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1. Changes in end-plate channel properties resulting from substitution of Sr2+ for Ca2+ in the Ringer solution have been analysed at the voltage clamped frog end-plate, by recording m.e.p.c.s and ACh induced noise variance. 2. In 2 mM-Sr2+--Ringer the peak size of m.e.p.c.s showed a very small increase, and the time constant of the decay phase (tau m.e.p.c.), at any given voltage, was increased by a factor of about two compared to control Ringer. The voltage dependence of tau m.e.p.c. was the same in both solutions. 3. Addition of increasing amounts of CaCl2 to 2 mM-Sr2+--Ringer produced a progressive shortening of tau m.e.p.c., with no change in voltage dependence. 4. Estimates of single channel properties from noise analysis showed that the elementary conductance appeared to be slightly increased in 2 mM-Sr2+--Ringer, whilst the mean channel life-time was prolonged by a factor of about two. These changes in single channel properties are sufficient to account for the observed changes in m.e.p.c.s. 5. Following inhibition of cholinesterase activity by neostigmine, similar effects on m.e.p.c.s and single channel properties were still observed on changing to 2 mM-Sr2+--Ringer. The shapes of m.e.p.c.s in Sr2+ + neostigmine Ringer were often altered, and showed flat 'plateaus'. 6. The observed effects of Sr2+--Ringer on channel life-time cannot be explained on the basis of changes in surface charge density on the membrane, and suggest that divalent cations have an additional, and more direct, influence on receptor channel properties.

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