Inositol 1.4,5-trtsphosphate (IP,) induces a release of Cal' from vacuolar membrane vesicles of Succharomyces cerewiue. The amount released is dependent on IP, concentration (concentration for half maximal effect, K,,,, apparent = 0.4 PM). Myo-inositol, and mositol 1,Cbisphosphate up to 50 PM have no effect on Ca" levels in the vesicles. The IP,-induced Ca*' release is blocked by dantrolene and 8-(N,N-diethylamino)-octyl 3.4.5-trimethoxybenzoate-HCI (TMB-8). which are known to block Ca" release from Ca" stores m ammal cells. IP,-induced release of Ca" also occurs when CaZC is accumulated by means of an artificial pH gradient, indicating that the effect of IP, is not due to an effect on the vacuolar H'-ATPase.The IP,-induced Ca'+ release IS not accompanied by a change m the pH gradient, which indtcates that it ts not due to a reversal of the Ca"/nH' antiport or to a decrease in dpH by IP,. The present results suggest that IP, may act as a second messenger in the mobilization of Ca'+ in yeast cells As in plant cells, the vacuolar membrane of yeast seems to contain a Cal+ channel, which can be opened by IP,. In this respect the vacuole could function as an IP,-regulated intracellular Ca" store, equivalent to the endoplasmicand sarcoplasmic reticulum in animal cells, and play a role m Ca"-dependent signal transduction in yeast cells.
