Myth-busting: Do heart cells really have their own ‘pulse’?

By Dr. Andy James, University of Bristol

The comedian, Jake Yapp, has been presenting on BBC Radio 4’s website a series of short comic histories of science entitled, “Everything We’ve Ever Known About …” Informative and entertaining, these are invariably funny. A recent edition concerned the heart, neatly covering the development of our understanding of heart function through the millennia, from the ancient Egyptians to the present day – where Jake concludes by commenting that if you isolate a heart muscle cell it will have its own pulse. Actually, strictly speaking, a pulse is the increase in pressure within the arteries as a result of beating of the heart. ‘Rhythm’ is a better word.

It’s a commonly held misconception, even amongst specialists in cell biology, that all heart muscle cells have their own rhythm that synchronises when the cells couple together (meaning they join and allow electricity to flow between them). Beautiful as this idea is, it simply isn’t true. The heart’s rhythm originates as electrical activity in a specialised area of the heart known as the sinus node- the heart’s physiological pacemaker. The heart muscle cells of the sinus node generate rhythmic electrical activity so that, when isolated, they contract regularly.

In the intact heart, a conduction system ensures that the electrical activity is passed to the chambers to produce a well-coordinated contraction. Many of the cells of the conduction system would also beat when isolated. But the cells from the walls of the chambers that produce the pressure to – as the English anatomist, William Harvey, would have it – propel the blood, do not. Chamber cells only need to receive the signal, but not generate it themselves.

The persuasive and attractive idea that all heart muscle cells show their own rhythm may have arisen from attempts to maintain heart muscle cells for several days in a petri dish. Heart muscle cells from adult mammalian hearts do not generally multiply under these conditions and while it is possible to keep the cells alive for a short time, after a few days, the cells die. However, cells from embryonic, or even neonatal, hearts do multiply in culture. They often show rhythmic contractions and, yes, they synchronise once they form connections with their neighbours.

So do heart cells really have their own pulse? It depends! Cells from adult hearts are programmed to behave in a fixed way- as a pacemaker cell, a conducting cell or a contractile cell from a chamber wall. Cells from immature hearts, on the other hand, retain the ability to change their properties and tend to develop pacemaker-like function in the petri dish.


Non-invasive imaging of transient biomolecular machines in vivo

pic2(This article was originally published in Physiology News 104, page 24-26)

While homeostasis is considered a steady state in physiology, it is by no means static. Instead, it is the sum of a concerted effect of sub-cellular events driven by molecular machines. These machines range from large enzymes to reactive small molecules, all of which exert an effect on the cell to alter its function for better or for worse.

Indeed, the over- or under-activity of these molecular machines can result in disease or injury (e.g. reactive oxygen species (ROS) and inflammation), or in an intended response to applied therapy (e.g. tumor casapase-3 (apoptosis) activation following radiation or chemotherapy). These active biomolecules rarely work in isolation, but are rather parts of integral networks within which any single sub-cellular machine is only transiently active or transiently present, often rapidly giving way to its successor in the signaling chain. This transiency makes these biomolecular machines elusive to analysis by traditional ex vivo methods in the context of the living organism.

However, measuring the activity of these molecules and overcoming the difficulty of target transiency can enable both the detection of disease prior to outward signs and symptoms, and the assessment of the success or failure of therapy prior to disease progression (Aboagye, 2006; Aboagye, 2010). Molecular imaging is a powerful tool for assessing a target biomolecular machine non-invasively and longitudinally over an entire volume of interest in living subjects (James & Gambhir, 2012). The ability to apply molecular imaging to interrogate these highly transient sub-cellular functions of fundamental importance in living subjects is provided by new chemistries for the design of activatable molecular imaging probes over a range of pre-clinical (i.e. fluorescent) to clinically-relevant (PET, MRI) imaging modalities.image1

Reactive oxygen species (ROS) are highly reactive oxygen-centered biomolecules underlying a broad range of diseases (Winterbourn, 2008). In the context of injury, ROS are very early effectors of the tissue response to the source of harm. Novel optical nanoprobes have been developed from semiconducting polymers that can sensitively report on the presence of specific ROS in living mice (Pu et al., 2013; Pu et al., 2014; Shuhendler et al., 2014). Our work with the nanoprobes has enabled the real-time detection of the very early response of tissues to mechanical or chemical injury in real-time and in living subjects.

The very early response to tissue injury (i.e. within the first 30 min) have been imaged with evidence pointing to the mast cell component of the innate immune system as being responsible for the very early ROS signaling (Fig. 1; previously unpublished data). Additionally, a dual optical channel probe has been developed capable of the real-time differentiation of hydrogen peroxide versus peroxynitrite mediated drug-induced hepatotoxicity in living subjects (Shuhendler et al., 2014).

‘The molecular imaging of these key sub-cellular machines is sure to continue to provide an unprecedented level of interrogation of physiology to both enhance our investigations of health and disease, and improve clinical outcomes’


Caspase-3 is a cysteine-aspartate protease whose activation signals the committal of the cell to die through apoptosis, a common death pathway induced by a variety of cancer chemotherapeutics and radiation. We designed a modular probe to undergo bioorthogonal macrocyclization upon activation by caspase-3, with the macrocycles self-assembling into nanoparticles in situ in tumor tissue (Shen et al., 2013; Shuhendler et al., 2015; Ye et al., 2014a; Ye et al., 2014b). Since caspase-3 activity correlated with the degree of probe retention in dying tumor tissue, this molecular imaging strategy was applicable to imaging through fluorescence, PET, and MRI. Importantly, this self-assembling molecular imaging probe was able to predict the degree of therapy outcome (i.e. the degree of tumor treatment effect) in mice following a single imaging session 48 hours after a single dose of therapy (Fig. 2).

Enhanced contrast production following therapy was not detected using a non-activatable control analog, illustrating the utility of our bioorthogonal self-assembly probe design for therapy response monitoring. Multiple sub-cellular markers for apoptosis have been identified each with molecular imaging probes targeting these sub-cellular changes, including annexin-V detecting phosphatidyl serine flipping from the inner to the outer cell membrane, ML-10 detecting depolarization of the cellular membrane, and fluorodeoxyglucose detecting the loss of metabolic function (i.e. reduced glucose utilization) (Witney et al., 2015). When these targets and respective probes were compared to the macrocyclization probe responding to caspase-3 activation, only the macrocyclizing probe accurately reported tumor response to therapy in living subjects as confirmed by ex vivo measurement of apoptosis (Witney et al., 2015). The ability of the macrocyclization agent to directly probe the effector of apoptotic cell death (i.e. caspase-3) and to significantly amplify the enzyme activity signal by being a substrate of this molecular machine may account for its performance in vivo.

The ability to investigate transient biomolecular machines in living subjects by molecular imaging can provide non-invasive, longitudinal, and predictive data about sub-cellular physiology within the intact organism. The molecular imaging of these key sub-cellular machines is sure to continue to provide an unprecedented level of interrogation of physiology to both enhance our investigations of health and disease, and improve clinical outcomes.


Aboagye EO (2006). Imaging in drug development. Clin Adv Hematol Oncol 4, 902–904

Aboagye EO (2010). The future of imaging: developing the tools for monitoring response to therapy in oncology: the 2009 Sir James MacKenzie Davidson Memorial lecture. Br J Radiol 83, 814–822

James ML & Gambhir SS (2012). A molecular imaging primer: modalities, imaging agents, and applications. Physiol Rev 92, 897–965

Pu K, Shuhendler AJ, Jokerst JV, Mei J, Gambhir SS, Bao Z & Rao J (2014). Semiconducting polymer nanoparticles as photoacoustic molecular imaging probes in living mice. Nat Nanotechnol 9, 233–239

Pu K, Shuhendler AJ & Rao J (2013). Semiconducting polymer nanoprobe for in vivo imaging of reactive oxygen and nitrogen species. Angew Chem Int Ed Engl 52, 10325–10329

Shen B, Jeon J, Palner M, Ye D, Shuhendler A, Chin FT & Rao J (2013). Positron emission tomography imaging of drug-induced tumor apoptosis with a caspase-triggered nanoaggregation probe. Angew Chem Int Ed Engl 52, 10511–10514

Shuhendler AJ, Pu K, Cui L, Uetrecht JP & Rao J (2014). Real-time imaging of oxidative and nitrosative stress in the liver of live animals for drug-toxicity testing. Nat Biotechnol 32, 373–380

Shuhendler AJ, Ye D, Brewer KD, Bazalova-Carter M, Lee KH, Kempen P, Dane Wittrup K, Graves EE, Rutt B & Rao J (2015). Molecular Magnetic Resonance Imaging of Tumor Response to Therapy. Sci Rep 5, 14759

Winterbourn CC (2008). Reconciling the chemistry and biology of reactive oxygen species. Nat Chem Biol 4, 278–286

Witney TH, Hoehne A, Reeves RE, Ilovich O, Namavari M, Shen B, Chin FT, Rao J & Gambhir SS (2015). A Systematic Comparison of 18F-C-SNAT to Established Radiotracer Imaging Agents for the Detection of Tumor Response to Treatment. Clin Cancer Res 21, 3896–3905

Ye D, Shuhendler AJ, Cui L, Tong L, Tee SS, Tikhomirov G, Felsher DW & Rao J (2014a). Bioorthogonal cyclization-mediated in situ self-assembly of small-molecule probes for imaging caspase activity in vivo. Nat Chem 6, 519–526

Ye D, Shuhendler AJ, Pandit P, Brewer KD, Tee SS, Cui L, Tikhomirov G, Rutt B & Rao J (2014b). Caspase-responsive smart gadolinium-based contrast agent for magnetic resonance imaging of drug-induced apoptosis. Chem Sci 4, 3845–3852pic2

Measuring a moving target: Symposium on Hormone Sensing

What do puberty, doping in athletics, and the meat industry have in common? The answer is hormones. Secreted into the blood by specialized organs and tissues, hormones communicate a bewildering array of signals to a myriad of target sites.

Two weeks ago, The Physiological Society brought together experts in physiology, endocrinology, chemistry, physics and engineering, to discuss how to produce a new generation of tools and methods for detecting hormones inside our bodies

“The biggest hurdle facing basic and clinical endocrinologists is how we can measure hormones inside the body,” said one of the symposium organizers, Timothy Wells, Senior Lecturer in Neuroscience at Cardiff University.

Addressed by UK and international experts in hormone-receptor interactions, light-based sensing and nanocarbon-based sensing, the Society’s symposium explored how these molecular interactions could be exploited to quantify the dynamic changes in circulating hormone levels.


Cutting edge work featured

One of the potential approaches was presented by Frank Vollmer and his colleagues from the Max Planck Institute for the Science of Light. He and his team are attempting to reach the ultimate limit of detection, by sensing single molecule interactions and the resultant changes in three-dimensional shape. Their new technique, which was published this week in Nature Photonics, may enable the detection of individual hormone molecules.

Thus, the day of talks highlighted just how far we’ve come since Ernest Starling coined the term hormone in 1905.

The event, titled “Novel approaches to Hormone Sensing, The Inaugural Bayliss-Starling Symposium,” was part of the society’s H3 symposia. The next symposium will be held on 15 November about one of the biggest discoveries in biotech, CRISPR. Visit our website for more info:

Prize Lecture Memoria – Edward Sharpey-Schafer

M0020242 Portrait of Sir E.A. Sharpey-Schafer

Sir Edward Albert Sharpey-Schafer (1850 –1935) was an English physiologist and Fellow of the Royal Society. Born Edward Schäfer, he studied under the physiologist William Sharpey and became the first Sharpey Scholar in 1873 at University College London (UCL). In 1874 he was appointed Assistant Professor of Practical Physiology at UCL where he went on to become Jodrell Professor. He was elected a Fellow of the Royal Society in 1878 at the age of just 28. Schäfer was appointed Chair of Physiology at the University of Edinburgh in 1899 where he would stay until his retirement. He was one of the nineteen founder members of the Physiological Society in 1876 and he also founded and edited [the Quarterly Journal of] Experimental Physiology from 1908 until 1933. Schäfer was knighted in 1913. He is renowned for his invention of the prone-pressure method or Schäfer method of artificial respiration. He was very active as a facilitator, mentor, coordinator, teacher and organiser through much of his career. He had started as a histologist and always emphasised the importance of structural knowledge. He was the co-discoverer (in 1894, with George Oliver) of adrenaline (as in the adrenal-derived, circulating hormone) and he coined the term ‘endocrine’ as the generic term for such secretions. He intuited (as did a few others, independently) that insulin must exist (i.e. a pancreatic hormone to account for diabetes mellitus) and coined the name (originally as ‘insuline’). (Banting and Best actually discovered what S-S and the others had predicted). Thus, he had a founding role in modern endocrinology. He also did important early work on the localisation of function (e.g. motor centres) to brain regions. After the death of his eldest son, John Sharpey Schafer, and in memory of his late professor William Sharpey, he changed his surname to Sharpey-Schafer in 1918. Sir Edward Albert Sharpey-Schafer died on 29 March 1935 aged 84. Funded by bequests from Sir Edward Sharpey-Schafer (1850–1935) and his daughter Miss GM Sharpey-Schafer and in memory of Sir Edward and his grandson Professor EP Sharpey-Schafer, The Physiological Society established the Sharpey-Schafer Prize Lecture. This is a triennial lecture given alternately by an established physiologist (preferably but not necessarily from abroad) and a young physiologist chosen by The Society.

Researcher in the Spotlight June 2016

Lisa at Merton

Dr Lisa Heather PhD, is a Diabetes UK RD Lawrence Fellow in the Department of Physiology, Anatomy and Genetics, University of Oxford. Her research revolves around metabolism and energy generation in the heart.

Lisa will give The Physiological Society Bayliss-Starling Prize Lecture ‘Cardiac metabolism in disease: All fuels are equal, but some fuels are more equal than others’ at our main meeting P16 in Dublin, Sunday 31 July 9:00 am.



What is your research about?

I study energy metabolism in the heart. Metabolism explains how we extract energy from the fuels we eat: how we convert glucose and fatty acids into ATP via a series of chemical reactions within the cell. When this process goes wrong the cell can become starved of energy, and ATP dependent processes – such as contraction – will be impaired. Abnormal cardiac energy metabolism occurs in a large number of diseases, including diabetes and heart failure. Understanding why these metabolic abnormalities occur and whether changing metabolism is beneficial for cardiac function is my area of research.

How did you come to be working in this field and was this something you always wanted to do?

My undergraduate degree was in Medical Biochemistry at the University of Surrey, and I had an amazing lecturer, Dr Jack Salway, teaching metabolism. He made the subject exciting and relevant, and made me want to pursue it further to become a ‘die-hard metabolist’. I moved to Oxford in 2003 and joined the lab of Professor Kieran Clarke, studying the effects of disease on cardiac metabolism. Kieran was (and still is) an excellent mentor, providing support whenever I needed it, but equally allowing me freedom to explore my own directions and stand on my own two feet.

When I first started in the field of metabolism it wasn’t a particularly fashionable field – everyone was focused on genetics, and metabolism was viewed as a subject where all the questions had already been answered. Scientific fashions change, and in the last 10 years metabolism has had a huge renaissance, mainly driven by discoveries in the cancer field. It’s an exciting time to be working in this area, new collaborations are emerging between diverse fields that have realised metabolism is influencing or being influenced by their disease or cellular process. Suddenly, having a good understanding of the fundamentals of metabolism is a powerful tool.

I have never considered leaving the field of metabolism as it’s the area I love, and when I set up my own group in 2011 I decided it was the field of diabetes, the ultimate metabolic disease, that I wanted to specialise in.

Why is your work important?

Metabolism underpins all cellular processes. It provides ATP for all active processes to occur, it provides the building blocks and intermediates for diverse chemical reactions, and provides substrates for post-translational modifications. Changes in metabolism have been implicated in many diverse diseases of all organs in the body. As stated by Steven McKnight in Science in 2010 “One simple way of looking at things is to consider that 9 questions out of 10 could be solved without thinking about metabolism at all, but the 10th question is simply intractable…. if you are ignorant about the dynamics of metabolism”.

Do you think your work can make a difference?

I really hope so. Understanding how a disease develops and progresses is the first step to working out how to prevent or reverse it.

What does a typical day involve?

A typical day can involve any combination of lab work, discussing data with students, planning new studies, writing and rewriting papers, teaching undergrads, and meetings. Each day is different and that’s one of the things I really enjoy about being an academic.

What do you enjoy most in your job?

I love the ‘Aha!’ moments. When you have been busy trying to work out why something has changed or the mechanism involved, and suddenly everything fits together and makes sense. When you have discovered something, however small, that wasn’t known before. It reminds me of those “magic eye” pictures, when you stare at it long enough that the blurry 2D pattern finally turns into a beautiful 3D image. The “Aha” moments are the reward for all those times the experiments didn’t work.

 What do enjoy the least?

On a day to day basis, I really hate having to collect liquid nitrogen from our outside cylinder! It’s the worst job! I generally really love my job and feel grateful that I get to do this every day.

Tell us something about you that might surprise us…

I really really really like designer shoes. If only Manolo Blahnik could make mitochondria-inspired pumps!

What advice would you give to students/early career researchers?

Do what you love. Being a scientist is a tough career, so you have to love it to deal with the challenges, such as paper rejections and lack of job security. Have faith in your own abilities. Be nice to people and help people when you can, people are then more likely to come to your assistance when you need them. Smile :)!