Ryanodine facilitates calcium-dependent release of transmitter at mouse neuromuscular junctions

Abstract

1. Quantal release of transmitter was measured intracellularly at mouse neuromuscular junctions in the presence and absence of ryanodine (Rnd). 2. Rnd at concentrations up to 1 μM did not significantly alter the frequency of miniature endplate potentials (m.e.p.ps) in the presence or absence of Ca2+, suggesting that Rnd is unlikely to alter the internal concentration of Ca2+ ([Ca2+](i)) at rest. 3. In a high-K+ (10 mM) bathing solution, Rnd further potentiated the facilitatory effect of Ca2+ on the frequency (F, s-1) of m.e.p.ps. Rnd shifted the relationship between ln(F) and ln[Ca2+](o) to lower concentrations. 4. In a high-Mg2+ bathing solution, Rnd did not affect the frequency of m.e.p.ps at any value of [Ca2+](o). However, Rnd slightly but significantly increased the quantal content (m) of e.p.ps. It shifted the relationship between ln(m) and ln[Ca2+](o) to lower concentrations. These results suggest that Rnd potentiates the quantal release of transmitter after depolarization of the membrane or nerve impulse, in keeping with the cooperativity of Ca2+ at the active site. 5. A series of two closely spaced nerve impulses produced a facilitation of transmitter release, as judged by the quantal content (m2) of the second response in relation to that of the first one (m1), m2/m1. Rnd did not change the ratio m2/m1. Thus Rnd is unlikely to affect the rapid phase of the sequestration of Ca2+ inside the nerve terminal. 6. High levels of K+ (5 mM) and caffeine (2 mM) potentiated both modes of transmitter release, in a manner dependent on [Ca2+](o). Caffeine did not potentiate facilitation of transmitter release. 7. These results indicate that Rnd facilitates the quantal release of transmitter presumably via an increase in [Ca2+](i) by a manner different from that of high-K+ or caffeine. The results suggest that Rnd probably affects calcium turnover in neuronal cells.