Effect of 1,25-dihydroxyvitamin D-3 on phosphate uptake into chick intestinal brush border membrane vesicles

The production of collagenase by human skin explants in culture is prevented by 10(-8) M dexamethasone, 5 . 10(-4) M dibutyryl cyclic AMP, or 2.5 . 10(-3) M theophylline. Decreases in collagenase activity are paralleled by reductions in the degradation of explant collagen during the culture period. Progesterone, which effectively inhibits collagenase production in rat uterine explant cultures, has no effect on human skin explants. The inhibition by cyclic AMP is nucleotide specific. When partially inhibitory concentrations of dexamethasone and dibutyryl cyclic AMP, or dexamethasone and theophylline, are added to culture medium together, the resultant inhibition is that predicted by additivity. Synergistic inhibition, as observed in rat uterus between progesterone and dibutyryl cyclic AMP, fails to occur. Dexamethasone inhibits the production of collagenase by cultured explants of rat uterus, with complete inhibition occurring at 10(-7) M steroid. Synergism between glucocorticoids and dibutyryl cyclic AMP or between dexamethasone and progesterone could not be demonstrated in the uterine culture system. These results suggest the existence of three regulatory systems for the control of collagenase production in mammalian tissues, and that cooperativity between systems may occur on a tissue-specific basis.     

Complex consequences of increased density for reproductive output in an invasive freshwater snail

Population density can have profound, often negative effects on fitness-related traits and population dynamics, and density dependence is of central importance to many prominent ecological and evolutionary hypotheses. Here, we used experimental manipulations of food, population density, and water conditioning to characterize the mechanisms underlying reproductive density-dependence in Potamopyrgus antipodarum. This New Zealand freshwater snail is a prominent model system for invasion biology, ecotoxicology, and the maintenance of sexual reproduction. We demonstrated that a primary source of negative density-dependence is food limitation, but surprisingly, we found that P. antipodarum reproductive output was much higher in high density versus low-density conditions when food was adequate. We then used manipulations of water environment to demonstrate that these positive effects of high density are likely caused by a waterborne substance produced by P. antipodarum. Altogether, these results indicate that there are strong and complex connections between food availability, density, and reproductive output in this important model system that could influence the dynamics of invasive populations, the costs and benefits of sex, and the approaches used for ecotoxicology studies.     

Escherichia coli minichromosomes: random segregation and absence of copy number control

Minichromosomes, i.e. plasmids that can replicate from an integrated oriC, have been puzzling because of their high copy numbers compared to that of the chromosomal oriC, their lack of incompatibility with the chromosome and their high loss frequencies. Using single cell resistance to tetracycline or ampicillin as an indicator of copy number we followed the development of minichromosome distributions in Escherichia coli cells transformed with minichromosomes and then allowed to grow towards the steady state. The final copy number distribution was not reached within 15 to 20 generations. If the minichromosome carried the sop (partitioning) genes from plasmid F, the development of the copy number distribution was further drastically delayed. We conclude that E. coli cells have no function that directly controls minichromosomal copy numbers, hence the absence of incompatibility in the sense of shared copy number control. We suggest that minichromosomes are subject to the same replication control as the chromosome but segregate randomly in the absence of integrated partitioning genes. This, combined with evidence that the lowest copy number classes are normally present despite high average copy numbers, can account for the high loss frequencies.     

Regulation of erythrocyte Ca2+ pump activity by protein kinase C

Using either inside-out vesicles (IOV) prepared from human erythrocytes or purified Ca2+-ATPase from the same source, the effects of protein kinase C (Ca2+/phospholipid-dependent enzyme) on Ca2+ transport and Ca2+-ATPase activity were measured. Incubation of IOV with protein kinase C in the presence, but not absence, of either 12-O-tetradecanoylphorbol-13-acetate or diolein led to a Ca2+-dependent stimulation of ATP-dependent calcium uptake. The effect was a 5-7-fold increase of Vmax without a significant change in the apparent Km for Ca2+. By comparison, the effect of calmodulin was a 14-fold stimulation of Vmax and a 4-fold reduction in apparent Km. The effect of protein kinase C and calmodulin on Ca2+ uptake were nearly additive. Stimulation of IOV Ca2+ transport by protein kinase C was entirely reversible by treatment of activated IOV with alkaline phosphatase. Incubation of purified Ca2+-ATPase with protein kinase C in the presence of 12-O-tetradecanoylphorbol-13-acetate or diolein led to a stimulation of Ca2+-dependent ATPase activity. These results indicate that protein kinase C stimulates the activity of the plasma membrane Ca2+ pump by a direct effect on the pump protein.