Gastrocnemius Preparation Research Articles

Insulin exits skeletal muscle capillaries by fluid-phase transport.

Before insulin can stimulate myocytes to take up glucose, it must first move from the circulation to the interstitial space. The continuous endothelium of skeletal muscle (SkM) capillaries restricts insulin's access to myocytes. The mechanism by which insulin crosses this continuous endothelium is critical to understand insulin action and insulin resistance; however, methodological obstacles have limited understanding of endothelial insulin transport in vivo. Here, we present an intravital microscopy technique to measure the rate of insulin efflux across the endothelium of SkM capillaries. This method involves development of a fully bioactive, fluorescent insulin probe, a gastrocnemius preparation for intravital microscopy, an automated vascular segmentation algorithm, and the use of mathematical models to estimate endothelial transport parameters. We combined direct visualization of insulin efflux from SkM capillaries with modeling of insulin efflux kinetics to identify fluid-phase transport as the major mode of transendothelial insulin efflux in mice. Model-independent experiments demonstrating that insulin movement is neither saturable nor affected by insulin receptor antagonism supported this result. Our finding that insulin enters the SkM interstitium by fluid-phase transport may have implications in the pathophysiology of SkM insulin resistance as well as in the treatment of diabetes with various insulin analogs.

Rebuttal from Carsten Lundby and David Montero.

Our opponent warrants the regulation of by diffusion of O2 from microvessels into active muscle fibres on the authority of two selected observations based on experiments conducted in isolated canine gastrocnemius preparations (Wagner, 2015). Of these observations only one is de facto, however (see below), and has already been rebutted by the bulk of canine evidence on this topic, which in line with unequivocal data in healthy humans does not indicate any regulatory role for O2 diffusion from microvessels into muscle at (Lundby & Montero, 2015). In fact Wagner (2015) omits that his own cited work demonstrates identical muscle O2 uptake despite having elevated P50 experimentally (Richardson et al. 1998).

Purification and Characterization of a Nonenzymatic Neurotoxin fromHippasa partita(Lycosidae) Spider Venom Gland Extract

India is a habitat for nearly one thousand four hundred forty-seven species of spiders under three hundred and sixty-five genera and sixty families. Our initial survey on toxic bite by spider revealed severe edema, itching, acute pain, and hemorrhage following tissue necrosis, which are the general symptoms of envenomation, but there are no reports of mortality. Significantly,Hippasa partitaspider, commonly called “funnel web spider,” which is endemic in hilly regions of the Western Ghats is responsible for envenomation. In this study, a nonenzymatic neurotoxin has been purified fromH. partitavenom gland extract. Gel filtration and ion exchange chromatography were used to purify the toxin into homogeneity as shown by SDS-PAGE, RP-HPLC, and MALDI-TOF. Neurotoxin is devoid of enzymatic activities but causes intense neurotoxic symptoms. Neurotoxin is found to inhibit the twitch response of sciatic nerve gastrocnemius muscle preparation and is found to be postsynaptic in action. Neurotoxin is devoid of coagulant activity, edema, and hemorrhage and is nonlethal to mice (up to 5 mg/kg body weight). In conclusion, a neurotoxin, which is a principle agent in whole venom responsible for induced neurotoxic symptoms, has been purified and characterized.

Distinguishing the effects of convective and diffusive O2 delivery on V̇o2 on-kinetics in skeletal muscle contracting at moderate intensity

With current techniques, experimental measurements alone cannot characterize the effects of oxygen blood-tissue diffusion on muscle oxygen uptake (Vo₂) kinetics in contracting skeletal muscle. To complement experimental studies, a computational model is used to quantitatively distinguish the contributions of convective oxygen delivery, diffusion into cells, and oxygen utilization to Vo₂ kinetics. The model is validated using previously published experimental Vo₂ kinetics in response to slowed blood flow (Q) on-kinetics in canine muscle (τQ = 20 s, 46 s, and 64 s) [Goodwin ML, Hernández A, Lai N, Cabrera ME, Gladden LB. J Appl Physiol. 112:9-19, 2012]. Distinctive effects of permeability-surface area or diffusive conductance (PS) and Q on Vo₂ kinetics are investigated. Model simulations quantify the relationship between PS and Q, as well as the effects of diffusion associated with PS and Q dynamics on the mean response time of Vo₂. The model indicates that PS and Q are linearly related and that PS increases more with Q when convective delivery is limited by slower Q dynamics. Simulations predict that neither oxygen convective nor diffusive delivery are limiting Vo₂ kinetics in the isolated canine gastrocnemius preparation under normal spontaneous conditions during transitions from rest to moderate (submaximal) energy demand, although both operate close to the tipping point.

Variation: use it or misuse it – replication and its variants

In a previous article (Drummond & Vowler, 2012a), we discussed variation between individuals, termed variance. When measurements are taken of subjects in separate groups, the variance of the measurements within the group can be compared with the variance that exists between groups. This allows us to estimate the likelihood that groups were drawn from the same population. Commonly, a treatment has been applied to these groups, and the aim would be to find if this had increased the variance between the groups, which might then indicate that the treatment had caused an effect (Fig. 1A).

Pharmacodynamic interaction between pantoprazole and vecuronium at neuromuscular junction

1 The effect of pantoprazole on vecuronium-induced neuromuscular blockade in in vivo has not been clearly defined. In this study, we demonstrate that chronic administration, but not acute administration, of pantoprazole alters the pattern of vecuronium-induced neuromuscular blockade. 2 This study was designed to evaluate the effect of acute and chronic administration of pantoprazole on vecuronium-induced neuromuscular blockade using the rat in vivo sciatic nerve-stimulated gastrocnemius preparation. 3 Vecuronium was administered as a slow intravenous infusion (29.41 μg kg(-1) min(-1)) until the gastrocnemius twitch response to sciatic nerve stimulation was completely abolished. The effect of acute (single dose, i.v.) and chronic administration (per oral for 21 days) of pantoprazole (3.64 mg kg(-1)) on vecuronium-induced blockade was assessed by comparing ED50 values, time required for 50% block, ED95 values, block duration and percentage of recovery with respect to control. 4 Acute administration of pantoprazole had no significant effect on any parameter of vecuronium-induced neuromuscular blockade. Chronic administration of pantoprazole significantly reduced vecuronium ED50 value, time for 50% block, ED95 value and percentage recovery from blockade compared with the control group (P<0.05). Reduction in the duration of vecuronium-induced blockade was not significantly affected by chronic treatment with pantoprazole compared with control. 5 On chronic administration, pantoprazole may produce earlier block, quick relaxation and reduces the recovery of vecuronium without affecting its duration of action.

In vivo pharmacodynamic interaction between pipecuronium and certain H2 blockers

Objective: To investigate the pharmacodynamic interaction of H2-receptor antagonists (i.e., famotidine and roxatidine acetate) with a neuromuscular blocker, pipecuronium using sciatic nerve stimulated gastrocnemius preparation of rats in vivo . Materials and Methods: The dose-response curve of pipecuronium (10-50 mg/kg i.v.) was elicited and the dose (ID50; 26.89 mg/kg i.v.) required to cause 50% of blockade of basal contractile twitch response was calculated. Benzyl alcohol (0.9 % v/v), famotidine (0.5, 2.0, 5.0 mg/kg i.v.) or roxatidine acetate (0.05, 0.2, 0.5 mg/kg i.v.) were administered 30 min prior to pipecuronium administration and their effects were studied on the dose-response curve of pipecuronium. Results: Famotidine did not alter the basal contractile twitch responses but with a dose of 2 mg/kg it significantly decreased, while with 5 mg/kg, it significantly increased the ID50 of pipecuronium. At higher dose (5.0 mg/kg) it significantly increased the time required for the onset of blockade without affecting the other parameters. Roxatidine acetate (0.2, 0.5 mg/kg) by itself produced significant neuromuscular blockade but did not alter the ID50 of pipecuronium, while with higher dose (0.5 mg/kg) it significantly decreased the same. Roxatidine (0.05 and 0.2 mg/kg) significantly increased the time required for onset of pipecuronium-induced neuromuscular blockade. At varying doses roxatidine also significantly increased the time required for peak effect and the recovery from the paralysis. Conclusion: Compared to roxatidine, famotidine produced less pharmacodynamic drug interaction with pipecuronium in rats. Such an interaction should be explored in clinical practice.

THE EFFECT OF RHYTHMIC SKELETAL MUSCLE CONTRACTIONS ON PEAK MUSCLE BLOOD FLOW

The purpose of this investigation was to examine the effect of rhythmic skeletal muscle contractions on peak muscle perfusion using surgically isolated spontaneously perfused in situ canine gastrocnemius preparations. Simultaneous pulsatile blood pressures were measured by means of transducers placed on the popliteal artery and vein, and pulsatile flow was measured with a flow-through type electromagnetic probe placed in the venous return line. Maximal vasodilation of the muscles' vascular beds was elicited by infusing a normal saline solution containing adenosine (29.3 mg/min) and sodium nitroprusside (180 μg/min) for fifteen sec, and then simultaneously occluding the popliteal artery and vein for five min. This technique trapped the vasodilators in the muscle vasculature during a period of hypoxia. The release of occlusion initiated a maximal hyperemic response, during which time four consecutive tetanic contractions were evoked with supramaximal voltage (6–8V, 0.2 msec stimuli for 200 msec duration at 50 Hz, 1/sec). In the face of constant arterial blood pressure (122 ± 8 mmHg), the contractions reduced venous pressure while muscle blood flow was reduced from 2056 ± 246 to 1738 ± 225 ml/kg/min when contractions were initiated. Blood flow increased to 1925 ± 225 ml/kg/min during the first five seconds after contractions were stopped. Virtual conductance decreased by 18% during the period of four tetanic contractions. We conclude that the mechanical action of rhythmic, synchronous, and maximal skeletal muscle contractions inhibits peak muscle perfusion during maximal vasodilation of the muscle's vascular bed. Supported by NIH Grant # 1R01AR-40342

O(2) delivery and the venous P(O(2))-O(2) uptake relationship in pump-perfused canine muscle.

Under the conditions of both an increased red cell affinity for O(2) at a constant rate of O(2) delivery (arterial O(2) content x flow) and a decrease in the rate of O(2) delivery induced by hypoxic hypoxia at constant blood flow, we have obtained a linear relationship between the partial pressure of O(2) in the muscle venous effluent (P(v,)(O(2))) and O(2) uptake (.V(O(2))). The relationship is described by the equation .V(O(2)) = D(a) x P(v,)(O(2)) + .V(O(2)conv)) where D(a) is the apparent O(2) diffusion capacity and .V(O(2)conv)) is O(2) delivery-limited .V(O(2)), and D(a) x P(v,)(O(2)) represents the O(2) diffusion-limited .V(O(2)) .V(O(2)diff)). From these observations, we propose the hypothesis that .V(O(2)) consists of two additive values, .V(O(2)conv)) and .V(O(2)diff)). The mechanism underlying the reduction in .V(O(2)) that is induced by reducing O(2) delivery to markedly below the .V(O(2)conv)) value has only been investigated using a model based on the single compartment of diffusion-limited .V(O(2)), and has not been investigated in terms of this additive .V(O(2)) model. The single compartment analysis appears to overestimate the role of O(2) diffusion in limiting the reduction of .V(O(2)) that occurs in response to a decrease in O(2) diffusion capacity, as reflected by the .V(O(2))/P(v,)(O(2)) ratio. To gain better insight into the mechanism involved, we altered the rate of O(2) delivery by changing arterial P(O(2)) from normoxia (with inhalation of air) to hypoxia (by inhalation of 10-11 % O(2)) and blood flow (with high and low flow rates (n = 7 for both groups), and very low and ischaemic flow rates (n = 4 for both groups)) in pump-perfused dog gastrocnemius preparations during tetanic isometric contractions at 1 Hz. As rates of O(2) delivery were reduced from 23.2 to 10.9 ml min(-1) (100 g)(-1), significant decreases in P(v,)(O(2)) and .V(O(2)) were observed (P < 0.05). From the data of P(v,)(O(2)) and .V(O(2)) values within this range of O(2) delivery rates, we obtained the regression equation .V(O(2)) = 0.22 x P(v,)(O(2)) + 8.14 (r = 0.58). From the equation, the intercept of the .V(O(2))-axis was significantly different from zero (P < 0.05), in accordance with the observation that the .V(O(2)) /P(v,)(O(2)) ratio (ml min(-1) (100 g)(-1) Torr(-1)) increased from 0.54 to 1.35 (P < 0.05). However, at extremely low rates of O(2) delivery (5.6 and 7.3 ml min(-1) (100 g)(-1) the .V(O(2))/P(v,)(O(2)) ratio was 1.51 and 2.80 (P < 0.05), respectively. This indicates a break in the linear .V(O(2))-P(v,)(O(2)) relationship as the rate of O(2) delivery was reduced to below the .V(O(2)conv)) value of the .V(O(2))-axis intercept. These results suggest that the reduction in .V(O(2)) caused by extreme reductions in the rate of O(2) delivery is not attributable to a reduction in O(2) diffusion capacity, as expected from the .V(O(2))/P(v,)(O(2)) ratio, but to a reduction in the O(2) delivery-limited .V(O(2)) component, as evaluated by the .V(O(2))-axis intercept of the linear .V(O(2))-P(v,)(O(2)) relationship.

Skeletal muscle VO2 on-kinetics: set by O2 delivery or by O2 utilization? New insights into an old issue.

Recent work conducted by our group has expanded knowledge on some basic issues related to pulmonary and skeletal muscle O2 uptake (VO2) on-kinetics. We demonstrated that, in exercising humans during transitions from unloaded pedaling to loaded pedaling below the ventilatory threshold, alveolar VO2 on-kinetics can be taken as a rather close approximation of skeletal muscle VO2 on-kinetics. Experiments conducted on the isolated in situ dog gastrocnemius preparation have shown that, during transitions from rest to contractions corresponding to approximately 70% of the muscle peak VO2, convective O2 delivery to muscle, intramuscular blood flow (Q) versus VO2 maldistribution, and peripheral O2 diffusion are not limiting factors for skeletal muscle VO2 on-kinetics. The latter, therefore, appears to be mainly determined by an intrinsic inertia of skeletal muscle oxidative metabolism, possibly related to acetyl group availability within mitochondria, to regulatory effects on intracellular respiration related to phosphocreatine splitting, and/or to other still not precisely identified control mechanism(s). Evidence from the literature suggests that the limiting factors for skeletal muscle VO2 on-kinetics may vary according to the intensity of muscular contractions or of exercise.

In vivo receptor characterization of neuropeptide Y-induced effects in consecutive vascular sections of cat skeletal muscle.

1. It has been suggested that the vasoconstrictor response to neuropeptide Y (NPY) is located in the microvessels and that it increases with reduced vessel diameter. The aim of the present study was to analyse quantitatively, on the cat gastrocnemius muscle preparation in vivo, the effects of NPY on total regional vascular resistance (RT) and its distribution to large-bore arterial resistance vessels (> 25 microns; Ra,prox), small arterioles (< 25 microns; Ra,micro) and the veins (Rv). Associated effects on capillary pressure (Pc,v) and fluid exchange were also studied. 2. Close-arterially infused NPY (1-32 micrograms kg-1 min-1) caused a dose-dependent, slowly developing vasoconstriction in all three vascular sections, yet with a preferential action in the small arterioles. At 32 micrograms kg-1 min-1, NPY raised RT by 133 +/- 22%, Ra,prox by 94 +/- 15%, Ra,micro by 277 +/- 104% and Rv by 81 +/- 11%. However, the veins (ED50 = 3.9 +/- 1.2 micrograms kg-1 min-1) were more sensitive to NPY than both large-bore arterial vessels (ED50 = 7.7 +/- 1.6) and small arterioles (ED50 = 7.0 +/- 1.4). NPY decreased Pc,v due to an increase in the pre-to post-capillary resistance ratio. 3. Close-arterial infusions of Pro34NPY and peptide YY evoked vasoconstrictor responses which did not differ from the response to NPY. In contrast, the Y2-preferring C-terminal fragments: Ac-[Leu28, Leu31]-NPY (24-36) and NPY(13-36) were without effect in the muscle vascular bed. The selective NPY Y1 receptor antagonist BIBP3226 (100 micrograms kg-1 min-1, i.a.) abolished the vascular response to NPY. 4. The present findings indicate that the vasoconstrictor response to NPY in skeletal muscle is preferentially located in the small arterioles and mediated via the Y1 receptor and, further, that Y2 and Y3 receptors do not play a significant role in the vasoconstrictor response to NPY in cat skeletal muscle. BIBP3226 was found to be an effective NPY antagonist in vivo and to lack agonist activity.

Effects of angiotensin-converting enzyme inhibition on arterial, venous and capillary functions in cat skeletal muscle in vivo.

The aim of the present study was to analyse quantitatively, on a cat gastrocnemius muscle preparation in vivo, the effects of local angiotensin-converting enzyme (ACE) inhibition by enalaprilat on total regional vascular resistance (tone) and its distribution to the large-bore arterial resistance vessels (> 25 microns), the small arterioles (< 25 microns) and the veins. Associated effects on capillary pressure and fluid exchange were also studied. Close-arterial infusion of enalaprilat (0.05-0.20 mg kg muscle tissue min-1) elicited a moderate dilator response in all three consecutive sections of the muscle vascular bed, an increase in capillary pressure and transcapillary fluid filtration. This dilation could be abolished by the selective bradykinin B2-receptor antagonist Hoe 140 (2 mg kg-1 min-1, i.a.), indicating that the dilator mechanism of ACE inhibition was an increased local concentration of bradykinin, and hardly at all a decreased concentration of angiotensin (AT) II. The generalized dilator response to ACE inhibition along the vascular bed suggested a relatively uniform distribution of ACE from artery to vein and this was further supported by the finding that a close-arterial infusion of AT I (0.04-0.32 microgram kg-1 min-1), which was vasoactive only after conversion to AT II by local ACE, elicited a generalized constrictor response in all three vascular sections. In contrast, infused AT II (0.01-0.16 microgram kg-1 min-1) constricted almost selectively the large-bore arterial vessels. The specific angiotensin AT1-receptor antagonist losartan (2 mg kg-1 min-1, i.a.) abolished the constrictor response to AT II but did not affect vascular tone under control conditions, indicating that AT II is not involved in the initiation of basal vascular tone in muscle. These results, taken together, indicate that under basal conditions vascular ACE contributes to the local control of vascular tone in skeletal muscle by degrading the endogenous dilator bradykinin, and not by converting AT I into vasoconstrictor AT II.

CANINE GASTROCNEMIUS MUSCLE IN SITU: ??VO2max367

The canine gastrocnemius (GP) muscle preparation is widely used as a model for the study of maximal metabolic rate in skeletal muscle. However, there is disagreement with regard to ˙VO2max in this model (Cain.Med. Sci. Sports Exerc. 27:60-64, 1995) with values ranging from 141 ± 3 (Hogan et al. J. Appl. Physiol. 73:728-736, 1992) to 201.8 ± 9.4 ml·kg-1·min-1 (Brechue et al.J. Appl. Physiol. 71:131-135, 1991). Therefore, it was the purpose of this study to reinvestigate ˙VO2max and maximal blood flow(˙Qmax) in the canine GP. The GP was surgically isolated in anesthetized dogs and the tendon was attached to a force transducer. Isometric muscle contractions were stimulated via the sciatic nerve; muscle blood flow was spontaneous. As previously reported, ˙VO2max during tetanic trains(0.2 msec stimuli for 200 msec duration at 50 Hz, once per sec) was significantly greater than ˙VO2peak in twitch contractions at 4 Hz(244 ± 23 vs. 134 ± 15 ml·kg-1·min-1; n = 4). Passive force required to stretch the GP to its optimal length (Lo) was 0.38 ± 0.04 N·g-1; n = 9. Hogan et al. stretched the GP with a passive force of 0.1 N·g-1 for contractions whereas Brechue et al. used a force of 0.4 N·g-1. When muscles in the present study were stimulated with tetanic trains (1 Hz) at Lo and care was taken to maintain the muscles at this length during the contractions, ˙VO2max averaged 266 ± 20 ml·kg-1·min-1 and ˙Qmax averaged 2.1 ± 0.2 L·min-1. These are the highest values of ˙VO2max and ˙Qmax ever reported for the canine GP during spontaneous, free flow conditions. We conclude that maintenance of the muscle at Lo during contractions may be a critical determinant of ˙VO2max and ˙Qmax.

Investigative approach to frog gastrocnemius laboratory: potential impact on animal use in teaching laboratories.

With growing concern over the use of animal experimentation in the teaching of physiology, many biology departments are reassessing the use of animal experiments in the teaching lab. However, it may be just as important to assess how animal experimentation is used in the undergraduate teaching laboratory rather than simply assessing if animal experimentation should be used at all. In our study, sophomore-level life science students enrolled in a core organismal biology course undertook a laboratory exercise designed to elucidate properties of muscles and neuromuscular communication following two protocols: 1) a standard demonstrational model wherein students were told to undertake the exercise as a means to understand physiological processes that they had been exposed to previously in lecture or 2) an investigative model wherein the use of the gastrocnemius preparation was a logical next step in an ongoing investigation, the content of which was driven by student-generated hypotheses. We have observed a significant decrease in a number of the negative comments concerning the use of animals in experimentation (25.6 vs. 3.6%) since the implementation of the investigative approach to the laboratory, suggesting that curricular approaches to the use of animals in the teaching laboratory may have an impact on student attitudes concerning animal experimentation.

Electrical stimulation and amino acid and ammonia metabolism in the canine gastrocnemius muscle.

This study examined the effects of electrical stimulation on amino acid and ammonia (NH3) metabolism in the isolated in situ canine gastrocnemius muscle preparation. Cut sciatic nerves of 10 mongrel dogs were stimulated at either 3 or 5 twitches/s (10 V, 0.2-ms duration) for 60 min. Muscle NH3 release dramatically increased on stimulation, and over 60 min the 3- and 5-Hz groups released 86.7 +/- 24.2 vs. 160.8 +/- 17.4 mumol.min-1.100 g-1 (P < 0.05) of NH3, respectively. Similarly, the intramuscular NH3 concentration was elevated (P < 0.05) above rest for both groups throughout stimulation, and it was higher (P < 0.05) at 5 min for the 5-Hz (82.7 +/- 2.4 mumol/100 g wet wt) than for the 3-Hz (67.4 +/- 7.4 mumol/100 g wet wt) group. Stimulation was also characterized by a large release of amino acids by both groups. The total amino acid release for 60 min was 415.4 +/- 64.9 vs. 193.3 +/- 56.2 (P < 0.05) mumol/100 g for the 3- and 5-Hz groups, respectively. However, there were no shifts or differences between groups in the intramuscular total amino acid pools. Glutamine (Gln) and alanine (Ala) dominated the amino acids released by muscle and together represented 35 and 46% of the total amino acids released over 60 min for the 3- and 5-Hz groups, respectively. The total release of Gln was higher (P < 0.05) for the 3-Hz (81.1 +/- 5.6 mumol/100 g) than for the 5-Hz (49.4 +/- 10.7 mumol/100 g) group, but there were no differences between groups in total Ala release. In contrast, both groups demonstrated an uptake of branched-chain amino acids (valine, isoleucine, and leucine) after 45 min of stimulation. These data show a stimulation-dependent production of NH3 and release of amino acid by the canine gastrocnemius muscle. These data further show that the degree of net muscle NH3 production is proportional to the frequency, whereas the degree of amino acid release is an inverse function of frequency.

Effects of glyceryl trinitrate, nitroprusside and nitric oxide on arterial, venous and capillary functions in cat skeletal muscle in vivo.

The aim of the present study was to analyse quantitatively, on a cat gastrocnemius preparation in vivo, the effects of i.a. or i.v. administered glyceryl trinitrate (GTN), sodium nitroprusside (SNP) or nitric oxide (NO dissolved in saline) on vascular resistance (tone) in the following consecutive vascular sections: Large-bore arterial resistance vessels (> 25 microns), small arterioles (< 25 microns), and the veins. Effects on hydrostatic capillary pressure (Pc,v) and transcapillary fluid exchange were simultaneously recorded. Close-arterially infused GTN (1-4096 micrograms kg tissue-1 min-1), SNP (0.5-32 micrograms kg tissue-1 min-1) and NO (0.14-0.82 mg kg tissue-1 min-1) elicited a generalized dose-dependent dilator response in all three sections, though with a preferential action on the arterial side. Further, these agents caused an increase in Pc,v and transcapillary fluid filtration. The sites of action along the vascular bed of these exogenous vasodilators differed from that previously established for endogenous EDNO. Infusion of GTN, SNP and NO during EDNO blockade (L-NAME) could, therefore, not restore the vascular resistance distribution to that prevailing in the initial control state. Myogenic vascular reactivity to standardized transmural pressure stimuli was clearly depressed by GTN and SNP. Intravenously infused GTN (4-512 micrograms kg body wt-1 min-1) and SNP (4-64 micrograms kg body wt-1 min-1) decreased arterial pressure and elicited, via reflex sympathetic activation, a dose-dependent vasoconstriction in skeletal muscle, a decrease in Pc,v, and net transcapillary fluid absorption. The constrictor response thus overruled the direct dilator effect of the drugs. The plasma volume expansion known to result from long-term systematic administration of nitrovasodilators seems in part to be caused by transcapillary fluid absorption in skeletal muscle.

Lactate and acid-base exchange during brief intense contractions of skeletal muscle in situ.

Our goal was to design a stimulation-contraction paradigm using an isolated in situ dog gastrocnemius muscle preparation that would provide an experimental model for brief intense intermittent (IC) exercise in humans. Second, acid-base and ion exchanges across the muscle were investigated using four 30-s bouts of isotonic tetanic contractions (2 s-1, 100-ms train, 50 impulses/s) with 4 min of rest between bouts. During the bouts, peak power output (W) was 18.2 mW/g in the first bout; it declined by 4.4% by the fourth bout and by 12-16% in each bout. Compared with repetitive continuous contractions (CC) at maximal O2 uptake (VO2), W was greater and VO2 (approximately 3.5 mumol.g-1.min-1) and CO2 production (approximately 4.5 mumol.g-1.min-1) were less with IC. Venous-arterial (v-a) differences and lactate output peaked immediately after each bout and were not different from the values reported for CC at maximal VO2. Thus, with IC, VO2/W was lower and the CO2 production/VO2 and lactate output/VO2 ratios were greater than those seen with CC. These differences suggest that this stimulation-contraction paradigm may be an appropriate model for brief intense exercise. The v-a [H+] difference was a direct result of the v-a PCO2 difference. The venous strong ion difference was always greater than or equal to the arterial strong ion difference because the v-a [Cl-] difference was opposite and greater than the v-a lactate concentration difference, whereas the v-a [Na+] and [K+] differences were small.(ABSTRACT TRUNCATED AT 250 WORDS)

L-(+)-lactate infusion into working dog gastrocnemius: no evidence lactate per se mediates VO2 slow component.

Constant-load exercise that engenders a sustained lactic acidosis (i.e., above the lactate threshold) is accompanied by a slow component of O2 uptake (VO2) kinetics that increases VO2 above rather than toward the predicted value. This response arises predominantly from within the exercising limbs and is temporally correlated with that of blood lactate. Lactate exerts a disproportionate metabolic stimulatory effect on gluconeogenic tissues, and there is a strong indication that lactate infusions may increase VO2 of resting tissues. To investigate the potential role of lactate in the VO2 slow component, we infused lactate in 20-min square-wave pulses (change of 10 mM) into the arterial blood supply of an electrically stimulated and surgically isolated dog gastrocnemius preparation (2 x 60-min bouts, approximately 30-40% peak VO2; n = 5) under iso-pH conditions at constant muscle temperature. With lactate infusions, intramuscular lactate concentration ([La]) rose proportionally with inflowing [La] (muscle [La] = 6.34 + 0.38 blood [La]; r = 0.642, P < 0.05) to approximately 80% of arterial blood [La], and neither blood (control, 7.39 +/- 0.01; high lactate, 7.40 +/- 0.01; P > 0.05) nor muscle (control, 7.02 +/- 0.03; high lactate, 7.00 +/- 0.04; P > 0.05) pH was changed. Compared with control values, lactate infusion decreased muscle VO2 from 5.1 +/- 0.3 to 4.1 +/- 0.2 ml.min-1.100 g-1 (P < 0.05). However, VO2 relative to tension remained constant. Notwithstanding the obvious differences between this preparation and the exercising human, this finding does not support a role for lactate per se in driving the VO2 slow component during intense exercise.

Postfreeze survival and muscle function in the leopard frog ( Rana pipiens) and the wood frog ( Rana sylvatica)

1. 1. Organismal survival and function of isolated muscle were assessed in the frogs Rana pipiens and R. sylvatica after freezing for various durations at 2°C. Maximal twitch and tetanic contractions were measured on gastrocnemius preparations made 3 h (both species) and 24 h ( R. sylvatica) after the onset of thawing at 3°C. 2. 2. Rana pipiens died from freezes lasting longer than 8 h. Muscle contaction strength was unimpaired when frogs were frozen for 6 h, whereas muscles generated only 10% of normal contraction strength when frogs were frozen for 24 h. 3. 3. Rana sylvatica survived freezing episodes lasting up to 48 h; howver, most frogs died when they were frozen for 96 h. Muscle contraction strength was an inverse function of freeze duration when frogs were thawed for 3 h but contractility was normal by 24 h. 4. 4. Despite surviving the initiation of internal ice formation, most vertebrates, including R. pipiens, seem best described as freeze intolerant since this ability is not a reliable means of protection from injury during natural freezes. In contrast, a few vertebrates, such as R. sylvatica, survive extensive internal freezing in a fashion that is consistent with their ecological needs.

The effect of carbon monoxide on respiration

In this review the effects of carbon monoxide on tissular oxygenation, at doses which are compatible with life, are considered. In a first section the relative CO-O2 affinity (M*) of various O2 carrying proteins is compared; M* is about 220 for hemoglobin, 20-25 for myoglobin and close to unity for cytochrome oxidases. Thus most of the acute CO toxicity should not be considered as due to malfunction of the intracellular respiratory chain. In addition the differences in M* are caused more by the changes in O2 affinity than by those in CO affinity. The second section deals with the changes in the O2 equilibrium curve (OEC) induced by the presence of HbCO in blood, i.e. the hyperbolization of this curve due to the progressive loss of allostery due to the preferential binding of CO to Hb. The functional importance of this phenomenon lies in the fact that the lower part of the OEC is shifted to the left, whereas the upper part is shifted to the right to an extent which depends upon the amount of HbCO. Thus the effects of the so-called CO anemia are considered to be due both to the reduction of functional Hb and to the reduced partial pressure in the hypoxic range of the OEC. The third section presents recent data concerning the effect of HbCO on the VO2max of the isolated gastrocnemius preparation. The results were obtained in hypoxia under conditions where perfusion and arterial O2 content, i.e. O2 delivery, were the same with and without 30% HbCO. The salient finding is a 26% reduction of VO2max under conditions of CO anemia as compared to hypoxia alone. Interestingly, the PO2 of the venous effluent of the muscle is found to be the same in both cases which leads to the interpretation that it is not the reduction of the mean capillary PO2 but rather a decrease of the blood-to-mitochondria O2 conductance which causes the fall in VO2max.