A novel parameter to estimate the minimum number of bound ligands needed to activate an ion channel.

We consider the Hill coefficient obtained from concentration-response curves and we stress the differences between the use of binding and the use of current for the response. We first show that in order to estimate the cooperativity of binding sites one must use the coefficient obtained from the binding curves and not that obtained from the current curves. For what concerns the estimation of the minimum number of bound ligands needed to activate the channel, while the coefficient obtained from the binding is simply not applicable, we show that the coefficient obtained from the current is not appropriate. We finally introduce a novel parameter that predicts the number of ligand molecules required to open the channel from current measurements at very low ligand concentration. The above considerations are exemplified by a few theoretical models. Copyright 1999 Academic Press.