Biochemical Foundations of Human Vital Activity - Volkov, N.I., Nesen, E.N. 2000

Biochemical Foundations of Human Vital Activity
Hormones as Metabolic Regulators
Mechanism of Hormone Action

There are Three Main Mechanisms of Hormonal Regulation of intracellular METABOLISM and Cell Functions through Changes in the rate of Protein Synthesis, The activity of intracellular metabolic Enzymes, and the permeability of cell membranes to various ions, metabolites, and Coenzymes.

Regulation of the Rate of protein synthesis. This action is exerted by steroid and THYROID Hormones: they penetrate The Cell and interact with specific receptors. The hormone-receptor complex enters The Nucleus, binds to Chromatin, and increases The rate of protein synthesis at the Gene level (Fig. 51). Active genes enhance the Synthesis of specific RNA, which leaves the nucleus, reaches the Ribosomes, and triggers the synthesis of new Proteins—which can be structural or contractile proteins of Muscles and other Tissues, as well as enzymes or hormones. This constitutes their anabolic effect. However, the rate of Protein synthesis in Cells is a relatively slow process, as it requires a large amount of energy and building blocks. Therefore, such hormones cannot provide rapid control over metabolic processes. Their primary function is to regulate the processes of growth, development, and Differentiation of the body's cells.

In athletes' bodies, Steroid Hormones enhance recovery processes and long-term biochemical adaptation. They promote Muscle mass growth and improve human strength qualities.

Introduction/15.html">Regulation of enzyme Activity. Certain protein-Peptide Hormones, as well as adrenaline and noradrenaline, do not penetrate inside cells. They regulate metabolism and the activity of many intracellular enzymes indirectly via secondary messengers, which may include Cyclic NUCLEOTIDES (cAMP, cGMP), Calcium Ions, diacylglycerol, Inositol, specific proteins, and other substances.

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Fig. 51 MECHANISM OF ACTION of steroid and Other Hormones that penetrate cell membranes

Hormones bind to specific receptors on the cell surface membrane and activate the G-protein located on the inner side of the membrane. This protein activates or inhibits the activity of the enzyme adenylate cyclase. Adenylate cyclase catalyzes the synthesis of cyclic AMP from ATP (Fig. 52). The action of cAMP (the "secondary messenger") inside the cell is realized through another enzyme, protein kinase (PK), which is inactive in the absence of cAMP. Subsequently, the cAMP-activated protein kinase catalyzes The transfer of phosphoric acid residues from ATP to various protein molecules within the cell. Enzymes responsible for The breakdown of fats, CARBOHYDRATES, and other body systems may undergo phosphorylation. In this case, ATP Synthesis in the cell is enhanced, the number of protein synthesis enzymes increases, and the functional activity of the cell changes. Cyclic AMP is degraded by the enzyme phosphodiesterase, resulting in the Termination of the hormone's action.

The regulation mechanism via secondary messengers is highly effective, as it significantly amplifies the hormonal signal and ensures a rapid biological response of the cell to an increase in hormone concentrations in the Blood. The adenylate cyclase system largely determines the activation of immediate mechanisms for restructuring intracellular metabolism under various influences, including physical exertion, and participates in providing rapid biochemical adaptation. As the body's fitness increases, the adenylate cyclase system of hormonal signal Transduction in skeletal muscles improves, which is manifested by an increased sensitivity of the adenylate cyclase enzyme to hormones. This allows for a more subtle regulation of intracellular metabolism during minor fluctuations in blood hormone levels.

Fig. 52 Action of Hormones on intracellular processes via secondary messengers (cAMP and other Transmitters)

The hormonal signal can be transmitted via the guanylate cyclase system, which synthesizes the intracellular messenger cGMP from GTP. The opposing effects of the same hormone can be mediated by different messengers—cAMP or cGMP.

The action of hormones on intracellular processes can also be mediated by calcium ions (Ca²⁺). When a hormone interacts with its receptor, transport systems for Ca²⁺ ions across the membrane are activated. Calcium enters the Cell Cytoplasm from the external environment or from intracellular stores. It binds to Ca²⁺-dependent proteins, one of which is calmodulin. The Ca²⁺-calmodulin complex enhances the activity of various intracellular enzymes, leading to altered biochemical processes and physiological functions.

Regulation of membrane permeability. Certain hormones and Neurotransmitters alter cell membrane permeability to a range of metabolite substances. An example is Insulin, which, upon binding to a receptor on The Plasma Membrane, drastically increases membrane permeability to glucose, Amino Acids, and certain ions, and enhances their influx into the cell. The Molecular Basis of this effect is not yet fully understood. Nevertheless, the enhanced uptake of specific substances into the cell affects biochemical processes, while the redistribution of ions across the membrane affects the cell's electrical potential and its contractile function.