Muscle ultrafiltration

On-line dialysis also separates the analyte from tissue matrix based upon molecular size, but in this case, the sample extract is passed over a membrane filter through which the analyte (and other low molecular weight compounds) is diffused into a second solvent on the other side of the membrane filter. Usually, the second solvent is then concentrated on to an SPE column to minimize the dilution effect that is caused by the dialysis process. Agasoester used on-line dialysis to separate oxytetracycline from muscle, liver, milk, and egg tissue matrix components. A problem encountered with on-line dialysis is the inability of analyte molecules that are bound to proteins in the sample extract to pass through the membrane filter. Problems with membrane clogging are reduced with on-line dialysis compared with ultrafiltration because no external force is being applied to bring the analyte across the membrane filter. 

Pathophysiology Muscle cramps can occur with up to 20% of dialysis sessions.48 The cause is often related to excessive ultrafiltration, which causes hypoperfusion of the muscles. Other contributing factors to the development of muscle cramps include hypotension and electrolyte and acid-base imbalances that occur during hemodialysis sessions. 

Treatment Nonpharmacologic treatments of muscle cramping that occurs during hemodialysis include decreasing the ultrafiltration rate and accurately determining the dry weight. Pharmacologic measures include vitamin E, which is administered at doses of 400 IU daily. Other options that are not as well studied include oxazepam and prazosin. 

Prior to analysis of -lactam residues in liquid foods such as milk, a pretreatment step for fat removal, accomplished by centrifugation (69-71), is usually required. In instances where milk is to be submitted to ultrafiltration, dilution with water/acetonitrile (72-76) or water/acetonitrile/methanol (77-79) is often needed. Milk filtration (80) or dilution with acetate (81, 82) or phosphate buffers (83) is sometimes essential prior to solid-phase extraction. Unlike milk, semisolid food samples such as muscle, kidney, and liver require normally more intensive sample pretreatment. Tissue break-up is mostly carried out by the combined use of a mincing apparatus and a tissue homogenizer. 

Infusions of AmB, intravenously or into the renal artery, induce short-term reduction in renal blood flow (RBF) and GFR, and an increase in renal vascular resistance, in both rats and dogs [83-85]. The effects of short term infusions of AmB on the renal microcirculation in rats revealed that the single nephron GFR was decreased by 2 mechanisms (Table 1) 1) a decrease in single nephron plasma flow, due to vasoconstriction of the afferent and efferent arterioles, and 2) a reduction in the glomerular capillary ultrafiltration coefficient (Kf), an effect probably mediated by mesangial cell contraction [86]. Previous micropuncture studies demonstrated a similar vasoconstriction of the afferent arteriole but also an increased permeability of the tubular epithelium to inulin [75]. Thus, the reduction in GFR after acute AmB infusions can be attributed to contraction of afferent smooth muscle cells, efferent smooth muscle cells and glomerular mesangial cells, as well as increased tubular permeability with back-leak... 

Cramps 5-20 Muscle hypoperfusion due to ultrafiltration and hypovolemia Hypotension Electrolyte imbalance Acid-base imbalance... 

Skeletal muscle cramps complicate 5% to 20% of hemodialysis treatments. Although the pathogenesis of cramps is multifactorial, plasma volume contraction and decreased muscle perfusion caused by excessive ultrafiltration is frequently the initiating event. Although there are no comparative data regarding the efficacy of nonpharmacologic and pharmacologic therapy, the former should be the first line of treatment because the adverse consequences are minimal (Table 45-5). 

The horse has been used as a model to study the distribution of intravenously infused amino acids and calcium from blood to the subcutaneous and muscle interstitial fluids. Subcutaneous ultrafiltrate levels of both amino acids and calcium were higher than blood concentrations under basal conditions and after infusion. After infusion of amino adds, subcutaneous ultrafiltrate concentrations tracked the rise and fall of plasma concentrations, but ultrafiltrate concentrations were greater than plasma concentrations at all time periods. Similarly, subcutaneous caldum ultrafiltrate concentrations tracked plasma concentrations after intravenous infusion of caldum. Subcutaneous concentrations were also slightly higher than plasma concentrations. Muscle concentrations of calcium were considerably higher than either plasma or subcutaneous concentrations, both in the basal state and after infusion. Caldum concentrations remained elevated for a longer time in muscle than in plasma or in subcutaneous tissue (Sojka et al., 1995 Spehar et al., 1995). These studies demonstrate the potential of using ultrafiltration to study distribution of nutrients in different tissues. 

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