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B Varnum

Publications and source records attributed to B Varnum.

25 records · Page 2Linked to original sources
PubMed1985

Insulin secretion and action in the hyperthyroid rat.

To gain insight into the mechanism of the altered carbohydrate metabolism in thyrotoxicosis, intravenous glucose tolerance tests (IVGTT) and pancreatic suppression tests (PST) were performed in hyperthyroid rats (0.1 mg/kg T4 X 5 days) to assess insulin secretion and action in vivo. Thyroid hormone injections significantly increased T4 levels (182.8 nM +/- 11.6 (SEM) versus 50.2 +/- 6.4; P less than 0.001) and baseline glucose concentrations (9.3 mM +/- 0.2 versus 7.1 +/- 0.2; P less than 0.001). Body weights, basal insulin concentrations, glucose concentrations during IVGTT, glucose disappearance rates and steady state plasma glucose levels (SSPG) were normal. Insulin concentrations during the glucose tolerance test and during the PST were significantly decreased. The metabolic clearance rate of insulin (ml/min/kg +/- SEM) was significantly (P less than 0.01) increased (54.4 +/- 3.5 versus 41.6 +/- 2.3) in the hyperthyroid rats. If the different baseline glucose values were subtracted from the glucose concentrations achieved during the 2 tests, both the glucose disappearance rate and the fall in SSPG levels were significantly enhanced in the T4-injected animals. Thus, in the hyperthyroid rat, insulin secretion is decreased, the clearance of insulin is increased and insulin sensitivity is either normal or possibly enhanced.

Animals↗
PubMed1985

Dictyostelium amebae alter motility differently in response to increasing versus decreasing temporal gradients of cAMP.

Using a perfusion chamber, we examined the behavior of individual amebae in increasing and decreasing temporal gradients of cAMP. We demonstrated that amebae respond to increasing temporal gradients of cAMP with stimulated motility and to corresponding decreasing temporal gradients with depressed motility. Depressed motility observed in decreasing temporal gradients corresponded to the inhibited levels observed when cAMP was applied at constant concentrations. These results were consistent with a simple model for the motile behavior of amebae in an early aggregation territory in which nondissipating waves of cAMP originate at the aggregation center and travel outward periodically. We conclude that chemotactically responsive amebae can assess whether a temporal gradient of chemoattractant is increasing or decreasing in the absence of a spatial gradient, and can adjust their motility accordingly.

Cell Aggregation↗
PubMed1984

Effects of cAMP on single cell motility in Dictyostelium.

The motility of individual, aggregation-competent amebae of Dictyostelium has been analyzed at different concentrations of cAMP under both nongradient and gradient conditions. The following is demonstrated: (a) concentrations of cAMP greater than 10(-8) M inhibit motility in a concentration-dependent fashion, decrease the frequency but not the degree of turning, and cause rounding in cell shape; (b) no concentration of cAMP stimulates motility, or positive chemokinesis; (c) concentrations of cAMP that stimulate a maximal chemotactic response do not affect motility and concentrations of cAMP that maximally inhibit motility do not stimulate chemotaxis under gradient conditions; and (d) the concentrations of cAMP that inhibit motility are identical under gradient and nongradient conditions.

Cell Movement↗
PubMed1983

An analysis of developmental timing in Dictyostelium discoideum.

A new method has been developed to assess the minimum complexity and relationships of those pathways (developmental timers) which time the consecutive stages of a developing system (Soll, 1983). This method has been applied to the morphogenetic program of Dictyostelium discoideum and has resulted in (1) a minimum estimate of the number of components comprising the timers for the first seven stages of morphogenesis, (2) a characterization of the temperature sensitivities of these components including demonstration of a reversible timer component, (3) detained temporal definition of a number of transition points between rate-limiting components including a major branch point for the onset of several independent timer components coincident with the onset of aggregation, and (4) a temporal model for the relationships between the timers of the seven consecutive morphogenetic stages, including several examples of parallel timers.

Biological Clocks↗
PubMed1981

Inhibition of a nutrient-dependent pinocytosis in Dictyostelium discoideum by the amino acid analogue hadacidin.

In the present study we examine the effects of the drug hadacidin (N-formyl-N- hydroxyglycine) on pinocytosis in the eukaryotic microorganism dictyostelium discoideum. At concentrations of up to approximately 8 mg/ml, hadacidin inhibited the rate of pinocytosis of fluorescein isothiocyanate (FITC) dextran in cells in growth medium in a concentration-dependent manner but had no effect on cells in starvation medium. Because hadacidin also inhibits cellular proliferation at this concentration, the relationship between growth rate and pinocytosis was studied further using another drug, cerulenin, to produce growth-arrest. These experiments showed no changes in the rate pinocytosis even after complete cessation of cellular proliferation. Other studies showed that the transfer of cells from growth to starvation medium reduced the rate of pinocytosis by approximately 50 percent. A reduction of similar magnitude occurred if cells were transferred from growth to starvation medium containing hadacidin. Also, no additional reduction in pinocytosis occurred when cells that had been treated with hadacidin were transferred to starvation medium containing hadacidin. These cells were able to take up [(14)C]hadacidin in the starvation medium. In contrast to the results with hadacidin-treated cells, cells in a cerulenin-induced state of growth-arrest when transferred to starvation medium exhibited the same 50 percent reduction in pinocytosis observed in cells not previously exposed to either drug. Cells treated with azide, in either growth or starvation medium, exhibited an immediate inhibition of all pinocytotic activity. After the transfer of log-phase cells to starvation medium supplemented with glucose, the reduction in rate was only approximately 10-15 percent. In contrast, a 50 percent reduction was observed after supplementation of starvation medium with sucrose, KCl, or concanavalin A. Maintaining the cells in growth medium containing hadacidin for as long as 16 h had no effect on the rate at which cells aggregated. These results are consistent with the conclusion that D. discoideum exhibits two types of pinocytotic activity: one that is nutrient dependent and the other independent of nutrients. This latter activity persists in starvation medium and is unaffected by hadacidin, whereas the nutrient-dependent activity is present in growth medium and is inhibited by hadacidin.

Antibiotics, Antineoplastic↗
PubMed1981

Chemoresponsiveness to cAMP and folic acid during growth, development, and dedifferentiation in Dictyostelium discoideum.

Chemoresponsiveness to cAMP and to folic acid are monitored in growing, developing, and dedifferentiating amebae of the cellular slime mold Dictyostelium discoideum. Two semiquantitative assays are employed, one measuring the directed movement of cells up a gradient of chemoattractant ('chemotaxis' assay) and the other measuring the outward spreading of cells in response to a chemical stimulant distributed equally throughout the substratum ('spreading' assay). Vegetative amebae possess relatively insignificant levels of chemotactic responsiveness to cAMP. Six h after the initiation of development, at approximately the same time as the onset of aggregation, cells rapidly acquire chemotactic responsiveness to cAMP. During 'erasure', a dedifferentiation induced by resuspending aggregating cells in fresh nutrient medium, chemotactic responsiveness to cAMP is lost just after the erasure event. By the same chemotactic assay, it is demonstrated that vegetative amebae possess a significant level of chemotactic responsiveness to folic acid. Two h after the initiation of development, cells completely lose chemotactic responsiveness to folic acid. During erasure, cells reacquire chemotactic responsiveness to folic acid at approximately the same time that they lose responsiveness to cAMP. Dramatically different results are obtained by the spreading assay. When cells lose chemotactic responsiveness to folic acid early in development and when erasing cells lose chemotactic responsiveness to cAMP, they retain the spreading response to the two stimulants, respectively. The different results obtained for chemoreception employing the two assays are discussed in terms of molecular mechanisms, and a testable hypothesis is proposed for the possible roles of chemoresponsiveness and erasure in late morphogenesis.

Chemotaxis↗