[Calcium inhibition, a new mode for Ca2+ regulation].

The intracellular concentration of Ca2+ ([Ca2+]i) in eukaryotic cells is low under resting conditions. Membrane stimulation can produce a transient increase in [Ca2+]i from submicromolar to micromolar levels. Classically, the effects of elevated [Ca2+]i on eukaryotic cell responses are excitatory: they induce muscle contraction or activate metabolic systems. However, micromolar concentration of Ca2+ has been found to inhibit the actin-myosin-ATP interaction of the lower eukaryote Physarum polycephalum, which is the first example of an effect transiently abolished by elevation of [Ca2+]i in response to extracellular stimulation (Kohama, K.: Trends Pharmacol. Sci. 22, 433-435, 1991). The inhibitory effects on the interaction are closely related to myosin phosphorylation. The activities of kinases to phosphorylate myosin was also inhibited by Ca2+. This article describes the inhibitory mode for Ca2+ regulation of the actomyosin system in Physarum and reviews this inhibitory mode discovered subsequently in many different cell types in both plants and animals.