GTP and Ca2+ Modulate the Inositol 1,4,5-Trisphosphate-Dependent Ca2+ Release in Streptolysin O-Permeabilized Bovine Adrenal Chromaffin Cells

The inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release was
studied using streptolysin O-permeabilized bovine adrenal
chromaffin cells. The IP3-induced Ca2+ release was followed by Ca2+
reuptake into intracellular compartments. The IP3-induced Ca2+
release diminished after sequential applications of the same amount
of IP3. Addition of 20 μM GTP fully restored the sensitivity to
IP3. Guanosine 5'-O-(3-thio)triphosphate (GTPγS) could not replace
GTP but prevented the action of GTP. The effects of GTP and GTPγS
were reversible. Neither GTP nor GTPγS induced release of Ca2+ in
the absence of IP3. The amount of Ca2+ whose release was induced by
IP3 depended on the free Ca2+ concentration of the medium. At 0.3
μM free Ca2+, a half-maximal Ca2+ release was elicited with ∼0.1 μM
IP3. At 1 μM free Ca2+, no Ca2+ release was observed with 0.1 μM
IP3; at this Ca2+ concentration, higher concentrations of IP3 (0.25
μM) were required to evoke Ca2+ release. At 8 μM free Ca2+, even
0.25 μM IP3 failed to induce release of Ca2+ from the store. The
IP3-induced Ca2+ release at constant low (0.2 μM) free Ca2+
concentrations correlated directly with the amount of stored Ca2+.
Depending on the filling state of the intracellular compartment, 1
mol of IP3 induced release of between 5 and 30 mol of Ca2+.