1. We have studied the effects of the nicotinic acetylcholine (ACh) receptor agonist dimethylphenylpiperazinium (DMPP) on the evoked release of ACh from motor terminals in the rat isolated hemidiaphragm using an electrophysiological approach. 2. DMPP (1-4 ìÒ) had no effect on the rate of spontaneous quantal ACh release but increased the number of quanta of ACh released per impulse during 50 Hz stimulation. The DMPPinduced increase in evoked ACh release was dependent on the frequency of stimulation, being absent when it was reduced to 0·5 Hz, but was not Ca¥ dependent, being unaffected at 50 Hz by a 4-fold decrease in the extracellular Ca¥ concentration. 3. The facilitation of evoked ACh release at 50 Hz by 2 ìÒ DMPP was abolished by 10 ìÒ of the calmodulin antagonist W7 (N-(6-aminohexyl)-5-chloro-1-naphthalenesulphonamide hydrochloride) and, in the presence of W7, 2 ìÒ DMPP depressed evoked ACh release at 0·5 Hz. The ability of the nicotinic ACh receptor antagonist vecuronium (1 ìÒ) to depress evoked ACh release at 50 Hz was also abolished by 10 ìÒ W7. 4. The present findings demonstrate, using an electrophysiological technique, that DMPP can produce changes in the evoked ACh release from rat motor nerve terminals that are consistent with the existence of facilitatory nicotinic ACh receptors on the motor nerve endings. Further, they indicate a role for calmodulin-dependent systems in this facilitatory effect of the compound.

Samples of frozen serum of human or bovine origin have been used as an additional standard for blood acid-base analysis. The serum was used to check and compare the performance of two pH electrodes and microequilibration systems. A mean buffer line for the serum was determined at the start of the series. Daily estimations of base excess and standard bicarbonate were made. When any analysis was outside two standard deviations from the mean values, the electrode system was not considered to be functioning correctly. It could be restored to normal function by suitable cleaning. The use of a control serum will demonstrate errors in the electrode system due to protein contamination which are not evident when the system is calibrated on buffer solutions.

The effects of primycin, a potent ionophore in biological membranes, have been studied at the neuromuscular junction of the garter snake. Primycin in concentrations greater than 2 × 10−7m produced a time‐ and concentration‐dependent depolarization of twitch muscle fibres. Primycin (10−7‐5 × 10−7m) produced an increased rate of quantal release of acetylcholine, which was not maintained, and a slight reduction in quantal size. Time to onset and to peak effect of primycin were concentration‐dependent whereas maximum frequency was not. Absence of extracellular Ca2+ produced a significant delay in the time to onset and to peak effect of primycin, but did not affect the peak miniature endplate potential (m.e.p.p.) frequency. Following 60 min exposure to primycin (5 × 10−7m), introduction of a high concentration of potassium (20 mm) produced no further increase in spontaneous release. In cut muscle preparations, exposure to primycin (10−7‐5 × 10−7m) reduced peak endplate current (e.p.c.) amplitude until nerve stimulation resulted in failures or the release of one or two quanta. E.p.c. amplitude was not restored with prolonged washing. The effects of primycin on the nerve terminal are considered to be consistent with its ability to increase the permeability of membranes to calcium ions resulting in an influx of extracellular calcium, an efflux of mitochondrial calcium and eventual depletion of synaptic vesicles.

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