Rat motor nerve terminals and the endplates they interact with exhibit changes to varying patterns of use, as when exposed to increased activation in the form of endurance exercise training. The extent to which these changes affect neuromuscular transmission efficacy is uncertain. In this study, the effects of habitual exercise on the electrophysiological properties of neuromuscular transmission in rat soleus muscle were investigated using a novel in situ approach. Consistent with previous reports, miniature endplate potential frequency was enhanced by habitual exercise. Other passive properties, such as resting membrane potential, miniature endplate potential amplitude, and "giant" miniature endplate potential characteristics were unaltered by the training program. Full-size endplate potentials were obtained by blocking soleus muscle action potentials with mu-conotoxin GIIIb. Quantal content values were 91.5 and 119.9 for control and active groups, respectively (P < 0.01). We also measured the rate and extent of endplate potential amplitude rundown during 3-s trains of continuous stimulation at 25, 50, and 75 Hz; at 50 and 75 Hz, we found both the rate and extent of rundown to be significantly attenuated (10--20%) in a specific population of cells from active rats (P < 0.05). The results establish the degree of activity-dependent plasticity as it pertains to neuromuscular transmission in a mammalian slow-twitch muscle.
Summary
K+ efflux from Umbilicaria muhlenbergii was not affected following the uptake of known amounts (< 20 μmol g‐1) of Ca2+, Mg2+, Sr2+ and Zn2+, but was increased in the cases of Cu2+ and Pb2+. In a number of samples that had taken up increasing amounts of Cu2+, a discontinuity in K+ release during metal incubation correlated with a decrease in subsequent 14C fixation. Some of the binding sites associated with the first phase of Cu2+ uptake (up to 12 μmol g‐1) are interpreted to occur near, on or within the algal cells. The larger K+ loss associated with higher Cu2+ uptake levels indicated that a component of the second Cu2+ uptake phase involved binding at or penetration of the fungal membranes. The uptake of Sr2+, Ni2+ and Zn2+ conferred some protection to samples subsequently exposed to 75 p.p.m. aqueous SO2 for 1 h, while Mg2+, and Ca2+ had no effect. The combined effects of Cu2+ uptake and SOS exposure (and also Pb2+ and SO8) were approximately cumulative. Finally, the results are explained by reference to a classification that separates metal ions into three chemically and biologically significant categories. It is concluded that class A metal ions (those with a preference for ionic interactions) and borderline metal ions with class A character tend to protect lichens against SO2 damage, while borderline metal ions with class B character (a preference for covalent interactions) have the opposite effect.
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