Guanosine 5'-triphosphate releases calcium from rat liver and guinea pig parotid gland endoplasmic reticulum independently of inositol 1,4,5-trisphosphate

AbstractGTP releases calcium from rat liver microsomes and guinea pig parotid gland microsomal subfractions independently of the presence of inositol 1,4,5-trisphosphate (IP3). Non-hydrolyzable guanine nucleotide analogues have no effect and inhibit the effect of GTP. The mechanism of GTP-mediated calcium release differs from IP3-mediated calcium release as indicated by the following findings: (i) GTP-induced calcium release depends on the presence of compounds which increase the viscosity of the medium (polyethylene glycol, polyvinylpyrrolidone, or bovine serum albumin); (ii) GTP-mediated calcium release is much slower; (iii) GTP-mediated calcium release is strongly temperature-dependent, whereas IP3-mediated calcium release is not; (iv) GTP-mediated calcium release is much more sensitive to a decrease of intravesicular free calcium than IP3-mediated calcium release