1. 45Ca efflux and tissue Ca content were examined in goldfish ventricles under conditions known to affect cellular Ca movements. 2. EGTA or Ca‐EGTA was added to the washout solutions in sufficient concentration (10 mM) to avoid retardation of the apparent tissue 45Ca efflux by extracellular 45Ca binding or backflux. 3. After a variable initial increase, the cellular Ca content usually stabilizes within 60 min when ventricles are immersed in Li‐ or K‐substituted saline containing 1.8 mM Ca0 (under these conditions the internal Ca2+ concentration is below 10(‐5) M). 4. 45Ca efflux is maximally activated by external concentrations of Ca2+ as low as 10(‐6) M, in both Na‐containing and Na‐free saline. 5. 45Ca efflux decreases in Na‐free solutions. It is reactivated by Na‐saline. The effect of different external Na concentration on 45Ca efflux is comparable at external Ca2+ concentrations between 10(‐6) M and 2 x 10(‐3) M. 6. Reactivation of Ca efflux after Na0 readmission is inhibited by metabolic poisoning, or in the presence of 10 mM‐caffeine. Loading with 45Ca at very low external Ca2+ concentration prevents the inhibition of Ca efflux in Na‐free medium. 7. Caffeine (10 mM) produces contractions of about equla size when K‐depolarized preparations are immersed in either Na‐ or Li‐saline. At the same time there is a similar increase in 45Ca efflux in absence of Na0 and in its presence. 8. In the virtual absence of Ca2+0 (10(‐5) M‐Ca, 10(‐2) M‐EGTA) and Na+0, the residual 45Ca efflux is reversibly inhibited by cyanide (2 mM). 9. The results are roughly compatible with the general concept of ATP‐dependent Na‐Ca exchange in internal Ca2+ homeostasis. However, this hypothesis should probably be modified to account for the fact that under physiological concentrations Na+0 and Ca2+0 do not compete for activating 45Ca efflux. Metabolic products may be involved in Na0‐ and Ca0‐dependent Ca efflux. It is therefore not excluded that a Na‐independent active mechanism co‐operates with Na‐Ca exchange in Ca extrusion.
Simmondsin, a glycoside extracted from jojoba meal (Simmondsia chinensis), causes a reduction in food intake after oral administration. To investigate the mechanism by which simmondsin reduces food intake, fasted and free-feeding rats were given simmondsin-supplemented food and simultaneously injected with devazepide, a specific antagonist of peripheral-type cholecystokinin receptors (CCKA receptors). In free-feeding rats, supplementation of food with 0.5% simmondsin caused a reduction in food intake of +/- 40% in the period of 4 h following food presentation. Intraperitoneal injection of 100 micrograms devazepide/kg body weight prevented this effect. In rats fasted for 20 h, the food intake in the 30 min after presentation of food supplemented with 0.15% or 0.50% simmondsin was reduced in a dose-related manner; this was also inhibited by simultaneous application of devazepide. It is suggested that peripheral CCKA receptors are involved in the effect of simmondsin on food intake. However, a direct effect of simmondsin on CCKA receptors has been excluded, since simmondsin was unable to cause contraction of the guinea-pig gallbladder in vitro.
Peripheral isolated populations may undergo rapid divergence from the main population due to various factors such as a bottleneck or a founder effect followed by genetic drift or local selection pressures. Recent populations of two economically important Copadichromis species in Lake Malombe, a satellite lake of Lake Malawi, were neither genetically nor morphometrically distinct from their source populations in the main lake. Evidence was found for a founder effect which had a different impact on the genetic composition of the two species. In addition, the increased fishing pressure in Lake Malombe may have led to a reduction of the body sizes of both species.
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