Ca(2+) is an essential ion in all organisms, where it plays a crucial role in processes ranging from the formation and maintenance of the skeleton to the temporal and spatial regulation of neuronal function. The Ca(2+) balance is maintained by the concerted action of three organ systems, including the gastrointestinal tract, bone, and kidney. An adult ingests on average 1 g Ca(2+) daily from which 0.35 g is absorbed in the small intestine by a mechanism that is controlled primarily by the calciotropic hormones. To maintain the Ca(2+) balance, the kidney must excrete the same amount of Ca(2+) that the small intestine absorbs. This is accomplished by a combination of filtration of Ca(2+) across the glomeruli and subsequent reabsorption of the filtered Ca(2+) along the renal tubules. Bone turnover is a continuous process involving both resorption of existing bone and deposition of new bone. The above-mentioned Ca(2+) fluxes are stimulated by the synergistic actions of active vitamin D (1,25-dihydroxyvitamin D(3)) and parathyroid hormone. Until recently, the mechanism by which Ca(2+) enter the absorptive epithelia was unknown. A major breakthrough in completing the molecular details of these pathways was the identification of the epithelial Ca(2+) channel family consisting of two members: TRPV5 and TRPV6. Functional analysis indicated that these Ca(2+) channels constitute the rate-limiting step in Ca(2+)-transporting epithelia. They form the prime target for hormonal control of the active Ca(2+) flux from the intestinal lumen or urine space to the blood compartment. This review describes the characteristics of epithelial Ca(2+) transport in general and highlights in particular the distinctive features and the physiological relevance of the new epithelial Ca(2+) channels accumulating in a comprehensive model for epithelial Ca(2+) absorption.

https://www.physiology.org/doi/pdf/10.1152/physrev.00003.2004

Calcium Absorption Across Epithelia

There are two main groups of ungulates or hoofed mammals living today, perissodactyls and artiodactyls. They are distinguished by a basic difference in foot morphology, which indicates that the two groups arose independently from a basal "proto-ungulate" stock (the order Condylarthra) in the early Tertiary. This difference involves the axis of symmetry of the foot. In perissodactyls it passes through the third metapodial, while in artiodactyls it passes between the third and fourth metapodials. The perissodactyls surviving today are horses, rhinos and tapirs. The recent artiodactyls are a much larger and more diverse group, broadly covering all "cloven-hoofed" animals such as pigs, hippos, camels, deer, giraffes, antelope, cattle and sheep. The perissodactyls must have evolved by the late Paleocene, as by the early Eocene four out of five of the known superfamilies are already distinguishable (Romer, 1966). Artiodactyls are also seen in the early Eocene (Romer, 1966), but the two major groups of ruminant artiodactyls, the tylopods and the pecorans, do not appear in the fossil record until the late Eocene (Gazin, 1955) (see Fig. 1). The perissodactyls were the dominant medium to largesized herbivores during the Eocene, and in the early Oligocene a reduction in their diversity coincided with the beginning radiations of the ruminant groups of artiodactyls. These latter animals were the dominant medium-sized herbivores throughout the remainder of the Tertiary, at least in terms of species diversity. The perissodactyls originated in North America; the origin of the artiodactyls, whether in North America or in Eurasia, is still in question (Olson, 1971). As the rise of the artiodactyls at the Eocene-Oligocene boundary coincides with the decrease in the diversity of the perissodactyls, theories have been advanced to explain this coincidence in terms of perissodactyls being competitively inferior to artiodactyls. One obvious difference between the two groups is the difference in limb morphology. Although the axis of symmetry of the foot seems an unlikely candidate for conferring a selective advantage, artiodactyls do possess a unique double trochleared tarsal or ankle joint. This structure has been regarded as a key feature of the group and contributory to their evolutionary success, on the grounds that it has enabled artiodactyls to achieve rapid acceleration for predator escape (e.g., Schaeffer, 1947; Romer, 1968; Colbert, 1969). While there is little doubt that this particular adaptation was of value to the artiodactyls themselves, I do not think, however, that it had a great deal to do with the relative competitive efficiencies of these two ungulate groups. For a start, this tarsus appeared in a highly advanced condition in the first artiodactyls in the early Eocene (Schaeffer, 1947) long before their radiation and diversification, and is found in noncursorial as well as cursorial forms. In addition, modern ecological work demonstrates that predation is not necessarily detrimental

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1558-5646.1976.tb00957.x

The evolutionary strategy of the equidae and the origins of rumen and cecal digestion.

It has been argued that environmental seasonality selects for larger body size among homeotherms (Brodie 1975; Downhower 1976; Boyce 1979). Indeed, strong correlations between body size and seasonality have been shown both within species of mammals (Brodie 1975; Boyce 1978) and among mammal faunas (Zeveloff 1982). Because of high mortality during the season of resource shortage. surviving individuals enjoy low competition and abundant resources during the growth season. Selection then favors rapid growth and attainment of large size because of enhanced survivorship (Lindsey 1966; Peters 1983; Calder 1984) and, in some taxa, increased fecundity (Sadleir 1969; Boyce 1981). Large individuals experience higher survival rates during the period of resource shortage because they metabolize somatic stores at a lower weight-specific rate. Thus, seasonality may produce two consequences to body-size evolution: (1) reduced densitydependent competition because of seasonal high mortality, known as Ashmole's hypothesis (Ashmole 1963; Ricklefs 1980) and (2) increased fasting endurance for individuals of large size (Lindsey 1966; Calder 1974). In this note we analyze data that substantiate the significance of large body size for enhancing fasting endurance, responding to the challenge of Clutton-Brock and Harvey (1983), who claim that further knowledge of \"food reserves\" is necessary to evaluate the seasonality hypothesis. There are now sufficient data to predict fasting endurance in mammals by dividing energy reserves by their rate of use (after Stahl 1962). The result, the scaling of fasting endurance, favors large body size. This relationship will, of course, be of primary significance in seasonal environments where organisms must endure occasional periods of fasting.

https://era.library.ualberta.ca/items/60e063a2-d8cf-42da-9930-031a57b4b824/view/ede13d82-5322-4cd0-9774-68d97fd19e59/AN_125_1985_873.pdf

Seasonality, fasting endurance, and body size in mammals

Thermal biology of the laboratory rat.

Theories Proposed for Control of Renin Release. 359 Introduction. 359 Baroreceptor theory. 361 Macula densa theory. 362 Sympathetic nervous system theory. 366 Hormonal control of renin release and role of angiotensin. 367 Analysis of Renin Release Data. 368 Introduction. 368 Methodological considerations. 368 Hemorrhage and systemic hypotension. 369 Acute local reduction of renal arterial pressure during oliguria or natriuresis ... 370 Acute sodium depletion by natriuretic drugs. 371

Control of renin release.

The rectal temperature of normal healthy camels at rest may vary from about 34°C to more than 40°C. Diurnal variations in the winter are usually in the order of 2°C. In summer the diurnal variation...

Body Temperature of the Camel and Its Relation to Water Economy

In a number of mammalian kidneys a close correlation was found between renal medullary thickness and ability to concentrate electrolytes in the urine, indicating that both outer and inner zone of t...

Bodil Schmidt-Nielsen

Papers

Body Temperature of the Camel and Its Relation to Water Economy

Structure and concentrating mechanism in the mammalian kidney

Water Metabolism of Desert Mammals

Urea Excretion in Mammals

Excretion in fresh-water turtle (Pseudemys scripta) and desert tortoise (Gopherus agassi)