Acetylcholine receptor channels and their block by clonidine in cultured bovine chromaffin cells.

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Abstract
1. Acetylcholine (10-50 .mu.M) applied to bovine chromaffin cells under whole-cell voltage clamp produced an inward current and an increase in the noise level of the current trace. At concentrations .gtoreq. 20 .mu.M the ACh current desensitized with time. Spectral analysis of the ACh-induced increase in current noise showed that it could be fitted by a single Lorentzian component with a time constant, .tau.noise = 11 .+-. 0.9 ms (Vm = -80 mV). 2. Clonidine (2-30 .mu.M) markedly reduced the size of the ACh-induced whole-cell current, and altered the shape of the noise spectrum. In the presence of clonidine ACh noise spectra were fitted by two Lorentzian components with time constants which varied with clonidine concentration. The single-channel conductance from noise (.gamma. - 24 pS) was unaltered by clonidine. 3. The reduction in the size of the whole-cell ACh current, produced by clonidine, was not mimicked by adrenaline (at concentrations up to 60 .mu.M). GABA-induced whole-cell currents and spectra of GABA noise were also unchanged suggesting that the effect of clonidine was specific for the ACh receptor channel. 4. When applied intracellularly (from the patch pipette) clonidine had no apparent influence on the whole-cell ACh current. Furthermore, when clonidine was perfused over the cell surface at a concentration sufficient to block the whole-cell ACh response, single ACh channels could still be recorded under the patch-pipette tip (i.e. in cell-attached patches, not exposed to clonidine). 5. Clonidine block showed little voltage dependence; the peak ACh current remained linearly dependent on clamp potential. In addition, the fast and slow time constants derived from ACh noise spectra in the presence of clonidine did not show the sort of dependence on antagonist concentration expected for simple channel block; this suggests that clonidine has a complex blocking action. 6. Single ACh channels recorded in outside-out patches had a conductance of .gamma. = 39 .+-. 0.7 pS, although a subpopulation of smaller and larger events also occurred in some patches. The mean single-channel conductance in outside-out patches was unaltered by clonidine. 7. Under normal conditions the kinetics of single ACh channels were more complex than suggested from noise analysis. Burst length distributions could be described by two exponential components with fast and slow time constants of .tau.f = 0.69 .+-. 0.17 and .tau.s = 9.51 .+-. 0.84 ms. Clonidine reduced the time constants of both components of the burst length distribution and also reduced the frequency of opening of ACh channels. 8. It is concluded that the action of clonidine on nicotinic ACh receptors in chromaffin cells may involve a complex blocking mechanism, perhaps related to agonist-induced desensitization.

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