and this reabsorptive process is stimulated by PTH and 1,25-dihydroxy-cholecalciferol [1,25(OH)2D3].
• To maintain Mg2+ balance, the kidneys must excrete daily the amount of Mg2+ absorbed by the intestines, which is about 120 mg. This amount is ~5% of the filtered load of Mg2+, so substantial reabsorption of Mg2+ must occur.
• Because of its low Mg2+ permeability, the proximal tubule reabsorbs less than 20% of the filtered load of Mg2+.
• More than 60% of the filtered Mg2+ is reabsorbed in the loop of Henle, primarily in the thick ascending limb by paracellular diffusion that is facilitated by paracellin-1 in the junctional complexes.
K+, Ca2+, and Mg2+ constitute the majority of the cations in the body. As is the case for Na+, homeostasis for each of these cations is maintained largely by regulation of its rate of renal excretion. Disturbances in the total body content and plasma concentrations of any of these cations can produce a wide variety of clinical manifestations. Of particular importance is the effect of the plasma concentration of these cations on neuromus-cular excitability and cardiac rhythm. In general, an increase in the plasma K+ concentration, or a decrease in the plasma Ca2+ or Mg2+ concentration, causes neuromuscular hyperexcitability. Some extreme examples are the potentially fatal cardiac arrhythmias produced by hyperkalemia and the tetany observed with severe hypocalcemia (low plasma Ca2+ concentration). Conversely, hypokalemia, hypercalcemia, or hypermagnesemia are associated with depression of neuromuscular excitability.
Much is known about the normal regulation of K+ balance by the kidney and about hormones that regulate the distribution of K+ between the intra- and extracellular compartments. Most of the body stores of K+ are intracellular, and total body K+ is an important determinant of normal cell volume and composition. Potassium deficiency is associated with muscle wasting, weakness, fatigue, and, in children, with a general failure to thrive.
Only in the past few years have researchers identified the most important transport mechanisms for Ca2+ and Mg2+ reabsorption and their regulation. Normal body stores of Ca2+ and Mg2+ are, of course, essential for bone development, and Mg2+ is an important cofactor for many enzyme systems, including mitochondrial adeno-sine triphosphatase (ATPase) and other transport ATPases.
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