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1.
Cells of Entamoeba histolytica accumulated K+ and extruded Na+ compared to the concentrations of those ions present in the growth medium. Pinocytic activity, measured by the uptake of horseradish peroxidase of 125I-polyvinylpyrrolidone, was high (up to 0.3 ml/ml cells per h). Upon addition of cytochalasin B, at a concentration (20 microM) that completely blocked pinocytosis, cells lost up to 40% of their Na+ content within 90 min; K+ content was not affected or increased slightly compared to control cells without the inhibitor. Cation loss was associated with cell shrinkage. The dose-response curves for the effects of cytochalasin B on pinocytosis and Na+ content were identical. These data provide direct evidence that pinocytosis is an important component of the homeostatic system for Na+. 相似文献
2.
As turkey erythrocytes were progressively depleted of ATP by preincubation with dinitrophenol, the (Na+ + K+ + 2Cl-)-cotransport system (assayed by the bumetanide-sensitive fraction of 86Rb+ influx) became less responsive to activation. The dependence upon intracellular ATP concentration was significantly steeper for transport activated by hypertonic shock (halfmaximal activity at 0.7 mM ATP) than for that activated by either epinephrine or cyclic AMP (halfmaximal activity at 1.7 mM ATP). Upon removal of epinephrine or cyclic AMP from cells that had been preincubated with those substances, bumetanide-sensitive transport activity declined sharply, even though the intracellular cyclic AMP concentration was still over 10-fold that required to maximally activate the transport system. These data are in agreement with the notion that the (Na+ + K+ + 2Cl-)-cotransport system in turkey erythrocytes is activated by cyclic AMP, presumably through the 'classical' pathway involving a protein kinase. They do however indicate that some other, as yet undefined aspect of cyclic AMP metabolism is important for the maintenance of transport activity. 相似文献
3.
The role of a pertussis toxin sensitive GTP-binding protein in mediating between cholecystokinin receptors and phosphatidylinositol 4,5-bisphosphate phosphodiesterase as well as in preventing cholecystokinin from increasing cellular cyclic AMP has been investigated using dispersed acini from rabbit pancreas. Pertussis toxin pretreatment (500 ng/ml, 2 h) did not affect cholecystokinin(octapeptide) (CCK-8)-induced increases in cytosolic free Ca2+ as judged from changes in fluorescence obtained from quin2-loaded acini. Although pretreatment with pertussis toxin was also without effect on resting acinar cell cyclic AMP levels, adenylate cyclase activity was increased, since inhibition of cyclic AMP phosphodiesterase activity by isobutylmethylxanthine (IBMX) resulted in an additional increase in cyclic AMP levels in toxin-treated acini, indicating that acinar cell adenylate cyclase activity is under some tonic inhibitory control by the pertussis toxin-sensitive inhibitory GTP-binding protein (Gi) of the adenylate cyclase system. CCK-8 gave an increase in cyclic AMP levels in both control (1.6-fold) and toxin-treated (2.3-fold) acini, leading to cyclic AMP levels in the toxin-treated acini 2-times as high as those in control acini. In the presence of IBMX, the cyclic AMP response to CCK-8 was again markedly enhanced in acini pretreated with the toxin (3.2- vs. 1.8-fold), resulting in cAMP levels in the toxin-treated acini 3.7-times those in the absence of IBMX, 2.5-times those in control acini in the presence of IBMX and 7.0-times those in control acini in the absence of IBMX. Neither the pretreatment with pertussis toxin, nor the presence of IBMX alone, nor the combination had an effect on basal amylase secretion. However, all three treatments potentiated the stimulatory effect of CCK-8 on amylase secretion and the amount of potentiation was proportional to the cyclic AMP levels reached. Our findings suggest that in the intact pancreatic acinar cell Gi inhibition of the catalytic subunit of the adenylate cyclase may largely be responsible for preventing cholecystokinin from increasing cellular cyclic AMP. They moreover show that cyclic AMP is a modulatory agent in rabbit pancreatic enzyme secretion, not able to stimulate secretion itself, but potentiating effects mediated by the phosphatidylinositol-calcium pathway. 相似文献
4.
Identification and localization of proteins encoded by two DIF-inducible genes of Dictyostelium 总被引:3,自引:0,他引:3
We show that pDd56 and pDd63, two related DIF-inducible genes of Dictyostelium, respectively encode the ST310 and ST430 polypeptides identified by Morrissey, Devine, and Loomis (1984, Dev. Biol. 103, 414-424). We localize the two proteins by immunoelectron microscopy to the extracellular matrix surrounding the stalk cells and the stalk tube. Coupled with their predicted amino acid sequence and biochemical properties, this suggests that they are structural proteins of the stalk. 相似文献
5.
6.
The bumetanide-sensitive (K+ + Na+ + 2Cl-)-cotransport system in turkey erythrocytes is activated by either of two treatments: addition of epinephrine or an increase in osmolarity. At elevated (20 mM) K+ concentration, cotransport activity induced by epinephrine slowly (within 90 min) declines to background level again. This time-dependent inactivation has been linked to bumetanide-sensitive cell swelling. We have compared both the initial rate of cotransport activity and its time dependence after induction by either epinephrine, increased osmolarity or a combination of the two treatments. As a measure of cotransport activity we took the bumetanide-sensitive fraction of 86Rb+ influx. Immediately after activation, several kinetic characteristics of this flux (Vmax; Km towards K+; Ki towards bumetanide; pH profile) were identical in cells activated by either treatment. By contrast, cotransport activated by hypertonicity was significantly more resistant towards subsequent inactivation. We show this to be due to the increase in intracellular ion concentrations brought about by hypertonic cell shrinkage. This tended to reverse the driving force for cotransport, and thereby prevented the bumetanide-sensitive swelling associated with inactivation. Our data support the notion that cell volume plays a key role both in the activation and in the time-dependent inactivation of bumetanide-sensitive transport. 相似文献
7.
Prostatic steroid-binding protein, whose expression is stimulated by androgens, consists of two subunits, one containing the polypeptides C1 and C3 and the other containing the polypeptides C2 and C3. We have isolated and sequenced cDNA clones specific for C3 mRNA and used them to isolate and characterize genomic clones for two C3 genes. Both genes are 3.2 kilobases with identical exon/intron arrangements, which is similar to the organization of the C1 and C2 genes, suggesting that they may have arisen by duplications of an ancestral gene. Finally, homologous human genes have not been detected. 相似文献
8.
The dnaK protein modulates the heat-shock response of Escherichia coli 总被引:49,自引:0,他引:49
E. coli bacteria respond to a sudden upward shift in temperature by transiently overproducing a small subset of their proteins, one of which is the product of the dnaK gene. Mutations in dnaK have been previously shown to affect both DNA and RNA synthesis in E. coli. Bacteria carrying the dnaK756 mutation fail to turn off the heat-shock response at 43 degrees C. Instead, they continue to synthesize the heat-shock proteins in large amounts and underproduce other proteins. Both reversion and P1 transduction analyses have shown that the failure to turn off the heat-shock response is the result of the dnaK756 mutation. In addition, bacteria that overproduce the dnaK protein at all temperatures undergo a drastically reduced heat-shock response at high temperature. We conclude that the dnaK protein is an inhibitor of the heat-shock response in E. coli. 相似文献
9.
During the main period of development of the oöcytes in Locusta migratoria migratorioides from 1.5 mm to 5.5 mm in length, a steady-state level of about 8 mg/ml VG (vitellogenin)2 is maintained. During this period, total uptake rate of VG by the terminal oocytes was calculated to increase linearly from about 0.05 mg/hr to about 0.5 mg/hr. This increase is matched by an increase in the rate of VG synthesis, as determined by 3H-leucine incorporation rates, while the rate of synthesis of nonvitellogenic proteins at the same time remains constant. 相似文献
10.
The hormonal and second messenger regulation of plasminogen activator (PA) activities in avian granulosa and theca cells has been documented. However, the physiological role(s) of PAs in the avian ovary remains poorly understood. The present studies were designed to evaluate PA activity in hen granulosa cells collected from the most mature (F1) preovulatory follicle at three discrete time points relative to a spontaneous ovulation and from follicles collected at various stages of follicular development. Levels of PA activity in the granulosa layer of the F1 follicle declined by greater than 90% as follicles were collected closer to their anticipated time of ovulation (e.g., from 17-16 h to 0.75-0.15 h; p less than 0.05). Timing of tissue collection was confirmed by evaluation of serum progesterone levels, which peaked as expected at the 6-5-h time point. During follicular development, PA activity was several times greater in rapidly growing follicles (6-12 mm, 1-3 wk prior to ovulation) than in slowly growing (1-5 mm) or preovulatory (F3 and F1) follicles (p less than 0.05). Granulosa cells of these rapidly growing follicles also incorporated significantly higher levels of 3H-thymidine than did granulosa cells of mature follicles (p less than 0.05), suggesting a higher level of DNA synthesis. Similarly, granulosa cells of the mitotically active germinal disc region of the F1 granulosa layer were found to possess at least 3-fold higher (p less than 0.05) levels of PA activity and a 2-fold greater level of 3H-thymidine incorporation than the more mature granulosa cells isolated from the remaining F1 granulosa layer.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献