Cyclic AMP suppresses expression of v-rasH oncogene linked to the mouse mammary tumor virus promoter

Clone 433.3 of NIH 3T3 cells is a stable carrier of the MMTV LTR:v-rasH chimeric DNA. Only in the presence of dexamethasone (a synthetic glucocorticoid), 433.3 cells exhibit an induced level of p21 transforming protein and phenotypic transformation. N6,O2'-dibutyryl cAMP (DBcAMP) antagonized the effect of dexamethasone in a time - and concentration - dependent manner. DBcAMP (5 X 10(-4)M) added 18 hr prior to the addition of dexamethasone (10(-7)M) almost completely blocked the hormone effect: cells contained levels of p21 20% of that in the cells treated with dexamethasone alone, and formed flat, contact inhibited monolayers. On the basis of these results together with our previous data on mammary carcinomas in vivo, we postulate that cAMP may be an intracellular suppressor acting at a regulatory locus of both cellular and viral ras genes.     

Comparison of uptake kinetics in freshly isolated suspensions and short-term primary cultures of rat hepatocytes

The apparent kinetics of uptake of various model substrates were examined for hepatocytes in suspension and primary culture up to 72 h. The ability of hepatocytes to take up taurocholate and ouabain was decreased in culture. Vmax for uptake of both substrates diminished rapidly with increasing time in culture. An increase in Km was observed in cultures 6 h after plating, but there was no further change with prolongation of culture time. The decrease of uptake of taurocholate and ouabain during culture may be due to the reduction in the number of transport carriers plus a decrease of affinity of the carrier to substrates. The nonsaturable component of cadmium uptake was much reduced in cultured cells compared with the suspensions. The saturable process was lower in 6 h culture but increased to a level comparable with the fresh cells at longer culture time. No significant change was found in the Km between suspensions and cultures. Uptake of alpha-aminoisobutyric acid was greater in culture while that of 3-O-methyl-D-glucose was relatively stable but about one-half that found in cell suspension. Thus, uptake of two substrates, taurocholate and ouabain, is clearly compromised with increasing time in primary culture, while uptake of the other substrates does not reflect such a dramatic decrease. It is therefore apparent that the cell preparation of choice in uptake studies depends on the substrate and the nature of the experiments.     

Peripheral chemoreflex responsiveness is increased at elevated levels of carbon dioxide after episodic hypoxia in awake humans.

We hypothesized that the acute ventilatory response to hypoxia is enhanced after exposure to episodic hypoxia in awake humans. Eleven subjects completed a series of rebreathing trials before and after exposure to eight 4-min episodes of hypoxia. During the rebreathing trials, subjects initially hyperventilated to reduce the partial pressure of carbon dioxide (Pet(CO(2))) below 25 Torr. Subjects then breathed from a bag containing normocapnic (42 Torr), low (50 Torr), or high oxygen (140 Torr) gas mixtures. During the trials, Pet(CO(2)) increased while a constant oxygen level was maintained. The point at which ventilation began to rise in a linear fashion as Pet(CO(2)) increased was considered to be the ventilatory recruitment threshold. The ventilatory response below and above the recruitment threshold was determined. Ventilation did not persist above baseline values immediately after exposure to episodic hypoxia; however, Pet(CO(2)) levels were reduced compared with baseline. In contrast, compared with baseline, the ventilatory response to progressive increases in carbon dioxide during rebreathing trials in the presence of low but not high oxygen levels was increased after exposure to episodic hypoxia. This increase occurred when carbon dioxide levels were above but not below the ventilatory recruitment threshold. We conclude that long-term facilitation of ventilation (i.e., increases in ventilation that persist when normoxia is restored after episodic hypoxia) is not expressed in awake humans in the presence of hypocapnia. Nevertheless, despite this lack of expression, the acute ventilatory response to hypoxia in the presence of hypercapnia is increased after exposure to episodic hypoxia.