Evidence for cooperativity between nicotinic acetylcholine receptors in patch clamp records

It is often assumed that ion channels in cell membrane patches gate indepen dently. However, in the present study nicotinic receptor patch clamp data o btained in cell-attached mode from embryonic chick myotubes suggest that th e distribution of steady-state probabilities for conductance multiples aris ing from concurrent channel openings may not be binomial, In patches where up to four active channels were observed, the probabilities of two or more concurrent openings were greater than expected, suggesting positive coopera tivity. For the case of two active channels, we extended the analysis by as suming that 1) individual receptors (not necessarily identical) could be mo deled by a five-state (three closed and two open) continuous-time Markov pr ocess with equal agonist binding affinity at two recognition sites, and 2) cooperativity between channels could occur through instantaneous changes in specific transition rates in one channel following a change in conductance state of the neighboring channel. This allowed calculation of open and clo sed sojourn time density functions for either channel conditional on the ne ighboring channel being open or closed. Simulation studies of two channel s ystems, with channels being either independent or cooperative, nonidentical or identical, supported the discriminatory power of the optimization algor ithm. The experimental results suggested that individual acetylcholine rece ptors were kinetically identical and that the open state of one channel inc reased the probability of opening of its neighbor.