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L C Maxwell

Publications and source records attributed to L C Maxwell.

At least 37 records · Page 2Linked to original sources
PubMed1987

Evaluation of an infrared laser-Doppler blood flowmeter.

Several laser-Doppler blood flowmeters are now commercially available; however, only one utilizes an infrared laser diode (Laserflo, TSI, St. Paul, MN). Because of this and other unique features such as its microprocessor-based signal analyzer, we evaluated this device's ability to measure tissue perfusion. Initially, we determined whether laser illumination directly affected the microvasculature. Intravital microscopic observations in the hamster cremaster muscle indicated that neither He-Ne nor infrared laser light affected the diameters of arterioles that were responsive to vasoactive agents. To test the flowmeter for linearity and repeatability, we used a rotating disk to simulate a light-scattering, flowing medium. The "flow" signal was highly correlated (r = 0.99) with the rotational velocity of the disk, was consistent among flow probes, and showed a high degree of reproducibility. The second model consisted of microsphere suspensions pumped through cuvettes. The laser-Doppler velocimeter (LDV) flow signal was linear with respect to pump output. With red blood cells in the perfusate, we examined the effects of blood oxygenation on the flowmeter's performance. The LDV flow signal was unaffected by changes in blood oxygenation. We evaluated linearity in vivo in isolated, perfused rat livers and in isolated canine gastric flaps. We observed linear relationships between total flow and laser-Doppler flow measured on the surface of the liver (r = 0.98) and in the gastric mucosa (r = 0.98), but the slopes of the relationships between total and local LDV flow showed considerable variability not noted in the in vitro studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1986

Estimation of the distribution of blood vessel diameters from the arteriovenous passage of microspheres.

We present an estimator of the probability density of capillary diameters based on a simple probabilistic model of microsphere passage through a microvascular bed. With this model, an estimate of the density is obtained as the solution of an integral equation. The density is approximated by a linear combination of B-splines with the coefficients restricted to be nonnegative. Equations for maximum likelihood estimates of the coefficients are obtained. The proposed method is applied to data from the dog intestinal circulation.

Animals↗
PubMed1985

Microsphere passage through intestinal circulation: via shunts or capillaries?

Significant quantities of 9-micron microspheres (20-30%) are not trapped in the intestine following intracardiac or intra-arterial injection, but reach venous blood. Some investigators propose that the passage of 9-micron spheres measures blood flow through noncapillary connections. Because frequency distributions of intestinal capillary diameters and 9-micron spheres overlap, microspheres could simply pass through capillaries. Therefore, we developed simple probabilistic models to predict both the size distribution and the percentage of injected spheres [9 +/- 1 (SD) micron] that should appear in venous blood. Chief assumptions in models are that microsphere delivery and sphere diameter are independent and that microspheres pass through capillaries of equal or larger size. The passage predicted by the models was consistent with values in canine intestinal circulation, demonstrating that passage through capillaries [7.38 +/- 1.4 (SD) micron] adequately accounts for spheres in venous blood. Because the diameters of nominal 9-micron spheres were distributed too narrowly to show a marked sieving effect on passage through the intestinal circulation, we also injected microspheres varying from 5 to 20 micron in diameter. This mixture demonstrated a marked sieving effect. The predicted frequency distribution for microsphere diameters in venous blood agreed with the observed distribution. Our models demonstrate that the passage of 9-micron spheres through capillaries, rather than through "shunts," adequately accounts for the appearance of spheres in venous blood and suggests that the frequency distribution of venous microspheres can provide an in vivo method for estimating the frequency distribution of intestinal capillary diameters.

Animals↗
PubMed1984

Growth of regenerating skeletal muscle fibers in cats.

Extensor digitorum longus (EDL) muscles of cats were grafted heterotopically or orthotopically, and either with or without prior denervation. The autografted muscles were studied at times from 4 to 518 days after grafting. Muscle weight, fiber cross-sectional area, and ultrastructural, histochemical, and biochemical characteristics of regenerating muscle fibers were determined. Prior denervation reduced the mass of muscle at the time of grafting, but had no significant effect on the characteristics of the regenerating muscles. Orthotopic and heterotopic autografts achieved similar recovery of structure. Mean fiber area reached control values. Differentiation into fiber types occurred, but compared to control muscles, autografts had fewer Type I and Type IIA fibers. Electron microscopic analysis of regenerating muscle fibers revealed centrally located nuclei, but otherwise normal ultrastructure. The bimodal distribution of Z-band width was consistent with differentiation into fiber types. Mean data of some morphological variables did not stabilize.

Animals↗
PubMed1984

Muscle fiber regeneration in nerve-intact and free skeletal muscle autografts in cats.

Extensor digitorum longus (EDL) muscles of adult cats were transplanted as free autografts (FRA) with the nerve severed or as nerve-intact autografts (NIA) with the nerve retained. Histochemical and contractile properties of NIA and FRA were analyzed at selected times from 1 to 14 wk after surgery. Regeneration was qualitatively similar in NIA and FRA. Regenerating fibers were observed in both NIA and FRA within 2 wk. After 14 wk there were fewer type I fibers in both NIA and FRA than in control EDL muscles. Capillarity was greater in NIA than FRA, but both types of autografts had significantly reduced capillarity relative to control muscles. Mean fiber area, muscle mass, and absolute tension development were greater in NIA than FRA but did not reach control muscle values. Muscle mass, mean fiber area, and contractile properties, but not the proportion of type I fibers, develop toward control values more quickly in autografts with the nerve left intact.

Animals↗
PubMed1984

Selective vasodilators redistribute intestinal blood flow and depress oxygen uptake.

In this paper we report the effects of adenosine and isoproterenol on intestinal oxygen consumption. Using the recently developed technique of laser-Doppler velocimetry (LDV), we also studied the effects of these two agents on regional blood flow in the canine intestinal mucosa and muscularis. In isolated gut loops perfused at constant blood flow, both vasodilators depressed O2 uptake, but isoproterenol increased rubidium extraction while adenosine depressed rubidium extraction. Under constant pressure perfusion both dilators depressed O2 uptake but increased total flow. Isoproterenol increased the capillary filtration coefficient (Kf), but adenosine depressed Kf. LDV flowmetry showed adenosine increased muscularis flow and depressed mucosal flow. Both LDV flowmetry and radioactive microspheres showed isoproterenol increased mucosal but depressed muscularis flow. Our data indicate that the deleterious effects of isoproterenol and adenosine in depressing intestinal oxygen uptake can be explained by the ability of these agents to selectively vasodilate one of the regional circulations within the gut wall and by a "vascular steal" mechanism to reduce blood flow in the other vascular bed not as sensitive to the agent in question. The reduction in the regional blood flow thus depresses the O2 consumption of the gut as a whole. By comparing our regional laser-Doppler blood flow data with a simple analog model, we show that the seemingly paradoxical effect of vasodilators in depressing a regional blood flow while increasing total perfusion is explicable by a "vascular steal" mechanism.

Adenosine↗
PubMed1984

Temporal changes after death in primate diaphragm muscle oxidative enzyme activity.

We have previously reported that muscle fibers in fresh samples of the diaphragm muscle of prenatal baboons have moderate to high oxidative capacity and are resistant to fatigue in vitro. These conclusions are in conflict with those of others on diaphragmatic muscle fibers studied in autopsy specimens from human infants. Reasons for these divergent interpretations could include species difference and the use of autopsy specimens rather than fresh tissue samples. We have, therefore, tested whether characteristics of human infant diaphragm muscle fibers differ from those of premature baboons, and whether the use of autopsy specimens alters interpretations of histochemical results. Samples obtained from premature, newborn, or adult baboons were quick-frozen immediately after death or after storage for as long as 24 h. Samples were obtained at autopsy from human infants at 4 to 24 h after death. Histochemical assay for NADH-TR activity was performed on cross sections. Samples from baboons at any age showed deterioration with storage, but the muscles from premature and newborn animals were considerably more susceptible to damage than those of adults. Fibers in human infant diaphragm obtained within 10 h of death looked remarkably similar to those of the infant baboons. However, samples obtained at later times after death showed deterioration and loss of oxidative enzyme activity. We conclude that diaphragmatic muscle fibers of humans and nonhuman primates are similar in enzymatic profile, but that elapsed time after death can reduce the intensity of mitochondrial enzyme assays. The decrement in tissue preservation with elapsed time after death is less pronounced in more mature muscles.

Animals↗
PubMed1983

Effects of ascorbic acid and EDTA on vascular concentration-response to catecholamines.

Concentration-response curves are often used as indicators of vascular reactivity and receptor sensitivity. Some investigators use ethylenediaminetetraacetate (EDTA) or ascorbic acid (ASC) to prevent catecholamine oxidation but many others do not. This difference may affect estimates of reactivity. We tested the effect of EDTA or ASC on contractile responses of hamster cremaster arterioles and rat aortic strips to epinephrine (EPI) or norepinephrine (NOR). Comparable contractile responses were elicited by lower EPI or NOR concentrations in the presence than in the absence of EDTA or ASC. Individual responses to EPI or NOR were maintained in the presence of EDTA or ASC, but rapidly declined if neither were present. Rapidly prepared and administered solutions of EPI or NOR elicited similar initial responses in the presence or absence of EDTA or ASC. However, catecholamine solutions prepared without these compounds lose effectiveness within minutes. We conclude that oxidation of EPI and NOR reduces apparent vascular reactivity and that EDTA or ASC prevents or delays the reduction.

Animals↗
PubMed1983

Capillarity and fiber types in the cremaster muscle of rat and hamster.

We determined muscle fiber type and capillarity in cremaster muscle samples from rats and hamsters of different ages. Histochemical estimation of oxidative capacity was made from the activity of either nicotinamide dinucleotide tetrazolium reductase (NADH-TR) or succinic dehydrogenase (SDH), and fibers were termed fast or slow from myofibrillar ATPase activity. Fibers were classified as type I (low ATPase, high NADH-TR/SDH), type IIa (high ATPase, high SDH/NADH-TR), type IIb (high ATPase, low SDH/NADH-TR), or type IIc (no acid reversal of ATPase, high NADH-TR). Type IIb fibers accounted for 60-80% of the muscle area in both species at all ages. The principal change with maturation was muscle fiber hypertrophy. Mean cross-sectional fiber area increased from 488 +/- 70 (SE) and 453 +/- 19 micron2 in young hamsters and rats, respectively, to 1,255 +/- 99 and 1,540 +/- 101 micron2 in adults. Capillary density (no. of capillaries/mm2 tissue) paralleled fiber hypertrophy; it decreased significantly with maturation from 684 +/- 60 (SE) to 228 +/- 26/mm2 in hamsters and from 341 +/- 15 to 213 +/- 15/mm2 in rats. In vitro estimates of capillary density are compared with previously obtained in vivo data (31), and sources of error are identified. We conclude that reported differences in microvascular function in the cremaster muscle in vivo during maturation or between species cannot be ascribed to changes in muscle composition.

Abdominal Muscles↗
PubMed1983

Development of histochemical and functional properties of baboon respiratory muscles.

We assessed morphological, histochemical, and physiological characteristics of respiratory muscles of a non-human primate, Papio cynocephalus, from midgestation through adult life. Samples were taken of diaphragm muscles for histochemical analysis, electron microscopy, and assessment of contractile properties and fatigability. Histochemical analyses were also performed on samples of intercostal muscles. Initially, developing fibers are type IIc but differentiate into types I and IIa fibers by term. We observed no IIb fibers in respiratory muscles of premature baboons. Beginning late in gestation, muscle fibers grew rapidly. After term, IIb fibers were found, and fiber size ranked by increasing mean fiber area became types I, IIa, and IIb. After term, we rarely observed type IIc fibers. In electron micrographs we observed large numbers of interfibrillar mitochondria in all muscle fibers of premature baboons but not in all IIb fibers of adults. Histochemical observations were supported by contractile properties. Muscles of premature baboons had significantly longer contraction and relaxation times than adult muscles. Muscles from premature baboons were more resistant to fatigue than those of adult baboons. We conclude that the fibers of respiratory muscles are high in oxidative capacity and are resistant to fatigue during gestation. Fatigue of the respiratory muscle fibers secondary to low oxidative capacity is not a likely cause of respiratory distress in premature baboons.

Animals↗
PubMed1982

Translocation of the temporalis muscle for treatment of facial paralysis.

Our purpose was to characterize in the rhesus monkey the structure and function of vascularized temporalis muscle flaps innervated by the facial nerve after translocation into the site of the denervated zygomaticus muscle. Animals were killed at 28 to 120 days following translocation. Control data were obtained from the contralateral side. Twenty-eight days after translocation, the time to reach peak twitch tension and one-half relaxation time were 170% of control zygomaticus muscle. Contraction times decreased with time and reached control values by 100 days. Absolute isometric tetanic tension was not different between the flap (4.29 +/- 1.28 newtons; X +/- SEM) and control zygomaticus (3.95 +/- 0.80 newtons). Succinate oxidase activity of the flap decreased from 279 +/- 18 nl O2/mg protein/min to control zygomaticus values (98 +/- 18) by 110 days. The type 1 fiber cross-sectional area of the flap was 52% of control temporalis muscle and 150% of control zygomaticus muscle (P less than 0.05). The temporalis flap demonstrated viable structure and function and appeared useful in facial movements.

Animals↗
PubMed1982

Relationship among fibre type, myosin ATPase activity and contractile properties.

At least two types of skeletal muscle myosin have been described which differ in ATPase activity and stability in alkaline or acidic media. Differences in ATPase characteristics distinguish Type I and Type II fibres histochemically. In this study, ATPase activity of myosin from muscles of several species with known histochemical and contractile properties has been determined to test the hypothesis that (1) myosin ATPase activity, (2) histochemical determination of fibre types and (3) maximum shortening velocity, all provide equivalent estimates of contractile properties in muscles of mixed fibre types. Maximum shortening velocity appears to be proportional to ATPase activity as expected from previous reports by Barany. However, both myosin ATPase and the maximum shortening velocity exhibit curvilinear relationships to the fraction of cross-sectional area occupied by Type II fibres. Therefore, we reject the hypothesis and conclude that histochemically determined myofibrillar ATPase does not accurately reflect the intrinsic ATPase activity or shortening velocity in muscles of mixed fibre types. Our data are consistent with the presence of more than two myosin isozymes or with a mixture of isozymes within single muscle fibres.

Adenosine Triphosphatases↗
PubMed1982

Vasodilation or altered perfusion pressure moves 15-micrometers spheres trapped in the gut wall.

To determine whether the microsphere method for measuring the intramural distribution of intestinal blood flow is affected by perfusion pressure or vasodilation, we infused radioactive 9- and 15-micrometers spheres into adjacent segments of isolated canine small bowel. After sphere infusion the blood supply of the control loop was occluded, and the vasculature of the experimental loop was either dilated by infusing isoproterenol or was subjected to increased perfusion pressure. Intestinal segments were dissected into mucosal, submucosal, and muscularis samples. Venous blood was collected during sphere infusions and experimental perturbations. Accumulations of spheres in tissue samples and venous blood were assessed in a gamma radioactivity counter. Isoproterenol caused previously infused spheres to leave submucosa and redistributed them primarily to mucosa with few additional spheres reaching venous blood. An increase in perfusion pressure also dislodged spheres from submucosa, but these did reach venous blood. The combined estimate of mucosal plus submucosal blood flow was relatively unaffected by isoproterenol infusion but was significantly altered by increased perfusion pressure. These results have the following implications for microsphere studies of the intramural distribution of intestinal blood flow: 1) tissue must be sampled after each sphere infusion unless the possibility of sphere migration has been experimentally eliminated and 2) even a two-compartment fractionation of blood flow into muscularis and mucosal-plus-submucosal compartments is not valid under some experimental conditions.

Animals↗
PubMed1981

Adaptation of the masseter and temporalis muscles following alteration in length, with or without surgical detachment.

Histochemical properties, muscle fiber cross-sectional area, muscle fiber length, and the oxidative capacity of masticatory muscles of female rhesus monkeys were assessed following alteration in functional length by an intraoral appliance or by detachment of the muscle. Experimental groups received the appliance only (A); the appliance and subsequent detachment of the masseter (AD); the appliance and detached masseter, but with surgical reattachment of the masseter to the pterygomasseteric sling (ADR); no appliance, but detachment and reattachment of masseter (DR); or an appliance which was removed after 24 weeks to study posttreatment responses (PT). Animals were sacrificed and the muscles were studied at intervals from 4 to 48 weeks after initiation of the experimental period. The results of these studies led to the following conclusions: (1) Stretching the masseter and temporalis muscles within physiological limits did not significantly alter the proportion of fiber types, although oxidative capacity of the fibers was reduced. (2) Fibers with "intermediate" myofibrillar ATPase activity were no more prevalent in experimental than control muscles. (3) The cross-sectional area of Type I fibers of masseter muscles decreased following some experimental procedures, indicating that recruitment of these fibers is the most sensitive to altered jaw function. (4) Minimal alteration of muscle capillarity was induced by any of the experimental procedures. (5) The lengths of masseter muscle fibers in Group PT and of temporalis muscle fibers in groups AD and ADR were greater than in control animals.

Adaptation, Physiological↗
PubMed1981

Effect of microsphere size on apparent intramural distribution of intestinal blood flow.

Microspheres were used in earlier studies to determine the distribution of blood flow within the intestinal wall, but the selection of microsphere size was not systematic. In one study, a 5-micrometers difference in mean diameter markedly altered the submucosal vs. mucosal distribution of microspheres. Therefore, we examined the effect of size on microsphere distribution within the gut wall. Loops of canine small bowel were perfused at constant pressure, and microspheres were injected in the following order: 9 +/- 1, 15 +/- 1, and 26 +/- 2 micrometers. Gut segments were separated into mucosal, submucosal, and muscularis tissues. The microspheres were recovered from digested tissues and venous blood. The number and diameter of spheres in an aliquot of resuspended spheres were determined by digital image analysis. The ratio of mucosal to submucosal accumulation varied 60-fold for sphere diameters from 8 to 20 micrometers. The muscularis' share did not vary systematically with sphere size. Approximately 20% of spheres less than 11 micrometers reached venous blood, whereas few (less than 1%) of larger sizes did. We conclude: 1) sphere distribution within the gut wall is size dependent, 2) correction for microsphere shunting will be difficult, because the route taken by untrapped spheres is unknown, and 3) fractionating intramural flow among the three layers is not practical with commercially available spheres. However, under control conditions 15-micrometers spheres seem to distribute independently of size between muscularis and a lumped mucosal-submucosal compartment.

Animals↗
PubMed1981

Capillarity and blood flow of transplanted skeletal muscles of cats.

After transplantation, skeletal muscle fibers and the microvasculature degenerate and then regenerate. The structural and functional characteristics of autografted muscle change with time and ultimately stabilize. Capillarity, blood flow, or fatigability was measured in control (total n = 22) and in autografted (total n = 42) extensor digitorum longus muscles of cats. From 10 to 180 days after transplantation, the capillaries per square millimeter in peripheral areas of autografts averaged 65% of control, and from 180 to 518 days, 45% of control. Resting blood flow in autografts increased eventually from total ischemia at the time of transplantation to values 400% of control. After 400 days, resting blood flow returned to control values. The blood flow of autografts increased in response to twitch stimulation, reaching maximal value (24.4 +/- 1.4 ml . 100 g-1 . min-1) at 1 twitch . s-1 . The maximal blood flow of autografts was comparable to the submaximal blood flow at 1 twitch . s-2 of controls and was 30% of the maximal flow of controls obtained at 4 twitches . s-1. The maximal blood flow in autografts did not increase from 80 to 460 days. When stimulated with repetitive twitches, autografts fatigued more rapidly than controls.

Action Potentials↗
PubMed1980

Lack of 'acid reversal' of myofibrillar adenosine triphosphatase in masticatory muscle fibres of rhesus monkeys.

Myofibrillar adenosine triphosphatase (ATPase) activity was demonstrated in sections of masseter and temporalis muscles and of selected limb muscles of adult rhesus monkeys. Incubations were performed either with no pre-treatment or after prior incubation in alkaline media (pH 10.2-10.4) or acidic media (pH 3.8-4.6). Without pre-treatment, fibres having high or low ATPase activity were observed in limb and masticatory muscles. Following alkaline pre-incubation the difference between high and low ATPase of limb muscle fibres is accentuated, whereas pre-incubation in acidic media (pH 4.3) results in inhibition of high and potentiation of low ATPase activities ('acid reversal'). While pre-incubation of masticatory muscle sections at pH 10.2 accentuates differences in ATPase activity, pre-incubation at pH 10.4 abolishes ATPase activity. In contrast, masticatory muscle fibres showed no reversal of ATPase activity following acidic pre-incubation (pH 4.3). Pre-incubation at pH 3.8 abolished the ATPase activity of both limb and masticatory muscle fibres. The biochemical basis for the differences in ATPase histochemistry between masticatory and limb muscles is not known.

Adenosine Triphosphatases↗