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Cellular thermogenic mechanisms : a comparative study of brown adipose tissue and skeletal muscle

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Matthias, A (1995) Cellular thermogenic mechanisms : a comparative study of brown adipose tissue and skeletal muscle. PhD thesis, University of Tasmania.

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Abstract

Thermogenic mechanisms in homeotherms appear to be largely controlled by
norepinephrine and the sympathetic nervous system. The thermogenic mechanism of
brown adipose tissue (BAT) has been well characterized by other researchers and
recent data from our laboratory have suggested a role for the vasculature in skeletal
muscle thermogenesis. The aim of this thesis was to compare the thermogenic
mechanisms of BAT and skeletal muscle. The contribution of the vasculature to BAT
thermogenesis and the possibility of a mitochondria' uncoupling mechanism in skeletal
muscle were investigated.
To examine the vascular contribution to BAT thermogenesis a technique for
the isolation and constant flow perfusion of the periaortic BAT deposit was
developed and characterized. In this preparation, the thermogenic response was
found to be entirely 0-adrenoreceptor mediated. Similar increases in oxygen
consumption were induced by norepinephrine, isoproterenol and BRL 35135A (a
specific 133-agonist). These induced responses were completely inhibited by
propranolol, but unaffected by either phentolamine or nitroprusside (a
nitrovasodilator). The lack of an a-mediated or nitroprusside-sensitive effect
suggested that the vasculature did not contribute significantly either directly (e.g. "hot
pipes") or indirectly (through alterations in blood flow) to BAT thermogenesis.
Initially, perfused rat hindlimbs were used in our laboratory to address the
question of uncoupling in skeletal muscle. When infused the rate of conversion of a
redox dye (MTT) was decreased by the addition of norepinephrine in association with
a stimulation of oxygen consumption. Similarly, measurement of muscle cell
mitochondria' membrane potential, using [311]-TPMT, suggested decreases were
occurring in the presence of norepinephrine. Together, these data implied that
norepinephrine could be causing uncoupling in skeletal muscle mitochondria. Thus, a
search for the mechanism responsible for this thermogenic process was conducted
using isolated subsarcolemmal skeletal muscle mitochondria.
Potential candidates that led to increased oxygen consumption and decreased
MTT conversion by subsarcolenunal skeletal muscle mitochondria were assessed in
conjunction with measurements of mitochondrial membrane potential (using
Rhodatnine 123). Mitochondria exhibited little respiratory control when succinate
was used as the substrate, suggesting that it may be able to act as an uncoupler. With succinate as the substrate, respiration was maximal despite an inhibition of the rate of
MTT conversion. The inhibition of MTT conversion was overcome by the transition
of the mitochondria from state IV to state III, and this ADP-regulated, succinateinduced
uncoupling appears specific for skeletal muscle mitochondria. Although
succinate seems a viable candidate for the uncoupling seen in skeletal muscle, it does
not decrease the mitochondrial membrane potential and this eliminates it as the
uncoupler which is acting in the perfused rat hindlimb.
As glycerol release has been noted from the perfused rat hindlimb on addition
of norepinephrine, fatty acids and their uncoupling action were investigated in the
isolated mitochondria. Fatty acids gave responses with isolated mitochondria similar
to that seen on the addition of uncouplers such as FCCP. Respiration was increased,
while both membrane potential and MTT conversion were decreased.
In conclusion, the development and examination of a novel perfused BAT
preparation suggested that the vasculature does not contribute significantly to
thermogenesis in a manner similar to that seen in skeletal muscle. In skeletal muscle,
however, fatty acids are a candidate for an uncoupler that can act in a manner similar
to that seen in BAT. With other researchers having shown that the induced loss of
mitochondrial membrane potential can be prevented if ADP and Mg2+ are present,
fatty acids may be able to induce uncoupling that is readily reversed with the onset of
exercise.

Item Type: Thesis (PhD)
Keywords: Body temperature, Body temperature
Copyright Holders: The Author
Copyright Information:

Copyright 1995 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1995. Includes bibliographical references (p. 108-130)

Date Deposited: 19 Dec 2014 02:27
Last Modified: 11 Mar 2016 05:55
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