Excitationcontraction coupling in skeletal and caudal heart muscle of the hagfish Eptatretus burgeri Girard
1 The Ine Marine Laboratory of National Institute for Physiological
Sciences, Ine, Kyoto 626-0424, Japan
2 Institute for Enzyme Research, Tokushima University, Tokushima 770-8503,
Japan
3 Laboratory of Biology, Graduate School of Commerce and Management,
Hitotsubashi University, Kunitachi, Tokyo 186-8601, Japan
4 Marine Biological Association of UK, Plymouth PL1 2PB, UK
* Author for correspondence (e-mail: iinoue{at}ier.tokushima-u.ac.jp)
Accepted 21 August 2002
![]() |
Summary |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Key words: excitationcontraction coupling, Ca2+, skeletal muscle, caudal muscle, hagfish, Eptatretus burgeri
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
In the wide spectrum of invertebrate and protochordate twitch muscle fibres
so far examined, influx of extracellular Ca2+ is necessary for
EC coupling under physiological conditions
(Hagiwara et al., 1971; Inoue
et al., 1994
,
1996
,
1997
; Bone et al.,
1997
,
1999
;
Tsutsui et al., 2000
).
However, as the vertebrate group evolved, skeletal muscle gained a new unique
function, and EC coupling in vertebrates is different: external
Ca2+ is not required (Armstrong
et al., 1972
). The fibres contract in response to electrical
excitation of the muscle membrane using only intracellular Ca2+
stored in the sarcoplasmic reticulum (SR). Depolarisation of the transverse
tubules directly triggers Ca2+ release from the SR, known as
depolarisation-induced Ca2+ release (DICR)
(Ríos and Brum, 1987
).
Hence this `vertebrate type' of EC coupling is in striking contrast to
the EC coupling of invertebrate twitch muscles.
An interesting question is where in phylogeny this change in the mechanism
of skeletal muscle EC coupling took place, i.e. when the DICR function
was acquired. We have previously shown that in the skeletal muscle of the
lamprey Lampetra planeri, EC coupling is of the `vertebrate
type', whilst in the acraniate Branchiostoma lanceolatum, EC
coupling is of the `invertebrate type', requiring influx of extracellular
Ca2+ (Inoue et al.,
1994). Based on these studies, we proposed that the DICR function
was acquired somewhere in phylogeny between the acraniate and agnathan levels
(Inoue et al., 1994
).
Unfortunately there is no living intermediate between the two levels; however
from molecular analyses, both lampreys and hagfish should be regarded as a
natural group of Agnatha, forming the sister group to the Gnathostomata. In
order to test our hypothesis, we examined whether EC coupling of the
hagfish skeletal muscle is of the `vertebrate type' (no requirement for
Ca2+ influx) or the `invertebrate type' (requiring Ca2+
influx), bearing in mind that the body fluid of hagfishes is iso-osmotic to
seawater (Alt et al., 1981
) and
contains a much higher concentration of Ca2+ than the body fluid of
any other vertebrate. We studied two different types of skeletal muscle
fibres, one with white (fast) fibres and the other with red (slow) fibres.
The hagfishes possess a unique circulation system, using a secondary caudal
heart. The caudal heart is a bilaterally symmetrical organ below the
notochord, consisting of two chambers on each side of a median caudal
cartilage. Both sides of the caudal heart muscle contract alternately, bending
the caudal cartilage to the contracting side. These rhythmic movements cause
the chamber of the other side to expand and flatten, so that blood is returned
to the central veins (see reviews by
Johansen, 1963;
Lomholt and Franko-Dossar,
1998
). The caudal heart muscle is extrinsic to each heart chamber.
The muscle fibres are long and cross-striated, and innervated from the spinal
cord. The hagfish Myxine glutinosa the caudal heart muscle was
examined by Retzius (1890
,
1892
), who found that it is
derived from parietal muscle and innervated by spinal nerves terminating in
similar nerve endings to those of red parietal muscle fibres. The fibres of
the caudal heart muscle are smaller in diameter than any in the myotomes. Our
study reveals that EC coupling in the caudal heart muscle as well as in
both white and red skeletal muscle fibres is of the `vertebrate type'.
![]() |
Materials and methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
White skeletal muscle fibres were dissected from the musculus tubulatus that protrudes the dental plate cartilage from the mouth. Approximately ten fibres (approx. 400 µm diameter and 3-5 cm long) were dissected under a binocular microscope. The fibres were separated (except for a portion at both ends) using a pair of fine needles. The tendon at one end of the fibres was pinned down to the Sylgard base of a small chamber. The other end of the fibres was connected to a strain gauge (SensoNor N801, Norway) via a small dissecting pin (Seirin Hinaishin No 2, Japan) as described below. The volume of the chamber was 4 cm3, and the bathing solution continuously flowed at a rate of 10 ml min-1. A sheet of red skeletal muscle fibres (100-150 µm thick, approx. 3 mm wide and 7 mm long) was sliced off from the parietal muscle using a vibrotome (Campden Instruments Vibroslice, UK), and was pinned down to the Sylgard base of the chamber under the binocular microscope.
Caudal heart tissues were dissected using a small pair of scissors. The caudal heart muscle was carefully dissected out from each heart and pinned down to the Sylgard base of the chamber under the binocular microscope. Epithelial tissue and connective tissue were removed. The caudal muscle fibres were 40-50 µm diameter and 5-7 mm long.
Aqueous solutions
Artificial seawater (ASW) consisted of 450 mmol l-1 NaCl, 9 mmol
l-1 KCl, 10 mmol l-1 CaCl2, 50 mmol
l-1 MgCl2 and 15 mmol l-1 Hepes-Na buffer, pH
7.8. Nominally Ca2+-free ASW (0Ca2+-ASW) was made by
replacing CaCl2 with MgCl2. Na+-free ASW
(0Na+-ASW) consisted of 450 mmol l-1 choline chloride, 9
mmol l-1 KCl, 10 mmol l-1 CaCl2, 50 mmol
l-1 MgCl2 and 15 mmol l-1 Tris-HCl buffer, pH
7.8.
Chemicals
Tetrodotoxin (TTX) (Sankyo, Japan), acetylcholine (ACh) (Sigma, USA) and
d-tubocurarine (d-TC) (Sigma, USA) were dissolved in ASW to make 1 mmol
l-1, 100 mmol l-1 and 1 mmol l-1 stock
solutions, respectively. Each stock solution was kept at 4°C in the dark,
and used by adding appropriate amounts to the bathing solutions.
CoCl2 was added to bathing solutions to block ionic currents
through calcium channels.
Stimulation and recordings of the intracellular potential and muscle
twitches
Fibres of either skeletal muscle or caudal heart muscle were stimulated
electrically using a pair of electrodes connected to the isolating unit of an
electric stimulator (Nihon Kohden SEM3201, Japan). One of the electrodes was a
suction electrode made of fine polyethylene tubing (100 µm inner diameter).
A microelectrode (10-20 M) filled with a 3 mol l-1 KCl
solution connected to a head stage of an electrometer (World Precision
Instruments Duo773, USA) was inserted in a fibre whose twitches had been
observed in response to electrical stimuli. For direct stimulation of a caudal
heart muscle fibre, a microelectrode was inserted to the fibre and
transmembrane current was applied to the fibre through the microelectrode
using the current injection circuit of the electrometer.
Twitches were monitored with a CCD camera and recorded on videotape (Video recorder: Sony FS10, Japan). Records of the twitches were obtained subsequently by placing a CdS photocell on the video monitor screen on the twitching muscles. The photocell signal was recorded through a low-pass filter (<33 Hz). Force generation of skeletal muscle fibres in response to electrical stimulation was measured with a strain gauge. A dissecting pin fixed to the gauge was inserted in the tendon at one end of the fibres.
Measurements of intracellular Ca2+ transients
Each muscle bundle was pinned down to a silicone pad in a culture dish. The
fibres were loaded with the Ca2+ indicator dye fluo-3 by incubating
them in each test solution containing 2 µmol l-1 fluo-3-AM
(Dojin, Japan) for 1.5 h at 14°C. After washing off the fluo-3-AM, the
chamber was placed on the inverted microscope fitted with an epi-illumination
apparatus, and a laboratory-built fluorescence photometer system
(Bone et al., 1997). A small
area of the specimen (100x100 µm2) was illuminated by
excitation wavelength light (470 nm with half-bandwidth of 20 nm), and the
intensity of emitted light (520-600 nm wavelength) was measured with a
photomultiplier tube (Hamamatsu Photonics R268, Japan). The detecting area
(10x10 µm2) was determined by a slit, and background level
measured on the area off the muscle.
Signals from the photocell, the strain gauge, and the photomultiplier were recorded with a VHS video system via a laboratory-modified PCM converter (Sony PCM501ES, Japan) at a sampling frequency of 15 kHz. Data were analysed offline using an IBM/PC compatible computer and pCLAMP software.
Collection of blood plasma
After decapitation of the animals, blood was collected (1-2 ml from each),
and centrifuged at 500 revs min-1 for 5 min. The supernatant was
taken and centrifuged at 3,000 revs min-1 for 10 min. 250 µl of
the supernatant was used for measurements of the osmolarity with an osmometer
(Fiske Mark 3, USA), and the remainder for measurements of the electrolyte
concentrations with a biochemical analyser (Fuji Film FDC3500, Japan).
Numerical values are presented as means ± S.D. Experiments were carried out at a room temperature of 21±1°C.
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
Electro-mechanical properties of skeletal muscle
Membrane potentials were recorded from white muscle fibres of the
musculus tubulatus using microelectrodes. The resting potentials in
ASW recorded from 16 fibres were -55 to -80 mV, mean -67.4±7.0 mV. This
is similar to that recorded from the white muscle fibres of the musculus
longitudinalis linguae in Myxine glutinosa, which is
used for retracting the dental plate into the mouth
(Andersen et al., 1963;
Nicolaysen, 1966
;
Flood, 1998
).
Electrical stimulation with a suction electrode at the tendon evoked a propagating action potential and a muscle twitch. A propagating action potential recorded from one of those twitching fibres overshot to +40 mV, followed by undershoot beyond resting potential (Fig. 1A). Externally applied TTX (1 µmol l-1) blocked both action potential and twitch (not shown). In contrast, both action potential and twitch were maintained after switching the external ASW to 0Ca2+-ASW that contained 10 mmol l-1 Co2+ (N=6). Electrical stimulation in ASW, using a bundle with seven fibres, evoked a transient force reaching 3x10-5 N (Fig. 1Bi). The same force was evoked even 10 min after switching the external ASW to 0Ca2+-ASW containing 10 mmol l-1 Co2+ (Fig. 1Bii). Hence, external Ca2+ was not involved in EC coupling.
|
Although we did not measure the membrane potential of the red fibres, we
assumed that, like the red fibres of Myxine glutinosa, they do not
propagate overshooting action potentials
(Jansen and Andersen, 1963)
and, like them, operate by slow and graded depolarisations. We mimicked this
effect with K+ depolarisation. To test the mechanism of EC
coupling, bundles of red fibres were pretreated with 0Ca2+-ASW
containing 10 mmol l-1 Co2+ for 1.5 h, then pinned down
in the chamber filled with the same solution. Contraction of red fibres was
observed under the binocular microscope when the external K+
concentration was increased to 100 mmol l-1, to be sure that the
function of DICR by intracellular Ca2+ release (see
Fig. 5) is definitively present
in the red fibres. The force generated by the K+ depolarisation was
measured with a strain gauge, and an example of the results obtained from six
specimens is shown in Fig.
2.
|
|
Caudal heart muscle properties
Membrane potentials were recorded from caudal heart muscle fibres with
microelectrodes. The resting potential was between -30 and -60 mV, mean
-43.6±8.9 mV (N=53).
An electrical stimulus supplied by a suction electrode evoked a twitch of several fibres simultaneously. Subthreshold stimuli evoked graded depolarising responses that lasted >50 ms. However, the subthreshold stimuli did not evoke muscle twitches. When stimulation exceeded the threshold, a fibre generated an action potential in an all-or-none manner (Fig. 3A), and the fibre twitched.
|
When a train of subthreshold stimuli at the same intensity was applied through a suction electrode, subthreshold responses were intensified until they reached a constant height (Fig. 3Bi). The amplitude of the subthreshold responses was different between fibres. These subthreshold responses were reversibly suppressed by addition of a number of agents, such as TTX (1 µmol l-1, N=6), d-TC (20 µmol l-1, N=8), or Co2+ (5 mmol l-1, N=7), and by a substitution of Na+ with choline+ (N=1), or Ca2+ with Ba2+ (N=4). Fig. 3Bii,iii shows the reversible suppression of subthreshold depolarisations by d-TC. Depolarisation was also induced by extracellular application of ACh (N=3, not shown). The subthreshold depolarisations represent junction potentials responding to neuromuscular transmitters released from nerve endings by electrical stimulation of the nerves innervating the muscle fibre. Hence, action potential and associated twitch were evoked when the junction potential exceeded the threshold for the Na+ spike.
In order to stimulate a muscle fibre directly, electric current was applied to a fibre through a microelectrode inserted to the fibre. Application of a 0.2 ms outward-directed current pulse evoked twitches in an all-or-none manner when the pulse intensity exceeded a threshold level (N=14). No other fibres twitched, indicating that there is no electrical coupling between fibres. The force generated by one fibre in the bundle of fibres was insufficient to be detected by the strain gauge. We detected muscle twitches from a video monitor screen using a CdS photocell. Fig. 4 shows the effects of 0Ca2+-ASW containing 10 mmol l-1 Co2+ and of 0Na+-ASW on twitches in response to electrical stimulation applied every 2s. Twitches were maintained in 0Ca2+-ASW containing 10 mmol l-1 Co2+ (Fig. 4B). In contrast, twitches declined gradually in the 0Na+-ASW, and eventually disappeared at 15 min after the Na+ substitution (Fig. 4C). The twitches recovered to near the original level within a short time (5 min) after replacing the external 0Na+-ASW with ASW (Fig. 4D). These results indicate that the twitch does not require influx of extracellular Ca2+, but does require Na+ for action potential generation.
|
To examine the presence of DICR further, changes in the intracellular Ca2+ level in response to depolarisation by increased external K+ concentration were measured in fibres loaded with fluo-3 (N=4). The fibres were bathed in 0Ca2+-ASW containing 30 mmol l-1 Co2+ for 30 min before fluorescence measurements were done. On increasing the external K+ concentration to 100 mmol l-1, the fluorescence signal increased (Fig. 5A), indicating that the K+ depolarisation had induced intracellular Ca2+ release without the influx of extracellular Ca2+.
Fig. 5B shows the effect of external K+ concentration on the membrane potential in caudal heart muscle fibres. Each point indicates mean ± S.D. obtained from 10 fibres. Between 9 and 200 mmol l-1 K+, the membrane potential changed linearly against log[K+]. The slope was approximately 35 mV per tenfold change, indicating that the membrane does not behave as a pure K+ electrode, and permits permeation of other ions. Therefore, the intracellular K+ concentration could not be estimated in this way.
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Skeletal muscles in vertebrates are large and multinucleated. The SR is
coupled in a regular way to the transverse (T)-tubular membrane. The couplings
comprise end-feet consisting of dihydropyridine (DHP) receptors in the
T-tubular membrane and ryanodine receptors in the SR membrane
(Inui et al., 1987;
Block et al., 1988
;
Bers and Stiffel, 1993
). In the
EC coupling of skeletal muscle, depolarisation of the T-tubular
membrane is sensed by a voltage sensor in the DHP receptor, and directly
transferred to the ryanodine receptor (the intracellular Ca2+
release channel) via the end-feet to trigger Ca2+ release
(Schneider and Chandler, 1973
;
Huang 1989
;
Ríos and Pizarro,
1991
). Hence EC coupling does not require influx of
external Ca2+ (Armstrong et al.,
1972
). The voltage sensor is thought to be a part of the
1
subunit of the DHP receptor, a superfamily of L-type calcium channels
(Tanabe et al., 1987
;
Brum et al., 1988
). Thus the
DHP receptor of skeletal muscle possesses a dual function: as a voltage sensor
for EC coupling and as an L-type calcium channel.
So far as is known, influx of external Ca2+ is indispensable for
physiological EC coupling in invertebrates and protochordates including
the acraniate Branchiostoma lanceolatum
(Hagiwara et al., 1971; Inoue
et al., 1994
,
1996
,
1997
; Bone et al.,
1997
,
1999
;
Tsutsui et al., 2000
).
Ca2+ enters the fibres through voltage gated L-type calcium
channels (Melzer, 1982
; Inoue
et al., 1994
,
1996
,
1997
; Bone et al.,
1997
,
1999
;
Tsutsui et al., 2000
). The
activation of those L-type calcium channels in protochordates and
invertebrates is fast (<1 ms), whereas that of vertebrate skeletal muscle
is much slower (>50 ms) (e.g. Inoue et
al., 1994
). This suggests that the evolution of EC coupling
may be accompanied by a molecular evolution of the DHP receptor to enhance its
function as a voltage sensor. However, at present little is known about the
molecular mechanism of the phylogeny of EC coupling. Future studies
from the point of molecular phylogeny may reveal the steps involved in the
striking change in the mechanism of skeletal muscle EC coupling.
EC coupling of caudal heart muscle
Our experiments revealed that the caudal heart muscle fibres possess the
DICR function. Therefore, the caudal heart is a `vertebrate type' skeletal
muscle, perhaps unsurprisingly since it is derived from parietal muscle and
similarly innervated.
Na+ spikes were evoked in an all-or-none manner when the
stimulation intensity exceeded a threshold level
(Fig. 3). Twitches also
occurred in an all-or-none manner when the stimulating current intensity
delivered by a microelectrode inserted into the fibre exceeded a threshold
level (Fig. 4). These suggest
that the sodium spike propagates along the fibre, although there is no direct
experimental evidence. Although the resting potential of the caudal heart
muscle fibres (-43.6±8.9 mV) is similar to that of red muscle fibres of
Myxine glutinosa (Andersen et al.,
1963; Flood,
1998
), the electrical properties of the caudal heart are different
from those of the slow fibres. In red fibres of Myxine glutinosa,
membrane depolarisation is produced by neuromuscular junction potentials,
never overshoots, and does not propagate along the fibres
(Andersen et al., 1963
;
Nicolaysen, 1966
;
Flood, 1998
). The electrical
responses of the caudal heart muscle fibres rather resemble intermediate
fibres of parietal muscle (Flood,
1998
). The resting potential of the intermediate fibres is
approximately -60 mV, and action potentials overshoot and propagate
(Andersen et al., 1963
;
Flood, 1998
). In the dogfish,
multiply-innervated red fibres propagate action potentials, though these do
not overshoot (Bone et al.,
1994
).
In living animals, at rest, the rate of contraction of the caudal heart is
slow (0.1-0.5 Hz), and fatigue-resistant. However, rather strong and fast
contractions appear within a few minutes after swimming. Contractions of the
caudal heart stop when they are swimming, as has been reported by Greene
(1900). Surprisingly these
caudal heart activities are maintained even 24 h after decapitation. The
swimming movements of the trunk muscle are also maintained after decapitation;
they begin responding to touch stimulation to the skin and continue for 10-30
s. These observations suggest that the caudal heart activities are linked with
the swimming movements of the trunk muscle and are principally regulated by
cycles of spinal chord reflexes.
![]() |
Acknowledgments |
---|
![]() |
References |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Ahlberg, P. E. (ed.) (2001). Major events in early vertebrate evolution. palaentology, phylogeny, genetics and development. Systematics Association, special volume series, 61. London: Taylor and Francis.
Alt, J. M., Stolte, H., Eisenbach, G. M. and Walvig, F. (1981). Renal electrolyte and fluid excretion in the Atlantic hagfish Myxine glutinosa. J. Exp. Biol. 91,323 -330.
Andersen, P., Jansen, J. K. S. and Løyning, Y. (1963). Slow and fast muscle fibres in the Atlantic hagfish (Myxine glutinosa). Acta Physiol. Scand. 57,167 -179.[Medline]
Armstrong, C. M., Bezanilla, F. and Horowicz, P. (1972). Twitches in the presence of ethylene glycol-bis(b-aminoethyl ether)-N,N'-tetraacetic acid. Biochim. Biophys. Acta 267,605 -608.[Medline]
Bers, D. M. and Stiffel, V. M. (1993). Ratio of
ryanodine to dihydropyridine receptors in cardiac and skeletal muscle and
implications for EC coupling. Am. J. Physiol.
264,C1587
-1593.
Block, B. A., Imagawa, T, Campbell, K. P. and Franziani-Armstrong, C. (1988). Structural evidence for the direct interaction between the molecular components of the transverse tubule/sarcoplasmic reticulum junction in skeletal muscle. J. Cell Biol. 107,2587 -2600.[Abstract]
Bone, Q., Curtin, N. A. and Woledge, R. C. (1994). Action potentials in red muscle fibres from dogfish. J. Physiol. 479,P4 .
Bone, Q., Inoue, I. and Tsutsui, I. (1997). Contraction in the absence of a sarcoplasmic reticulum: muscle fibres in the small pelagic tunicate Doliolum. J. Mus. Res. Cell Motil. 18,375 -380.[Medline]
Bone, Q., Carré, C., Tsutsui, I. and Inoue, I. (1999). Calycophoran siphonophore muscle fibres without any sarcoplasmic reticulum but with tubular invaginations morphologically analogous to a T-system. J. Mar. Biol. Assn. UK 79,1111 -1116.
Brodal, F. and Fänge, R. (ed.) (1963). The Biology of Myxine. Oslo: Universitetsforlaget.
Brum, G., Fitts, R., Pizarro, G. and Ríos, E. (1988). Voltage sensors of the frog skeletal muscle membrane require calcium to function in excitation-contraction coupling. J. Gen. Physiol. 398,475 -505.
Flood, P. R. (1998). The skeletal muscle types of Myxine glutinosa. In The Biology of Hagfishes (ed. J. M. Jørgensen, J. P. Lomholt, R. E. Weber and H. Malte), pp. 173-202. London: Chapman and Hall.
Forey, P. and Janvier, P. (1993). Agnathans and the origin of jawed vertebrates. Nature 361,129 -134.
Greene, C. W. (1900). Contributions to the
physiology of the California hagfish, Polistotrema stouti. I. The
anatomy and physiology of caudal heart. Am. J.
Physiol. 3,366
-382.
Hagiwara, S., Henkart, M. P. and Kidokoro, Y. (1971). Excitation-contraction coupling in amphioxus muscle cells. J. Physiol. (Lond.) 219,233 -251.[Medline]
Huang, C. L.-H. (1989). Intramembrane charge
movement in skeletal muscle. Physiol. Rev.
68,1197
-1247.
Inoue, I., Tsutsui, I., Bone, Q. and Brown, E. R. (1994). Evolution of skeletal muscle excitation-contraction coupling and the appearance of dihydropyridine-sensitive intramembrane charge movement. Proc. Roy. Soc. Lond. B 255,181 -187.
Inoue, I., Tsutsui, I., Brown, E. R. and Bone, Q. (1996). A phylogenetic approach to understanding the molecular mechanism of excitation-contraction coupling in striated muscle. In Basic Neuroscience in Invertebrates (ed. H. Koike, M. Kidokoro, K. Takahashi and T. Kanaseki), pp. 45-57. Japan Scientific Societies Press, Tokyo.
Inoue, I., Tsutsui, I., Brown, E. R. and Bone, Q. (1997). Evolution of skeletal muscle excitation-contraction coupling. In Toward Molecular Biophysics of Ion Channels, Progress in Cell Research 6 (ed. M. Sokabe, A. Auberbach and F. Sigworth), pp. 111-124. Amsterdam: Elsevier Scientific Press.
Inui, M., Sails, A. and Fleischer, S. (1987).
Purification of the ryanodine receptor and identity with feet structures of
junctional terminal cisternae of sarcoplasmic reticulum from fast skeletal
muscle. J. Biol. Chem.
262,1740
-1747.
Jansen, J. K. and Andersen, P. (1963). Anatomy and physiology of the skeletal muscles. In The Biology of Myxine (ed. F. Brodal and R. Fänge), pp.161 -194. Oslo: Universitetsforlaget.
Johansen, K. (1963). Cardiovascular system of Myxine glutinosa L. In The Biology of Myxine (ed. F. Brodal and R. Fänge), pp. 289-316. Oslo: Universitetsforlaget.
Jørgensen, J. M., Lomholt, J. P., Weber, R. E. and Malte, H. (ed.) (1998). The Biology of Hagfishes. Chapman and Hall, London.
Lomholt, J. P. and Franko-Dossar, F. (1998). The sinus system of hagfishes-lymphatic or secondary circulatory system? In The Biology of Hagfishes (ed. J. M. Jørgensen, J. P. Lomholt, R. E. Weber and H. Malte), pp.259 -272. Chapman and Hall, London.
Mallatt, J., Sullivan, J. and Winchell, C. J. (2001). The relationships of lampreys to hagfishes: a spectral analysis of ribosomal DNA sequences. In Major Events in Early Vertebrate Evolution. Palaeontology, Phylogeny, Genetics and Development. Systematic Association special volume series, 61 (ed. P. E. Ahlberg), pp.106 -118. London: Taylor and Francis.
Melzer, W. (1982). Twitch activation in Ca2+-free solutions in the myotomes of the lancelet (Branchiostoma lanceolatum). Eur. J. Cell Physiol. 28,219 -225.
Nicolaysen, K. (1966). On the functional properties of the fast and slow cranial muscles of the Atlantic hagfish. Acta Physiol. Scand. 68Suppl. 277, 142(Abstract).
Retzius, G. (1890). Ein s.g. Caudalherz bei Myxine glutiosa. Biologische Untersuchungenm NF 1, 94-96.
Retzius, G. (1892). Zur Kentnisse der motorischen Nervendingungen. Biologische Untersuchungenm NF 3,41 -52.
Ríos, E. and Brum, G. (1987). Involvement of dihydropyridine receptors in excitation-contraction coupling. Nature 325,717 -720.[Medline]
Ríos, E. and Pizarro, G. (1991). Voltage
sensor of excitation-contraction coupling in skeletal muscle.
Physiol. Rev. 71,849
-908.
Schneider, M. F. and Chandler, W. K. (1973). Voltage dependent charge movement in skeletal muscle: a possible step in excitation-contraction coupling. Nature 242,244 -246.[Medline]
Tanabe, T., Takeshima, H., Mikami, A., Flockerzi, V., Takahashi, H., Kangawa, K., Kojima, M., Matsuo, H., Hirose, T. and Numa, S. (1987). Primary structure of the receptor for calcium channel blockers from skeletal muscle. Nature 328,313 -318.[Medline]
Tsutsui, I., Inoue, I., Carré, C. and Bone, Q. (2000). Activation of locomotor and grasping spine muscle fibres in chaetognaths; a curious paradox. J. Muscle Res. Cell Motil. 21,91 -97.[Medline]
Zardova, R. and Meyer, A. (2001). Vertebrate phylogeny: limits of inference of mitochondrial genome and nuclear rDNA sequence data due to an adverse phylogenetic signal/noise ratio. In Major Events in Early Vertebrate Evolution. Palaeontology, Phylogeny, Genetics and Development. Systematic Association special volume series, 61 (ed. P. E. Ahlberg), pp. 135-155. London: Taylor and Francis.