ISSN:
1432-1351
Quelle:
Springer Online Journal Archives 1860-2000
Thema:
Biologie
,
Medizin
Notizen:
Summary 1. In Na+-free media, 2 min glucose influx(J G i ) was sharply reduced. Li+ activated J G i to a small extent in the absence of Na+, while K+, choline and tris did not. When Na+ was varied with tris, choline, or Li+ substituted for deleted Na+, J G i was a hyperbolic function of Na+ concentration. When K+ was substituted, J G i was a linear function of Na+ concentration (Fig. 1). When K+ was varied at a low, constant Na+ concentration, K+ behaved as a fully competitive inhibitor of Na+ activation of J G i (Fig. 2). 2. At various fixed concentrations of Na+, J G i obeyed Michaelis-Menten kinetics. With decreasing Na+, there was a decrease in maximal glucose influx (J G i max) and no significant change in the apparent Michaelis constant (Kt) (Fig. 3, Table 2). Similar results were obtained with fixed glucose concentrations and varying Na+ (Fig. 4, Table 2). 3. Simultaneous determinations of22Na+ and14C-glucose influxes at 15 sec intervals showed that zero order kinetics pertained over a 2 min incubation period (Fig. 5). Influx coupling coefficients (J na i /J G i ) were determined in double labeling experiments with constant Na+ and varying glucose concentrations or with constant glucose and varying Na+. The experimentally determined coefficient was higher than 1.8 indicating a true coefficient of 2 or more. This coefficient was independent of Na+ and glucose concentrations (Fig. 6, Table 3). 4. After 30 min preloading with glucose, worms were incubated for 30 min in media with modified cation compositions containing 10.5 mM glucose. In low Na+, low K+ media, accumulation of glucose continued. In low Na+, high K+ media, there was no change in tissue glucose. In Na+-free, high K+ media, there was a net glucose efflux against a concentration difference (Table 4). Efflux of22Na+ from worms previously equilibrated in 250 mM22Na+ was not affected by the presence of glucose in the external medium. 5. The data are consistent with Crane's Na+-gradient hypothesis for the transport and accumulation of sugars.Calliobothrium is compared with some other Na+-glucose coupled animal systems and the data are discussed in terms of a model showing first order dependence on concentration and a coupling coefficient of 2 or more.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1007/BF00696634
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