The source, time course and stoichiometry of cytosolic free Ca2+ ([Ca2+]i) during contraction were examined in smooth muscle cells isolated from the guinea pig and human stomach. Contraction by receptor-linked agonists (eg, acetylcholine, cholecystokinin octapeptide and Met-enkephalin) was preceded by stoichiometric increases in [Ca2+]i and net 45Ca2+ efflux that were maintained in the absence of extracellular Ca2+ or in the presence of a Ca2+ channel blocker (13600). The intracellular Ca2+ store could be depleted by repeated stimulation with all agonists in Ca2+-free medium or in the presence of 13600 resulting in loss of contractile response; response was restored by re-exposure of the cells to Ca2+.
The source of intracellular Ca2+ an the signal for its release were examined in saponin-permeabilized muscle cells. The cells retained their ability to contract in response to receptor-linked agonists and developed an ability to contract in response to inositol trisphosphate (IP3). The cells accumulated Ca2+ to the same extent as intact muscle cells, but only in the presence of ATP. IP3 caused a prompt, concentration-dependent increase in contraction, [Ca2+]i and net 45Ca2+ efflux. These effects were maximally similar to those produced by CCK-8 alone or in combination with IP3: Depletion of the Ca2+ store by repeated stimulation of single muscle cells in Ca2+-free medium with IP3, acetylcholine or CCK-8 separately resulted in loss of contractile response to all three agents; the response was restored by re-exposure of the muscle cell to a cytosol-like perfusate (Ca2+ 180 nM).
The studies demonstrate that a product of membrane phosphoinositide hydrolysis is capable of mobilizing Ca2+ from a depletable, non-mitochondrial intracellular store that is utilized by receptor-linked agonists. The magnitude of IP3-induced Ca2+ release is correlated with contraction.
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Makhlouf, G.M. Ca2+ release and contraction induced by IP3 and contractile agonists in mammalian gastric smooth muscle. Mol Cell Biochem 82, 137–143 (1988). https://doi.org/10.1007/BF00242529
- inositol trisphosphate
- muscle cells