David R. Gross's research while affiliated with University of Illinois, Urbana-Champaign and other places

Most naturally occurring congenital cardiovascular entities found in humans have been identified in one or more species of animals but the utility of these naturally occurring models as research subjects is not well established. Many of the congenital diseases are associated with noncardiovascular defects and some of these may result in infertility, impotence, and other reproductive problems that preclude the breeding of these animals to obtain adequate numbers for research purposes. The advent of sophisticated genetic testing has made the identification of specific genes responsible for specific defects more practical, and this has led to the creation of specific transgenic animal models, knock-ins and knock-outs, that have advanced our understanding of both congenital defects and genetic predisposition for a variety of cardiovascular diseases.

The reader is referred to two standard veterinary textbooks for in-depth information about the pharmacodynamics, pharmacokinetics, and practical use of anesthetic and analgesic agents in animals.1, 2 The preanesthetic, anesthetic, chemical restraint, and analgesic regimens cited in this chapter have received approval from an Institutional Animal Care and Use Committee (IACUC) or similar body with the same responsibilities or have been published by board certified veterinary anesthesiologists. There is increasing pressure, primarily a response of Institutional Animal Care and Use Committees to external pressure, to require that any procedure conducted in animals that could be considered to cause pain or distress to a human be conducted with the benefit of anesthesia or analgesia. This policy can and does result in some anesthetic deaths, stress associated with the anesthetic procedure, and/or confounding drug-induced changes in the animal that may be more severe than the procedure to be conducted. In some instances, proper prior training and acclimation of the animals, particularly of the larger species, to the procedure can result in fewer problems than tranquilizing, sedating, or anesthetizing the animal. It might still be necessary to provide chemical restraint for a variety of reasons including humane considerations. For most circumstances, the same agents used for anesthesia and/or analgesia can be used for chemical restraint.

The normal cardiovascular system is a finely tuned pump and delivery system functioning together, with optimal efficiency, to match cardiac output to the integrated metabolic demands of the entire corpus. The inherent physical properties of the two systems contribute to the efficiency, and the physiological coordination of central and local control mechanisms assure that all tissues are supplied with appropriate blood supply. Under normal circumstances, local demands for blood flow are met by locally controlled adjustments and diversion of flow to metabolically active tissues without the need to increase pump function. When metabolic demands increase significantly, cardiac output must increase to meet the demand. If the heart is unable to meet the demand, the corpus is, by definition, in heart failure. The problem for the physician treating patients and for the researcher studying the mechanisms and/or the treatment of cardiovascular disease is to be able to understand and quantify changes in or loss of function.

Studies investigating the integrative central control of the locomotor and cardiovascular system have mostly been conducted in rats. These studies have shown that control of cardiovascular responses is located in neurons in close proximity, if not overlapping or possibly identical to, neurons responsible for respiratory and locomotor control. In rats cardiorespiratory and locomotor centers have been identified in the periaqueductal gray (PAG), posterior hypothalamic area (PHA), nucleus tractus solitarius (NTS), rostral ventrolateral medulla (rVLM), and the cuneiform nucleus (CnF). Of these, the PH has been clearly identified as both a locomotor and cardiovascular center.1 The CnF, with the pedunculopontine nucleus, has been identified as the mesencephalic locomotor center.2,3 The spinal cord and the lateral tegmental field (LTF) have been identified as integration sites for cardiorespiratory and locomotor responses.4 Interestingly, exercise training induced attenuation of dendritic fields of neurons in the exercising rat model.1

Ischemic heart disease is the most common cause of heart failure in humans. Ventricular dilation, hypertrophy, biochemical alterations, and edema formation are all consequences of the poor pumping capacity of the damaged myocardium. Two very different types of ischemia are studied. Global ischemia is associated with cardiac arrest usually iatrogenic during cardiopulmonary bypass surgery or from ventricular fibrillation although during the initial period of fibrillation coronary flow increases. Regional ischemia is associated with a localized myocardial infarction. The two types of ischemia differ significantly in their biochemical and electrophysiological characteristics.1

The normal values found in this chapter are provided to serve as a quick and easy reference. Investigators might find these values useful to be able to compare to the data they acquire. Manuscripts that report results from animals or preparations significantly different than these ranges could be problematic (Tables 3.1-3.6).

Previous chapters have described a large number of transgenic animal models used to study specific cardiovascular syndromes. This chapter will fill in some gaps. Many of these transgenic animals were developed to study normal and/or abnormal physiological responses in other organ systems, or to study basic biochemical and molecular reactions or pathways. These models were then discovered to also have effects on the cardiovascular system, some of them unanticipated. A word of caution, particularly when highly inbred mouse strains are used to develop transgenic models - not all strains of a particular species are created equal. When cardiovascular parameters of age- and sex-matched A/J and C57BL/6J inbred mice were compared the C57BL/6J mice demonstrated eccentric physiologic ventricular hypertrophy, increased ventricular function, lower heart rates, and increased exercise endurance.1

Human essential hypertension is a multifactorial and complex disease involving several genes that have, thus far, defied complete characterization. Congenic models such as the spontaneously hypertensive stroke-prone rats are genetically homogeneous and thus aid the search for causative genes. The identification of quantitative trait loci (QTL) responsible for blood pressure regulation by genome-wide scanning is most commonly employed;1 however, the identification of a QTL is just the first step to identify the gene(s) of interest. Congenic strains must be produced to verify the QTL and identify the chromosomal region that must then be reduced to an appropriate size for positional cloning of the gene(s). So-called “speed congenic strategies” have been used to confirm blood pressure QTLs on rat chromosome 2.2 The number of articles published each year suggests that the various animal models of hypertension are probably the most used in cardiovascular research. The volume of significant, i.e., leading to effective treatment of the human condition, research conducted using these models verifies their importance.

Animals used in meaningful experiments must be held long enough in approved facilities to insure that they are not incubating infectious diseases. If the animal care facility does not require this, responsible investigators must still make certain these precautions are taken. Animals should be vaccinated, or tested and certified to be free of the diseases most likely to cause a problem in that species or that can be transmitted to personnel working with them. They should be verified free of both internal and external parasites. Prior to use they should be given a complete physical exam. Normal physiological parameters for the most commonly used species are in this chapter. Depending upon the experimental design, it might be prudent to collect blood samples and establish normal hematological and/or serum enzyme levels for each individual animal used. Although these precautions can be time consuming and expensive, the costs are minimal when compared with the overall cost of conducting experiments where the results are suspect because the physical condition or general health of the subject animals is a problem. One of the most neglected aspects of a physical examination conducted on animals used in research protocol is a history. Significant information can be gleaned by talking with observant animal care personnel about the appetite of the animal(s) and the character of the urine and feces. It is the responsibility of the principle investigator to ascertain that the animal care personnel are, in fact, observant and have been properly trained to record their observations appropriately for each individual animal or cage of grouped animals.

Cardiomyopathy is a general term applied to a wide variety of conditions that result in myocardial lesions not related to specific disease states. The term encompasses a wide variety of conditions initiated by numerous etiologies. The disease usually presents as either hypertrophic or dilated cardiomyopathy (HCM or DCM). Naturally occurring cardiomyopathy seems to be caused by mutations in one or more sarcomeric proteins. Inherited DCM can result from mutations in the genes encoding cardiac troponin T, troponin C, and alpha-tropomyosin, while different mutations in the same genes cause HCM. DCM mutations depress myofibrillar function as a result of thin filament mutations while HCM has the opposite effect.1 Familial HCM is described as an autosomal dominant disorder that manifests as cardiac hypertrophy with myocyte disarray. Mutations in five different loci result in the disease. Beta cardiac myosin heavy-chain, alpha tropomyosin, and cardiac troponin T have been identified as distinct disease genes.2–6 Liu et al.7 compared the morphological features of spontaneously occurring HCM in 38 humans, 51 cats, and 10 dogs. They found that asymmetric hypertrophy of the ventricular septum, marked disorganization of myocardial cells, abnormal intramural coronary arteries, and myocardial fibrosis were common to all three species. Transgenic mice expressing cardiac troponin T (cTnT)-Q92 develop HCM characterized by enhanced systolic function and have higher levels of cardiac troponin I, cardiac alpha-actin, cardiac alpha-tropomyosin, and cardiac troponin than wild type.8