Identification and properties of a Gs protein in catfish liver membranes

Compartmentalization of non-adenine nucleotides in anoxic cardiac myocytes

Loss of 5'-nucleotides from cardiac myocytes is a distinguishing feature of myocardial ischemia. Previous work has documented dislocations of metabolic processes mediated by both purine and pyrimidine nucleotides, especially the adenine nucleotides. This study was designed to establish the extent of anoxia-induced depletion of non-adenine nucleotides in the cytosolic compartment of heart muscle cells. Cardiac myocytes were incubated aerobically (O(2)) or anoxically (N(2)) for 30 or 60 min; anoxic cells at both time points were reoxygenated for 10 min. Roughly 85-90% of cytosine triphosphate (CTP) and uridine triphosphate (UTP) were cytosolic under aerobic conditions, compared with 62% of guanosine triphosphate (GTP) and 90% of adenosine triphosphate (ATP) under similar conditions. Similarly, the total cytidine and uridine nucleotide pool of aerobic myocytes was 70-90% cytosolic vs. 61% of total guanine nucleotides and 78% of total adenine nucleotides. After the onset of anoxia, cytosolic nucleotides (principally the triphosphate forms) were quickly degraded. Reoxygenation of anoxic myocytes for 10 min allowed some recovery of ATP, GTP, and CTP, but very little recovery of UTP. The recovered nucleotide appeared almost exclusively in the cytosol. These results support the concept that non-adenine nucleotides could reach critically low levels in anoxic or ischemic heart in advance of adenine nucleotides. The importance of the depletion of non-adenine nucleotides is discussed in terms of the energetic needs of the myocyte, and the need for the cell to drive G-protein-coupled reactions, lipid synthesis, and glycogenesis.

Adenylyl cyclase activity and its modulation in the gills of Mytilus galloprovincialis exposed to Cr6+ and Cu2+

The adenylyl cyclase (AC)/cAMP system regulates a large number of physiological functions in bivalve mussels, although its basal properties and the potential effects of environmental pollutants are scarcely studied. We characterized some properties of AC and measured both the enzyme activity and the cAMP levels in the gills of the filter-feeding sea mussel Mytilus galloprovincialis. Basal AC activity was 5.6+/-0.8 pmol cAMP 10 min(-1) mg protein(-1) and showed a Km value of 0.82+/-0.06 mM for ATP in the presence of 5mM Mg2+. It was stimulated up to 2.5- and 3.5-fold by 5-HT and GTPgammaS, respectively. Similarly to what was found in other bivalves, forskolin is a poor activator that reached significant stimulation only at 100 microM. Both basal and 5-HT-stimulated AC activity were significantly increased in the gills of mussels exposed for 7 days in aquaria to Cr6+ (10 ng/l) and Cu2+ (5 microg/l). The cAMP content of the gill under these conditions was also significantly higher than in control animals. In vitro exposure of gill membrane preparations to Cr6+ and Cu2+ induced a bimodal effect. Cu2+ significantly stimulated AC activity at nanomolar concentrations, but a strong inhibition was displayed in the micromolar range. A similar bell-shaped curve was obtained in the presence of Cr6+, with maximal AC stimulation at 10(-8)M and inhibition at 10(-5)M. Overall, these data suggest that the mussel AC/cAMP system can be affected with different patterns by heavy metals. AC activity is strongly affected by acute exposure to heavy metals in vitro, probably through a direct interaction of the pollutants with the enzyme molecule, while AC activity and cAMP content increase in organisms exposed for 7 days in vivo, probably as a defense response to acclimate the physiological functions to the environmental challenge.

G proteins immunodetection and adrenergic transduction pathways in the liver of Anguilla anguilla

G proteins are members of a highly conserved superfamily of GTPases, which includes heterotrimeric (alpha, beta, gamma) proteins acting as critical control points for transmembrane signaling. In ectothermal vertebrates, knowledge about these proteins is scarce, and our work provides the first demonstration that G(s), G(q), and G(i) proteins are all present in the liver of a fish. G(q)alpha subunits of about 42 kDa have been identified in European eel (Anguilla anguilla) liver membranes, supporting previous reports about the existence of hormone transduction pathways coupled to inositol 1,4,5-trisphosphate/Ca(2+) enhancement in fish hepatocytes. Although two G(s)alpha proteins of about 45 and 52 kDa have been reported in mammals, a single isoform of approximately 45 kDa has been recognized in eel liver. G(s)alpha and G(q)alpha proteins are involved in the epinephrine transduction pathway, leading to cAMP and Ca(2+) intracellular increments, respectively. Interestingly, both messengers significantly stimulated glucose release from eel hepatocytes but with a different time course. In fact, the Ca(2+)-dependent glucose output preceded the cAMP-mediated release by about 7 min. G(i)alpha subunits of about 40 kDa were also immunodetected, suggesting the presence of hormone receptors leading to adenylyl cyclase inhibition in eel liver; however, alpha(2)- adrenoreceptor ligands were ineffective on both enzyme activity and glucose release.