Hypertonicity facilitates activation of Ca²⁺-induced Ca²⁺ release at the frog motor nerve terminals in Ca²⁺ free solutions

Abstract

Repetitive Ca²⁺ entries through voltage-gated Ca²⁺ channels gradually prime β type-ryanodine receptors (RyRs) and activate Ca²⁺-induced Ca²⁺ release (CICR) which enhances both spontaneous and evoked transmitter exocytosis. The time course of priming, subsequent activation and inactivation of CICR was seen as a slow increase and subsequent decrease (hump) in the frequency of miniature end-plate potentials (MEPPs) during continuous tetanus. MEPP-hump was reduced in magnitude by reducing external Ca²⁺, and abolished in a Ca²⁺ free solution. In this study, hypertonicity produced a similar slow rise and fall of MEPP frequency even in a Ca²⁺ free, EGTA solution containing a small amount of Mg²⁺. These changes in MEPP frequency were blocked by ryanodine (20 μM), thus being equivalent in property to MEPP-hump induced by tetanus. The hypertonicity-induced MEPP-hump increased in magnitude with the extent of hypertonicity, but decreased with a rise in Mg²⁺ concentration and abolished at more than 2 mM Mg²⁺. The results suggest that hypertonicity facilitates spontaneous activation of Ca²⁺ release via RyRs even at the low basal Ca²⁺ level. Since Murayama et al. (1998) reported that hypertonicity increased the binding of ryanodine to α- and β-RyR of frog skeletal muscle, it may be that hypertonic solution sets up the condition for the activation of RyRs even at a low cytosolic Ca²⁺ level. [Jpn J Physiol 55 Suppl:S142 (2005)]