Permeability of sarcoplasmic reticulum membrane. The effect of changed ionic environments on Ca2+ release

Summary Permeability properties and the effects of a changed membrane potential on Ca²⁺ release of sarcoplasmic reticulum vesicles of rabbit skeletal muscle were investigated by Millipore filtration. The relative permeability of sarcoplasmic reticulum to solutes determined under conditions of isotope exchange at equilibrium and/or under conditions of net flow of solute and water into the vesicles was as follows: sucrose, Ca²⁺, Mn²⁺–, choline⁺, Tris⁺–+, Na⁺, Li⁺, Cl^–. Transient membrane potentials were induced by rapidly changing the ionic environment of the vesicles. Knowledge of the relative permeation rates of the above ions allowed prediction of the direction and extent of membrane polarization. Osmotic effects in the polarization measurements due to the rapid influx of solute and water into the vesicles were minimized by using media containing a fast (K⁺ or Cl^–) and a relatively slow (gluconate^– or choline⁺) penetrating ion.⁴⁵Ca²⁺ efflux from vesicles derived from different parts of the sarcoplasmic reticulum structure was not appreciably changed when vesicles were made more positive inside (choline chloride potassium gluconate) or more negative inside (potassium gluconate choline chloride). These studies suggest that part or all of the ion-induced changes in sarcoplasmic reticulum membrane permeability, previously interpreted to indicate depolarization-induced Ca²⁺ release, may be due to osmotic effects.     

Effect of photoperiods on feed intake rhythms of domestic fowl

Pinealectomized and control groups of Single Comb White Leghorn pullets were housed in individual laying cages within an experimental room maintained at a temperature of 22 ± 2°C. Hourly feed intake data were collected on the birds subjected to single 3-h changes in the duration of light or dark periods at either auroral (lights-on) or vespertine (lights-off) time of a 14L:10D lighting cycle. Feed intake reached an acrophase at either the 12th or 13th hour of the light period then declined until the onset of darkness. Vespertine changes in the light cycle were more effective in shifting the intake acrophase than the auroral changes. This observation was consistent irrespective of the direction of the change. When the laying hens were subjected to a 26-h lighting rhythm, the strength of cyclic light as a zeitgeber for feed intake rhythm was again demonstrated. The feed intake rhythm developed a 26-h duration with an acrophase consistently 22–23 hours post-vespertine. Pinealectomy did not effect the ability of hens to adjust to new lighting regimens.     

Hormonal regulation of hepatic P-enolpyruvate carboxykinase (GTP) during development.

Hepatic gluconeogenesis in the rat does not begin until birth. The enzyme P-enolpyruvate carboxykinase appears initially at birth and is the final enzyme in the gluconeogenic sequence to develop. The appearance of this enzyme in the cytosol of rat liver is caused by the stimulation of enzyme synthesis, probably due directly to an increase in the hepatic concentration of cAMP. Enzyme degradation does not begin until 36 hours after birth. Studies with fetal rats in utero have shown that dibutyryl cAMP or glucagon will stimulate P-enolpyruvate carboxykinase synthesis and that this effect can be blocked by insulin. Insulin is known to depress the synthesis of P-enolpyruvate carboxykinase in adult rat liver and in Reuber H-35 liver cells in culture. The glucocorticoids are without effect on the synthesis of the enzyme in fetal rat liver. Work by Girard et al. (J. Clin. Invest. 52: 3190, 1973) has established that the molar ratio of insulin to glucagon drops from 10 immediately after birth, to 1 after one hour. This is due to both a rise in glucagon and a fall in insulin concentrations at birth. These studies, together with our work on the synthesis of P-enolpyruvate carboxykinase, indicate that the sharp drop in the concentration of insulin may relieve the normal inhibition of enzyme synthesis. This would allow the initial stimulation of enzyme synthesis by the glucagon-mediated rise in the concentration of CAMP.     

Infulence of hormones and medium composition on the degradation of phosphoenolpyruvate carboxykinase (GTP) and total protein in Reuber H35 cells.

Reuber H35 cells were pulse-labeled with radioactive leucine and the influence of hormones, serum, and amino acids on protein degradation was investigated during a subsequent chase period. Radioactive, immunoprecipitable phosphoenolpyruvate carboxykinase (GTP) (EC 4.1.1.32) had a half-life of 5 to 6 hours which was not influenced by either N6, O2-dibutyryl adenosine 3':5'-monophosphate, dexamethasone, or insulin. The rate of phosphoenolpyruvate carboxykinase degradation was the same under steady state conditions as during the approach to a new steady state following hormonal induction or deinduction of the enzyme. Therefore, hormonal regulation of enzyme activity in vivo is the result of changes in the rate of enzyme synthesis. The rate of proteolysis for total cell proteins was increased under nutritional step-down conditions produced by the removal of serum or amino acids, or both, from the medium. This effect was completely prevented by insulin. Cycloheximide and puromycin, but not actinomycin D or cordycepin, inhibited protein degradation under step-down conditions but did not further decrease the basal rate of proteolysis measured in the presence of either insulin or serum plus amino acids. There was a good correlation between changes in proteolysis produced by serum and amino acids and changes in the degradation rate of phosphoenolpyruvate carboxykinase. Also, inhibition of proteolysis with cycloheximide and puromycin was accompanied by a decrease in the degradation rate for enzyme antigen. It is suggested that nutritional step-down leads either to the synthesis or activation of a proteolytic system.     

Water in normal muscle and muscle with a tumor

The total water content, the amount of non-freezable water, and the Na⁺ and K⁺ contents in the gastrocnemius muscle of albino mice with and without a solid tumor were determined. The spin-lattice relaxation time (T₁) for the water protons in the two kinds of muscle were measured at six resonance frequencies ranging from 4.5 to 60 MHz over the temperature range +37 to −65°C. Quantitatively calculated T₁ values are given. The difference in T₁ for the two types of muscle at temperatures above −5°C is attributed to the difference in the distribution ratio of water between hydration and free states, and bears no direct relation to the concentration of Na⁺.     

Indomethacin alters the effects of substance-P and VIP on isolated airway smooth muscle

Both substance-P and vasoactive intestinal peptide (VIP) have previously been demonstrated to contract and relax, respectively, the isolated guinea pig trachea. In addition, substance-P and VIP have been localized within the pulmonary innervation of various species. In the present studies, substance-P was found to cause a concentration-related contraction of isolated lung parenchymal strips of the guinea pig, as well as isolated tracheal strips. VIP caused a significant concentration-related relaxation of the isolated tracheal strip, but not the lung parenchymal strip. Indomethacin, a prostaglandin synthetase inhibitor, potentiated the contractile response of the trachea to substance-P and inhibited the VIP- and isoproterenol-induced relaxation. These studies are potentially important in understanding the pathogenesis of bronchospastic disorders, since alterations in prostaglandin biosynthesis may result in hyperreactivity of airways to contractile agonists such as neurotransmitters, as well as an inhibition of relaxation induced by endogenous substances such as VIP or β agonists.     

Life‐history constraints in grassland plant species: a growth‐defence trade‐off is the norm

Plant growth can be limited by resource acquisition and defence against consumers, leading to contrasting trade‐off possibilities. The competition‐defence hypothesis posits a trade‐off between competitive ability and defence against enemies (e.g. herbivores and pathogens). The growth‐defence hypothesis suggests that strong competitors for nutrients are also defended against enemies, at a cost to growth rate. We tested these hypotheses using observations of 706 plant populations of over 500 species before and following identical fertilisation and fencing treatments at 39 grassland sites worldwide. Strong positive covariance in species responses to both treatments provided support for a growth‐defence trade‐off: populations that increased with the removal of nutrient limitation (poor competitors) also increased following removal of consumers. This result held globally across 4 years within plant life‐history groups and within the majority of individual sites. Thus, a growth‐defence trade‐off appears to be the norm, and mechanisms maintaining grassland biodiversity may operate within this constraint.     

The effect of insulin and glucocorticoids on the synthesis and degradation of phosphoenolpyruvate carboxykinase (GTP) in rat adipose tissue cultured in vitro.

1. Epididymal adipose tissue from the rat was maintained in culture for periods of up to 96h. 2. After an initial decrease in protein synthesis during the first 24h of culture, the adipose tissue recovered its capacity to synthesize and accumulate proteins of a relatively large size. 3. The activity of phosphoenolpyruvate carboxykinase decreased in a parallel manner, but increased again after 24h of incubation of the tissue in culture, to a value twice that noted in the tissue in vivo. This increase in enzyme activity was due to an increase in its rate of synthesis. 4. Both insulin and dexamethasone (9alpha-fluoro-16alpha-methyl-11beta,17,-21-trihydroxypregna-1,4-diene-3,20-dione) inhibited phosphoenolpyruvate carboxykinase synthesis, but dexamethasone also decreased total protein synthesis. 5. The half-life of phosphoenolpyruvate carboxykinase in adipose tissue cultured in vitro was 5--7h and was not altered by insulin or dexamethasone. 6. It is concluded that both insulin and glucocroticoids lower the activity of phosphoenolpyruvate carboxykinase in rat adipose tissue by decreasing its rate of synthesis.     

Restricted plant species on sub-Antarctic Macquarie and Heard Islands

The recent distributional history of two Macquarie Island vascular plant species, Carex trifida, Poa litorosa, and the Heard Island vascular plant, Ranunculus crassipes is examined. C. trifida is known from only one small population on the north west coast of Macquarie Island. Four populations of P. litorosa were first recorded in the 1980s; we believe however, that it was first observed, but misidentified in the 1950s. R. crassipes was first discovered on Heard Island in the late 1980s. We argue that all three species are indigenous and arrived on their respective islands within the last 200 years by natural processes, most likely from warmer neighbouring islands, where these species have more extensive distributions. There have been small-scale changes in distribution of all species, mainly expansion. Further expansion of all three species is expected as a response to warming climate. Feral rabbit grazing is having a confounding negative influence on populations of P. litorosa.