Many agonists induce "quantal" Ca2+ release or adaptive behavior in muscle ryanodine receptors

Ryanodine receptors have recently been shown to undergo an unusual kind of inactivation process termed adaptation, which bears similarities to the transient calcium releases induced in other systems by successive incremental additions of inositol-1,4,5-trisphosphate. Such releases are sometimes termed "quantal". In this study we report that many agonists induce similar behavior in muscle sarcoplasmic reticulum and that the responses depend not on the calcium pumps therein but rather on the ryanodine receptors. The chemical diversity of these agonists makes it very unlikely that adaptation simply affects the sensitivity of the receptor to agonists at any one binding site. More likely, this result indicates that adaptive behavior of ryanodine receptors results whenever the ryanodine receptor is activated and that this process affects the action of most, if not all, agonists. Evidence is presented suggesting that the releases observed do not represent all-or-none releases from vesicle subpopulations (true quantal behavior) but rather seem to involve partial release from more homogeneously sensitive stores, a process referred to here as adaptation or increment detection.