limitation, OERG was only 50% of that normally associated with ischemia in this tissue.

ENDOTHELIAL AND SYMPATHETIC REGULATION OF VASCULAR TONE IN CANINE SKELETAL MUSCLE

In vitro studies have shown that production of the vasoconstrictor endothelin-1 (ET) is inhibited by NO in porcine aorta (Boulanger & Luscher, 1990) while in vivo studies have shown that the increase in total peripheral resistance following nitric oxide synthase (NOS) inhibition in anesthetized rats is blunted by blockade of endothelin receptors (Nafrialdi et al., 1994; Richard et al., 1995). In order to assess the role of endothelin in the regulation of resting vascular tone in skeletal muscle, it was first necessary to establish that we could effectively inhibit the vasoconstrictor actions of endothelin in our experimental preparation. Endothelin-1 binds to both ETA receptors on vascular smooth muscle producing vasoconstriction (Barnes, 1994) and to ETB receptors on endothelial cells to induce transient vasodilation through stimulation of NO production (Fujitani et al., 1993; Sakurai et al., 1992). The contribution of ETB receptors in the vasoconstrictor response to ET is minimal, but some studies have demonstrated that the ETA receptor antagonists BQ123 and FR139317 were unable to fully prevent or reverse the vasoconstrictor effect of endothelin in (Bird & Waldron, 1993; McMurdo et al., 1993). Because the dilatory action of ET is transient, we elected to focus on the vasoconstrictor action of ET.

O2 Transport and Uptake in Dogs during CO Hypoxia with and without β-Block

At levels of carboxyhemoglobin greater than 40% (CO hypoxia), cardiac output increases in anesthetized dogs (Einzig et al., 1980; Sylvester et al., 1979). At the same time, both sympathetic activity (Fitzgerald et al., 1976) and circulating levels of catecholamines are increased (Sylvester et al., 1979). In addition to these facts, Scharf et al. (1975) reported that left ventricular contractile performance, as measured by dp/dt, increase when isolated dog hearts were perfused with blood from donor dogs that had inhaled carbon monoxide. The increased ventricular dp/dt was abolished following β-adrenergic blockade; These findings suggest that β-adrenergic receptor activity is an important component in the cardiac response to CO hypoxia. With respect to the peripheral circulation, Cain and Chapler (1979) showed that β-adrenergic vasodilation contributed to the rise in hindlimb skeletal muscle blood flow which occurred in both anemic and hypoxic hypxia. To identify the role of β-adrenergic receptors during CO hypoxia, we have measured some of the cardiovascular and metabolic responses of the whole body and hindlimb skeletal muscle with and without propranolol to block β-adrenergic receptors.

Critical oxygen extraction in dog hindlimb after inhibition of nitric oxide synthase and cyclooxygenase systems.

When anesthetized dogs were given the α-adrenergic blocking agent phenoxybenzamine and then made hypoxic, their ability to extract oxygen from a supply that was limiting to oxygen uptake was significantly less than in unblocked animals (Cain, 1978). This was evident by a lower slope of the line relating O2 uptake to oxygen delivery as O2 uptake became linearly dependent upon O2 delivery. The reason for the lesser efficiency in extracting oxygen by the α-blocked animals was postulated to depend upon the loss of vasoconstrictor tone. The hypothesis that was offered stated that a vigorous constrictor tone was necessary in hypoxia so that blood flow in excess to need would not occur in any organ system or tissue. The constrictor tone in areas where O2 demand exceeded O2 supply would then be offset by the production of vasodilator metabolites in proportion to the imbalance of supply and demand. In this manner, blood flow and O2 delivery would be matched to local O2 need so that O2 would not be shunted through areas that were overperfused relative to their O2 uptake.

Central Reflex Effects of Hypoxia on Muscle Oxygenation

We postulated that whole body hypoxia causes O2 demand to increase in skeletal muscle because hypoxia stimulates sympathetic release of catecholamines which are calorigenic. Such an effect would be masked in a typical hypoxia model in which regional O2 uptake would also be limited by O2 availability. Our first goal in these experiments was to circumvent this difficulty and demonstrate that our postulate was correct. The second goal for these experiments was to determine the strength and duration of centrally mediated vasoconstriction in muscle in the absence of local hypoxia. Previous studies (Cain and Chapler, 1979; 1980) have shown that muscle vascular resistance increased less than 20% in severe whole body hypoxia and that after 30 min of hypoxia, local dilatory factors had overcome any vasoconstriction. To meet both these goals, we maintained normoxic regional perfusion to hindlimb skeletal muscles of anesthetized dogs by use of a pump and membrane oxygenator while ventilating the animal with an hypoxic gas mixture. To show the role of innervation in regional hypoxic reactions, we compared metabolic and hemodynamic events in innervated and denervated limbs.

Muscle O2 Uptake While Perfused at Constant Pressure with Normoxic Blood During Global Hypoxia

When the isolated hindlimb muscles of anesthetized dogs were perfused at constant flow (CF) with normoxic blood while the whole animal was ventilated with 9% O2 in N2, limb O2 uptake (VO2) increased 25% instead of decreasing with whole body VO2 (Bredle et al., 1988a). This hypoxia-induced increase in limb O2 demand was attributed to the calorigenic effects of catecholamines liberated at sympathetic nerve endings and from the adrenal medulla as a result of increased chemoreceptor activity during hypoxia (Blatteis and Lutherer, 1974; Cain, 1969; Sylvester et al., 1979). The increase in hindlimb VO2 was prevented by β2-adrenergic receptor blockade (Bredle et al., 1988a) and was largely dependent on intact innervation to the hindlimb (Bredle et al., 1988b). Our goal in the present study was two-fold: 1) to ascertain whether hypoxic vasoconstrictor tone was sufficiently potent to prevent any increase in limb VO2 even if it were perfused with normoxic blood, and 2) to observe whether any autoregulatory escape would occur if that were the case. With respect to the second goal, as the ratio of O2 supply to demand decreases, then local vasodilator factors should accumulate to relieve local vasoconstriction (Cain and Chapler, 1979). To accomplish these goals, we pump-perfused the limb muscles of anesthetized dogs with normoxic blood at constant perfusion pressure (CP) while ventilating them with an hypoxic gas mixture. In this manner, blood flow varied inversely with the level of sympathetic vasoconstrictor tone.

The Role of Beta-Adrenergic Receptors in the Cardiac Output Response During Carbon Monoxide Hypoxia

Cardiac output increases during carbon monoxide hypoxia (COH) in anesthetized dogs when the level of carboxyhemoglobin exceeds 40% (Einzig et al., 1980; King et al., 1984; King et al., 1985; Sylvester et al., 1979). This compensatory response partially offsets the decrease in whole body oxygen delivery which results from the reduced oxygen content; oxygen uptake is maintained in spontaneously breathing anesthetized dogs at both 50 and 65% carboxyhemoglobin (King et al., 1984). The mechanisms underlying the cardiac output response during COH are not fully understood. It has been shown that nonselective β1 and β2-adrenergic blockade with propranolol resulted in lower values for cardiac output at 30 minutes of COH than in unblocked animals (King et al., 1985; Villeneuve et al., 1985). The effect of propranolol could have resulted from blockade of β1, β2 or a combination of the β1 and β2-adrenergic receptor sub-types. In the present study, the effects of the selective β2 blocker ICI 118,551 on cardiac output and whole body oxygen uptake responses were observed during severe COH (62% decrease in arterial oxygen concentration) in anesthetized, spontaneously breathing dogs. The data were compared to our earlier results obtained during COH in dogs treated with propranolol.

Cardiovascular and metabolic responses to carotid clamping in anemic dogs.

It has been observed that blood flow to the hindlimb was maintained or even increased in proportion to cardiac output during isovolemic hemodilution of anesthetized dogs with dextran (Grupp et al., 1972; Cain and Chapler, 1978; 1981). This was true even when the decrease in total O2 transport limited whole body O2 uptake. In one study, we infused norepinephrine to increase vasoconstrictor tone. The results suggested that the reduction in viscosity prevented any measurable increase in hindlimb resistance during anemia so that blood flow could not be redistributed to areas more essential than resting skeletal muscle. In these new experiments we have used a more physiologic cardiovascular challenge, clamping of the carotid arteries, to see if lowered blood viscosity alters the response to baroreceptor stimulation. As will be shown, the buffering effect of aortic baroreceptors prevented much change in the peripheral resistance but carotid clamping did alter O2 uptake.

The Role of Endothelium-Derived Relaxing Factor (EDRF) in the Whole Body and Hindlimb Vascular Responses during Hypoxic Hypoxia

We and others have postulated that the most efficient oxygen extraction from a diminished oxygen supply is achieved when locally-generated vasodilators promote blood flow to areas of greater O2 demand while others remain under vasoconstrictor tone (Cain and Chapler, 1980; Granger and Shepherd, 1979). Previous experiments in our laboratory have shown that administration of Nω-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase (NOS) inhibitor, doubles both total and hindlimb peripheral resistance in the anesthetized dog. The current study was undertaken to test the hypothesis that the greater levels of whole body and hindlimb resistance during NOS inhibition would limit the ability of the animal to effectively utilize the reduced oxygen supply available during severe hypoxic hypoxia (HH). In particular, we wished to assess the functional significance of endothelium derived relaxing factor on oxygen utilization and vascular resistance during severe hypoxic hypoxia in the whole body and skeletal muscle.