The chief constituent of the rat heart and the hearts of other mammals is cardiac muscle. Though striated in appearance, cardiac muscle controls an involuntary activity, the beating of the heart, whereas the contraction of most striated muscles is voluntary. Each cardiac muscle cell contains a single nucleus and will contract in a steady rhythm in the absence of any input. When multiple cardiac cells are in contact with one another, the first cell to contract instigates the contraction of the others. The rate at which the heart contracts is typically determined by specialized pacemaker cells, which are regulated by chemical signals from the nervous system.

The rhythmic contraction of the heart muscle pushes blood through its chambers and into connecting blood vessels. Oxygenated blood is transported away from the heart to other regions of the body via arteries, while veins are responsible for carrying oxygen-depleted blood to the organ. The sinoatrial node controls the rhythm of the heart and is commonly referred to as the pacemaker. On any given day, the rhythm of the heart may change several times, generally slowing when an individual is relaxed and at rest and speeding up during exercise or moments of anxiety.

The hearts of laboratory rats are commonly used for studies of human heart diseases and conditions. They also are heavily utilized in research on the affects of various materials on heart muscle, including such diverse substances as garlic, insulin, calcium, and adenyl compounds. Information gained from work with the laboratory animals has been critical to the development of the modern understanding of the mammalian heart and may eventually lead to better preventatives and treatments for human heart problems. For example, in late 2004 heart researchers successfully utilized gene therapy to restore failing rat hearts back to normal activity levels, an achievement that could be an important step on the way to developing gene therapy protocols for human patients with chronic heart failure.

The sample of rat heart tissue presented in the digital image above was labeled with Tetramethylrhodamine conjugated to wheat germ agglutinin, a lectin that selectively binds to N-acetylglucosamine and sialic acid residues, and with Alexa Fluor 488 conjugated to phalloidin, a phallotoxin that binds filamentous actin. The specimen was counterstained for nuclear DNA with 4',6-diamidino-2-phenylindole (DAPI). Images were recorded in grayscale with a 12-bit digital camera coupled to either a Nikon E-600 or Eclipse 80i microscope equipped with bandpass emission fluorescence filter optical blocks. During the processing stage, individual image channels were pseudocolored with RGB values corresponding to each of the fluorophore emission spectral profiles.

Rat Heart Tissue Labeled with Alexa Fluor 568 Conjugated to a Phallotoxin and Wheat Germ Agglutinin - A common animal model for various human maladies, the rat has been widely employed in the study of coronary heart disease, the single leading cause of death in the United States today. Coronary heart disease is characterized by an insufficient supply of blood to the myocardium, usually due to the build up of plaques in the arteries.