Twitch and tonic muscle fibers of snake skeletal muscle differ in their synpatic as well as mechanical properties. These experiments were aimed at detemining the basis of the difference in vesicular release properties of nerve terminals at twitch and tonic endplates. Miniature endplate currents (MEPCs) were recorded from voltage-clamped garter snake muscle fibers depolarized by high K+ in either a control Ca2+ or high-Ca2+ solution. MEPC frequency increased at twitch and tonic endplates and remained elevated for 8 h during depolarization in control Ca2+. At twitch endplates depolarized in the presence of high Ca2+, an increase in MEPC frequency was followed by a progressive decline. In contrast, MEPC frequency remained elevated in high Ca2+ at tonic endplates. The observed decrease in MEPC frequency at depolarized twitch endplates in high Ca2+ was not a function of the level of depolarization or initial MEPC frequency, nor was it due to a reduction in MEPC amplitude and loss of MEPCs in baseline noise. An optical assay of presynaptic function in which the activity-dependent dye FM1-43 was used confired that quantal release differs at twitch and tonic endplates. Most twitch nerve terminals were labeled by FM1-43 during prolonged depolarization with control Ca2+ or after brief depolarization with high Ca2+. In contrast, the number of twitch nerve terminals and the degree to which they were stained was greatly reduced after prolonged exposure to high K+ and high Ca2+, whereas depolarized tonic endplates were well stained by FM1-43 during brief and prolonged exposure to high Ca2+. FM1-43 staining also revealed variable levels of quantal release between individual boutons at twitch endplates after prolonged depolarization in high-Ca2+ solution. The observed reduction in presynaptic function at twitch nerve terminals after prolonged depolarization in high-Ca2+ solution was reversible and therefore not due to irreversible damage to terminal boutons. MEPC frequency increased at both twitch and tonic endplates when either Sr2+ or Ba2+ was substituted for high Ca2+ during K+-induced depolarization. Over time, in Sr2+ or Ba2+ solutions, MEPC frequency remained elevated at tonic endplates but declined at twitch endplates with a time course similar to that observed in high Ca2+. MEPC amplitudes at both endplates remained constant. We conclude that the regulation of quantal release differs in nerve terminals innervating twitch and tonic endplates and postulate that differential intraterminal accumulation of Ca2+ may underlie the observed difference in presynaptic function.
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