Effect of caffeine and reduced temperature (20 degrees C) on the organization of the pre-Golgi and the Golgi stack membranes

Caffeine is known to inhibit the enzyme phosphodiesterase (Butcher and Sutherland, 1962) , thus resulting in elevated levels of cAMP inside the cells. Forskolin, which is known to elevate cAMP levels inside cells, did not affect the movement of SFV glycoproteins (data not shown). More recently caffeine has been shown to release Ca 2 § ions from the intracellular stores other than the inositol triphosphate-sensitive store (Burgoyne et al., 1989) . Also the temperature dependence of the caffeine-induced inhibition of membrane traffic follows interestingly well the conditions used in studies of the GTP-mediated Ca 2+ release (Gill et al., 1986) . If the effect of caffeine on the membrane traffic and on the calcium release are in fact related, this could indicate that the ability of the membranes in the ER-to-Golgi interface to recycle escaped, resident ER proteins back to the ER might be regulated by calcium.