Gracilis Muscle Preparation Research Articles

The value and limitation of iloprost infusion in decreasing skeletal muscle necrosis

Iloprost has been shown to minimize skeletal muscle necrosis when administered before the onset of ischemia in animal experiments, possibly by preventing neutrophil activation. Since patients with acute limb ischemia are seen after the process has begun, we investigated whether iloprost can be protective when given only during reperfusion. After anesthesia, 18 adult mongrel dogs underwent a standard isolated gracilis muscle preparation. In six control animals (group I) the gracilis muscle was subjected to 6 hours of ischemia followed by 48 hours of reperfusion. Group II animals (n = 6) received intravenous infusion of iloprost at a dose of 0.45 μg/kg/hr beginning 1 hour before the onset of muscle ischemia and throughout the experiment (6 hours of ischemia and 1 hour of reperfusion). In addition to the continuous infusion, they received 0.45 μg/kg intravenous boluses of iloprost 10 minutes before the induction of ischemia and 10 minutes before reperfusion. Group III animals (n = 6) had a similar ischemic interval, but were given a bolus of iloprost of 0.45 μg/kg at end ischemia followed by continuous infusion of 0.45 μg/kg/hr for 48 hours during reperfusion. Muscle biopsies were obtained at baseline and after 1 hour of reperfusion in all groups. Additional biopsies were obtained at 48 hours of reperfusion in groups I and III. Myeloperoxidase activity, a marker of neutrophil activation, was measured in all muscle biopsies. At the end of reperfusion, the gracilis muscle was harvested in all animals and weighed. Muscle necrosis was estimated by serial transection, nitroblue tetrazolium histochemical staining followed by computerized planimetry. Iloprost decreased the percentage of muscle necrosis from 58% ± 7% in control animals to 34% ± 3% in group II animals (p < 0.04). However, there was no difference in the percentage of muscle necrosis when iloprost was given during the reperfusion phase only (55% ± 8% vs 58% ± 7% in group III and group I, respectively). Myeloperoxidase activity at 1 hour of reperfusion was higher than the baseline in group II animals (0.10 ± 0.02 vs 0.02 ± 0.01 units/mg tissue protein, p < 0.05). The muscle weight increased significantly at the end of reperfusion in all three groups. However, the percentage increase was not significantly different between the three groups (34% ± 8% vs 32% ± 7% vs 39% ± 12%). Although we confirmed that iloprost decreases muscle necrosis when started before the onset of ischemia, we were unable to show a protective effect when given only during reperfusion. Since there was a statistically significant increase in myeloperoxidase activity after 1 hour of reperfusion, it appears that the protective effect of iloprost is not related to inhibition of neutrophil activation. Contrary to previous belief, iloprost did not prevent a rise in vascular permeability.

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.

Leukocyte activation in ischemia-reperfusion injury of skeletal muscle

Polymorphonuclear leukocyte (PMN) participation in the pathophysiology of the reperfusion injury following skeletal muscle ischemia has become recognized. We measured the activation of PMNs as evidenced by production of superoxide anion (O 2 −) in the isolated canine gracilis muscle preparation of ischemia-reperfusion injury. PMNs were isolated from the gracilis muscle venous effluent and central venous blood after 6 hr of bilateral gracilis ischemia and 1 hr of reperfusion in five dogs. Baseline samples were obtained prior to ischemia from the central venous circulation. Liberation of O 2 − from PMNs and from PMNs stimulated by opsonized zymosan was determined by measuring ferricytochrome reduction. Results are expressed as nanomoles of O 2 produced/2 × 10 6 PMN ± SEM. O 2 − production by unstimulated cells was increased from 0.33 ± 0.15 nmole in the baseline samples to 0.96 ± 0.08 nmole in the central venous sample ( P < 0.01). With stimulation by zymosan, production increased from 10.3 ± 1.4 nmole in the baseline samples to 15.2 ± 1.1 nmole in the central venous sample ( P < 0.05) and to 15.5 ± 0.9 nmole in the gracilis venous sample ( P < 0.01). These increases in superoxide production were not seen in the three sham-operated animals. Mean infarct size determined by planimetry was 55%. O 2 − produced by PMNs from central venous blood correlated with infarct size ( r = 0.934, P = 0.02). These data imply that PMNs are activated by muscular ischemia, and the degree of activation is directly related to the extent of the muscle infarction.

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.

The rate and distribution of muscle blood flow after prolonged ischemia

The magnitude and distribution of muscle blood flow in the lower extremity after relief of an acute arterial occlusion may influence the extent of the resulting necrosis. The object of this study was to document the distribution of blood flow in the resting state and after prolonged periods of complete ischemia, and to assess the relationship between the degree of reactive hyperemia and subsequent necrosis. The isolated bilateral canine gracilis muscle preparation that we have previously characterized was used for microsphere studies. Total blood flow was measured by means of timed venous collections, and the distribution of flow was determined by means of a multiple microsphere injection technique. Measurements of microsphere distribution and blood flow were made before ischemia and during the initial 48 minutes of reperfusion after both 4 and 5 hours of normothermic ischemia, which resulted in 46.7% ± 6% and 71.2% ± 7% necrosis, respectively. The muscle was harvested and sectioned transversely into six slices approximately 1.5 cm thick, and the extent of necrosis was quantified by means of nitroblue tetrazolium staining 48 hours after reperfusion. Blood flow distribution during the early reperfusion phase was determined in each muscle slice and in both the alive and dead portions of each slice by use of the microsphere injection technique. Preischemic blood flow was distributed homogeneously throughout the muscle and was 4.5 ± 0.8 ml/100 gm/min (mean ± SEM, n = 8). On reperfusion total flow was 6 to 10 times higher than it was before ischemia and was distributed predominantly to the middle slices (p < 0.05, n = 12). In both the 4- and 5-hour ischemia groups there was no correlation between blood flow in each slice and the extent of necrosis (p > 0.1). The degree of reactive hyperemia was greater after 4 hours of ischemia than after 5 hours (20.3 ± 2.5 ml/100 gm/min vs 12.5 ± 1.6 ml/100 gm/min, p < 0.05). On reperfusion, blood flow is not distributed homogeneously throughout the muscle and the extent of reactive hyperemia is inversely related to the length of ischemia. Hyperemic blood flow reaches the maximum rate in the center portions of the muscle belly closest to the inflow and does not seem to be related to the distribution of necrosis.

Inhibitors of Prostaglandin Synthesis Reverse the Effects of Chronic β-Receptor Blockade to Attenuate Adrenergic Neurovascular Transmission in Dogs

The effects of acute and chronic treatment with beta-adrenergic receptor blocking drugs on peripheral adrenergic neurovascular transmission were investigated. Experiments were performed using the blood perfused gracilis muscle preparation in control dogs and in animals treated with single or repeated doses of beta-receptor antagonists. After 7 days of d,l-propranolol, l-propranolol, or atenolol administration, the arterial pressor response to sympathetic nerve stimulation was significantly reduced in treated dogs (p less than 0.05) when compared with control- or d-propranolol-treated animals. In comparison, acute beta blockade produced by a single intravenous dose of propranolol had no effect on the pressor response to nerve stimulation. Inhibition of fatty acid-cyclooxygenase activity by indomethacin or piroxicam enhanced the vasoconstrictor response to sympathetic nerve stimulation in chronic d,l-propranolol-, l-propranolol-, and atenolol-treated dogs, but had no effect on vascular neurotransmission in control-, chronic d-propranolol-, or acutely d,l-propranolol-treated animals. The vasoconstrictor response to intraarterial phenylephrine was not significantly altered by chronic propranolol treatment, and measurement of norepinephrine overflow during sympathetic nerve stimulation failed to reveal any difference in neurotransmitter release between control- and propranolol-treated dogs. The results indicate that chronic but not acute beta-adrenergic receptor blockade alters signaling at the neurovascular synapse to diminish adrenergic transmission. This effect does not appear to result from a change in postsynaptic alpha 1 receptors or from a decrease in norepinephrine release.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurogenic histaminergic vasodilation in canine skeletal muscle: mediation by alpha 2-adrenoceptor stimulation.

This study examines the neurogenic effect of alpha 2-adrenoceptor stimulation on skeletal muscle vascular resistance and its relation to the level of background sympathetic activity. The isolated, separately perfused, neurally intact canine gracilis muscle preparation was used since it permits deliberate and quantifiable alterations in background sympathetic activity, as measured by skeletal muscle vascular resistance. Systemic intravenous UK-14304, a highly selective alpha 2-adrenoceptor agonist, produced a precipitous, neurogenic vasodilation that lowered vascular resistance below the subsequently denervated resistance, thus indicating that an active vasodilation was involved. The overall magnitude of the vasodilation was much greater in animals that had been hemorrhaged to elevate background sympathetic activity than in animals that had been transfused to lower background activity. The neurogenic vasodilation was unaffected by baroreceptor and cardiopulmonary receptor denervation and by prior cholinergic-receptor blockade of the gracilis muscle. Prior H1- and H2-histaminergic-receptor blockade, on the other hand, eliminated the active vasodilation but not a vasodilation down to the subsequently denervated resistance. Prior alpha 1-adrenoceptor blockade lowered resistance down to the subsequently denervated resistance and greatly attenuated the active vasodilation. The present study demonstrates that withdrawal of sympathetic activity by alpha 2-adrenoceptor stimulation produces an active vasodilation resulting from histamine release in skeletal muscle as well as a passive vasodilation resulting from lysis of peripheral vasoconstrictor tone.

Quantitation of postischemic skeletal muscle necrosis: Histochemical and radioisotope techniques

Skeletal muscle necrosis will result from prolonged periods of ischemia. The purpose of this study was to develop a method to estimate the extent of necrosis using nitroblue tetrazolium staining and technetium scanning. The bilateral canine gracilis muscle preparation with total vascular isolation was exposed to 4 hr of complete normothermic ischemia followed by reperfusion. After 45 hr of reperfusion 99mTc pyrophosphate (PYP) was injected and 3 hr later the muscles were harvested, cut into six slices, and stained with nitroblue tetrazolium. Biopsies were taken from tetrazolium-positive and -negative areas for electron microscopy to confirm the ability of the stain to distinguish viable from necrotic muscle. Computerized planimetry of the staining pattern was used to estimate the extent of necrosis as a percentage of the total muscle. Electron microscopy confirmed the validity of nitroblue tetrazolium to discriminate between viable and necrotic skeletal muscle in this experimental model. After 4 hr of ischemia the percentage necrosis was 30.2 ± 6.1% (mean ± SEM, n = 12), there was no difference in the extent of necrosis in left vs right paired muscles, using tetrazolium staining or technetium PYP uptake. There was a statistically significant correlation between the percentage necrosis and the density of 99mTc PYP uptake per muscle ( r = 0.83, P < 0.001) and per slice ( r = 0.94, P < 0.001). This study demonstrates the ability of tetrazolium staining to accurately differentiate between viable and necrotic skeletal muscle and provides a reproducible method for estimating the extent of necrosis in the gracilis muscle model.

Skeletal muscle insulin unresponsiveness during chronic hyperdynamic sepsis in the dog.

Recent reports from our laboratory and others have documented changes in insulin unresponsiveness and electrolyte and hormonal changes characteristic of hypodynamic shock states in anesthetized animals. Since most acute shock protocols do not adequately mimic the clinical profile of sepsis, the present study was undertaken to document the hemodynamic and metabolic changes associated with chronic hyperdynamic peritonitis in dogs. Mongrel dogs of either sex weighing 20 +/- 2 kg were surgically instrumented with an electromagnetic aortic flow probe for monitoring cardiac output determinations, and aortic and central venous catheters for withdrawing blood for blood pressure and chemical analyses. Following a recovery period (7-10 days) control hemodynamic and metabolic measurements were made. Sepsis was induced (peritoneal abscess) by implanting a 4" X 4" gauze sponge, previously inoculated with human fecal bacteria, amid the intestines. Experimental (N = 18) and pair-fed control (N = 6) animals were monitored daily for 21 days or until death. During the septic protocol, cardiac index increased from a control value of 3.4 L/min/m2 to 5.5 L/min/m2 by the end of the experimental period. Mean arterial blood pressure, total peripheral resistance index, body weight, and plasma Ca++ fell below control values during the experimental period. Body temperature, plasma glucose, insulin, glucagon, and Mg++ were all elevated with sepsis. At the end of the chronic experimental period, skeletal muscle insulin responsiveness was assessed in the isolated, innervated, constantly perfused gracilis muscle preparation. Pair-fed control animals responded to various concentrations of local insulin infusion by increasing glucose uptake by the gracilis muscle. However, septic animals had a blunted response to local insulin infusion resulting in a decrease in the maximum dose response effect. These data demonstrate that: chronic, hyperdynamic peritonitis in the dog more closely mimics the human clinical profile of sepsis; and hyperdynamic sepsis is associated with a state of skeletal muscle insulin unresponsiveness which results from a post-receptor defect.

Skeletal muscle metabolism and insulin resistance during endotoxin shock in the dog

The effect of locally infused endotoxin on gracilis muscle glucose uptake was determined in anesthetized mongrel dogs. The effects of infusion of small amounts of Escherichia coli endotoxin into the arteries of isolated, innervated, constant flow perfused gracilis muscles on glucose uptake and other metabolic variables were determined. Locally infused endotoxin consistently caused a significant and substantial increase in skeletal muscle glucose uptake with no alterations in muscle arteriovenous difference of insulin, oxygen, carbon dioxode, or pH, or in venous blood hematocrit or temperature. These data demonstrate that endotoxin can act locally to increase glucose uptake by skeletal muscle, independent of the action of insulin or other metabolic factors. During natural (free flow) conditions, glucose uptake by the muscle increased markedly during six hours of shock. Increased glucose uptake occurred concomitantly with muscle ischemia and hypoxia. However, when muscle blood flow was held constant, thereby preventing local muscle ischemia and hypoxia, glucose uptake by the gracilis muscle did not change during shock. These results implicate local muscle ischemia and/or hypoxia as the mediator(s) of the increased muscle glucose uptake during shock. Further studies demonstrated that local muscle hypoxia was the stimulus for increased glucose uptake by skeletal muscle during endotoxin shock, and muscle ischemia per se did not after muscle glucose uptake. Since approximately 50% of body mass is composed of skeletal muscle, the contribution of this organ system to the hypoglycemia of endotoxin shock in the dog may be substantial. The ability of insulin to promote glucose diffusion into skeletal muscle before and during gram-negative endotoxin shock was studied in mongrel dogs anesthetized with sodium pentobarbital. The in vivo, isolated, innervated, constant flow perfused gracilis muscle preparation was used. Prior to shock induction, close intra-arterial insulin infusion resulted in a 320% increase in muscle glucose uptake. However, at one, two, and three hours of endotoxin shock, gracilis muscle glucose uptake was unaltered by insulin infusion. This loss of responsiveness to insulin occurred with no alteration in gracilis muscle oxygen uptake, muscle venous P O 2 , or muscle blood flow. During control experiments, however, the muscle response to intra-arterial infusion of insulin (increased glucose uptake) was unaltered during the three-hour control period. These data demonstrate that skeletal muscle insulin resistance develops early and is maintained during three hours of endotoxin shock in the dog.

Rat gracilis muscle preparation for combined macro- and microvascular research.

A skeletal muscle preparation for the study of single vessel and whole organ vascular responses is presented. After surgical isolation from anesthetized rats, the gracilis muscle preparation is autoperfused via cannulation circuits. This allows the measurement of, and experimental control over, such macrovascular parameters as arterial and venous pressures and total muscle blood flow. In addition, the preparation is thin enough to allow the simultaneous study of microvessels by transilluminated microscopy. Such dual capabilities allow the preparation to be used in a wide variety of investigations and, in particular, as a primary tool in correlating microcirculatory responses with those of the whole organ in experiments designed to elucidate local control mechanisms of skeletal muscle vasculature. An example of such is demonstrated for reactive hyperemia responses simultaneously obtained at arteriolar and muscle venous effluent locations.

Prostaglandin modulation of adrenergic vascular control during hemorrhagic shock

This study was designed to evaluate 1) whether the initial compensatory skeletal muscle vascular constriction induced by hemorrhagic hypotension is primarily the result of increased adrenergic neural tone rather than circulating vasoconstrictor agents, and 2) whether the secondary skeletal muscle decompensatory vasodilation is caused by inhibitory action of prostaglandins on peripheral adrenergic nervous system. A constant-flow vascularly isolated double canine gracilis muscle preparation in which one muscle served as innervated control for the contralateral muscle was used. Dogs were subjected to standard stepwise hemorrhagic shock protocol. In series 1, perfusion pressures of control muscles were compared to denervated muscles with the result that innervated muscle perfusion pressures increased initially from 105 to 175 mmHg but subsequently fell significantly (P less than 0.05) to 147 mmHg. Only modest increases in perfusion pressures with no significant secondary fall were noted in denervated muscles. Series 2 compared innervated control perfusion pressures to pressures perfusing muscles pretreated with prostaglandin-synthesis inhibitor sodium meclofenamate (MCF). The MCF-treated muscle perfusion pressures rose to 260 mmHg where they remained without the secondary fall noted in control muscles. These data support the two hypotheses tested.

Normal resting and exercising muscle blood flow during acute ethanol infusion.

We have examined the effect of ethanol on muscle blood flow at rest and during electrically stimulated exercise in five normal dogs using the isolated gracilis muscle preparation. Under pentobarbital anesthesia and mechanical ventilation, ethanol (15% in 0.9% NaCl) infused at 4 mL/min for 2 hr produced an arterial blood concentration of 268 +/- 10 mg/dL (X +/- SEM). Resting muscle blood flow was 6.3 +/- 0.9 mL/min/100 gm in 8 dogs infused with saline alone. During stimulated contraction using supramaximal voltage at a rate of 5 stimuli/sec, respective mean flows for ETOH and saline dogs at 1, 5, 10, and 30 min were 35.5, 19.9, 27.7, 22.2, and 22.0, 20.0, 23.6, and 19.1 mL/min/100 gm. The 1-min flow rate for ethanol infused animals was significantly greater (P < 0.05) than that observed in saline infused animals. The remainder of the values were not significantly different. Potassium release from contracting muscle was normal. These observations do not support the theory that ethanol induces vasoconstriction in the vascular bed of skeletal muscle nor do they support the postulation that muscle cell ischemia plays a role in alcoholic myopathy.

Central and peripheral receptor areas in the reflex response to acute experimental hyperosmolality

Acute experimental hyperosmolality has been shown to initiate reflex vascular adjustments in peripheral tissue. This study attempted to further investigate the general receptor areas which might initiate these reflexes in anesthetized dogs.Injections of 50 ml. of 50% glucose were made over 30 seconds in random sequence into the right atrium, pulmonary artery, left ventricle, and distal thoracic aorta. Systemic arterial pressure fell in all animals with no significant differences observed between injection sites. The vascular resistance of the isolated, innervated, constant flow perfused gracilis muscle preparation exhibited a biphasic reflex response characterized by initial reflex vasodilatation and followed by reflex vasoconstriction. The reflex responses to right atrial, pulmonary artery, and left ventricular injections were quantitatively and qualitatively similar. The reflex muscle vasodilation response was, however, significantly diminished following the injection in the descending aorta.Local α-adrenergic blockade of the muscle abolished the reflex response in the muscle. Systemic β-adrenergic blockage resulted in accentuated hypotension and enhanced reflex muscle vasoconstricition. Isotonic Ringer's solution produced no response from any injection site.The reflex response to acute experimental hyperosmolality appears to be complex with multiple receptor areas, central and peripheral, participating. These experimental observations are clinically relevant during central administration of plasma expanders as well as various angiographic procedures.

A gracilis muscle preparation for quantitative microcirculatory studies in the rat

A description of the preparatory procedure of the rat gracilis muscle is given. The muscle consists anatomically of two parts, an anterior and a posterior venter, and is supplied by branches of the femoral vessels. As a hemodynamic isolated preparation the gracilis muscle is shown to be applicable to qualitative microcirculatory studies as well as to quantitative measurements on the arteriolar and capillary level. The data presented have been obtained by videomicroscopy, by indirect videophotometric methods, and also by direct techniques like servo-null micropuncture. The use of the preparation described and the results presented are discussed and compared with different methods applied in microcirculation research.

Components of the hemodynamic response to elevated cerebrospinal fluid pressure

The cardiovascular effects of elevated cerebrospinal fluid (CSF) pressure were studied in adrenalectomized dogs with and without ganglionic blockade. A significant increase in vascular pressures and cardiac output occurred in those without ganglionic blockade but was absent or markedly blunted in those with ganglionic blockade. This indicated that an intact sympathetic nervous system was required for the pressor response to elevated cerebrospinal fluid pressure but the adrenals were not required. A betaadrenergic effect was noted in dogs with ganglionic blockade and adrenalectomy. The effects of elevated CSF pressure were also studied using a perfused gracilis muscle preparation in 12 animals with beta-adrenergic blockade, with and without adrenalectomy. Animals with intact adrenals and denervated gracilis muscle showed an increase in aortic pressure and gracilis muscle resistance. Adrenalectomized animals with innervated gracilis muscle demonstrated an increase in aortic pressure and gracilis muscle resistance. Elevated CSF pressure with intact adrenals and alpha-adrenergic blockade demonstrated a decrease in aortic pressure but no change in gracilis muscle resistance. The presence of a nonadrenal circulating beta agonist that increases cardiac output and decrease peripheral vascular resistance is indicated by these studies. Also, the Cushing reflex involves an adrenal alpha-adrenergic component affecting skeletal muscle resistance and another alpha-adrenergic component dependent on an intact sympathetic nervous system.

Effects of altering O2 delivery on VO2 of isolated, working muscle.

Maximal oxygen consumption (VO2 max) and muscle blood flow (Q max) were measured in an isolated gracilis muscle preparation before and after alteration in perfusion pressure (BP), arterial oxygen saturation (SaO2), and hemoglobin concentration (Hb). Q varied directly with BP and inversely with Hb (viscosity) but was unaffected by changes in arterial SaO2. VO2 max varied directly with oxygen delivery under all conditions. These results indicate that VO2 max is normally limited by oxygen delivery rather than any intrinsic limiting of oxygen consumption of the muscle.

A Computer for the Calculation of Skeletal Muscle Vascular Resistance

On-line dynamic experiments on skeletal muscle vascular resistance require an extensive number of simple calculations in order to derive the data. A low-cost analog computer is described which utilizes perfusion pressure, venous flow, and muscle weight to calculate vascular resistance per 100 gm of muscle weight. This computer has been in use with the isolated perfused canine gracilis muscle preparation and has been shown to have considerable merit in reducing calculations and permitting immediate evaluation of the experimental data. A drop rate meter is incorporated in the device for the calculation of venous flow; perfusion pressure is obtained from a standard commercial transducer. A block diagram is included, and typical data are shown which illustrate the usefulness of continuous on-line calculation of resistance.

On the mechanism of rhabdomyolysis in potassium depletion.

Rhabdomyolysis and myoglobinuria occur commonly in men who sustain environmental heat injury during intensive physical training in hot climates. These also occur in patients with potassium depletion. Since physical training in hot climates may be accompanied by serious losses of body potassium, the possibility was considered that performance of strenuous exercise when potassium deficient might enhance susceptibility to rhabdomyolysis. Potassium is released from contracting skeletal muscle fibers and its rising concentration in interstitial fluid is thought to dilate arterioles thereby mediating the normal rise of muscle blood flow during exercise. If potassium release from deficient muscle were subnormal, exercise would not be accompanied by sufficient muscle blood flow and rhabdomyolysis could occur by ischemia. This hypothesis was examined by comparing the effect of electrically stimulated exercise on muscle blood flow, potassium release, and histology of the intact gracilis muscle preparation in normal and potassium-depleted dogs. In normal dogs, muscle blood flow and potassium release rose sharply during exercise. In contrast, muscle blood flow and potassium release were markedly subnormal in depleted dogs despite brisk muscle contractions. Although minor histologic changes were sometimes observed in nonexercised potassium-depleted muscle, frank rhabdomyolysis occurred in each potassium-depleted animal after exercise. These findings support the hypothesis that ischemia may be the mechanism of rhabdomyolysis with exercise in potassium depletion.

Glucose uptake at rest and during contraction in isolated dog skeletal muscle.

AbstractCostin, J. C., B. Saltin, N. S. Skinner, Jr. and G. Vastagh, Glucose uptake at rest and during contraction in isolated dog skeletal muscle. Acta physiol. scand. 1971. 81. 124–137.In an in situ isolated canine gracilis muscle preparation the a‐v difference for glucose, oxygen, and lactate has been studied at rest and during stimulation. Both free flow (systemic blood flow) and constant flow perfusion (blood from reservoir) have been applied and in both sets of experiments the blood flow has been determined. In the constant flow experiments no significant difference in the glucose uptake could be detected between stimulation as compared with rest. In the free flow experiments the same stimulation frequency produced a significant uptake of glucose (≅ 20 μg glucose per g muscle and minute), which could account for approximately 10 per cent of the total energy output and 20–30 per cent of the total carbohydrate utilization. It is suggested that the difference between the free flow and the constant flow experiments was due to the fact that, in the free flow experiments where the muscle is perfused with the systemic blood flow, some humoral factor is present which is necessary for the glucose to penetrate the cell barrier. It was also possible to demonstrate a transient glucose uptake in the muscle in the constant flow experiments, i.e. when the muscle was suddenly perfused with blood containing a high glucose concentration (≅ 150 mg/100 ml blood).