© 2000 by European Society of Cardiology
Copyright © 2000, European Society of Cardiology
The elusive coypu: the importance of collateral flow and the search for an alternative to the dog
Cardiovascular Research, Rayne Institute, St Thomas’ Hospital, London SE1 7EH, UK
* Tel.: +44-171-922-8133; fax: +44-171-928-0658
KEYWORDS Blood flow; Collateral circulation; Coronary circulation; Coronary disease; Infarction; Ischemia
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1 Introduction |
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 Top 1 Introduction 2 Infarct size reduction?...3 It's negative, we...4 Understand your model...5 Collaterals -- the...6 Problem solved? The...7 Defining species difference...8 So what about...References |
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There are several ‘villains’ in this story: (i) researchers who convinced themselves that myocytes could stay alive without blood; (ii) authors who discarded (or journal editors who refused to publish) negative studies; (iii) dogs that had too much and too variable a collateral flow; (iv) legislation and animal suppliers that made the use of canine preparations (and an intriguing alternative, see Fig. 1) impossible or prohibitively expensive; and (v) a UK government plot to exterminate the coypu (nutria).
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2 Infarct size reduction? no problem! |
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Top1 Introduction2 Infarct size reduction?...3 It's negative, we...4 Understand your model...5 Collaterals -- the...6 Problem solved? The...7 Defining species difference...8 So what about...References |
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For anyone interested in myocardial injury and protection, the 1970s and 1980s were both confusing and exciting. Almost every copy of every journal contained a new paper in which yet another drug reduced infarct size in a canine model of coronary artery occlusion and myocardial infarction. The interventions ranged from the well known (beta blockers, calcium antagonists, vasodilators and glucose) through the surprising and sometimes disasterous (steroids) to the bizarre (rutosides, cobra venom and hyaluronidase). Opinion leaders in the field vigorously promoted the concept that just because myocardial tissue lay within the distribution of a recently occluded coronary artery did not mean that it was necessarily condemned to death — despite the absence of reperfusion. For those of us who believed that early reperfusion was an absolute prerequisite for the salvage of severely ischemic tissue this seemed like heresy!
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3 It's negative, we won’t publish it.... unless it's a clinical trial! |
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Top1 Introduction2 Infarct size reduction?...3 It's negative, we...4 Understand your model...5 Collaterals -- the...6 Problem solved? The...7 Defining species difference...8 So what about...References |
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Astounding claims resounded from the dog labs but, despite this, clinical trials were consistently disappointing and no single intervention was ever adopted for widespread clinical use as an anti-infarct agent in patients with myocardial infarction. Clearly, something was wrong — possibly the design of the trials, the choice of experimental models or the concept upon which tens of thousands of experimental animals (mostly dogs) met an untimely end. There were, in fact, many negative studies but these were often consigned to the sponsors’ archives or were very difficult to get published and those that were published usually appeared in lesser journals. However, one laboratory in particular (that of Robert Jennings and Keith Reimer) did manage to publish negative studies in reputable journals and indeed, with a few exceptions, every drug they studied appeared to have no beneficial effect on infarct size.
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4 Understand your model and its pathology before you use it! |
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Top1 Introduction2 Infarct size reduction?...3 It's negative, we...4 Understand your model...5 Collaterals -- the...6 Problem solved? The...7 Defining species difference...8 So what about...References |
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With hindsight we now know that the Jennings and Reimer laboratory was one of the few that: used adequate numbers of animals, measured infarct size after several days, used a morphological assessment of infarct size and took full account of the baseline variables influencing the rate and extent of infarction. Ironically the understanding of the vital importance of these baseline variables (particularly collateral flow) received major impetus from a well publicised case of scientific fraud that came to light in a multi-centre canine infarct size reduction trial (the AMPIM study). This trial [1] rapidly became a landmark publication, representing the cornerstone of our understanding of the factors which determine the amount of tissue that will infarct within an ischemic risk zone. It precipitated some long overdue realism in relation to the (in)ability of drugs to protect against cell death in severe sustained ischemia. Researchers began to acknowledge the key distinction between the undoubted ability of some drugs to slow the rate of cell death as opposed to their questionable ability to reduce its ultimate extent [2–4]. To make the critical distinction between injury slowing and injury delaying required eventual infarct size to be measured several days after an episode of ischemia and reperfusion (as Jennings and Reimer) and not after a few hours as practised by many investigators.
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5 Collaterals — the key to the problem |
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Top1 Introduction2 Infarct size reduction?...3 It's negative, we...4 Understand your model...5 Collaterals -- the...6 Problem solved? The...7 Defining species difference...8 So what about...References |
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Of the many variables that can influence the rate of evolution of infarction the extent and distribution of collateral or residual flow delivered to the ischemic zone is undoubtedly the most important. Unfortunately, the dog (which was the favoured species of most infarct size reducers) has a very variable and often substantial collateral flow (see Fig. 2) — particularly in the case of the ‘failed greyhound’ (regular losers at the greyhound race track who were wanted no more!). This strain, with its enormous hypertrophied heart, often exhibited collaterals capable of providing 30% or more of normal flow to the perfusion bed of the LAD despite a total occlusion. The variability in collateral flow (with high levels slowing down the infarction process greatly), the failure to distinguish injury slowing versus injury preventing and the frequent use of inadequate animal numbers often made the outcome of studies more a matter of chance than good science. The need to interpret results in the light of collateral flow gave rise to the ‘Reimer-gram’ — the acid test of the ability of a drug to reduce infarction. As shown in Fig. 3, collateral flow in a typical dog heart study may vary between 2% and 40% and infarct size may occupy 90% to 40% of the area at risk. It is only on the rare occasions when the regression line for the treated group falls significantly below that of the controls that drug-induced infarct size reduction can be claimed with any degree of confidence.
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6 Problem solved? The challenge of the $1000 dog |
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Top1 Introduction2 Infarct size reduction?...3 It's negative, we...4 Understand your model...5 Collaterals -- the...6 Problem solved? The...7 Defining species difference...8 So what about...References |
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The application of rigorous methodology and the distinction between delaying and reducing infarction led to a severe decline in the number of papers claiming miraculous drug-induced infarct size reductions. At last we were in a position to study infarct size limitation properly — but unfortunately, for most investigators animal legislation and soaring costs now made canine studies impossible. The search was on for an alternative to the dog — pigs were used quite often and offered the advantage of being devoid of collaterals (see Fig. 4) thereby effectively eliminating the most troublesome variable from the experiment and at the same time (not surprisingly), leading to many negative studies. However, pigs are not the most delightful of experimental subjects, anaesthesia and arrhythmias can be a problem and costs were again very high. Surely small mammalian hearts could be used for the rapid, cheap and effective screening of the ability of drugs to limit the evolution of infarction.
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7 Defining species difference in collateral flow |
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Top1 Introduction2 Infarct size reduction?...3 It's negative, we...4 Understand your model...5 Collaterals -- the...6 Problem solved? The...7 Defining species difference...8 So what about...References |
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Many workers switched to the rat and rabbit for their studies, some even undertook infarct size studies in the mouse. However, before the St Thomas’ group were prepared to switch from our previously used dog, pig and baboon models, we felt it imperative to be fully aware of the cardiac collateral status of various small mammalian heart. Hence the study cited in this special issue of Cardiovascular Research in which we characterised the extent and the distribution of collateral flow within zones of regional ischemia in isolated perfused hearts from the guinea pig, dog, cat, rat, ferret, baboon, rabbit and pig. In the light of the classical work by Schaper [5], our findings (Fig. 5) were not unexpected with the dog and the cat having extensive collateral connections which delivered flow preferentially to the epicardial tissue, thus explaining, in part, the greater vulnerability of the endocardium to necrosis and the phenomenon of the ‘wave front of cell death’ [6]. Interestingly, the guinea pig heart was found to be totally collateralised making it impossible to induce infarction in this species — how great it would be if the human had the coronary artery anatomy genes of the guinea pig!
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8 So what about the coypu? |
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Top1 Introduction2 Infarct size reduction?...3 It's negative, we...4 Understand your model...5 Collaterals -- the...6 Problem solved? The...7 Defining species difference...8 So what about...References |
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Whilst small mammalian hearts can undoubtedly be used as a fast and economical test bed for assessing infarction, they do suffer from the difficulty of reliably measuring flow (and its all important regional distribution) in a pretty small piece of meat. Surely, we thought, there must be a low cost, large, readily available mammalian heart. Eureka! What about the coypu?
In 1939, myocastor coypus molina (known in some countries as nutria) was introduced from South America to the UK for fur farming — a giant rat-like beast (an aquatic rodent, related to the guinea pig and the porcupine) about 60 cm long and weighting up to 9 kg (see Fig. 1). By 1939 there were 40 coypu farms in the UK and inevitably some inmates escaped. By the 1960s they had infested much of the UK, ravaging crops and done great damage to river banks. At this stage the government instituted a control and eradication programme — it became illegal to own or breed a coypu but surely we could use them for infarct sizing? Just imagine the potential — big hearts, easy to breed (gestation period of 132 days, litters of up to 9 born with their fur and their eyes open) and inexpensive, maybe even free from the trappers who were receiving a bounty for their capture. Enter bureaucracy — transporting a coypu turned out to be more difficult than moving a nuclear warhead or international terrorist through the centre of London. Thus, despite relentless efforts, we were never able to study the collateral circulation of the coypu — for better or for worse, the coypu is now eliminated from the UK (other than a stuffed one in a Cambridge museum!) and the cardiological community has been spared what might have been a series of papers in Cardiovascular Research extolling the virtues of the coypu for studies of infarct size.
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References |
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Top1 Introduction2 Infarct size reduction?...3 It's negative, we...4 Understand your model...5 Collaterals -- the...6 Problem solved? The...7 Defining species difference...8 So what about...References |
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- Reimer K.A., Jennings R.B., Cobb F.R., et al. Animal models for protecting ischemic myocardium: results of the NHLBI cooperative study. Circ Res (1985) 56:651–665.
[Abstract/Free Full Text] - Hearse D.J. Calcium antagonists and cardiovascular disease. Opie L.H, ed. (1984) New York: Raven Press. 129–145.
- Hearse D.J. The protection of the ischemic myocardium: surgical success versus clinical failure? Progress in Cardiovascular Diseases (1988) 30:381–482.[CrossRef][Web of Science][Medline]
- Hearse D.J., Yellon D.M. Therapeutic approaches to myocardial infarct size limitation. Hearse D.J., Yellon D.M., eds. (1984) New York: Raven Press. 17–41.
- Schaper W. Therapeutic approaches to myocardial infarct size limitation. Hearse D.J., Yellon D.M., eds. (1984) New York: Raven Press. 79–90.
- Reimer R.A., Jennings R.B. The ‘wavefront phenomenon’ of myocardial ischemic cell death. Transmural progression of necrosis within the framework of ischemic bed size (myocardium at risk) and collateral flow. Laboratory Investigation (1979) 40:633–644.[Web of Science][Medline]
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