Biomedical subjects

Colleen A McMullen

Publications and source records attributed to Colleen A McMullen.

11 recordsLinked to original sources

PubMed2005

Lactate is a metabolic substrate that sustains extraocular muscle function.

Lactic acid is considered the end product of glycolysis and is a major cause of muscle fatigue. However, the lactate dehydrogenase (LDH) reaction is bidirectional: Lactate can be oxidized to pyruvate and used as a substrate for the Krebs cycle. Therefore, our hypothesis was that lactate sustains the contractile function of rat extraocular muscles during periods of increased activity. The study used extraocular and extensor digitorum longus (EDL) muscles from adult Sprague-Dawley rats to determine LDH isoform expression, total LDH activity, and contractile function in vitro. To evaluate the role of lactate on fatigue, we tested the effect of cinnamate, a blocker of lactate transport, and exogenous lactate on fatigue resistance. Cinnamate accelerated fatigue in the extraocular muscles: Endurance and residual force decreased significantly. Conversely, cinnamate did not affect the endurance or residual force of EDL muscles. Replacing glucose with exogenous lactate increased EDL fatigability but had no effect on the extraocular muscles. However, the extraocular muscles fatigued faster when exposed to exogenous lactate combined with cinnamate. The LDH-A and LDH-C isoforms were expressed at lower levels in extraocular muscle; LDH-B was equally abundant in the EDL and extraocular muscles. Total LDH activity in the extraocular muscles was only approximately 32% of the level in EDL. These results support the hypothesis that lactate sustains the contractile performance of the extraocular muscles.

Animals↗

PubMed2005

Contractile dysfunction and altered metabolic profile of the aging rat thyroarytenoid muscle.

The larynx and its muscles are important for ventilation, coughing, sneezing, swallowing, Valsalva's maneuver, and phonation. Because of their functional demands, the intrinsic laryngeal muscles have a unique phenotype: very small and fast fibers with high mitochondrial content. How aging affects their function is largely unknown. In this study, we tested the hypothesis that an intrinsic laryngeal muscle (thyroarytenoid muscle, a vocal fold adductor) would become weaker, slower, and fatigable with age. Muscles from Fischer 344 x Brown Norway F1 hybrid rats (6, 18, and 30 mo of age) were used for in vitro contractile function and histology. Thyroarytenoid muscles generated significantly lower twitch and tetanic forces at 30 mo vs. 6 and 18 mo. Maximal shortening velocity decreased by 20% at 30 mo (vs. 6 mo), and velocity of unloaded shortening was slower at 18 and 30 mo by 19 and 27% vs. 6 mo. There was no histochemical evidence of altered myosin ATPase activity at 18 or 30 mo of age. Fatigue resistance was significantly decreased at 18 and 30 mo. We also found abundant mitochondrial clusters and ragged red fibers in the muscles of 30-mo-old rats, and there was an age-related increase in glycogen-positive fibers. We conclude that rat thyroarytenoid muscles become weaker, slower, and more fatigable with age. These functional changes are not due to alterations in myosin ATPase activity, but a switch in the expression of myosin isoforms remains a possibility. Finally, the alterations in mitochondrial and glycogen content indicate a shift in the metabolic characteristics of these muscles with age.

Aging↗

PubMed2005

Fatigue resistance of rat extraocular muscles does not depend on creatine kinase activity.

BACKGROUND: Creatine kinase (CK) links phosphocreatine, an energy storage system, to cellular ATPases. CK activity serves as a temporal and spatial buffer for ATP content, particularly in fast-twitch skeletal muscles. The extraocular muscles are notoriously fast and active, suggesting the need for efficient ATP buffering. This study tested the hypotheses that (1) CK isoform expression and activity in rat extraocular muscles would be higher, and (2) the resistance of these muscles to fatigue would depend on CK activity. RESULTS: We found that mRNA and protein levels for cytosolic and mitochondrial CK isoforms were lower in the extraocular muscles than in extensor digitorum longus (EDL). Total CK activity was correspondingly decreased in the extraocular muscles. Moreover, cytoskeletal components of the sarcomeric M line, where a fraction of CK activity is found, were downregulated in the extraocular muscles as was shown by immunocytochemistry and western blotting. CK inhibition significantly accelerated the development of fatigue in EDL muscle bundles, but had no major effect on the extraocular muscles. Searching for alternative ATP buffers that could compensate for the relative lack of CK in extraocular muscles, we determined that mRNAs for two adenylate kinase (AK) isoforms were expressed at higher levels in these muscles. Total AK activity was similar in EDL and extraocular muscles. CONCLUSION: These data indicate that the characteristic fatigue resistance of the extraocular muscles does not depend on CK activity.

Adenylate Kinase↗

PubMed2005

Eliminating the Ant1 isoform produces a mouse with CPEO pathology but normal ocular motility.

PURPOSE: The adenine nucleotide transporter 1 gene (ANT1) encodes an inner mitochondrial membrane protein that transports ATP into the cell. Mutations within ANT1 produce a syndrome of chronic progressive external ophthalmoplegia (CPEO) in humans. Ant1 knockout (Ant1-/-) mice develop cardiomyopathy and mitochondrial myopathy of limb muscles. Because the extraocular muscles (EOM) are preferentially affected in human CPEO, the objective of this study was to determine whether Ant1-/- mice also exhibit an EOM mitochondrial myopathy. METHODS: ANT isoform expression of isolated EOMs, EOM morphology and mitochondrial content, mitochondrial structure and function, ocular motility in intact mice, and contractile performance in isolated muscle preparations were examined. RESULTS: Ant1-/- EOMs had the typical appearance of mitochondrial myopathy, including increase in mitochondrial size, number, and oxidative phosphorylation (OXPHOS) staining. However, there were no measurable ocular motor abnormalities in intact Ant1-/- mice, and their isolated EOMs did not show evidence of increased fatigability. EOMs of wild-type mice exhibited higher levels of Ant2 mRNA compared with hindlimb muscle, which may compensate for the Ant1 loss in mutant mouse EOMs and account for the normal EOM function. CONCLUSIONS: The Ant1-/- mice provide a model in which to study CPEO pathology and compensatory mechanisms.

Adenine Nucleotide Translocator 1↗

PubMed2005

Mitochondria are fast Ca2+ sinks in rat extraocular muscles: a novel regulatory influence on contractile function and metabolism.

PURPOSE: The ultrafast extraocular muscles necessitate tight regulation of free cytosolic Ca2+ concentration ([Ca2+]i). Mitochondrial Ca2+ influx may be fast enough for this role. In the present study, three hypotheses were tested: (1) Mitochondrial Ca2+ uptake regulates [Ca2+]i and production of force in extraocular muscle; (2) mitochondrial content correlates with their use as Ca2+ sinks; and (3) mitochondrial content in extraocular muscle is determined by the transcription factors and coactivators that initiate muscle adaptation to aerobic exercise. METHODS: Extraocular and extensor digitorum longus (EDL) muscles from adult Sprague-Dawley rats were used to examine how the Ca2+ release agonists caffeine and 4-chloro-3-ethylphenol (CEP), calcimycin (a Ca2+ ionophore) and carbonyl cyanide m-chlorophenyl hydrazone (CCCP; a mitochondrial uncoupler) alter [Ca2+]i and force transients. Mitochondrial volume density and capillary density were analyzed by stereology and citrate synthase and cytochrome c oxidase by biochemical assays. Real-time PCR measured mRNAs of genes involved in mitochondrial biogenesis. RESULTS: Caffeine, CEP, and calcimycin increased resting [Ca2+]i to a greater extent in EDL. Peak tetanic [Ca2+]i increased in extraocular muscle with caffeine and CEP. CCCP augmented peak tetanic and submaximum [Ca2+]i and force significantly more in extraocular muscles. Mitochondrial volume density and capillary density were three times greater, and citrate synthase and cytochrome c oxidase were only approximately 2-fold higher in extraocular muscle. Calcineurin Aalpha, calcineurin B, and peroxisome proliferator activated receptor (PPAR)gamma were more abundant in extraocular muscle. CONCLUSIONS: These data support the hypothesis that mitochondria serve as Ca2+ sinks in extraocular muscles. The high mitochondrial content of these muscles may partly reflect this additional function. It is likely that mitochondrial Ca2+ influx increases the dynamic response range of the extraocular muscles and matches metabolic demand to supply.

Animals↗

PubMed2004

Long-term age-dependent behavioral changes following a single episode of fetal N-methyl-D-Aspartate (NMDA) receptor blockade.

BACKGROUND: Administration of the N-methyl-D-aspartate (NMDA) antagonist ketamine during the perinatal period can produce a variety of behavioral and neuroanatomical changes. Our laboratory has reported reliable changes in learning and memory following a single dose of ketamine administered late in gestation. However, the nature of the drug-induced changes depends on the point during embryonic development when ketamine is administered. Embryonic day 18 (E18) rat fetuses pre-treated with ketamine (100 mg/kg, i.p. through the maternal circulation) and taught a conditioned taste aversion (CTA) learn and remember the CTA, whereas E19 fetuses do not. The current study sought to determine if long-term behavioral effects could be detected in animals that received ketamine or a saline control injection on either E18 or E19. Rat behavior was evaluated on two different measures: spontaneous locomotion and water maze learning. Measurements were collected during 2 periods: Juvenile test period [pre-pubertal locomotor test: Postnatal Day 11 (P11); pre-pubertal water maze test: P18] or Young-adult test period [post-pubertal locomotor test: P60; post-pubertal water maze test: P81]. RESULTS: Water maze performance of ketamine-treated rats was similar to that of controls when tested on P18. Likewise, the age of the animal at the time of ketamine/saline treatment did not influence learning of the maze. However, the young-adult water maze test (P81) revealed reliable benefits of prenatal ketamine exposure - especially during the initial re-training trial. On the first trial of the young adult test, rats treated with ketamine on E18 reached the hidden platform faster than any other group - including rats treated with ketamine on E19. Swim speeds of experimental and control rats were not significantly different. Spontaneous horizontal locomotion measured during juvenile testing indicated that ketamine-treated rats were less active than controls. However, later in development, rats treated with ketamine on E18 were more active than rats that received the drug on E19. CONCLUSION: These data suggest that both the day in fetal development when ketamine is administered and the timing of post-natal behavioral testing interact to influence behavioral outcomes. The data also indicate that the paradoxical age-dependent effects of early ketamine treatment on learning, previously described in fetuses and neonates, may also be detected later in young adult rats.

3',5'-Cyclic-GMP Phosphodiesterases↗

PubMed2004

Dynamic processing of taste aversion extinction in the brain.

While substantial advances have been made in discovering how the brain learns and remembers, less is known about how the brain discards information, reorganizes information, or both. These topics are not only relevant to normal brain functioning but also speak to pathologies in which painful memories do not wane but are evoked time and again (e.g., post-traumatic stress disorder; PTSD). Here, we measured brain activity (as indicated by the regional expression of c-Fos protein) in rats during acquisition and throughout extinction of a conditioned taste aversion (CTA). We compared that brain activity with animals that had intact CTA memories or those that experienced an explicitly unpaired (EU) conditioned stimulus (CS; saccharin, SAC) and unconditioned stimulus (US; lithium chloride, LiCl). The data show a dynamic and nonuniform pattern of c-Fos protein expression in brain nuclei known to mediate gustation and CTAs. In particular, brainstem nuclei (e.g., nucleus of the solitary tract; NTS) and the basolateral nucleus of the amygdala (BLA) are active early as CTAs are formed and as extinction of the learned response begins. Later in the extinction process, the BLA reduces c-Fos expression relative to nonextinguished controls. Finally, as almost full reacceptance of the taste is achieved, the gustatory neocortex (GNC) expresses enhanced levels of c-Fos protein. Thus, extinction of a CTA is not represented by a simple reversal of the c-Fos activity evoked by CTA conditioning. Rather, the data demonstrate that extinction of conditioned responses is a dynamic process in which the activity levels of particular nuclei along the brain's taste pathway change depending on the extent to which the conditioned response has been extinguished.

Analysis of Variance↗

PubMed2004

Carbonic anhydrase isoform expression and functional role in rodent extraocular muscle.

Carbonic anhydrase (CA) accelerates contractile function, particularly in fast-twitch skeletal muscles. Since the extraocular muscles are considered to be amongst the fastest skeletal muscles in mammals, this study tested two hypotheses: (1) CA is expressed at higher levels in rat extraocular muscles than in extensor digitorum longus (EDL, a fast limb muscle), and (2) inhibition of CA activity increases twitch duration and force in the extraocular muscles to a greater extent than in EDL. By real-time quantitative PCR we determined that the expression of CA3 isoform, typically high in skeletal muscles, is significantly depressed in extraocular muscles. Message levels for the CA2 and CA4 isoforms were higher in the extraocular muscles, while CA5 expression was equivalent in both muscles. Strong CA activity was demonstrated by histochemistry in frozen EDL muscle sections, in particular along the sarcolemma and in capillaries. By contrast, extraocular muscle had very low sarcolemmal or cytosolic CA activity. CA inhibition with 6-ethoxyzolamide (ETZ) reversibly increased twitch duration and force in EDL muscle bundles. In the extraocular muscles, ETZ did not alter twitch kinetics. Based on these results, we reject our initial hypotheses and conclude that CA does not influence the fast contractile kinetics characteristic of the extraocular muscles.

Animals↗

PubMed2004

Functional and genomic changes in the mouse ocular motor system in response to light deprivation from birth.

Previous studies have suggested that abnormal visual experience early in life induces ocular motor abnormalities. The purpose of this study was to determine how visual deprivation alters the function and gene expression profile of the ocular motor system in mice. We measured the effect of dark rearing on eye movements, gene expression in the oculomotor nucleus, and contractility of isolated extraocular muscles. In vivo eye movement recordings showed decreased gains for optokinetic and vestibulo-ocular reflexes, confirming an effect of dark rearing on overall ocular motor function. Saccade peak velocities were preserved, however, arguing that the quantitative changes in these reflexes were not secondary to limitations in force generation. Using microarrays and quantitative PCR, we found that dark rearing shifted the oculomotor nucleus transcriptome to a state of delayed/arrested development. The expression of 132 genes was altered by dark rearing; these genes fit in various functional categories (signal transduction, transcription/translation control, metabolism, synaptic function, cytoskeleton), and some were known to be associated with neuronal development and plasticity. Extraocular muscle contractility was impaired by dark rearing to a greater extent than expected from the in vivo ocular motility studies: changes included decreased force and shortening speed and evidence of abnormal excitability. The results indicate that normal development of the mouse ocular motor system and its muscles requires visual experience. The transcriptional pattern of arrested development may indicate that vision is required to establish the adult pattern, but it also may represent the plastic response of oculomotor nuclei to abnormal extraocular muscles.

Animals↗

PubMed2003

Paradoxical absence of M lines and downregulation of creatine kinase in mouse extraocular muscle.

The M lines are structural landmarks in striated muscles, necessary for sarcomeric stability and as anchoring sites for the M isoform of creatine kinase (CK-M). These structures, especially prominent in fast skeletal muscles, are missing in rodent extraocular muscle, a particularly fast and active muscle group. In this study, we tested the hypotheses that 1). myomesin and M protein (cytoskeletal components of the M lines) and CK-M are downregulated in mouse extraocular muscle compared with the leg muscles, gastrocnemius and soleus; and 2). the expression of other cytosolic and mitochondrial CK isoforms is correspondingly increased. As expected, mouse extraocular muscles expressed lower levels of myomesin, M protein, and CK-M mRNA than the leg muscles. Immunocytochemically, myomesin and M protein were not detected in the banding pattern typically seen in other skeletal muscles. Surprisingly, message abundance for the other known CK isoforms was also lower in the extraocular muscles. Moreover, total CK activity was significantly decreased compared with that in the leg muscles. Based on these data, we reject our second hypothesis and propose that other energy-buffering systems may be more important in the extraocular muscles. The downregulation of major structural and metabolic elements and relative overexpression of two adenylate kinase isoforms suggest that the extraocular muscle group copes with its functional requirements by using strategies not seen in typical skeletal muscles.

Adenylate Kinase↗

PubMed2002

Simple behavioral methods to assess the effect of drugs or toxins on sensory experience.

When behavioral pharmacologists/toxicologists study conditioned taste aversions (CTAs), or other conditioned responses, as a means to investigate the effects of various drugs or toxins on a learned response, failure to discover a CTA is frequently attributed to the treatment's influence on the associative process. This kind of analysis may fail to identify drug-induced sensory changes that may influence conditioned stimulus (CS) or unconditioned stimulus (US) saliency. The current paper outlines a simple method by which a drug's influence on CS or US sensation may be determined. Further, illustrative data are provided regarding how N-methyl-D-aspartate (NMDA) receptor blockade modulates taste and the sensation of malaise. Ketamine (an NMDA receptor antagonist) has been reported to block CTAs in both neonatal and adult rats. The current experiments evaluated ketamine's ability to modulate the taste of a frequently employed CS (saccharin HCl=SAC) or the aversive aspects of a common US (Lithium Chloride=LiCl). Rats normally exhibit a preference for 0.3% SAC over 0.6% SAC and will suppress consumption of these liquids following an injection of LiCl. We report that ketamine did not markedly antagonize these consummatory patterns nor did it disrupt spontaneous locomotor movements. Taken together, these findings point to ketamine's limited ability to change the sensory capacities required for CTA formation. Investigators interested in determining the underlying causes of drug-induced CTA blockade may choose to employ paradigms similar to the one used here.

Animals↗