Discontinuously reinforced metal matrix composites are susceptible to reinforcement particle cracking, which reduces tensile elongation. However, many of the particles experience multiple cracking episodes, which would seem unfavourable in a matrix filled with similar particles. Simulations of an aluminium matrix composite reinforced with 9 vol.-%SiC particles have been performed using a non-linear axisymmetric finite element model. Reinforcement particles with and without defects were cracked in the simulation in order to study the subsequent behaviour of the composite. Particle defects were found to decrease the impact of reinforcement particle cracking on the stress in the surrounding matrix. Multiple cracking of reinforcement particles was shown to be feasible because of the load bearing ability of fractured particles.MST/5066

Inbred Dahl/Rapp salt-sensitive and salt-resistant rats differ in their blood pressure response to dietary salt. We studied sodium-hydrogen (Na-H) exchanger kinetics in renal brush border membrane vesicles prepared from both strains on either a 1% or 8% NaCl diet. Kinetics measurements were made with the acridine orange fluorescence quenching technique in vesicles prepared at pH 6.0. The initial Na-H exchange rate was measured using preparations with similar initial quench values. The maximal transport rate (Km,, fluo-rescence units per second per milligram protein [±SEM]) in salt-sensitive rats on a 1% NaCl diet was significantly lower than that in salt-resistant rats (36.9±4.4 versus 51.8±5.5, respectively, P<.0005). With the 8% NaCl diet for 1 week, the V^ of salt-resistant rats decreased and became similar to that of salt-sensitive rats. The affinity for sodium (K m , millimoles per liter [±SEM]) was also lower in salt-sensitive rats than in salt-resistant rats while on a 1% NaCl diet (11.8±1.0 versus 19.6±2.3, respectively, P<.002). These values converged when T he sodium-hydrogen (Na-H) exchangers make up a family of integral membrane proteins present in nearly all mammalian cells. Using the energy stored in the inwardly directed sodium gradient, these transport proteins mediate the electro-neutral exchange of extracellular sodium for intracellu-lar protons. Na-H exchangers are involved in a number of cellular homeostatic processes and are thought to play a central role in the control of intracellular pH and cell volume. 1-2 It has been postulated that an abnormality in cellular sodium uptake is an important factor in the development of essential hypertension. 3 Since Na-H exchange mediates a large fraction of cellular sodium uptake, the role of Na-H exchangers in the development of human hypertension, particularly of the salt-sensitive variety, has been an area of intense research over the past several years. Several studies have examined plate-let and leukocyte Na-H exchange as well as red blood cell sodium-lithium exchange in humans and have found enhanced exchange rates in patients with essential hypertension. 48 both strains were fed an 8% NaCl diet for 1 week. Inhibition by 25 ^imol/L amiloride was less in salt-sensitive rats than in salt-resistant rats on the 1% NaCl diet. These results show that salt-sensitive rats have lower renal apical membrane Na-H exchange activity than salt-resistant rats on a 1% NaCl diet. Salt-sensitive rats do not modulate renal apical membrane Na-H exchange in response to an 8% NaCl diet, whereas salt-resistant rats show a more physiologically appropriate response to increased dietary salt. The maladaptive response by salt-sensitive rats on a high salt diet may well contribute to the development of severe hypertension under these conditions. The differences between these two strains in sodium affinity and amiloride inhibition suggest structural differences in the renal apical membrane Na-H exchangers at the sub-strate and amiloride binding sites or altered cellular regula...

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