No abstract

A method is described here for making multiple fast external solution changes at the tip of a patch pipette. The time for the change, 0.2 ms, has been established by measuring changes in liquid junction potential at the tip of an open patch pipette. This technique of producing an abrupt change in solution allows agonist/receptor reactions to be studied under non-equilibrium conditions. We have applied this technique to the nicotinic receptors in outside-out patches from skeletal muscle cell line C2 (Jaffe and Saxel 1979) and from bovine adrenal chromaffin cells. The application of step changes in acetylcholine concentration produces current traces with a characteristic shape, which may be compared with the predictions of established models for the activation and desensitisation of the nicotinic receptor. The results of making single steps and also short pulses in acetylcholine concentration are demonstrated. The direct comparison of two different cholinergic agonists is demonstrated.

expressing either the fetal (Q‐F18) or the adult (Q‐A33) complement of nicotinic acetylcholine receptor subunits derived from mouse skeletal muscle. Pulses of ACh were applied to outside‐out patches of cell membrane by means of a fast perfusion system, at concentrations from 100 nM to 10 mM. We obtained current records with intracellular potentials of ‐60 and +40 mV. The goal of this study was to estimate the channel opening rate. By fitting sums of exponentials to averaged responses, we estimated the rate of development of the current on the application of acetylcholine. The rate constant of the predominant exponential component (the on‐rate) ranges over 3 orders of magnitude, from around 100 s−1 (fetal) at low concentrations of ACh to over 100 000 s−1 (fetal and adult) at the highest concentrations. We establish that our measurement of the on‐rate is not limited by technical constraints, and can therefore be related to the rate constants of a kinetic scheme. Our observations are consistent with a model having a rate‐limiting channel opening step with a forwards rate constant (β) of 80 000 s−1 on average for adult receptors and 60 000 s−1 for fetal receptors, and a minimum opening to closing ratio (β/α) of around 33 (adult) or 50 (fetal). The channel opening rate, β, varies from around 30 000 s−1 to well over 100 000 s−1 for different patches. The large variation cannot all be ascribed to errors of measurement, but indicates patch to patch variation.

No abstract

Work over the past ten years has greatly increased our understanding of both the structure and function of the muscle nicotinic acetylcholine receptor. There is a strongly supported general picture of how the receptor functions: agonist binds rapidly to sites of low affinity and channel opening occurs at a rate comparable to the agonist dissociation rate. Channel closing is slow, so the channel has a high probability of being open if both agonist-binding sites are occupied by ACh. Results of expression studies have shown that each subunit can influence AChR activation and have given a structural basis for the major physiological change known for muscle AChR, the developmental change in AChR activation. These general statements notwithstanding, there are still major areas of uncertainty which limit our understanding. We have emphasized these areas of uncertainty in this review, to indicate what needs to be done. First, the quantitative estimates of rate constants are not as strongly supported as they should be. The major reasons are twofold--uncertainties about the interpretation of components in the kinetic data and difficulties of resolving brief events. As a result, any inferences about the functional consequences of structural alterations must remain tenuous. Second, the functional behavior of individual AChRs is not as well understood as it should be. The kinetic behavior of an individual receptor clearly can be complex (section II). In addition, there is evidence that superimposed on this complexity there may be stable and kinetically distinguishable populations of receptors (section III). Until the basis for the kinetically defined populations is clarified, kinetic parameters for receptors of defined structure cannot be unambiguously obtained. Finally, it is not surprising that the studies of AChR of altered structure have not given definitive results. Two reasons should be apparent from the preceding points: there is not a fully supported approach for kinetic analysis, and the "normal" population may not be clearly defined. An additional complication is also emerging, in that the available data support the idea that specific residues distributed over all subunits may influence AChR activation. This possibility renders the task of analysis that much more difficult. The muscle nicotinic AChR has served as a prototype for the family of transmitter-gated membrane channels, which includes the muscle and neuronal nicotinic receptors, the GABAA, the glycine and possibly the non-NMDA excitatory amino acid receptor (Stroud et al., 1990). It is interesting to note that the functional properties of the GABAA receptor, probably the best-studied of the other members of the family are rather similar.(ABSTRACT TRUNCATED AT 400 WORDS)