On the optimality principle in a bio-flow absorption system: an analysis of the Jmax-Kt relationship in the proximal tubule of the kidney.
The absorption process of sugars and amino acids in the proximal tubule of the kidney is one of the typical examples of operation of bio-flow absorption systems in the living body. The relationship between two kinetic parameters of the Michaelis-Menten equation, Jmax and Kt, which are the main determinants of the reaction velocity in the absorption process, was investigated by using experimentally obtained data for several actively transported substances and a mathematical model of the proximal tubule. When examined the data of the parameters for sugars and amino acids obtained from both amphibian (Triturus) and mammalian (rat) kidneys, a distinct and common tendency was found such that a high affinity system has a low capacity and vice versa. For all substances tested in this study, the calculated concentration profile along the tubule monotonically reduced to zero at around the middle portion of the tubule, although each substance has considerably different values of Jmax and Kt. From the digital simulation, the observed Jmax-Kt relationship among the tested substances could be explained in terms of the optimality principle, by which all the systems could accomplish absorption with a sufficient reserve capacity within a limited length of the tubule. The importance of such an elaborated arrangement of the transport systems in the renal proximal tubule is discussed from the viewpoints of evolution and functional speciality of the organ.
Yokoyama
R
R
Department of Computer Science, Faculty of Engineering, Iwate University, Japan.
Hoshi
T
T
D000042:Absorption; D000818:Animals; D001692:Biological Transport; D003198:Computer Simulation; D007687:Kidney Tubules, Proximal / Q000502:physiology*; D008954:Models, Biological; D014319:Triturus