Stimulus-secretion coupling processes in brain: Dependence upon extracellular calcium concentration

The relationship between [Ca²⁺]₀ and secretion was investigated in crude synaptosomal fractions from the cerebral cortex and the corpus striatum of the rat. The sigmoid nature of the relationship was analyzed with a linearizing modification of the Michaelis-Menten equation. Higher [K⁺]₀ increased calcium-dependent release of [³H]dopamine from striatal synaptosomes. Log releaselog [Ca²⁺]⁰ curves were shifted upward and to the left. Higher [K⁺]₀ also increased the slope, or sigmoidicity, of the relationship. In the presence of 56 mM [K⁺]⁰, release of [³H]dopamine was greater than that of [¹⁴C]γ-aminobutyrate from striatal synaptosomes at all the [Ca²⁺]₀ tested. The differences in release resulted simply from an upward shift of the curves, as no differences were observed in either the apparent K_m for calcium or the sigmoidicity of the relationship. Release of [¹⁴C]γ-aminobutyrate from cortical and striatal synaptosomes was also investigated. KCl-facilitated, calcium-dependent release of γ-aminobutyrate from cortical synaptosomes was greater than release from striatal synaptosomes. In addition to an upward shift of the log release-log [Ca²⁺]₀ plot, the curve for cortical synaptosomes exhibited less sigmoidicity. No differences in the K_m for calcium were observed. The similarity of K_m values for calcium of the secretion systems for different transmitters and for different regions suggests that a similar rate-limiting process(es) may exist in the different secretion systems. However, the differences in sigmoidicity and maximal rates of secretion suggest that the systems do differ in at least some aspects.