Passive transport, calcium

Calcium Uses Ca + ATPase and Na+-Ca + exchanger Re-absorbed Re-absorbed, ascending loop via passive transport ... 

The level of free calcium in the cytosol is affected by multiple mechanisms of active and passive transport. These are summarized in Figure 6.1. While the level of calcium in the cytosol varies in time, it is always much lower than in the extracellular space, or than in the mitochondria and ER. These two organelles function as intracellular buffers or reservoirs of calcium. Accordingly, calcium transport systems operating in both directions exist not only at the cytoplasmic membrane but also at the mitochondrial (iimer) membrane and the ER. 

The sarcoplasmic reticulum vesicles start to release calcium passively when the activity of the calcium pump is blocked, either by inhibition of the transport enzyme or by depletion of energy-yielding substrates [45]. The rate of passive calcium efflux from vesicles loaded in the absence of calcium-precipitating agents, is approx. 20... 

Cells constantly, either actively or passively, transport chemicals across the plasma membrane and communicate with surrounding cells, generating chemical diffusion gradients around them. Measurement of such activities within gradients enables a better understanding of the cellular transport mechanisms. For example, calcium is regulated to nanomolar values in the cytosol by a complex interaction of cellular and plasma membrane pumps, reporters, channel activity, regional sequestration, and chemical... 

Calcium crosses the intestinal mucosa by both active and passive transport. The active process is saturable, transcellular, and occurs throughout the... 

The passive transport pathway is nonsaturable and paracellular. It occurs throughout the small intestine and is unaffected by calcium status or parathyroid hormone (PTH). It is relatively independent of 1,25(0H)2D3, although this metabolite has been foimd by some investigators to increase the permeability of the paracellular pathway. A substantial amoimt of calcium is absorbed by passive transport in the ileum due to the relatively slow passage of food through this section of the intestine. The amoimt of calcium absorbed by passive transport will be proportional to the intake and bioavailability of calcium consumed. 

Lactose improves calcium absorption in young infants, in whom absorption of calcium is predominantly by passive transport. In adults, the presence of lactose in the diet has little effect on the efficiency of calcium absorption. 

Additional cellular events linked to the activity of blood pressure regulating substances involve membrane sodium transport mechanisms Na+/K.+ ATPase Na+fLi countertransport Na+ -H exchange Na+-Ca2+ exchange Na+-K+ 2C1 transport passive Na+ transport potassium channels cell volume and intracellular pH changes and calcium channels. 

For compounds not metabolized by the gut wall, liver, or affected by transporters, a direct relationship between oral absorption and bioavailability should be observed. The calculated oral absorption, using PSA as a measure for passive membrane permeability reflecting the absorption step, relates to the in vivo observed bioavailability for three classes of compounds - angiotensin-converting enzymes (ACE) inhibitors, P-blockers, and calcium antagonists - is shown below [25],... 

Calcium Adequate intake (Table 3-1) in divided doses Absorption-predominantly active transport with some passive diffusion, fractional absorption 10-60%, fecal elimination for the unabsorbed and renal elimination for the absorbed calcium... 

In sum, the natural tendency will be for sodium, calcium, and chloride ions to flow into the neuron and for potassium ions to flow out, and in so doing to reduce the membrane potential to zero. In reality, this is not so easy. The plasma membrane of the neuron is not very permeable to these ions. If it were, it would be impossible to sustain concentration gradients across it. The rate of passive diffusion of these ions across this membrane is very slow, though not zero, and different for each ion. So how do ions get across the neuronal plasma membrane rapidly There are two ways gated channels and active transport by pumps. 

Absorption - Calcium is absorbed from the Gl tract by passive diffusion and active transport. Calcium must be in a soluble, ionized form for absorption to occur. Vitamin D is required for calcium absorption and increases the absorptive mechanisms. 

As active transport uses a carrier system, it is normally specific for a particular substance or group of substances. Thus, the chemical structure of the compound and possibly even the spatial orientation are important. This type of transport is normally reserved for endogenous molecules such as amino acids, required nutrients, precursors, or analogues. For example, the anticancer drug 5-fluorouracil (Fig. 3.6), an analogue of uracil, is carried by the pyrimidine transport system. The toxic metal lead is actively absorbed from the gut via the calcium transport system. Active uptake of the toxic herbicide paraquat into the lung is a crucial part of its toxicity to that organ (see chap. 7). Polar and nonionized molecules as well as lipophilic molecules may be transported. As active transport may be saturated, it is a zero-order rate process in contrast to passive diffusion (Fig. 3.3). 

A number of suggestions have been made that calcium may be transported because it is coupled to the movement of other ions or because it moves passively down an electrochemical gradient established by the movement of some other ions600. Thus, a sodium-induced potential has been found which was sufficient to account for the passive movement of calcium into the shell gland of the domestic fowl during egg shell formation. In the mollusc, the shell side of the mantle is normally positive relative to blood and a potential of this type would, of course, tend to move calcium away from the extrapallial fluid. A potential of this orientation could be produced by the movement of an anion into the animal (mollusc) and the low chloride concentration of the extrapallial fluid could be accounted for on this basis. 

The transport of calcium into the mitochondrion can lower external Ca2+ to levels of 1 to 0.1 jumole/1. This has, therefore, been interpreted as a basic mechanism in maintaining intracellular calcium at these levels. Only about 3 % of the calcium which passively diffuses into the cell is expelled by a calcium pump into the plasma membrane, whereas the remaining 97 % is sequestered into the mitochondria. This occurs because both processes have similar rate constants, but the total mitochondrial surface is some 30 times larger than that of the plasma membrane. This argument presupposes, however, that the calcium which enters the cell is equally available to both sets of membranes. 

The carrier-mediated active transport system of calcium is responsible for the relaxation of muscle. However, the rate of efflux from sarcoplasmic reticulum membranes during reversal of the transport process is 102 to 104 orders too low to account for the massive calcium release from sarcoplasmic reticulum in stimulated muscle. Instead, passive diffusion of calcium across the sarcoplasmic reticulum membrane will proceed during excitation of muscle178,179,186. The rate of calcium release observed during excitation is 1.000-3.000 p moles/mg protein/min which is an increase of about 104 to 10s over the resting state. 

This is the case for magnesium and calcium electrodes whose cations are bivalent. The surface films formed on such metals in a wide variety of polar aprotic systems cannot transport the bivalent cations. Such electrodes are blocked for the metal deposition [28-30], However, anodic processes may occur via the breakdown and repair mechanism. Due to the positive electric field, which is the driving force for the anodic processes, the film may be broken and cracked, allowing metal dissolution. Continuous metal dissolution creates an unstable situation in the metal-film and metal-solution interfaces and prevents the formation of stable passivating films. Thus, once the surface films are broken and a continuous electrical field is applied, continuous metal dissolution may take place at a relatively low overpotential (compared with the high overpotential required for the initial breakdown of the surface films). Typical examples are calcium dissolution processes in several polar aprotic systems [31]. 

Both the active and passive modes of calcium transport are increased during pregnancy and lactation. This is probably due to the increase in calbindin and serum PTH and 1,25-dihydroxyvitamin D concentrations that occur during normal pregnancy. Intestinal calcium absorption is also dependent on age, with a 0.2% per year decline in absorption efficiency starting in midlife. The fractional absorption of calcium depends on the form and dietary source. Absorption rates are 29% for the calcium in cow s milk, 35% for calcium citrate, 27% for calcium carbonate, and 25% for tricalcium phosphate. Other factors that limit the bioavailability of calcium in the intestine are oxalates and phy-tates, which are found in high quantities in vegetarian diets and which chelate calcium. 

OH)2D3 binds with high affinity (1010 to 10" 1 M) and specificity to its nuclear receptor, VDR, which in turn functions to modulate gene expression. The net effect of l,25-(OH)2D3 is to increase intestinal absorption of calcium. At a normal calcium intake of 1000 mg about 15% of calcium absorption occurs by passive diffusion. The amount of calcium absorbed via this mechanism is not sufficient to sustain calcium balance, even when the intake of calcium is significantly increased. The major component of calcium absorption occurs by active transport, a process that is activated solely by... 

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