Introduction

Although this chapter is devoted to a discussion of myocardial metabolism, it should be understood that all energy-requiring processes in the biosphere are dependent on capture of solar energy and its entrapment in molecules that can be used to fuel biological processes such as cardiac contraction. The synthesis by plants of molecules such as glucose from CO2 and H2O is powered by the sun via the process of photosynthesis. Photons radiated by the sun are trapped by chlorophyll and used to drive the synthesis of adenosine triphosphate (ATP). ATP has two phosphate bonds with high levels of stored chemical energy (Fig. 1).

The energy released by breakdown of these high-energy phosphate bonds is captured and used to drive the synthesis of glucose. Hence, a portion of the energy derived from the sun is ultimately stored in the form of chemical bonds resident in the chemically stable glucose molecule. The chemical energy stored in glucose can be released in a controlled fashion by enzyme-catalyzed reactions to drive the synthesis of other species of molecules, including fatty acids (another convenient storage molecule for chemical energy), as well as for resynthesis of ATP.

Because animals are unable to convert solar energy to a storable form of chemical energy, all animal life is ultimately

From: Handbook of Cardiac Anatomy, Physiology, and Devices Edited by: P. A. Iaizzo © Humana Press Inc., Totowa, NJ

powered by the photosynthetic process in plants. The ingestion of plants supplies animals with usable forms of stored chemical energy. Even carnivores ultimately depend on energy storage compounds that have passed up the food chain from plants.

The general aim of this chapter is to discuss processes involved in the transfer of chemical bond energy contained in the ingested energy storage molecules that we call food into ATP, which is the common energy currency of the heart and all other biological tissues.

In the myocardium, as in other biological tissues, most energy-requiring processes use ATP as the immediate source of energy. Hydrolysis of the terminal phosphate bond of ATP releases energy that can be captured and used to drive energy-requiring processes, such as protein synthesis, muscle contraction, and ion transport. The ability of the heart to pump blood to the pulmonary and systemic circulations is dependent on the availability of ATP. Because tissue stores of ATP are very modest, continuous synthesis is required to maintain ATP levels sufficient to drive energy-requiring processes. The energy required for ATP synthesis is derived from the controlled breakdown of the chemical bonds in carbohydrates (glucose) and fatty acids.

The purposes of this chapter are to: (1) describe mechanisms for delivery of carbon substrate and oxygen to the heart; (2) describe the biochemical pathways that transfer chemical bond energy in carbon substrates to ATP in the normal heart; and (3) discuss some of the alterations in these processes that

Fig. 1. Adenosine triphosphate (ATP) is the major source of chemical energy used to power the reactions that support contractile and other processes in myocardium and all other tissues. The ~ symbol is used to designate a phosphate bond with a very high level of stored chemical energy. Although ATP contains two ~ phosphate bonds, only hydrolysis of the terminal phosphate bond releases the energy that directly powers cellular processes.

Fig. 1. Adenosine triphosphate (ATP) is the major source of chemical energy used to power the reactions that support contractile and other processes in myocardium and all other tissues. The ~ symbol is used to designate a phosphate bond with a very high level of stored chemical energy. Although ATP contains two ~ phosphate bonds, only hydrolysis of the terminal phosphate bond releases the energy that directly powers cellular processes.

occur in the diseased heart. It should be understood that this relatively brief summary of myocardial metabolism is of necessity a superficial overview of the individual topics discussed. A list of references (primarily current topical reviews and research reports, plus several standard texts) is provided at the end of the chapter for more detailed review of the major topics presented here.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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