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The book describes a method of transferring Information within vertebrates. Such communication is necessary in order to coordinate physiological processes with each other and to the happenings in the external environment. Even unicellular organisms synchronize their various internal life processes. In such small creatures, however, local accumulations of metabolites may exert a direct control on biochemical reactions, whereas external Stimuli have relatively widespread effects so that specialized pathways for communication may not be as necessary. Therefore, when the distances involved are short, physical processes such as conduction, convection, and diffusion may be adequate for the integration of the physiological processes. Nevertheless, even unicellular organisms possess specific coordinating Systems such as that seen in the protozoan Tetrahymena (Blum, 1967), which possesses epinephrine. This hormone has similar metabolic actions in this protozoan to those that it has in vertebrates.
The problems of communication and coordination are greater in multicellular than in unicellular organisms. There are several reasons for this, especially their larger size. As the linear distances between the different parts of an animal increase, simple physical Communications become relatively slower and less precise, and so not as effective. In multicellular organisms, the cells are usually specialized and perform different functions that, in combination, are essential for the animal's life. Thus, some tissues may be concerned with the formation of reproductive germ cells, several others with the preparation of suitable nutritive materials, and yet others with building morphological structures. The ultimate successful completion of these processes will be determined by the effectiveness of the communication between the tissues themselves and the external environment.
The transfer of information in animals
There are three principal ways by which cells in multicellular organisms can communicate with each other. First, when they are in close juxtaposition and are only separated by narrow fluid-filled Spaces, direct electrical and chemical interactions can occur. Cells also maintain some structural connections with each other and secrete Special excitants by which they may also communicate.
In a mammal the endocrine glands secrete more than 40 distinct hormones. In addition, different species may form many hormones that although structurally analogous nevertheless display chemical differences. Such natural variants are usually characteristic of a single species and represent a polymorphism of the excitant's molecular structure. This change has a genetic basis. It may only be the Substitution of a single amino acid residue in the molecule of a peptide hormone or it may be much more extensive. The biological effects of such differences can be considerable or negligible.
Vertebrate hormones belong to two principal classes of chemical compound. Some are made from cholesterol. These are the steroid hormones from the adrenal cortex and the gonads. The others are made up of amino acids and range in complexity from those, like epinephrine, that are derived from a single tyrosine molecule, to others like the pituitary growth hormone that contain about 190 such units. The molecular weights can vary from about 200 to 30 000.
What properties do these molecules have that make them suitable to be hormones? What characteristics may be important for their utilization as such? Armed with considerable hindsight about endocrine physiology some answers can be offered. The basic requirements will not be the same for all hormones but will depend on what they do. The steroid hormones are poorly soluble in water but readily soluble in lipids. This will facilitate their penetration into the cell and fixation at intracellular sites. Such lipid solubility will also be important if a hormone is to penetrate the blood-brain barrier. Transport in the blood is essential for a hormone to fulfill its physiological role so that, if they are hydrophobic molecules, they must either be effective at very low concentrations or be attachable to protein components that carry them to their sites of action. This binding is especially prominent among the steroid and thyroid hormones but also may involve proteinaceous hormones. An ability to interact with other biological molecules is also important for “triggering” the excitant effects of hormones.
This edition has been extensively revised and contains nearly 1000 new references and over 60 new or modified figures. This proliferation reflects advances in our knowledge of hormones and their actions, and a persistent interest in the application of this knowledge to the domain of comparative endocrinology. Both of the dedicatees of this book are now deceased. When they first became interested in this subject over 60 years ago they utilized contemporary pharmacology to help to lay the foundations of the discipline. Its recent propagation largely reflects the use of the techniques of molecular biology and the unravelling of the genome to decipher the interrelationships of hormones in both vertebrates and their invertebrate progenitors.
It would not have been possible for me to prepare this edition without the collaboration of The University of Western Australia, particularly the Physiology Department and Professor Trevor Redgrave. All members of the department helped by making me feel welcome there. The Biological Sciences Library is the principal repository of the new information that I have used. Its comprehensive collection and helpful staff provided a pleasant venue for many hours of searching.
The use of hormones for the purpose of coordination involves a complex series of physiological events. Such a life history begins with the formation of the excitant by the endocrine glands and concludes with the response of a target, or effector tissue, and the hormone's ultimate destruction or its excretion from the body. The events that determine the action of a hormone are shown in Fig. 4.1. This basic pattern persists throughout the vertebrates, though, as will be described, certain differences exist.
The formation of hormones
Although the formation of all hormones is determined at the genetic level, it can be either a relatively direct translational procedure or, alternatively, occur as a result of the prior formation of enzymes that mediate synthesis. Enzymes are also involved in the former process, however.
The synthesis and processing of hormones
Hormones, like other proteins, are synthesized by the process of DNA transcription to produce RNA, followed by translation into a primary protein that may be subsequently modified to produce a mature hormone (see, for instance, Andrews, Brayton, and Dixon, 1987; Conlon, 1989). Such processes occur in the endocrine glands but may exhibit differences depending on the particular type of cell where the gene is being expressed. Thus, the products of the proopiomelanocortin (POMC) genes are different in the pars distalis and pars intermedia in the pituitary. The glucagon gene also produces different polypeptides when it is expressed in the pancreatic A-cells and the intestinal L-cells. Different species may also process hormone precursor proteins differently and contribute to the diversity of the endocrine System. The primary RNA transcript of the genomic DNA is spliced to remove introns, cleaved, and polyadenylated, resulting in the formation of mature mRNA. Such splicing can, however, follow more than one pattern (“alternate splicing”) to produce multiple mRNA species encoding different polypeptides. For instance, the calcitonin gene can produce calcitonin and calcitonin gene-related peptide (Chapter 3). The translation of the nudeotide sequence of the mRNA occurs on the ribosomes. The N-terminal signal sequences of amino acids targets the nascent polypeptides to the endoplasmic reticulum during the translation process in preparation for the hormone's secretion. Cleavage of the signal peptide in the endoplasmic reticulum converts the preprohormone to the prohormone.
About 70% of the body weight of animals is water, in which is dissolved a variety of solutes, the presence of many of which is vital for life. Within the body, the Solutions inside the cells differ from those that bathe the outside, and the composition of each of these Solutions must be maintained so as to provide an environment with an electrolyte content and osmotic concentration suitable for life. These intra- and extracellular fluids provide the framework in which life exists.
The physiochemical properties of the body fluids in animals usually differ greatly from those of their external environment. Animals continually suffer exposure to the whims of the exoteric conditions and this will tend to change the composition of their body fluids. In addition, although the intra- and extracellular fluids have identical osmotic concentrations, there are qualitative differences in the solutes they contain, and equilibration, through diffusion, will tend to occur. Such animals, however, maintain the gradients between their body fluids and the environment, an equilibrium that is maintained as a result of a complex pattern of physiological events. These processes involve the cells, and Special tissues and organs that are concerned with osmoregulation. The integration of the functions of these homeostatic tissues relies largely on hormones. The nervous System makes little direct contribution to such regulatory processes, though at the cellular level itself considerable autoregulation, independent of hormones, exists. Hormones do ultimately influence some cellular processes, of course, but they generally appear to do this in effector tissues like the kidney, gills, and gut, which are especially concerned with the overall osmoregulation of the animal. For a more complete account of the role of hormones in osmoregulation the book by Bentley (1971) and the more recent compendium collected by Pang and Schreibman (1987) could be consulted. Animals occupy diverse osmotic environments; the major ones are the sea, fresh water, such as rivers and lakes, and dry land. Differences exist between the availability of water and salts within these environments and this is particularly apparent to animals that lead a terrestrial life. Water may be relatively freely available to some terrestrial species that live in areas where rainfall is high, and lakes, ponds, and rivers exist in close proximity to where they live.
Animals require a continual supply of food in order to sustain life. Such nutrients, in the first instance, are obtained from the external environment. These materials are used as an energy supply, as building blocks for growth and reproduction, and also as a source of certain essential chemicals necessary to the adequate functioning of the metabolic machinery in the body. The processes involved are therefore basic to life and are regulated to a considerable extent by hormones.
The foods that animals obtain from the environments where they live are usually chemically far more complex than can be used by their cells. The original nutrients are transformed in the body into compounds that may sometimes be immediately metabolized by the cells, or they may be converted into substances that can be stored for subsequent transformation into such compounds.
Hormones play an important role in regulating the interconversions of nutrients to metabolic Substrates and their stored forms. The endocrine secretions may help to regulate the levels of nutrients by contributing to the control of their absorption from the gut, their levels in the blood, the nature and rate of their storage, their release from tissues, and their assembly into the structural elements of the body.
Animals lead diverse lives in a plethora of environmental conditions. The definitive metabolic processes are basically similar in all animals and lead to the utilization of ATP, for the supply of energy, and the building of cells. Nevertheless, the physiological processes leading to these accomplishments may differ considerably. Such processes are dictated by numerous circumstances and events.
The chemical nature of the foodstuffs that animals obtain from their environments may differ greatly. In their feeding habits, animals may be carnivorous, herbivorous, or omnivorous. Even within these categories considerable differences exist in the types of food animals eat. Some animals may feed principally on invertebrates such as insects, molluscs, and worms that live in terrestrial, fresh water, or marine environments. Other animals feed on vertebrates. Plants from equally diverse situations are also used for food.
The skin and gills of vertebrates constitute the major external interface between the animal and its environment. This integument is physiologically and anatomically a very important tissue that exhibits considerable diversity reflecting the differences that exist in the physiochemical gradients between the vertebrates and their environments (Bereiter-Hahn, Matoltsy, and Richards, 1986). The integument may, therefore, play a role in the animal's osmoregulation, thermoregulation, and respiration. In addition, the integument provides signs and signals that can promote social and sexual contact and can help the animal to blend in with its surroundings and so protect it from predators, or help it catch its food. Of primary importance is the skin's role as an integumental skeleton by which it contains the animal in a condition that facilitates its locomotion. The relative importance of these various roles of the integument varies in different species and the structure varies accordingly also.
In fishes and larval amphibians, the gills, which function as organs of respiration, make up a large part of the animal's external surface. Exchanges of oxygen and carbon dioxide readily occur across these highly vascularized tissues, which are also the sites of considerable movements of water and salts. Many fishes contain Special cells in their gills and skin called “chloride cells” or ionocytes, which are the site for active extrusion of salts. The endocrine control mechanisms influencing the permeability of the gills are described in Chapter 8.
The skin is the major nonbranchial interface between the animal and its environment. In its simplest form, the skin consists of two major layers of tissue: an outer epidermis, which has several strata of cells, and an inner dermis. However, such a simple arrangement does not exist in nature, as various other structures are also included in the skin that modify its properties. These structures indude scales, hair, feathers, pigment cells, secretory glands, and certain sense organs. Such accessories contribute to the particular physiological properties exhibited in the integument of each species.
It is over 6 years since the first edition of this book went to press. Interest in comparative endocrinology has not wanted in that time, as shown by the steady stream of papers and the organization of meetings and symposia on this subject. Several new hormones have been identified and described in the interim. Information about the synthesis of proteins that act as prohormones has provided enlightenment about the existence of more “hormone families” with consequent speculation about their evolution. An increased utilization of radioimmunoassays and immunohistochemistry has promoted many of these advances. There has also been an increased appreciation of commonalities of the endocrine and nervous Systems, as described in the discipline of neuroendocrinology. However, because the basic information about the endocrine System has not really changed, it has been unnecessary to alter significantly the conclusions at the end of each chapter.
In view of the great expansion of the literature, the preparation of this edition has been especially challenging. I have generally refrained from substituting new references for old ones, a practice that would ignore the seniority of discoveries and distort the historical perspective of the subject. There are thus many more references in the text. I hope that this does not distract the students for whom this book is primarily intended. They should “read around” the references and use them as a source if necessary. More senior readers may find the expanded bibliography more useful. Finally, I would like to apologize to the many endocrinologists whom it has not been possible to quote but without whose discoveries our knowledge of this subject would be much poorer.
Calcium is vital for animal life. In vertebrates, this divalent ion is present at various sites, including the body fluids and structural parts of the cell (especially the mitochondria and endoplasmic reticulum), and in most species it is also a major component of the endoskeleton. The outer shell of the eggs of birds and many reptiles also consists principally of calcium. The physiological role of calcium appears to be the result of a rather unique set of physicochemical properties. In aqueous solution, calcium can exist in a soluble form, which is important for its mobility in the body, and yet, equally essential to its role, many of its salts, including phosphates and carbonates, have a low solubility so that a physicochemical equilibrium may exist between its solid and aqueous forms. The quantity of calcium free in solution is thus restricted in the presence of certain anions. In addition, such relatively insoluble salts in certain of their crystalline forms, principally calcium phosphate and calcium carbonate, can contribute to the mechanical support and stability of biological structures. Calcium also has a ready propensity to associate and combine with proteins. Such combinations are seen in the body fluids, where a considerable portion of the calcium is bound to serum proteins, and in cells, where it contributes to their structural stability by helping maintain essential ionic bridges at vital points in protein molecules; therefore when tissues are placed in calcium-free Solutions they tend to disintegrate and the cells swell and fall apart.
Calcium plays an essential role in coordinating many events in the body that may reflect those general properties described above. Calcium stabilizes membranes and this effect can be seen in the hyperactivity of nerve fibers placed in Solutions with low calcium concentrations. Such instability and the repetitive electrical depolarization of the nerve cell membrane result in tetanic contractions of the muscles they supply. Muscle contraction requires the presence of calcium; when released from the sarcoplasmic reticulum within the cell, calcium couples the initiating electrical depolarization of the cell membrane to those processes that initiate changes in the contractile proteins.
This book has been written primarily for use as a textbook by undergraduate, as well as graduate, students. It is hoped that it may serve as a basis for course work in comparative endocrinology and also as an auxiliary text to aid in the teaching of comparative animal physiology. In order to gain the most from this book, the reader should have a basic knowledge of zoology and animal physiology. I have nevertheless attempted to put the endocrinology that is described into a broader biological framework by relating it to the animal's physiology, ecology, and evolutionary background. This is one of the reasons why I have departed from the more usual format of previous textbooks in this area, which generally deal with each endocrine gland in succession, chapter by chapter. Instead, I have attempted to describe certain broad and basic biological processes, the functioning of which is often coordinated by the secretion from several endocrine glands.
No attempt has been made to describe invertebrate endocrinology, as the rapid growth of this area really justifies a separate textbook. The book by K. G. Highnam and L. Hill (Comparative Endocrinology of the Invertebrates, Elsevier: Amsterdam, 1970) deals admirably with this subject.
It has not been possible in a book of this nature to give a complete list of original references. There are far too many of these, and many of the earlier observations are already a part of the “classical literature”. Instead, I have attempted to refer the reader to more recent papers and reviews that contain references to the material described and can act as useful “starting points” for the students who wish to study the subject further. In order to keep abreast of developments in the various subject areas described, the current literature should be consulted. The principal Journals where papers on these subjects are published are General and Comparative Endocrinology, Journal of Endocrinology, Endocrinology, and Comparative Biochemistry and Physiology. In addition, many papers appear in the Standard physiological Journals, espeäally Journal of Physiology and American Journal of Physiology.
Every language has operations that adjust the relationship between semantic roles and grammatical relations in clauses. Such devices are sometimes referred to as alternative voices. For example, the passive operation in English when applied to most transitive verbs places the PATIENT in the subject role and the AGENT in an oblique role. The more normal arrangement for transitive verbs is for the AGENT to bear the subject relation and the PATIENT the object relation:
(1) a. Active
Orna baked these cookies. AGENT = subject
PATIENT = object
b. Passive
These cookies were baked by Orna. PATIENT = subject
AGENT = oblique
In this chapter we will discuss a range of structures that adjust the relationship between grammatical relations and semantic roles in terms of valence. Not all of these would be considered in traditional grammar under the heading of “voice,” but because of their functional similarity and because many languages treat them in structurally comparable ways, it is often convenient to group some or all of these operations together in a single chapter of a grammar or grammar sketch.
Valence can be thought of as a semantic notion, a syntactic notion, or a combination of the two. Semantic valence refers to the number of participants that must be “on stage” (see section 0.2.3) in the scene expressed by the verb. For example, the verb eat in English has a semantic valence of two, since for any given event of eating there must be at least an eater and an eaten thing. In terms of predicate calculus, the concept EAT is a relation between two variables, x and y, where x is a thing that eats and y is a thing that undergoes eating. This semantic relationship would be represented in predicate calculus notation as EAT(X, y) (see below).
Grammatical valence (or syntactic valence) refers to the number of arguments present in any given clause. A syntactic argument of a verb is a nominal element (including possibly zero, if this is a referential device in the language) that bears a grammatical relation to the verb (see chapter 7). So, for example, a given instance of the verb eat in English may have a syntactic valence of one or two.