Compare the parts of hen and humming bird in taking in food note down the similarities and difference
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Histidine containing dipeptides (HCDs) have numerous ergogenic and therapeutic properties, but their primary role in skeletal muscle remains unclear. Potential functions include pH regulation, protection against reactive oxygen/nitrogen species, or Ca2+ regulation. In recognition of the challenge of isolating physiological processes in-vivo, we employed a comparative physiology approach to investigate the primary mechanism of HCD action in skeletal muscle. We selected two avian species (i.e., hummingbirds and chickens), who represented the extremes of the physiological processes in which HCDs are likely to function. Our findings indicate that HCDs are non-essential to the development of highly oxidative and contractile muscle, given their very low content in hummingbird skeletal tissue. In contrast, their abundance in the glycolytic chicken muscle, indicate that they are important in anaerobic bioenergetics as pH regulators. This evidence provides new insights on the HCD role in skeletal muscle, which could inform widespread interventions, from health to elite performance.
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Introduction
The histidine containing dipeptide (HCD) carnosine, and its related methylated analogues (anserine and balenine), are formed by β-alanine and L-histidine, and are purported to have pleiotropic ergogenic and therapeutic effects. These include the enhancement of high-intensity exercise performance1, protection against the effects of senescence2, neuro-protection3,4 and tumour inhibition5. These influences may occur due to a number of mechanisms, namely intracellular proton buffering, protection against reactive species and/or the regulation of Ca2+ transients and sensitivity6,7. Their primary role, however, is unknown. This renders targeted intervention difficult, meaning that the full therapeutic and ergogenic potential of these dipeptides remains unexploited. The abundance of HCDs (carnosine) in human skeletal muscle (approximately 20–30 mmol.kgDM−1 in m. vastus lateralis8), along with evidence of a positive influence on exercise performance1, implies an important role in skeletal muscle metabolism. Elucidation of their primary role in skeletal muscle is challenging, however, as all potential processes to which they may contribute up-regulate concurrently and cooperatively in response to high-intensity activity. Similarly, many of the conditions in which HCDs have been reported to convey a therapeutic benefit are multi-factorial, therefore potentially benefitting from most, or all, of the HCDs purported biological functions.
In recognition of the challenge of isolating biological processes in-vivo, we employed a comparative physiology approach to provide new insight into the primary mechanism of HCD action in skeletal muscle metabolism. More specifically, we selected two physiologically distinct avian species (namely hummingbirds and chickens), whose unique skeletal muscle types represented the extremes of biological processes in which HCDs are most likely to exert their primary influence. An overview of the contrasting characteristics of the hummingbird and chicken m. pectoralis are described in Fig. 1. Of particular relevance to this investigation, is the hummingbird’s remarkable skeletal muscle characteristics, which gives rise to its unique locomotive ability. Hummingbirds have an outstanding capacity to accelerate and to alter flight speed, trajectory and body orientation9,10. They are also capable of beating their wings with sufficient frequency to hover. This characteristic is unique among avian species, and occurs due to their extraordinarily high wing-beat frequency, which is the fastest of all vertebrates11,12. This is achieved through highly developed contractile properties, and requires optimised regulation of Ca2+ transients and sensitivity13. Hummingbirds also have the highest mass-specific metabolic rate of all vertebrates14, achieved through an outstanding capacity to deliver, uptake and utilize oxygen9, allowing a constant supply of aerobically generated energy to the working muscles15,16. This extremely advanced oxidative system, must be accompanied by an equally well-developed system to neutralise the metabolic by-products of oxidative phosphorylation, namely reactive species1
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