Control of Intracellular Calcium Redistribution by Guanine Nucleotides and Inositol 1,4,5-Trisphosphate in Permeabilized GH4C1 Cells*

Abstract

In GH4C1 rat pituitary cells, a GTP-binding protein appears to be involved in signal transduction between the TRH receptor and phospholipase C. In certain other cell types, another role for GTP has been reported, namely regulation of Ca2+ translocation from one intracellular pool to another. Using digitonin-permeabilized GH4C1 cells, we have investigated whether an analogous process occurs in pituitary cells. In permeabilized GH4C1 cells, TRH, inositol 1,4,5-trisphosphate (IP3), and nonhydrolyzable GTP analogs guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) and 5'-guanylyl imidodiphosphate each increased free Ca2+ concentration [( Ca2+]). Unlike several other systems, GTP did not increase [Ca2+]. Guanosine 5'-O-(2-thiodiphosphate) inhibited Ca2+ release induced by both TRH and GTP gamma S. Heparin abolished IP3-induced Ca2+ release but did not prevent Ca2+ release induced by TRH or GTP gamma S, suggesting a mechanism for their actions that did not depend solely on IP3 production. Neomycin inhibited GTP gamma S-induced Ca2+ release, but it did not prevent TRH- or IP3-induced Ca2+ release. In the absence of ATP, GTP gamma S did not elevate [Ca2+], although TRH and IP3 did, suggesting that ATP-dependent sequestration of Ca2+ was necessary for the action of GTP gamma S in this system, but not for TRH and IP3. Repeated additions of IP3 resulted in an attenuation of the response to IP3- GTP gamma S, which itself increased [Ca2+] after IP3 attenuation, restored the attenuated Ca2+ response to IP3. We conclude that, in permeabilized GH4C1 cells, GTP gamma S as well as TRH cause intracellular Ca2+ release; however, their mechanisms of action are, at least in part, distinct. Furthermore, the IP3-depletable Ca2+ pool can be refilled from a GTP gamma S-sensitive compartment via Ca2+ transport through the cytosol.