Kinetic modelling of coupled transport across biological membranes.

In this report, we have modelled a secondary active co-transporter (symport and antiport), based on the classical kinetics model. Michaelis-Menten model of enzyme kinetics for a single substrate, single intermediate enzyme catalyzed reaction was proposed more than a hundred years ago. However, no single model for the kinetics of co-transport of molecules across a membrane is available in the literature. We have made several simplifying assumptions and have followed the basic Michaelis-Menten approach. The results have been simulated using GNU Octave. The results will be useful in general kinetic simulations and modelling.

Transport of Tetraethylammonium in Renal Cortical Endosomes of Cadmium-Intoxicated Rats

Effects of cadmium (Cd) intoxication on renal endosomal accumulation of organic cations (OC ) were studied in rats using 14C-tetraethylammnium (TEA) as a substrate. Cd intoxication was induced by s.c. injections of 2 mg Cd/kg/day for 2-3 weeks. Renal cortical endosomes were isolated and the endosomal acidification (acridine orange fluorescence change) and TEA uptake (Millipore filtration technique) were assessed. The TEA uptake was an uphill transport mediated by H /OC antiporter driven by the pH gradient established by H -ATPase. In endosomes of Cd-intoxicated rats, the ATP-dependent TEA uptake was markedly attenuated due to inhibition of endosomal acidification as well as H /TEA antiport. In kinetic analysis of H /TEA antiport, Vmax was reduced and Km was increased in the Cd group. Inhibition of H /TEA antiport was also observed in normal endosomes directly exposed to free Cd (but not Cd-metallothionein complex, CdMt) in vitro. These data suggest that during chronic Cd exposure, free Cd ions liberated by lysosomal degradation of CdMt in proximal tubule cells may impair the endosomal accumulation of OC by directly inhibiting the H /OC antiporter activity and indirectly by reducing the intravesicular acidification, the driving force for H /OC exchange.

Effect of cisplatin on Na+/H+ antiport in the OK renal epithelial cell line

Cis-diamminedichloroplatinum II (cisplatin), an effective antitumor agent, induces acute renal failure by unknown mechanisms. To investigate direct toxic effects of cisplatin in the renal proximal tubular transport system, OK cell line was selected as a cell model and Na+/H+ antiport activity was evaluated during a course of cisplatin treatment. The cells grown to confluence were treated with cisplatin for 1 hour, washed, and incubated for up to 48 hours. At appropriate intervals, cells were examined for Na+/H+ antiport activity by measuring the recovery of intracellular pH (pHi) after acid loading. Cisplatin of less than 50 muM induced no significant changes in cell viability in 24 hours, but it decreased the viability markedly after 48 hours. In cells exposed to 50 muM cisplatin for 24 hours, the Na+-dependent pHi recovery (i.e., Na+/H+ antiport) was drastically inhibited with no changes in the Na+-independent recovery. Kinetic analysis of the Na+-dependent pHi recovery indicated that the Vmax was reduced, but the apparent Km was not altered. The cellular Na+ and K+ contents determined immediately before the transport measurement appeared to be similar in the control and cisplatin group, thus, the driving force for Na+-coupled transport was not different. These results indicate that cisplatin exposure impairs the Na+/H+ antiport capacity in OK cells. It is, therefore, possible that in patients treated with a high dose of cisplatin, proximal tubular mechanism for proton secretion (hence HCO3- reabsorption) could be attenuated, leading to a metabolic acidosis (proximal renal tubular acidosis).