Subplasmalemmal Ca2+ stores of probable relevance for exocytosis in Paramecium. Alveolar sacs share some but not all characteristics with sarcoplasmic reticulum.

Isolated subplasmalemmal Ca2+ stores ('alveolar sacs') from Paramecium tetraurelia cells sequester 45Ca2+ depending on ATP concentration. 45Ca2+ uptake is sensitive to SERCA-type Ca(2+)-ATPase inhibitors. They cause a slow release of 45Ca2+, as does caffeine. Of some importance are also the negative results we obtained with ryanodine, inositol 1,4,5-trisphosphate (InsP3), cyclic adenosinediphosphoribose (cADPR), 3',5'-cyclic guanosine monophosphate (cGMP, +/- beta-nicotinamide-adenine dinucleotide) or with increased [Ca2+]. These data were corroborated by experiments in vivo, including microinjection studies. Again ryanodine, InsP3, cADPR or cGMP did not trigger exocytosis, the trigger effect of SERCA inhibitors was sluggish, whereas caffeine induced exocytosis in a dose-dependent fashion. We then tested 45Ca2+ release also with isolated cell cortices (cell fragments containing cell membranes with stores and secretory organelles still attached). Under conditions which initiate exocytosis in vitro (depending on [ATP], reduction of [Mg2+] in presence of Ca2+; c.f. Lumpert et al. 1990, Biochem. J. 269, 639) we observed significant 45Ca2+ release with cortices as with isolated alveolar sacs. Our interpretation is as follows. (a) Alveolar sacs have a SERCA-type Ca(2+)-pump. (b) They have some sensitivity to caffeine, but none to ryanodine, InsP3 or cADPR. (c) There might be a direct functional coupling of these subplasmalemmal Ca2+ stores to the plasmalemma to which they are connected via feet-like structures; also like the SR, activation of this store is modulated by Mg2+ and ATP.