Ca2+ release and contraction induced by IP3 and contractile agonists in mammalian gastric smooth muscle

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.