Down‐regulation of quantal size at frog neuromuscular junctions: Possible roles for elevated intracellular calcium and for protein kinase C

Abstract

Previous work showed that quantal size can be at least doubled at the frog neuromuscular junction by pretreatment with hormones or hypertonic solutions, primarily by the release of more acetylcholine (ACh) per quantum. Once increased, quantal size slowly declined over hours. Quantal size was measured from miniature endplate potentials (MEPPs) or currents (MEPCs). In the present experiments, preparations in which quantal size had been increased were exposed to 17–25 mM [K⁺], quantal size decreased within minutes. Release of comparable numbers of quanta by nerve stimulation did not decrease size. K⁺-solutions did not decrease size if Ca²⁺ was omitted or replaced with Sr²⁺. The phosphokinase C (PKC) activators phorbol 12,13-diacetate (PDA) and 1-oleoyl-2-acetyl-rac-glycerol (OAG) also decreased quantal size within minutes when applied in a hypertonic solution that increased the rate of spontaneous release. Phorbol 12,13-dideconate, which does not activate PKC, did not decrease quantal size. The size decrease triggered by K⁺-solutions or PKC activators was blocked by 100 μM 1-(5-isoquinolinyl-sulfonyl)-2-methyl-piperazine (H7), a protein kinase inhibitor. Apparently, in creasing [K⁺]elevated intracellular [Ca²⁺], which activates PKC, and which leads to the down-regulation of quantal size. During the period in which size is decreasing, there appears to be large and normal subpopulations of MEPP sizes, with normal gradually replacing large. This suggests that large quanta are formed by adding additional ACh to preformed quanta shortly before they are available for release.