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 Km 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 Km for calcium were observed. The similarity of Km 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.
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