Pre- and postsynaptic effects of the calcium channel blocker verapamil at neuromuscular junctions.

Experiments on the frog sartorius muscle were used to study the effects of the L-type calcium channel blocker verapamil on endplate currents. Verapamil had no effect on the amplitudes of miniature and multiple-quantum endplate currents, the synchronicity of transmitter secretion, or repeat activity in nerve endings. Verapamil had no effect on the decay of miniature currents, but accelerated that of multiple-quantum currents. This effect was sharply increased after inhibition of cholinesterase activity. In conditions of inhibited cholinesterase activity, verapamil depressed currents during rhythmic stimulation. This depression was more marked in synapses with high quantal compositions and in conditions of membrane depolarization. Thus, the sensitivity of neuromuscular junction calcium channels to verapamil was unrelated to the release of transmitter from the motor nerve ending either at physiological levels of secretion or when secretion was potentiated by potassium channel blockers. At the postsynaptic level, the effect of verapamil was insignificant in relation to cholinoreceptors in the resting and active states, though verapamil could cooperatively enhance the transition of postsynaptic receptors into the desensitized state in conditions of prolonged transmitter action.

The role of desensitisation in decay time of miniature endplate currents in frogs Rana ridibunda and Rana temporaria.

A new comparative characteristic of endplate microphysiology has been introduced. It is the feasibility of receptors to become desensitised as demonstrated on two frog species, Rana temporaria and Rana ridibunda: the decay times (tau(MEPC)) of single quantum miniature endplate currents (MEPCs) in the sartorius muscles of both species were about 1 ms and were not affected by the desensitisation-promoting agent proadifen when AChE was active. However, when the desensitisation was induced by anticholinesterase neostigmine and promoted by proadifen, the prolongation of tau(MEPC) from 1 ms was almost twice as great in Rana temporaria (tau(MEPC) = 4.4 ms) than in Rana ridibunda (tau(MEPC) = 3.1). This indicates that desensitisation reduces the number of available receptors and lowers the number of available ACh molecules for repetitive binding by trapping them by desensitised, high-affinity receptors significantly more in Rana ridibunda than in Rana temporaria. The application of proadifen, a promoter of desensitisation, decreased the prolongation of MEPCs in both species, but this shortening was more rapid in Rana ridibunda than in Rana temporaria. It is concluded that the desensitisation-induced reduction in the density, and the number of postsynaptic receptors is significantly higher at Rana ridibunda than in Rana temporaria endplates.

Rapid relief of block by mecamylamine of neuronal nicotinic acetylcholine receptors of rat chromaffin cells in vitro: an electrophysiological and modeling study.

The mechanism responsible for the blocking action of mecamylamine on neuronal nicotinic acetylcholine receptors (nAChRs) was studied on rat isolated chromaffin cells recorded under whole-cell patch clamp. Mecamylamine strongly depressed (IC(50) = 0.34 microM) inward currents elicited by short pulses of nicotine, an effect slowly reversible on wash. The mecamylamine block was voltage-dependent and promptly relieved by a protocol combining membrane depolarization with a nicotine pulse. Either depolarization or nicotine pulses were insufficient per se to elicit block relief. Block relief was transient; response depression returned in a use-dependent manner. Exposure to mecamylamine failed to block nAChRs if they were not activated by nicotine or if they were activated at positive membrane potentials. These data suggest that mecamylamine could not interact with receptors either at rest or at depolarized level. Other nicotinic antagonists like dihydro-beta-erythroidine or tubocurarine did not share this action of mecamylamine although proadifen partly mimicked it. Mecamylamine is suggested to penetrate and block open nAChRs that would subsequently close and trap this antagonist. Computer modeling indicated that the mechanism of mecamylamine blocking action could be described by assuming that 1) mecamylamine-blocked receptors possessed a much slower, voltage-dependent isomerization rate, 2) the rate constant for mecamylamine unbinding was large and poorly voltage dependent. Hence, channel reopening plus depolarization allowed mecamylamine escape and block relief. In the presence of mecamylamine, therefore, nAChRs acquire the new property of operating as coincidence detectors for concomitant changes in membrane potential and receptor occupancy.