Interference of Ha-ras with inositol trisphosphate-mediated Ca(2+)-release

Calcium-regulated GTPase activity in the calcium-binding protein calexcitin

Calexcitin (CE) is a calcium-binding protein, closely related to sarcoplasmic calcium-binding proteins, that is involved in invertebrate learning and memory. Early reports indicated that both Hermissenda and squid CE also could bind GTP; however, the biochemical significance of GTP-binding and its relationship to calcium binding have remained unclear. Here, we report that the GTPase activity of CE is strongly regulated by calcium. CE possessed a P-loop-like structure near the C-terminal similar to the phosphate-binding regions in other GTP-binding proteins. Site-directed mutagenesis of this region showed that Gly(182), Phe(186) and Gly(187) are required for maximum affinity, suggesting that the GTP-binding motif is G-N-x-x-[FM]-G. CE cloned from Drosophila CNS possessed a similar C-terminal sequence and also bound and hydrolyzed GTP. GTPase activity in Drosophila CE was also strongly regulated by Ca(2+), exhibiting over 23-fold higher activity in the presence of 0.3 microM calcium. Analysis of the conserved protein motifs defines a new family of Ca(2+)-binding proteins representing the first example of proteins endowed with both EF-hand calcium binding domains and a C-terminal, P-loop-like GTP-binding motif. These results establish that, in the absence of calcium, both squid and Drosophila CE bind GTP at near-physiological concentrations and hydrolyze GTP at rates comparable to unactivated ras. Calcium functions to increase GTP-binding and GTPase activity in CE, similar to the effect of GTPase activating proteins in other low-MW GTP-binding proteins. CE may, therefore, act as a molecular interface between Ca(2+) cytosolic oscillations and the G protein-coupled signal transduction.

Effect of cortisol on norepinephrine-mediated contractions in ovine uterine arteries

Cortisol potentiated norepinephrine (NE)-mediated contractions in ovine uterine arteries (UA). We tested the hypothesis that cortisol regulated alpha(1)-adrenoceptor-mediated pharmacomechanical coupling differentially in nonpregnant UA (NUA) and pregnant UA (PUA). Cortisol (10 ng/ml for 24 h) significantly increased contractile coupling efficiency of alpha(1)-adrenoceptors in NUA, but increased alpha(1)-adrenoceptor density in PUA. Cortisol potentiated NE-induced inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] synthesis in both NUA and PUA, but increased coupling efficiency of alpha(1)-adrenoceptors to Ins(1,4,5)P(3) synthesis only in NUA. Carbenoxolone alone did not affect NE-mediated Ins(1,4,5)P(3) production, but significantly enhanced cortisol-mediated potentiation of NE-stimulated Ins(1,4,5)P(3) synthesis in PUA. In addition, cortisol potentiated the NE-induced increase in Ca(2+) concentration in PUA, but increased NE-mediated contraction for a given amount of Ca(2+) concentration in NUA. Collectively, the results indicate that cortisol potentiates NE-mediated contractions differentially in NUA and PUA, i.e., by upregulating alpha(1)-adrenoceptor density leading to increased Ca(2+) mobilization in PUA while increasing alpha(1)-adrenoceptor coupling efficiency and myofilament Ca(2+) sensitivity in NUA. In addition, the results suggest that pregnancy increases type 2 11 beta-hydroxysteroid dehydrogenase activity in the UA.

P2Y purinoceptors in normal NIH 3T3 and in NIH 3T3 overexpressing c-ras

The ability of purinergic agonists to induce Ca2+ responses has been tested in two lines of murine fibroblasts: normal NIH 3T3 fibroblasts and NIH 115.14, a clone expressing high levels [1] of the c-ras protooncogene. Both kinds of cells are responsive to ATP in the range 1 microM-1 mM; ADP and ATP gamma S are almost as potent as ATP, while AMP is unable to elicit a response. Ca2+ measurements performed in single cells by image analysis show great variability among cells but in each individual responding cell the Ca2+ rise occurs in an all-or-none fashion. The transient Ca2+ response does not depend on influx from the extracellular medium. Electrophysiological experiments reveal the activation of an outward current (at -50 mV) by ATP, probably due to Ca(2+)-activated K+ channels, confirming the absence of a substantial Ca2+ influx. Finally, stimulation by ATP produces a small but significant increase in the production of inositol phosphates. These results indicate that these cell lines possess purinergic receptors which are not integral membrane channels and which are coupled to InsP3 formation and may be therefore classified as P2Y.