Calcium-activated potassium channel-mediated arteriolar relaxation during endotoxic shock.

The role of calcium-activated potassium (KCa) channels in the in vivo relaxation of arterioles was investigated before endotoxin shock (Pre-ENDT) and during endotoxin shock at 180 min (Post-ENDT). Diameters of 2nd and 3rd order (A2 and A3) arterioles in the left cremaster muscle of male Sprague-Dawley rats anesthetized with pentobarbital sodium were measured using videomicroscopy. Adenosine (ADO) at 534 micrograms intraarterially, topical ADO at 10(-3) M, and the endothelium-dependent agonist topical acetylcholine (ACH) at 10(-4) M significantly dilated both A2 and A3 arterioles Pre-ENDT and Post-ENDT. Topical tetraethylammonium chloride (TEA) at 1 mM blocked ADO (intraarterially and topical)-induced A2 and A3 arteriolar dilations Pre-ENDT and Post-ENDT. Arteriolar dilation to ACH was maintained Pre-ENDT, but was blocked by TEA in A2 and A3 arterioles Post-ENDT. The endothelium-independent agonist sodium nitroprusside (10(-5) M), when topically applied, caused maximal arteriolar dilation Pre-ENDT and Post-ENDT in the presence of TEA. The data show that vascular smooth muscle KCa channels are a significant factor in ADO-induced relaxation of cremaster microvessels and are not significantly affected by ENDT. The results also suggest that the mechanism for endothelium-dependent ACH vasodilation changes from a non-KCa channel-mediated mechanism Pre-ENDT to a KCa-mediated mechanism Post-ENDT.

The influence of sympathoadrenal activation on skeletal muscle oxygen extraction during endotoxemia.

We have previously shown a direct relationship (r = .97) between the fall in arterial blood pressure and the increase in skeletal muscle oxygen extraction (MVO2) during canine endotoxemia. Since it is well known that hypotension activates the sympathetic system, the primary aim of these experiments was to determine if the increase in MVO2 during endotoxemia is a result of elevated levels of catecholamines due to increased sympathetic neural and/or humoral activity (sympathoadrenal system). Canine gracilis muscles were vascularly isolated and perfused in situ at a constant flow (6-7 ml/min/100 g). Endotoxemia was induced by a 30 min intravenous infusion of Escherichia coli endotoxin (2 mg/kg), which induced a 50% reduction in arterial pressure. Perfusion pressure, mean arterial pressure, and arteriovenous oxygen difference (a-v O2) were continuously measured. We found 1) no significant difference between the amount of O2 extracted by an innervated or a denervated muscle during endotoxemia; 2) the intra-arterial infusion of norepinephrine or epinephrine into a denervated gracilis muscle (plasma molar concentrations of; 10(-11), 10(-9), 10(-7), and 10(-5) failed to increase MVO2 to the level observed during endotoxemia; 3) pretreatment of a muscle with propranolol to block skeletal muscle beta-adrenergic receptors, did not suppress the endotoxin-induced rise in MVO2. We concluded that the increase in MVO2 seen after the administration of endotoxin is not due to either increased sympathetic nerve activity or elevated levels of circulating catecholamines. We speculate that the increased MVO2 during endotoxemia is caused by nonadrenergic mediators released by endotoxin rather than the hypotensive stimulus.

Endotoxin induces metabolic dysregulation of vascular tone.

The goals of this study were to determine 1) if endotoxin alters vascular responsiveness to metabolic stimuli and 2) if the decompensatory loss of skeletal muscle vascular tone that occurs during endotoxemia is induced by increased muscle metabolism. Vascularly isolated and denervated canine gracilis muscles were perfused in situ at a constant flow. In the first set of experiments, gracilis muscle O2 extraction (MVO2) and perfusion pressure were continuously measured during direct electrical stimulation of the muscle mass. Endotoxemia was induced by a 30-min intravenous infusion of Escherichia coli endotoxin (2 mg/kg), and the stimulations were repeated 60 min postendotoxemia. Compared with the nonendotoxic control, the endotoxemic muscle stimulation resulted in a decreased MVO2, and the vascular response (dilation) was potentiated. In the second set of experiments, the MVO2 of the experimental muscle (GMe) was lowered by cooling the temperature of the blood perfusing the muscle to 22-24 degrees C while maintaining the temperature of the contralateral control muscle (GMc) at 34-35 degrees C. After the administration of endotoxin, arterial pressure fell and the GMc showed a progressive increase in MVO2 and loss of vascular tone (decompensation). Coincidently, the GMe showed no significant change in MVO2 and did not vasodilate. The major findings of this study are 1) endotoxin induces the vasculature to become more reactive to metabolic vasodilation, and 2) the decompensatory vasodilation typically observed during endotoxemia can be abolished if MVO2 (i.e., metabolism) is kept low by cooling the muscle. The data suggest that endotoxemia increases vascular sensitivity to vasodilatory metabolites, which allows local mechanisms to dominate extrinsic nonneural forces and control vascular tone, thus inducing vasodilation.

Oxygen extraction and vascular dilation are dependently increased in skeletal muscle during canine endotoxemia.

The principal aim of these experiments was to evaluate the ability of a skeletal muscle to extract oxygen during endotoxemia, and determine if the decompensatory decrease in skeletal muscle vascular resistance that occurs after exposure to endotoxin is related to muscle oxygen uptake (VO2). A vascularly isolated denervated canine gracilis muscle was perfused in situ at a constant flow (5-7 mL/min/100 g). Endotoxemia was induced by a 30-min intravenous infusion of Escherichia coli endotoxin (2 mg/kg). Perfusion pressure and arteriovenous oxygen difference (a-v O2) were continuously measured, and muscle O2 extraction was calculated (VO2 = flow x a-v O2). These studies found that gracilis muscle oxygen uptake increased from a resting value of 0.30 mL O2/min/100 g to 0.63 mL O2/min/100 g (111% increase) by 90 min post-endotoxin. The arterial conductance (i.e., arterial dilation) increased 58% during this time. The amount of oxygen the muscle extracts was found to be directly related to the degree of vasodilation (r = .97), and inversely correlated to mean arterial pressure (r = .97). Pre-dilating the muscle with sodium nitroprusside did not alter oxygen extraction. However, after the introduction of endotoxin, a pre-dilated muscle increased VO2 94% by 90 min. These observations support the concept that endotoxin causes an increase in VO2 without producing a defect in the ability of muscle to extract oxygen. The vasodilation typically observed in skeletal muscle during endotoxemia is not the cause of the increased oxygen uptake. It seems likely that mediators released by endotoxin metabolically stimulate skeletal muscle cells, which increases oxygen demand, thus promoting vasodilation.

Hemodynamic responses to changes in carotid sinus pressure after endotoxin and ibuprofen.

The hemodynamic responses to changes in carotid sinus pressure (CSP) were evaluated in nine pentobarbital-anesthetized dogs during control, endotoxin-treatment, and ibuprofen (after endotoxin) treatment periods. Both carotid sinuses were isolated and perfused at varying pressures with oxygenated blood in the vagotomized animal. Alterations in carotid sinus pressure and the resultant responses were measured at 15-min intervals during a 30-min control period, for 60 min after 1 mg/kg endotoxin, and for 60 min after 10 mg/kg ibuprofen given after endotoxin. The results showed a reduction in calculated gain for mean arterial pressure (MAP) (change in arterial pressure/change in CSP), heart rate, and peripheral resistance (TPR) after endotoxin, without a corresponding reduction in cardiac output (CO) gain. These gain changes were accompanied by a decrease in absolute MAP, CO, and TPR. An indicator for cardiac performance gain also increased. Relatively, arterial pressure was partially maintained by an increase in CO despite a loss in ability to vasoconstrict. Ibuprofen failed to correct the MAP gain, and only partially restored MAP, but shifted a greater relative response to peripheral resistance. To test if TPR would also decrease if the decrease in CO was prevented, three additional animals were studied with a pump in series with the heart to maintain CO; TPR again dropped after endotoxin. The results indicate a loss of peripheral arterial tone after endotoxin, partially restored by ibuprofen. The CO response indicates a peripheral vascular failure rather than a central or carotid sinus failure mechanism.

Effect of ibuprofen upon denervated skeletal muscle resistance and compliance vessels during endotoxemia.

The primary aim of these studies was to specifically evaluate the non-neural role of the cyclooxygenase products on the peripheral vascular decompensation associated with endotoxemia. The constant-flow perfused, vascularly isolated, denervated double-canine gracilis muscle preparation in which one muscle is used as a control for the contralateral side was employed. The experimental muscle (GMi) received ibuprofen while the control (GMc) was infused with the vehicle. The results of these studies suggest that endotoxin increases the arterial conductance (i.e., arterial dilation) by 100% and venous compliance (i.e., venoconstriction) by 40%. These observations, which are consistent with venous pooling, were not significantly altered by the continuous intra-arterial infusion of ibuprofen at a peripheral blood concentration of 160 microM. Ibuprofen caused a small but statistically significant increase in the conductance/compliance ratio at 60, 75, and 90 min post endotoxin, suggesting that cyclooxygenase products may play a minor role in the non-neural regulation of capillary fluid dynamics during endotoxemia. Consequently, these studies do not provide convincing evidence that would support a non-neural cyclooxygenase role in the peripheral vascular decompensation reported to occur during systemic endotoxemia.

Contribution of extravascular compression to reduction of maximal coronary blood flow.

Effects of ventricular compression on maximally dilated left circumflex coronary blood flow were investigated in seven mongrel dogs under pentobarbital anesthesia. The left circumflex artery was perfused with the animals' own blood at a constant pressure (63 mmHg) while left ventricular pressure was experimentally altered. Adenosine was infused to produce maximal vasodilation, verified by the hyperemic response to coronary occlusion. Alterations of peak left ventricular pressure from 50 to 250 mmHg resulted in a linear decrease in total circumflex flow of 1.10 ml.min-1 x 100 g heart wt-1 for each 10 mmHg of peak ventricular to coronary perfusion pressure gradient; a 2.6% decrease from control levels. Similar slopes were obtained for systolic and diastolic flows as for total mean flow, implying equal compressive forces in systole as in diastole. Increases in left ventricular end-diastolic pressure accounted for 29% of the flow changes associated with an increase in peak ventricular pressure. Doubling circumferential wall tension had a minimal effect on total circumflex flow. When the slopes were extrapolated to zero, assuming linearity, a peak left ventricular pressure of 385 mmHg greater than coronary perfusion pressure would be required to reduce coronary flow to zero. The experiments were repeated in five additional animals but at different perfusion pressures from 40 to 160 mmHg. Higher perfusion pressures gave similar results but with even less effect of ventricular pressure on coronary flow or coronary conductance. These results argue for an active storage site for systolic arterial flow in the dilated coronary system.

Influence of histaminergic receptors on denervated canine gracilis muscle vascular tone during endotoxemia.

The purpose of this study was to determine if endogenously released histamine and its non-neural interaction with the H1- and H2-histaminergic receptors in the peripheral vasculature can account for the decompensatory loss of peripheral vascular tone associated with the hypotension occurring during endotoxemia. A denervated in situ constant flow double canine gracilis muscle preparation that permitted one muscle to serve as a control (GMc) for the contralateral experimental muscle (GMe) was used. Endotoxemia was induced by intravenous infusion of 2 mg.kg-1.30 min-1 endotoxin. The specific H1 and H2 antagonists diphen-hydramine and cimetidine were infused either together or separately in both high and low dosages into the GMe. Blockades were validated by intra-arterial injection of histamine or the specific agonists betahistine for H1 and dimaprit for H2 receptors. The results suggest that the high-dose diphenhydramine produced a nonspecific dilation not seen with the lower dose. Because both the blocked and unblocked vascular beds exhibited the same degree of vasodilation after endotoxin, these studies do not support the hypothesis that endogenously released histamine is responsible for the loss of vascular tone. These studies do verify, however, that a nonneurally mediated loss of skeletal muscle vascular tone is an important factor to consider in the overall cardiovascular hypotension occurring during endotoxin shock.

Systemic and local effects of endotoxin on canine gracilis muscle vascular conductance.

To define the site and mechanism of action that endotoxin has on the peripheral vasculature, an in situ constant-flow double-canine gracilis muscle (GM) preparation was utilized. During systemic endotoxemia, one GM was innervated and the other was denervated during a 30-min intravenous infusion of 2 mg/kg endotoxin. Significantly increased vascular conductance (URP) in the denervated GM (106 +/- 26%) occurred compared with the innervated GM (50 +/- 7%), which suggests that decompensation is not totally dependent on neural depression. During local endotoxemia, with both GMs either intact or denervated, one GM was infused intra-arterially for 30 min with a dose of endotoxin calculated to provide a blood concentration similar to that achieved during systemic endotoxemia, whereas the other GM was infused with the vehicle. The URPs did not change significantly in either the saline or endotoxin GMs. Therefore, endotoxin does not act directly on peripheral vasculature or totally through depression of the autonomic nervous system. It apparently interacts with a systemically dependent mechanism to release a vasodepressor substance that is transported to the peripheral vasculature causing relaxation of vascular tone.

Effect of systemic endotoxin on skeletal muscle vascular conductance during high and low adrenergic tone.

The objective of this study was to determine the role of adrenergic tone on the peripheral vascular decompensation reported to occur during systemic endotoxemia. An in situ constant-flow double-canine gracilis muscle (GM) preparation allowed one GM to serve as an innervated control (GMc) for the contralateral denervated muscle (GMe). Group I (n = 9): normal inherent vascular tone; Group II (n = 7): adrenergic tone elevated by bilateral common carotid artery ligation. The GMc Group II vascular conductance was significantly lower than GMe at .0485 +/- .004 ml/min/100 g/mm Hg and .0636 +/- .005 ml/min/100 g/mm Hg respectively. GM denervation had no significant effect on Group I conductance suggesting a low level of baseline intrinsic adrenergic tone; however, denervation did increase the vascular conductance by more than 30% from .0485 +/- .004 to .0638 +/- .008 ml/min/100 g/mm Hg in Group II. A 2 mg/kg dose of endotoxin was infused i.v. over 30 min and data collected over an additional 60 min. The endotoxin caused a decrease in MAP from 129 +/- 5 to 73 +/- 6 mm Hg in Group I and from 177 +/- 16 to 92 +/- 14 mm Hg in Group II. both the GMc and GMe Group I GMs showed an initial increase in conductance to 115 +/- 9 and 117 +/- 8% respectively at 5 min followed by a reduction to 82 +/- 9 and 101 +/- 10% at 60 min. Group II (GMe) showed a significantly increased conductance following denervation to 125% which increased insignificantly to 140 +/- 15% at 60 min, while conductances either remained at about 100% or decreased slightly to 88 +/- 7% in the innervated Group II GMc. The data suggest that the baseline level of vascular tone may not be an important factor when evaluating the effect of systemic endotoxemia on the skeletal muscle peripheral vasculature.

Needle holder damage to surgical needles.

Clamping surgical needles between the jaws of needle holders with teeth markedly weaken the needles, making them prone to breakage. In contrast, clamping surgical needles between either smooth needle holder jaws or jaws embedded with tungsten carbide particles did not alter the ductility of surgical needles.

Influence of cutting edge configuration on surgical needle penetration forces.

A standardized test for measuring the needle penetration forces has been developed that can be easily replicated in any laboratory. Using this test, conventional cutting edge needles utilized in the test produced lower penetration forces than reverse cutting edge needles. The lower penetration forces encountered by the conventional cutting edge needles imply that the physician should be able to handle these needles with more dexterity and precision than the reverse cutting edge needle.

The effect of allopurinol and catalase on cardiovascular hemodynamics during hemorrhagic shock.

The primary objective of this study was to determine the effect that the xanthine oxidase inhibitor allopurinol (ALLO) and the hydrogen peroxide scavenger catalase (CAT) have on the cardiovascular compensatory ability of the dog to respond to severe hemorrhagic hypotension. Twenty-four mongrel dogs were anesthetized with sodium pentabarbitol and surgically prepared to monitor 1) average arterial blood pressure (AAP), 2) central venous pressure (CVP), 3) heart rate (HR), 4) cardiac index (CI = CO/kg), and hindlimb skeletal muscle blood flow (MBF). Total body vascular conductance (TBC) and skeletal muscle vascular conductance (MVC) were calculated by dividing the CI or MBF by the difference between the AAP and CVP. Eight animals were placed into each of the following three groups, bled over a 1-hr period of time to an AAP of 50 mm Hg and monitored for an additional 2 hr. Group I controls received an intravenous volume of lactated Ringer's equivalent to that volume given to groups II and III. Group II was pretreated 24 hr prior to hemorrhage with 100 mg/kg ALLO orally and received a bolus injection of 25 mg/kg 15 min prior to hemorrhage plus an intravenous infusion of 5 mg/kg/hr over the 3-hr study. Group III was given the same ALLO treatment as group II plus an additional 5-mg/kg/hr intravenous infusion of CAT throughout the duration of the 3-hr study. The results show that the intense compensatory increase in total body vascular tone which occurs during severe hypovolemia is significantly reduced at the 60-, 120-, and 180-min periods in the ALLO/CAT group; however, when ALLO alone was used this effect lasted only through the 120-min period. A similar, but statistically less convincing, picture was seen in the skeletal muscle vascular bed. Thus, the ALLO/CAT group seemed to inhibit some free radical mechanisms better than the ALLO group during and immediately following hemorrhage. Allopurinol alone lost its effectiveness before the 3 hr, which suggests that a free radical mechanism may play an early role in the pathophysiologic shock sequence. As shock continues, however, other factors seem to override the free radical mechanism. One possible explanation for this early tissue protective action of allopurinol and catalase is the inhibition of the oxygen free-radical-induced microvascular swelling and plugging.

Efficacy of military antishock trousers in compensatory and decompensatory hemorrhagic hypotension.

The compensatory cardiovascular response to hemorrhage includes a baroreceptor-induced activation of the sympathetic nervous system resulting in an attempt to reestablish MAP through peripheral vasoconstriction. If the hypotension is not reversed this compensatory vasoconstriction will progress to a loss of vascular tone known as vascular decompensation. The primary purpose of the present study was to compare the effectiveness of military antishock trousers (MAST) applied during the compensatory and decompensatory stages of hemorrhagic hypotension. MAST pressures of 30, 50, 70, and 90 mm Hg were applied during control, compensation, and decompensation. The results showed that MAST pressures up to 90 mm Hg were ineffective at raising mean arterial blood pressure (MAP) when applied to normotensive dogs; MAP increased 62% when MAST were applied during compensation as the result of a significant augmentation of cardiac output (stroke volume and heart rate) with no change in TPR; and a modest increase in MAP from 40 to 55 mm Hg occurred when MAST pressure was increased to 70 mm Hg during decompensation, which was accounted for entirely on the basis of an increased total peripheral resistance with no significant change in CO.

Intrinsic versus extrinsic regional vascular control during hemorrhagic hypotension and shock.

An analysis of both intrinsic (eg, autoregulatory) and extrinsic adrenoreceptor regulation of the vascular smooth muscle within skeletal muscle (SM), cutaneous (C), and mesenteric (M) tissues obtained during local tissue hypotension (LH), hemorrhagic hypotension (HH), and shock (S) is presented. A series of pressure/conductance curves show that the intrinsic regulation of vascular tone remains down to LH values of 40 mm Hg in M to 60 mm Hg in SM and does not occur in C; all three vascular beds respond to HH by exhibiting strong extrinsic vasoconstriction, the elevated tone persists throughout HH in C but lasts only a few minutes in M while SM vasoconstriction may last up to 45 min; and during the terminal phase of S, vascular tone was best maintained in C. In vitro studies suggest that the prehemorrhage alpha 1 adrenoreceptor control is greatest in M and least in SM. During compensatory and early decompensatory HH, alpha 1 receptors are depressed in SM. M vessels show this alpha 1 receptor hyposensitivity only during compensatory HH. All vessels show strong responsiveness to NE during all stages of HH and S, yet M vessels demonstrate a progressive increase in the NE concentrations required to elicit and ED50, suggesting some degree of adrenoreceptor desensitization or down regulation. This is in contrast to the adrenoreceptor hypersensitivity noted in SM during both compensatory and decompensatory stages. C vessels show this pattern in all stages except S. These data verify that each vascular bed has its own unique set of vascular control mechanisms that can act independently during HH and S.

Protective effect of physical training on cardiovascular responses to hemorrhagic hypotension and shock.

The primary objective of this study was to determine if a measurable degree of protective cardiovascular adaptation to hypovolemic shock is developed in response to aerobic training. Twelve rats were trained (T) by running on a rodent treadmill 60 min/day, 5 days/wk at 30 m/min on a 5 degree incline for a period of 13-17 weeks. Elevated levels of SDH activity in the vastus intermedius muscles of the trained group (T) verified physiological training. Each T rat was weight matched with a sedentary untrained (UT) control, anesthetized with sodium pentobarbital, and subjected to a modified Wiggers hemorrhagic shock protocol. The parameters monitored were the maximum reduction in vascular capacitance (ie, maximum blood shed) when MAP was lowered to 30 mm Hg by hemorrhage; the time necessary to achieve maximum blood loss at 30 mm Hg (compensation time); and the time between maximum and 20% uptake of the shed volume from the reservoir (decompensation time). The data show that the initial MAPs for the UT group were significantly higher than the T group (133 +/- 3 mm Hg vs 121 +/- 4 mm Hg). The maximum blood loss normalized to body weight and pressure drop was .268 +/- .012 ml/kg/mm Hg for UT and .343 +/- .02 ml/kg/mm Hg for T (P less than .001), suggesting that T had a better ability to reduce total vascular capacitance. Also, both the compensation and decompensation times were greater in the T than UT groups. These data suggest that treadmill exercise-conditioned rats have a greater inherent cardiovascular compensatory ability than untrained rats.

The influence of hypertension upon the normal cardiovascular responses to hemorrhagic hypotension and shock.

The data suggest that rats genetically inbred to be hypertensive (SHR) are less able to compensate for hemorrhage and shock than their normotensive controls (WKY). Two reasons for this genetic dysfunction are: 1) SHRs seem to depend more on innervated alpha 1 than noninnervated alpha 2 adrenoreceptors for vasoconstriction; and 2) the vascular smooth muscle hypertrophy noted in SHRs may interfere with effective vasoconstriction.

Vascular adrenergic interactions during hemorrhagic shock.

The objective of this paper is to review the sequence of vascular events that follows severe hemorrhage. The initial cardiovascular imbalance is a fall in the volume/vascular capacity relationship that leads to reductions in cardiac output and mean arterial pressure (MAP). Peripheral sensors detect the fall in MAP and changes in blood chemistry that cause withdrawal of the normal inhibitory tone from the cardiovascular control centers in the central nervous system. The resulting increased sympathetic activity initiates a series of events that include stimulation of peripheral adrenergic nerves and the adrenal medulla. The magnitude of the compensatory vasoconstriction that follows is the net result of the interaction of the epinephrine (E) from the adrenal medulla and norepinephrine (NE) from the peripheral nerves on the peripheral vascular adrenoreceptors as well as other nonadrenergic mechanisms not discussed here (i.e., angiotensin endogenous opiates). By using pharmacological blocking agents, these adrenoreceptors have been subclassified as: innervated postsynaptic alpha 1; presynaptic alpha 2 (Ps alpha 2); and extrasynaptic alpha 2 (Es alpha 2) adrenoreceptors. The action of E and NE on the alpha 1 and Es alpha 2 receptors initiates the compensatory vasoconstriction, whereas action of these catecholamines on the Ps alpha 2 located on the presynaptic membrane inhibits further release of NE from peripheral nerve terminals, thereby reducing the effect of the innervated alpha 1 receptors. This autoinhibition together with a similar action by prostaglandin E on NE release is thought to be, at least in part, responsible for the vascular decompensation known to occur in the skeletal muscle after hemorrhage. Thus, one of the factors determining survival after hemorrhage may be related to the relative dominance of alpha 1 and Es alpha 2 receptors during the initial compensatory response.