Relative contribution of superficially bound and extracellular calcium to activation of contraction in isolated rat portal vein.

The spontaneous electrical and mechanical activity of the isolated rat portal vein is abolished after only 2-3 min in nominally Ca-free medium, and after 5-6 min there is no contractile response to depolarizing (122 mM K+), Ca-free solution. In the present study we have examined the electrical and mechanical responses of the portal vein to depolarization with simultaneous readministration of Ca2+ (2.5 mM) after periods of variable length in Ca-free standard solution. After 30 to 60 min of Ca depletion a slow contracture occurred in response to the high-K+ solution with 2.5 mM Ca2+. When the period in Ca-free medium was reduced below 30 min an early, faster phase appeared in the contracture response, and this phase was more rapid the shorter the time of Ca depletion. It is suggested that the slow contracture obtained after 30 min or more uses mainly extracellular Ca for activation and that the faster phase seen after shorter periods of Ca depletion is due to release of superficially bound Ca. This latter pool of tissue bound Ca does not alone produce contraction in response to depolarization, suggesting that extracellular Ca is required to trigger the release perhaps through a regenerative process.