Low doses of biologically active substances: effects, possible mechanisms, and features.

Some substances can produce a response in biological systems at extraordinarily low concentrations. Often called the 'low' or 'small' dose effect, the meaning or definition of what is meant by 'low' in this context is discussed. For the purposes of this article the expression is taken to be a concentration substantially below the equilibrium dissociation constant of the effector-target complex, but extremely low concentrations (<10(-19) m) are excluded from consideration. The main features of very low or 'small' dose action are described. Possible mechanisms of the effects of very low doses are suggested. The main thrust of the paper concerns the explanation of bimodal and polymodal dose-effect curves. A general formal kinetic model for such curves and its application will be discussed.