Aspects of the physiology and anatomy of cardiac and skeletal muscles in South Polar notothenioid fish
Abstract
Aspects of the physiology and anatomy of cardiac and
skeletal muscle in south polar notothenioid fish have been
investigated.
Relative ventricle weights for the haemoglobinless
Chaenocephalus aceratus (family Channichthyidae) were
approximately three times greater than for sympatric
'red-blooded' species. The ventricle in the channichthyid
was 'sac-like' in shape and had an entirely trabecular
myocardium: these characteristics were associated with low
maximum myocardial power outputs (1.46mW(g ventricle
weight) ⁻¹). The blood supply to the ventricle was through
the venous lacunary circuit and vessels in the
subepicardium.
The myoglobin-poor ventricular myocardium in C.aceratus
was composed of myocytes and granulated non-contractile
cells. Myocardial cytochrome oxidase activity (34 μmoles
g⁻¹min⁻¹) at O°C was similar to that of warmer-water species
with myoglobin-rich ventricles. This suggests that the
channichthyid has achieved compensation for the rate
depressing effects of the low temperatures and low myoglobin
concentrations on oxidative metabolism. Morphometric
analyses of the myocytes in C.aceratus indicated that they
had higher mitochondrial (0.43) and lower myofibrillar
(0.31) volume densities than other teleosts. It was proposed
that the proliferation of mitochondria serves to maintain
high aerobic capacities by reducing oxygen diffusion
distances between the lacunae and the myocytes. The anatomy of the pectoral fin muscles in N.neglecta
(family Nototheniidae) has been described. Six muscles
(2.49% of total body weight) articulated the fin blade.
These were composed of a core of small diameter fibres
(18-99μm) which stained intensely for markers of aerobic
metabolism. Overlying these was a layer of larger diameter
fibres (24-156μm) which stained poorly for these markers. The
two fibre types were also differentiated on the basis of
their mechanical properties: demembranated preparations of
the larger diameter fibres generated approximately twice the
tensions and had twice the unloaded contraction velocities
of the small diameter fibres. Despite these differences
however, the fibres could not easily be differentiated on
the basis of the pH stabilities of their myosins.
At 1°C the maximum isometric tension (Po) generated by
both pectoral fibre types in N.neglecta demonstrated
incomplete temperature compensation compared to published
values for homologous fibres in warm-water teleosts.
Unloaded contraction velocity (Vmax) was not temperature
compensated. In addition to Po, adaptive modifications in
the curvature of the force-velocity curves were identified
in the nototheniid which served to enhance power output at
low temperatures. Between -5 and +10°C Po was relatively
temperature independent (R₁₀ = approximately 1.2) whereas
the temperature coefficent of Vmax was high
(Q₁₀ = approximately 2). Following activations in excess of
12°C both fibre types failed to relax completely. This
suggests that adaptations in the contractile proteins which
conferred high power outputs at low temperatures were
associated with a limitation in the temperature range over
which they could maintain function. Simultaneous measurements were made of force generation
and myofibrillar ATP hydrolysis during isometric
contractions in demembranated white trunk fibres isolated
from teleosts adapted to different thermal environments. ATP
hydrolysis was quantified using a novel technique which
allowed the fibres to be activated in the presence of an ATP
regenerating system based on phosphocreatine and creatine
phosphokinase. ATP activity was determined by measuring the
increase in free creatine in the activating solution. The
results obtained indicated that the economy of contraction
(tension/ATP hydrolysed) was substantially higher in the
fibres from Notothenia neglecta than those from warmer-water
species at their preferred body temperatures.
Type
Thesis, PhD Doctor of Philosophy
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