Effects of Growth Hormone on the Function of β‐Adrenoceptor Subtypes in Rat Adipocytes

Objective: The influence of growth hormone (GH) on the regulation of lipolytic response to specific agonists to β‐adrenoceptors and several post‐receptor steps in the lipolytic cascade were investigated. Research Methods and Procedures: Adipose tissues from rats were incubated with or without GH (1.38 nM). After a 24‐hour incubation, isolated adipocytes were prepared for different assays. Rats were hypophysectomized. One week after operation, l‐thyroxine and hydrocortisone acetate was given to hypophysectomized rats. One group of rats was treated with GH (1.33 mg/kg, daily). After 1 week of hormonal treatment, adipose tissues were removed for different studies. Results: GH treatment increased both basal lipolysis and lipolytic sensitivity to dobutamine and CGP 12177 in adipocytes. The lipolytic sensitivity to terbutaline was not influenced by GH treatment. GH treatment increased the maximal lipolytic response to dobutamine and CGP 12177, but not to terbutaline as determined with absolute values of lipolysis. Forskolin‐induced lipolysis was increased by addition of GH to tissues. Moreover, GH treatment resulted in enhanced expression of hormone‐sensitive lipase. GH treatment in hypophysectomized rats influenced neither the expressions of Gαs protein and cholera toxin‐catalyzed adenosine diphosphate‐ribosylation of Gαs protein, nor cholera toxin‐induced 3′, 5′‐cyclic adenosine monophosphate accumulation. However, the expression of Gαi protein was decreased after GH treatment. Discussion: These and previous results suggest that GH increases lipolysis in rat adipocytes partly through the β‐adrenergic system, including increases in both β₁‐ and β₃‐adrenergic receptor function, and partly through enhanced adenylate cyclase function, and expression of hormone‐sensitive lipase, perhaps via a decrease in Gαi protein expression.     

An Assessment of the Potential for Selenium to Impair Reproduction in Bull Trout, Salvelinus confluentus, from an Area of Active Coal Mining

Selenium (Se) is an essential nutrient, but in higher concentrations can reduce recruitment in fish populations by increasing rates of deformities during early development. Recent work has identified elevated levels of Se in water and biota collected downstream from coal mining activity in Alberta's northeast slopes region. We also recently identified increased incidence of terata and edema in rainbow trout and brook trout with elevated tissue Se from this area. However, there is currently no information regarding the potential for Se to contribute to declining stocks of bull trout, a species of concern in the area. The present study provides an assessment of the potential for Se to contribute to low recruitment in bull trout downstream from coal mining activity. Non-destructive muscle biopsy sampling and a sensitive atomic fluorescence analysis technique are used to determine Se. Results indicate that most bull trout (>90%) captured immediately downstream from coal mining activity in the region have concentrations of Se that would be expected to impair recruitment. Additional work is required to determine the extent of Se's contribution to low recruitment in bull trout.     

Cytoplasmic free calcium concentration in porcine platelets: Regulation by an intracellular nonmitochondrial calcium pump and increase after thrombin stimulation

Mechanisms are assumed to exist in the resting platelet which maintain the concentration of cytoplasmic free calcium below that level required to activate cellular responses. To assess such processes the porcine platelet plasma membrane was selectively lysed with digitonin and the uptake (or flux) of free calcium monitored by an extracellular calcium electrode. Lysis resulted in an immediate lowering of the extracellular free calcium, due to the action of intracellular organelle(s) acting on the extracellular space through the permeabilized plasma membrane. In resting platelets, the rate of calcium uptake was first order with respect to the extracellular prelytic calcium concentration, and hence the cytoplasmic free concentration was found to be 1·10^?7 M by extrapolation to a point of zero flux (i.e., the null point). This approach could not be used with thrombin-stimulated platelets, as external calcium was required for both secretion of ATP + ADP and aggregation. Nevertheless, evidence for an increase in cytoplasmic free calcium after thromin stimulation was obtained. Metabolic inhibitors and agents known to inhibit calcium uptake by mitochondria had no effect on the calcium flux following lysis, indicating different mechanisms for calcium homeostasis in the platelet when compared with other cell types (e.g., liver). Levels of ionophore A23187, which caused platelet aggregation, gave a massive release of the nonmitochondrial pool of calcium into the cytoplasmic space. Thus, in porcine platelets an intracellular energy-requiring calcium pump, which sequesters calcium in a nonmitochondrial membranous compartment, is crucial for intracellular calcium homeostasis.