"... While varying extracellular or intracellular Ca2+ concentration assesses the intrinsic biochemical Ca2+ cooperativity of
neurotransmitter release, varying the number of open Ca2+ channels using pharmacological channel block or the tail current
titration probes the cooperativity between individual Ca2+ channels in triggering exocytosis.
Here we provide a detailed analysis of the Ca2+ sensitivity measures probed by these experimental
protocols, present simple expressions for special cases, and demonstrate the distinction between the Ca2+ current cooperativity, defined
by the relationship between exocytosis rate and the whole-terminal Ca2+ current magnitude, and the underlying Ca2+ channel cooperativity,
defined as the average number of channels involved in the release of a single vesicle.
Further, we use three-dimensional computational modeling of buffered Ca2+
diffusion to analyze these distinct Ca2+ cooperativity measures, and demonstrate the role of endogenous Ca2+ buffers on such measures.
We show that buffers can either increase or decrease the Ca2+ current cooperativity of exocytosis, depending on their concentration and
the single-channel Ca2+ current."
Matveev V, Bertram R, Sherman A (2009) Ca2+ current versus Ca2+ channel cooperativity of exocytosis. J Neurosci 29:12196-209 [PubMed]