Cultured cells from renal cortex of hibernators and nonhibernators. Regulation of cell K+ at low temperature

Cold- and ouabain-resistance of renal Na,K-ATPase in cold-exposed and hibernating jerboas (Jaculus orientalis)

The temperature dependence and the ouabain sensitivity of Na,K-ATPase was examined in the nephron of normal, cold-exposed, and hibernating jerboas. The transport and hydrolytic activity of renal Na,K-ATPase displayed similar temperature dependence in rats and normal jerboas. Cold-resistance of Na,K-ATPase appeared in cold-exposed jerboas and further increased during hibernation. Three subpopulations of Na,K-ATPase displaying very high (Ki approximately 10(-13) M), high (Ki approximately 10(-9) M) and low sensitivity to ouabain (Ki approximately 10(-6) M) were detected in the thick ascending limb and collecting duct of jerboas. In thick ascending limbs, the subpopulation of very high sensitivity to ouabain disappeared in cold-exposed animals, which accounted for the previously reported decrease in Na,K-ATPase activity. In collecting ducts of cold-exposed animals, the subpopulation of very high sensitivity to ouabain also disappeared, but the resulting decrease in activity was overbalanced by the appearance of the subpopulation of high sensitivity.

Effects of cold exposure and hibernation on renal Na,K-ATPase of the jerboa Jaculus orientalis

Changes in activity and abundance of renal Na,K-ATPase were evaluated during cold exposure and hibernation of the jerboa Jaculus orientalis by measuring the hydrolytic activity, the number of units and the transport activity of Na,K-ATPase in isolated nephron segments. As compared to controls, jerboas exposed to cold (6 degrees C) for 4-5 weeks displayed mild diuresis, decreased urinary osmolality and increased kaliuresis. In cold-exposed jerboas, Na,K-ATPase hydrolytic activity was reduced in the medullary thick ascending limb and enhanced in the cortical and outer medullary collecting duct, whereas it was not altered in other nephron segments. The number of Na,K-ATPase units and the activity of Na,K-pump, determined by [3H]-ouabain binding and by ouabain-sensitive rubidium uptake respectively, changed in parallel with the hydrolytic activity in the medullary thick ascending limb and cortical collecting duct. The maximal rate of activity (Vmax) of Na,K-ATPase was not modified further during hibernation. Thus, cold exposure, but not the onset of hibernation, induces segment-specific changes in the abundance and activity of Na,K-ATPase units which are likely to be related to the entry into hibernation, but not to the maintenance of some renal functions during deep hibernation.

Membrane function in mammalian hibernation

For homeotherms the maintenance of a high, uniform body temperature requires a constant energy supply and food intake. For many small mammals, the loss of heat in winter exceeds energy supply, particularly when food is scarce. To survive, some animals have developed a capacity for adaptive hypothermia in which they lower their body temperature to a new regulatory set-point, usually a few degrees above the ambient. This process, generally known as hibernation, reduces the temperature differential, metabolic activity, as well as the energy demand, and thus facilitates survival during winter. Successful hibernation in mammals requires that the enzymatic processes are regulated in such a manner that metabolic balance is maintained at both the high body temperature of the summer-active animal (37 degrees C) and the low body temperature of the winter-torpid animal (approx. 5 degrees C). This means that the cellular membranes have thermal properties capable of maintaining a balanced metabolism at these extreme physiological temperatures. The available evidence indicates that, for some tissues, preparation for hibernation involves an alteration in the lipid composition and thermal properties of cellular membranes. Marked differences in the thermal response of cellular membranes have been observed on a seasonal basis and, in some membranes, differences in lipid composition have been associated with the torpid state. However, to date, no consistent changes in lipid composition which would account for, or explain, the changes in membrane thermal response, have been detected. An important point to emphasize is that the process of 'homeoviscous adaptation', which occurs in procaryotes and some poikilotherms during acclimation to low temperatures, is not a characteristic feature of most membranes of mammalian hibernators.

Inhibition of succinate oxidation and K+ transport in mitochondria during hibernation

Respiration of liver mitochondria of ground squirrels changes with physiological state. The inhibition of respiration at the level of dehydrogenases occurs during hibernation which is spontaneously removed during arousal. The main mechanism causing a decrease in respiration during hibernation seems to be the inhibition of succinate oxidation, induced by oxaloacetic acid. This is evidenced by the removal of the inhibition by glutamic and isocitric acids. A close correlation between the changes of K+ transport in mitochondria and of the physiological state of hibernator is observed. During hibernation the K+ transport rate decreases 3 times and during arousal it increases 1.5-fold in comparison with the active animals. The K+ content in mitochondria of hibernating and active ground squirrels is the same, whereas during arousal it increases 2-fold.

Metabolic responses of striped bass (Morone saxatilis) to temperature acclimation. I. Alterations in carbon sources for hepatic energy metabolism

The relative contribution of carbohydrate and lipid energy metabolism in liver tissue of temperature-acclimated striped bass was examined in vitro. Respirometry experiments were conducted to assess the role of various endogenous foodstuffs in providing reduced two-carbon fragments for aerobic metabolism. Liver composition was measured as a reflection of foodstuff flux and storage. Catabolism of 14C-labeled substrates to 14CO2 was monitored to estimate the tissue capacity for utilization of various classes of compounds. When measured at the temperature of acclimation, oxygen uptake (VO2) by liver slices shows near perfect compensation between 15 and 25 degrees C, whereas a 2.75-fold increase is found between 5 and 15 degrees C. Respiratory quotients (R.Q.) near unity are found at 5 degrees C and decrease to 0.85 and 0.82 at 15 and 25 degrees C, respectively. Inhibition of VO2 by iodoacetic acid, a glycolytic inhibitor, is near 60% at 5 degrees C and decreases to 30-40% at 15 and 25 degrees C. Evolution of 14CO2 from 14C-labeled glucoses and palmitate confirms a conservation of liver tissue capacity for carbohydrate utilization between acclimation temperatures of 5 and 15 degrees C, and an increasing capacity for utilization of fatty acids as temperature increases. Ratios of 14CO2 from 14C-6-and 14C-1-labeled glucoses indicate a relative increase in the participation of the pentose shunt in carbohydrate metabolism with cold acclimation. The results are consistent with an increasing reliance on carbohydrates for energy metabolism in the cold, whereas lipid substrates are utilized more at warm temperatures. These changes may be adaptive in partitioning energy reserves for seasonal activities of migration and reproduction.

Properties of Na-K pump in primary cultures of kidney cells

Activities related to Na-K transport were measured in cell cultures of ground squirrel kidney cortex in order to compare these cells with those of intact kidney and of continuous cell lines. A microsomal preparation containing plasma membrane Na,K-ATPase from fresh kidney showed twice the activity of a similar preparation from 72-hour cultured cells. Na,K-ATPase of homogenates of 72-hour cells showed one-third to one-fourth the specific activity of that from 6-hour cultured cells. The associated K-dependent phosphatase activity also declined as a function of time in culture. The ouabain-sensitive influx of K into 6-hour cultured cells was twice as great as the K influx into 72-hour cells. The number of sites binding 3H-ouabain in intact cultured cells declined 81% on a cell protein basis between 6 and 72 hours in culture. This decline in ouabain binding sites was relatively greater than that of K influx, so that the K turnover number increased over this same time period. The decline in ouabain-sensitive K influx during culture was complementary to an increase in furosemide-sensitive K influx. Measurements of unidirectional and net K fluxes showed that there were three components of K influx into 3-day cultured cells: ouabain-sensitive Na:K exchange, furosemide-sensitive K:K exchange, and K diffusion. In the 6-hour cultures, however, there was no furosemide-sensitive K:K exchange. Thus, after three days in culture ground squirrel kidney cells lose a feature characteristic of the original parent cells (high Na,K-ATPase activity), and gain a feature common to many undifferentiated cultured cells (furosemide-sensitive K:K exchange).

Temperature dependence of active K+ transport in cation dimorphic sheep erythrocytes

Arrhenius diagrams of K+ pump fluxes measured between 15 degrees C and 41 degrees C were discontinuous in high K+ but not in low K+ sheep red cells. Exposure of low K+ cells to anti-L caused a bimodal temperature response of K+ pump flux with a transition temperature, Tc, similar to that found in high K+ cells but with comparatively higher activation energies above Tc.

Na-K pump and Na-K-ATPase: disparity of their temperature sensitivity

As previously observed in red blood cells, ouabain-sensitive K influx of kidney cells grown in culture for 3 days was much less inhibited by cooling that Na-K-ATPase of the same cells. (At 5 degrees C K influx was 9.7% of that at 38 degrees C, Na-K-ATPase, 1--2%.) Resealed ghosts of erythrocytes of ground squirrels were made containing 24Na and ATP, and the Na efflux and ATP hydrolysis were measured simultaneously. Under these conditions there was no difference in the reduction of activity with cooling, and the amount of reduction was close to that of active K transport in intact cells. The high sensitivity to temperature, characteristic of broken membranes, could not be induced in intact cells or resealed ghosts by eliminating either the Na/K gradient or the ATP gradient nor by chelation of cellular and extracellular Ca. It could not be eliminated in broken membranes by protection with ATP or Mg. Structural reorganization of membrane during lysis may cause the increase in temperature sensitivity of Na-K-ATPase.

A comparison of the effects of prior cold incubation on cerebral cortex function in a hibernator (Cricetus auratus) and a non-hibernator (Cavia porcellus)--II. High energy phosphate levels in cerebral cortex slices after in vitro cold incubation

1. ATP and CP levels were measured in brain slices from golden hamster and guinea pig after varying periods of cold storage and subsequent incubation at 37 degrees C in the presence and absence of K+ salts. 2. ATP and CP levels were maintained at higher levels in hamster tissue. 3. The results are discussed in relation to the ability of a hibernator to transform and transport chemical energy at low temperatures.