The influence of salinity on oxygen consumption and plasma electrolytes in grass carp, Ctenopharyngodon idella Val.

Fingerling grass carp, Ctenopharyngodon idella , 90–160 mm t.l. , were acclimated to experimental salinities of < 1 (fresh water), 91, 195 and 317 mOsm/kg. Oxygen consumption rates of fish declined, as salinity increased, from 0.16 mg O₂/g-h in fresh water to 0.11 mg O₂/g-h at an ambient concentration of 317 mOsm/kg. Plasma electrolytes (Na⁺ and Cl⁻¹) and total plasma ionic concentrations increased slightly following 10 days of exposure to ambient salinities greater than 195 mOsm/kg. At 317 mOsm/kg fish appeared to lose control of plasma electrolyte concentrations. As the osmotic gradient was reduced between the fish and the external medium it might have become advantageous to the maintenance of the osmotic equilibrium to reduce blood circulation to the gills, thus imposing a reduction in metabolic rate.     

Salinity relationships in a freshwater population of eastern mosquitofish

Tolerance limit to elevated ambient salinity, and plasma osmotic regulatory capabilities were evaluated in a freshwater population of Gambusia holbrooki. The upper tolerance limit of ambient salinity was found to be 25‰. Plasma osmotic concentrations trended sharply upward with increasing salinity, but in a stepwise fashion, unlike patterns in related, but more euryhaline species. These laboratory observations are consistent with field observations that natural populations of G holbrooki are generally restricted to fresh and dilute brackish waters.     

Hysteretic behavior of the hepatic microsomal glucose-6-phosphatase system.

Carbamyl-P:glucose and PPi:glucose phosphotransferase, but not inorganic pyrophosphatase, activities of the hepatic microsomal glucose-6-phosphatase system demonstrate a time-dependent lag in product production with 1 mM phosphate substrate. Glucose-6-P phosphohydrolase shows a similar behavior with [glucose-6-P] less than or equal to 0.10 mM, but inorganic pyrophosphatase activity does not even at the 0.05 or 0.02 mM level. The hysteretic behavior is abolished when the structural integrity of the microsomes is destroyed by detergent treatment. Calculations indicate that an intramicrosomal glucose-6-P concentration of between 20 and 40 microM must be achieved, whether in response to exogenously added glucose-6-P or via intramicrosomal synthesis by carbamyl-P:glucose or PPi:glucose phosphotransferase activity, before the maximally active form of the enzyme system is achieved. It is suggested that translocase T1, the transport component of the glucose-6-phosphatase system specific for glucose-6-P, is the target for activation by these critical intramicrosomal concentrations of glucose-6-P.     

Carbamyl phosphate: glucose phosphotransferase and glucose-6-phosphate phosphohydrolase of nuclear membrane. Interrelationships between membrane integrity, enzymic latency, and catalytic behavior.

The presence of carbamyl-phosphate:glucose phosphotransferase in liver nuclei of five species of mammals and birds is demonstrated. The activity is confined to nuclear membranes and is due exclusively to multifunctional glucose-6-phosphatase-phosphotransferase (D-glucose-6-phosphate phosphohydrolase; EC 3.1.3.9). The nuclear enzyme constitutes approximately 16 to 19 percent of total hepatic glucose-6-phosphatase-phosphotransferase. Carbamyl-phosphate:glucose phosphotransferase and glucose-6-P phosphohydrolase activities of membrane of chicken liver nuclei are shown to be catalytically identical with the maximally activated microsomal enzyme. A correspondence is seen in two-substrate kinetic double reciprocal plots, K-m or apparent K-m values for the various substrates, K-i values for the competitive inhibitors P-i and ATP, and pH-activity profiles. Comparative studies were carried out with various intact, disrupted, and detergent-dispersed membranous preparations by a combination of enzyme kinetic and electron microscopic techniques. It is concluded that (a) intimate interrelationships exists between catalytic behavior of this enzyme and morphological integrity of membranes of which the enzyme is a part; (b) activities of the enzyme of nuclear membrane appear quite available for physiological phosphorylative functions; and (c) interrelationships between membrane morphology and catalytic behavior of this membrane-bound enzyme may well be involved in the bioregulation of this complex, multifunctional enzyme system.     

Stimulation of TM3 Leydig cell proliferation via GABA<Subscript>A</Subscript> receptors: A new role for testicular GABA

The neurotransmitter gamma-aminobutyric acid (GABA) and subtypes of GABA receptors were recently identified in adult testes. Since adult Leydig cells possess both the GABA biosynthetic enzyme glutamate decarboxylase (GAD), as well as GABA_A and GABA_B receptors, it is possible that GABA may act as auto-/paracrine molecule to regulate Leydig cell function. The present study was aimed to examine effects of GABA, which may include trophic action. This assumption is based on reports pinpointing GABA as regulator of proliferation and differentiation of developing neurons via GABA_A receptors. Assuming such a role for the developing testis, we studied whether GABA synthesis and GABA receptors are already present in the postnatal testis, where fetal Leydig cells and, to a much greater extend, cells of the adult Leydig cell lineage proliferate. Immunohistochemistry, RT-PCR, Western blotting and a radioactive enzymatic GAD assay evidenced that fetal Leydig cells of five-six days old rats possess active GAD protein, and that both fetal Leydig cells and cells of the adult Leydig cell lineage possess GABA_A receptor subunits. TM3 cells, a proliferating mouse Leydig cell line, which we showed to possess GABA_A receptor subunits by RT-PCR, served to study effects of GABA on proliferation. Using a colorimetric proliferation assay and Western Blotting for proliferating cell nuclear antigen (PCNA) we demonstrated that GABA or the GABA_A agonist isoguvacine significantly increased TM3 cell number and PCNA content in TM3 cells. These effects were blocked by the GABA_A antagonist bicuculline, implying a role for GABA_A receptors. In conclusion, GABA increases proliferation of TM3 Leydig cells via GABA_A receptor activation and proliferating Leydig cells in the postnatal rodent testis bear a GABAergic system. Thus testicular GABA may play an as yet unrecognized role in the development of Leydig cells during the differentiation of the testicular interstitial compartment.     

Dissolved oxygen and thermal regimes of a Ugandan crater lake

This paper quantifies the temporal pattern of thermal stratification and deoxygenation in Lake Nkuruba, a small (3 ha), deep (maximum depth = 38 m) crater lake in western Uganda. Dissolved oxygen penetrated to an average depth of 9 m and a maximum depth of 15 m below which the lake was permanently anoxic over the 2 years of study. Although surface oxygen levels were correlated with both surface water temperature and rainfall, seasonal cycles of dissolved oxygen were not well-defined and may have been obscured by the high frequency of short-term fluctuations and by inter-annual variations caused by shifts in rainfall. Surface water temperature averaged 23.3±0.7 °C (S.D.) and varied directly with air temperature. Both diurnal changes and top-bottom temperature differentials were small averaging 1.7±0.7 °C and 1.6±0.8 °C, respectively. Thermal stability ranged from 101.3 to 499.9 g-cm cm^-2 and was positively related to surface water temperature suggesting that this small protected lake responds rapidly to short-term meteorological changes. Because contribution to the annual heat exchange cycle was confined to upper waters, the lake's annual heat budget was low, 1,073.8 cal cm^-2 yr^-1. However, net primary productivity was relatively high averaging 1.3 g C m^-2d^-1. The region where Lake Nkuruba is situated experienced a very strong earthquake (6.2 on the Richter scale) on 4 February, 1994. Subsequently, water levels dropped markedly in the lake, falling 3.14 m over a 5-month period. This revised version was published online in September 2006 with corrections to the Cover Date.     

Expression of the alpha 1-proteinase inhibitor gene family during evolution of the genus Mus

alpha 1-Proteinase inhibitors (alpha 1-PIs) are members of the serpin superfamily of proteinase inhibitors, and are important in the maintenance of homeostasis in a wide variety of animal taxa. Previous studies have shown that in mice (genus Mus), evolution of alpha 1-PIs is characterized by gene amplification, region-specific concerted evolution, and rapid accumulation of amino acid substitutions. The latter occurs primarily in the reactive center, which is the region of the alpha 1-PI molecule that determines the inhibitor's specificity for target proteinases. The P1 residue within the reactive center, which is methionine in so-called orthodox alpha 1-PIs and an amino acid other than methionine in unorthodox alpha 1-PIs, is a primary determinant of inhibitor specificity. In the present study, we find that the expression of mRNAs encoding unorthodox alpha 1-PIs is polymorphic within Mus species, i.e., among individuals or inbred strains. This is in striking contrast to mRNAs that encode orthodox alpha 1-PIs, whose concentrations are relatively invariant. The intraspecies variations in mRNA expression represent polymorphisms in the structure of the alpha 1- PI gene family. The results, taken together with previously described aspects of alpha 1-PI evolution, indicate that the dissimilar levels of polymorphism exhibited by orthodox and unorthodox alpha 1-PIs, which likely have distinct physiological functions, may reflect different levels of selective constraint. The significance of this finding to the evolution of gene families is discussed.      

The influence of ambient salinity on routine metabolism in the teleost Cyprinodon variegatus Lacepède

Routine metabolism of the euryhaline cyprinodontid Cyprinodon variegatus Lacepede was measured at a series of ambient salinities ranging from fresh water through 100‰. Fish used had been sequentially acclimated to the test salinities (ambient temperature of 20±1° C and a 12: 12 L: D schedule).
Routine metabolic rates were highest at ambient salinities from 15 to 50‰. Metabolism was somewhat lower at ambient salinities less than 15‰, and showed a sequential decline at ambient salinities greater than 50‰. It is suggested that routine metabolism is depressed at elevated salinities by reduced O, transfer, a consequence of maintenance of hydromineral balance in hypersaline waters.