Metabolic and physical control of cell elongation rate: in vivo studies in nitella

Several levels of control of elongation rate are revealed through the detailed study of responses of the Nitella internode to abrupt shifts in turgor. The immediate response, which apparently reflects the physical state of the cell, is approximately described by the equation r = (P — Y)m where r is rate, P is pressure, Y is the wall's yielding threshold, and m is related to the wall's apparent fluidity (reciprocal viscosity). Because P and Y are in the range 5 to 6 atmospheres, and (P — Y) is roughly 0.2 atmosphere, elongation rate is initially extremely sensitive to changes in P. A small step-down in turgor (0.7 atmosphere) stops growth, and a similar rise greatly accelerates it. These initial responses are, however, soon (15 minutes) compensated by changes in Y. An apparent metabolism-dependent reaction (azide-sensitive) lowers Y; strain hardening (azide-insensitive) raises it. These two opposing processes, acting on Y, serve as a governor on (P — Y), tending to maintain it at a given value despite changes in P. This ability to compensate is itself a function of turgor. Turgor step-downs are less and less well compensated, leading to lower rate, as turgor falls from 5 atmospheres to about 2 atmospheres where growth appears not to resume. This is the lowest attainable yield value, Y₁. The turgor dependency of compensation reflects a turgor requirement of the Y-lowering (“wall-softening”) process. Thus the relation between steady state, r_s, and turgor is an indirect one, derived from time-dependent alterations of the cell wall. This relationship superficially resembles the instantaneously valid one in that, roughly, r_s = (P — Y₁)m_s. Y₁ and m_s, however, have much lower values than Y and m. The duality of the elongation rate versus turgor relation and the prominent role of Y in regulating rate are the major features of growth control in Nitella.     

The control of the differentiation of female embryonic germ cells in the bird

The left gonad from female chick embryos at 4–12 days of incubation was cultured in vitro as pieces of intact gonad, pieces of isolated cortex, and groups of pure germ cells. All cultures were maintained for a time equal to 17 days in ovo. At the end of the culture period, a cytological and quantitative study was made on the germ cells.The results show that some germ cells in pieces of intact 6-day gonad and pieces of 6-day cortex complete their normal developmental sequence and enter zygotene. This shows that the factors that control the differentiation of the germ cells reside in the cortex of the gonad and their expression does not depend upon the pituitary and the medullary estrogens after 6 days of incubation.Germ cells that are cultured as isolated cells do not attach to the tissue culture substrate, do not divide mitotically, and do not enter zygotene. Evidence is presented that suggests 12-day germ cells do enter zygotene when cultured with pieces of 12-day cortex. These data suggest the differentiation of the female germ cells is regulated by the somatic cells of the cortex.     

Calcitonin and CGRP block bombesin- and substance P-induced increases in airway tone

Calcitonin gene-related peptide (CGRP) and calcitonin (C) are two peptides that are cocontained and probably coreleased with the potent bronchocontrictors, bombesin (B) and substance P (SP), within the lung. Although CGRP and C have a wide intrapulmonary distribution, their actions have not been well defined. By the use of a computerized lung mechanics analyzer, changes in response to 10-min infusions of these agents were measured in spontaneously breathing, anesthetized guinea pigs. Infusion of 0.3 nmol.kg-1.min-1 CGRP and 2 nmol.kg-1.min-1 C caused little change in lung mechanics. Infusion of 0.06 nmol.kg-1.min-1 B and 0.3 nmol.kg-1.min-1 SP caused a marked increase in inspiratory, expiratory, and total pulmonary resistance (RT), from base-line values (P less than 0.02), with a maximal effect at 10 min postinfusion (PI) [RT = 326 +/- 20% (SE) (B), 490 +/- 73% (SP)]. Coinfusion of C or CGRP with B or SP at the above concentrations caused a marked reduction in SP - [RT = 189 +/- 28% (C), 142 +/- 16% (CGRP) at 10 min PI] and B - [RT = 157 +/- 18% (C), 158 +/- 10% (CGRP) at 10 min PI] induced changes in resistance (P less than 0.015). The mode of action of C and CGRP is unknown, but these peptides may antagonize the effects of B and SP via autonomic pathways by interfering with B- or SP-induced changes in intracellular calcium concentrations or by increasing intracellular cAMP levels by binding to specific cellular receptors linked to adenylate cyclase.     

Regulation of hepatic cholesterol and lipoprotein metabolism in ethinyl estradiol-treated rats

The regulation of hepatic cholesterol and lipoprotein metabolism was studied in the ethinyl estradiol-treated rat in which low density lipoprotein (LDL) receptors are increased many fold. Cholesterol synthesis was reduced at both its diurnal peak and trough by ethinyl estradiol. The diurnal variation in 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase was abolished, whereas that for acyl coenzyme A: cholesterol acyltransferase (ACAT) was retained. LDL receptor number did not vary diurnally. Feeding these animals a cholesterol-rich diet for 48 h suppressed cholesterol synthesis and reductase activities to levels similar to those found in cholesterol-fed control animals, but ACAT activity was unaffected. LDL receptors were reduced about 50%. Intravenously administered cholesterol-rich lipoproteins suppressed HMG-CoA reductase and LDL receptors in 2 h but had a variable effect on ACAT activity. Intragastric administration of mevalonolactone reduced reductase and increased acyltransferase activity but had little effect on LDL receptors when given 2 or 4 h before death. Although animals fed a cholesterol-rich diet before and during ethinyl estradiol treatment became hypocholesterolemic, free and esterified cholesterol concentrations in liver were high as was ACAT activity. HMG-CoA reductase was inhibited to levels found in control animals fed the cholesterol-rich diet. LDL receptors were increased to a level about 50% of that reached in animals receiving a control diet and ethinyl estradiol. These data demonstrate that key enzymes of hepatic cholesterol metabolism and hepatic LDL receptors respond rapidly to cholesterol in the ethinyl estradiol-treated rat. Furthermore, estradiol increases LDL receptor activity several fold in cholesterol-loaded livers.     

Co-operativity in protein-protein association. The structure and stability of the actin filament

Co-operative association, in which a protein subunit is held simultaneously by two bonds, is enormously more favorable than association forming either bond alone. A theoretical framework for calculating the effect of co-operativity is developed here, which should have a broad application to protein-protein and protein-DNA associations. The theory is applied in detail to actin. Fragmentation of an actin filament is extremely unfavorable: the association constant for annealing-fragmentation is estimated here to be at least 10(13) M-1. In contrast to these very strong bonds within the filament, subunits are loosely attached at the end, with an association constant of 2 x 10(5) M-1. The eight orders of magnitude difference between annealing-fragmentation and end association can be attributed to the co-operative formation of one additional protein-protein bond in the annealing reaction. This observation, and a quantitative analysis of the co-operativity, lead to an important conclusion: the longitudinal bond, which connects subunits in the long-pitch helix, must be substantially stronger than the diagonal bond, which connect subunits between these helices. This conclusion contradicts some recent models based on Fourier construction, in which the longitudinal bond is weak or absent. Prominent longitudinal bonds also require a rigidity of the actin filament that must be reconciled with previous reports of torsional flexibility. A hinge within the actin subunit is suggested, separating it into two flexibly attached domains. In one possible model the two domains are oriented radially: the inner domains are connected by longitudinal and diagonal bonds to form a relatively rigid helical backbone, and the outer domains are attached to this backbone by flexible hinges, permitting them to move through angles of 10 degrees to 20 degrees or more. Flexibility of the outer, myosin-binding domain should be functionally important, permitting attachment of myosin cross-bridges over a range of angles.     

Evidence for ram suspension feeding by the piscivore, Seriola dumerili (Carangidae)

Synopsis We have quantitatively analyzed a videotape of Seriola dumerili (Carangidae) displaying ram suspension-feeding behavior and ram ventilation in the field. This is the first report of facultative suspension feeding by a piscivorous carangid. The intraoral morphology of S. dumerili is not typical of ram suspension-feeding fishes in that closely-spaced, long gill rakers are lacking. While the mechanism of particle retention is not known for any ram suspension-feeding fish species, scanning electron microscopy revealed denticles on the branchial surfaces of S. dumerili that could play a role in particle entrapment.     

Maintenance of Weight Loss: A Needs Assessment

This study identified facilitators and obstacles to maintenance of weight loss following a very-low-calorie-diet and behavior modification program. A survey was mailed to a random sample of 178 program completers and received a 61% response rate; the most frequent follow-up period was more than 2 years. Twenty-nine percent reported weighing the same (within 10 lbs) or less than the end of their participation in the treatment program (maintainers), while 71% reported their present weight was a mean of 65% higher than their initial weight loss (regainers). Maintainers were significantly more likely to report engaging in regular aerobic exercise, attending a maintenance support group, and confidence in their ability to manage their weight in the future, while regainers were more likely to report stress and motivation as frequent weight management obstacles. Respondents consistently identified the need for low/no cost ongoing support. Maintainers and relapsers reported similar challenges in managing their weight, yet with different results, suggesting the need to identify subgroups for which different post-treatment support options could be applied.