MUSCLE CELL FUNCTION LABORATORY

David G. Allen

  

  

RESEARCH

PERSONNEL in 1994 and 1995

Dr David G. Allen Professor (in-charge) University 1989-
Lorraine Kerr Senior Technical Officer (0.5) NHMRC 1989-
Chris Balnave PhD student APA 1992-
Dr Eva Chin Postdoctoral Research Fellow NHMRC 1993-
Dr Anna Park Visiting Scholar 1993-

Current effective full-time personnel = 4.5

  
Research in this Laboratory is concerned with the regulation of intracellular ions, particularly calcium, sodium and protons, and with their effects on cell function.
  

PROJECTS in 1994

The distribution of ionic and myofibrillar changes in muscles damaged by stretch

Muscles which are stretched during contraction are susceptible to a form of injury which involves reduced force and a delayed onset muscle stiffness and soreness. With Chris Balnave the Laboratory has developed a single fibre model of stretch-induced muscle injury and it was shown that resting intracellular calcium is raised, while the intracellular calcium during tetani is reduced. It was shown that the reduced tetanic calcium is part of the cause of the reduced force in muscles injured by stretch. The distribution of these changes provides information about the potential mechanisms involved - for instance if localized damage to the surface membrane causes the elevated resting calcium, then the distribution of the elevation might also be localized. Therefore, the distribution of both resting and tetanic calcium was measured by digital fluorescence microscopy before and after a series of stretches which caused injury. It was found that both changes were uniformly distributed over the fibre. This result suggested that the injury mechanism is relatively uniformly distributed over the whole muscle fibre. In other approaches to this issue a collaboration with Dave Davey is examining the distribution of myofibrillar damage in the electron microscope. In addition, the distribution of sarcomere irregularities in fixed single fibres is being examined using protein fluorescence in the confocal microscope.

The mechanism of action of caffeine in skeletal muscle fibres

Caffeine is well known to increase tetanic force in skeletal muscle and to slow relaxation. The increased force is attributed to increased calcium release from the sarcoplasmic reticulum, while the mechanism of slowing of relaxation is not well understood. With Håkan Westerblad who was on a short visit from the Karolinska Institute in Sweden, it was shown that caffeine increases intracellular calcium, increases the calcium sensitivity of the contractile proteins and slows the sarcoplasmic reticulum calcium pump. Using the model, developed by the Lab., of intracellular calcium movements and force development, it was shown that these mechanisms acting in concert were adequate to explain the slowing of relaxation which occurred in the presence of caffeine.
The cause of reduced calcium release in fatigued skeletal muscle

In previous work by the Lab it was shown that during muscle fatigue the intracellular calcium release declines and that this makes an important contribution to the reduced force which characterizes muscle fatigue. Most of the reduced calcium release recovers quickly but a small component fails to recover and causes a long-lasting form of fatigue, known as low frequency fatigue. With Eva Chin, possible mechanisms for this long-lasting reduction in calcium release have been investigated. The starting hypothesis was that it might be caused by the elevation of intracellular calcium which inevitably accompanies the increased muscle activity which leads to fatigue. Some recent biochemical studies have suggested that a calcium-activated protease might selectively damage the calcium release channel in the sarcoplasmic reticulum when calcium is elevated. To test this hypothesis caffeine was used to increase intracellular calcium to about the same level as occurred in fatigue, leading to low frequency fatigue. The results showed that elevated calcium alone is capable of producing low frequency fatigue; so the hypothesis was supported.
The causes of slowing of relaxation in fatigued frog muscle

Repeated tetani in skeletal muscle lead to a reduction in force and a slowing of relaxation. In previous studies in mouse muscle the Lab showed that most of the slowing of relaxation was caused by slowing of crossbridge kinetics. In the present study the causes of slowing of relaxation in fatigued Xenopus single muscle fibres have been reinvestigated. Brief tetani were repeated at short intervals until force had declined to 85% of control and relaxation was substantially slowed. The figure shows intracellular calcium, Ca2+-derived force and force for the 1st (continuous line) and the 25th (intermittent line) tetanus. It is clear that the rate of decline of [Ca2+]i was substantially slowed in the 25th tetanus. Because the relation between calcium and force is conditions. These two relations were then used to convert the control and fatigued calcium signals to Ca2+-derived force. Measurements of time to 50% relaxation suggested that half of the slowing of relaxation was apparent in the Ca2+-derived force and was caused by the reduced rate of decline of intracellular calcium. The other half of the slowing of relaxation was seen only in the force records and represented changes in crossbridge kinetics.

Nice image!
Intracellular [Ca2+]i , Ca2+-derived force and force from Xenopus single muscle fibre. Continuous line shows a control tetanus; dashed line shows the 25th tetanus in a series when the muscle was fatigured.

The role of intracellular sodium and pH in ischaemic muscle damage in the heart

The Lab has shown that intracellular calcium is elevated in ischaemia and is associated with muscle damage when ischaemia is prolonged. With Anna Park, a visiting scholar from Gyeongsang National Univ., Korea, the Lab is currently exploring the mechanism of the rise of calcium. One hypothesis is that acidosis, caused by lactic acid accumulation, triggers sodium entry to the cell in exchange for protons and that subsequently calcium entry occurs in exchange for sodium. This hypothesis has been tested by measuring both intracellular pH and sodium during ischaemia. Sodium did not rise in ischaemia, despite a large acidosis, so it appeared that the sodium-hydrogen exchanger was blocked during ischaemia. Conversely, during reperfusion, sodium showed a transient rise which was inhibited by blockers of the sodium-hydrogen exchanger.
  

RESEARCH PLANNED for 1995

The mechanism of failure of calcium release in fatigue

Earlier work by the Lab showed a correlation between failure of calcium release and the time at which intracellular ATP starts to decline. Using caged ATP, the Lab will explore whether a reduced ATP concentration causes the failure of calcium release by suddenly increasing ATP.
Long term fatigue studies

All current work has been concerned with muscle fatigue which is induced by intense muscle activity and takes only 5-10 min to develop. Many forms of human activity lead to slower development of fatigue in which the main metabolic change is gradual consumption of glycogen. A series of experiments are planned to explore the cause of this slowly-developing muscle fatigue.
  

COLLABORATIONS

Muscle fatigue:
Dr Håkan Westerblad, Karolinska Institute, Sweden (1989-present).
  

FACILITIES

The Laboratory is located in rooms 348A-H of the Anderson Stuart Building. Prof. Allen's office is 348D. Four independent projects can be accommodated and each is based around a fluorescent microscope and accessory equipment. One microscope is adapted for digital fluorescent imaging. The Lab also has access to the confocal microscope located in Max Bennett's Laboratory. A comprehensive range of stimulating, recording and analysis equipment and software is available.
  

SCHOLARSHIPS

Australian Postgraduate Award

Chris Balnave, 1992-present.
  

OTHER RESEARCH ACTIVITIES in 1994

University committees

Chair, Univ. of Sydney Cancer Research Fund (1991-present).

Faculty of Medicine committees

Member, Research Committee.

Member, Anderson Stuart Refurbishment Project Control Group.

Editorial board of scientific journal

Circulation Research (1993-present).

Refereeing

Manuscripts:
for Journal of Physiology (2), Circulation Research (5), Pflügers Archiv European Journal of Physiology (5), Journal of Molecular and Cellular Cardiology (3), Cardiovascular Research (1), American Journal of Physiology (4), Clinical and Experimental Physiology and Pharmacology (1).

Grant applications:
for NHMRC (5), ARC (2), NHF (4).

Overseas grant applications:
for Israeli Science Foundation (1), US-Israel Binational Science Foundation (1), Wellcome Trust UK (2).

Theses examined:
for Faculty of Science DSc Committee (2).

International conferences

Oji International Conference on Calcium and the Heart, Osaka, Japan (invited speaker) (Oct).

Neural and Muscular Aspects of Fatigue, Miami, USA (invited speaker) (Nov).

National conferences

Eccentric Muscle Damage, Sydney (organizer and speaker) (Mar).

Seminars

Overseas

Stockholm Physiological Society, Karolinska Institute, Sweden (Apr).

Dept of Physiology, Jikei School of Medicine, Japan (Oct).

Local

Dept of Biological Sciences, Cumberland (May).

Neuroscience Group, Univ. of Newcastle (Jun).

Faculty of Medicine Research Conference, Leura (Oct)

Co-operative Research Centre for Cardiac Technology, Sydney (Dec).

News media

Sydney Morning Herald article, 'Chalk this one up to a sack race' (6 Aug).
  

5-YEAR RESEARCH PUBLICATIONSe>/A>

MOST RECENT TOTAL ANNUAL CITATIONS (for 1993): 409
  

FUNDING in 1994 and 1995

NHMRC The cause of failure of calcium release
in fatigued skeletal muscle
Allen DG 1993
1994 $67,703
1995 $68,786

NHMRC Intracellular calcium in skeletal
muscle during length changes
Allen DG 1994 $36,635
1995 $37,222
1996

NHF Role of changes of intracellular ions
during myocardial ischaemia
Allen DG 1993
1994 $35,000

NHF Regulation of intracellular ions
during cardiac ischaemia
Allen DG 1995 $35,100
1996

Total for 1994: $138,935 + share of confocal equipment grants

Total for 1995: $141,108

  

  

TEACHING

COURSES TAUGHT

Medicine 2

Lectures: 18, on the cardiovascular system, including 1 Question session, and 2 Distinction lectures.

Practical classes: 1, of 3 h, presented to 6 groups, on the cardiovascular system, involving demonstration of blood pressure measurement, ECG and heart sounds; students then designed their own experiment to investigate some aspect of cardiovascular function. Each was also given a follow-up session of 2 h, which included student presentations of results, tutorial and multiple-choice questionnaire.

Examination: Involved an unseen-data question, and a set of true/false questions.

Medicine 3 (Clinical Physiology)

Lectures: 4, on clinically-applied cardiovascular physiology.

Examination: One short-answer question and multiple choice questions.

Medical Science 3: Human Life Sciences 3

Lectures: 3, on intracellular ionic regulation.

Small group tutorials: 4, discussing ionic regulation in the heart during ischaemia.

Large group tutorials: 2, supervising student presentations on ionic regulation.

Examination: Long essay question.

Medical Science 3/Science 3 (Cardiovascular)

Lectures: 8 on electrophysiology of the heart and the limits to human exercise performance.

Tutorials: 5 in which students discuss scientific papers.

Examination: Long essay question.
  

TEACHING HOURS in 1994

Med 2 Med 3 MedSc 3
(CV)
HLS 3 Total
Lectures 18 4 8 3 33
Practical classes (no.) 24 (8) - - - 24
Practical discussion sessions 8 - - - 8
Tutorials 8 - 5 2 15

Total formal contact teaching time = 80 h

Additional time was devoted to lecture preparation, setting and marking of exams and discussions with colleagues on curriculum development.
  

  

ADMINISTRATION

(see also OTHER RESEARCH ACTIVITIES in 1994)

Service to Department

Head of Department (1991-early 1995).

University committees

Member, Academic Board.

Faculty of Medicine committees

Member, Heads of Departments Committee.

Member, Committee of Management.