Fluid movement in bone: theoretical and empirical

Evidence from the literature and from our laboratory demonstrates a pronounced and rapid flow of fluids and associated solutes through the extravascular spaces in bone. Minutes after injection, large molecules such as ferritin and horseradish peroxidase (HRP) have been localized throughout the osteocytic lacunae and canaliculi of cortical bone in the chick, rat and dog. Patterns of marker movement preclude diffusion as the mechanism for solute movement and suggest a centrifugal bulk flow of fluids. We have developed a computer model of bone fluid flow that has led to the conclusion that the pattern and rate of fluid movement is governed by the pressure differential across the bone, the vascular architecture, and the porosity of the mineralized matrix. The validity of simulations in which a substance is injected and monitored over time has been tested by comparisons with actual injections of markers in the rat. Evidence is presented for a relationship between blood flow and bone dynamics in growth, repair and pathology of bone. We employed the tail suspension model of weightlessness in the rat to test the effect of posture on the perfusion of cortical bone using injections of HRP. Data indicated that perfusion of the femur was reduced by this treatment. We propose a "rheostat" mechanism, which suggests that bone perfusion may set limits for bone growth and remodeling. Therefore, bone mass reflects the ability of the vasculature to supply oxygen and nutrients to the cells on and within the mineralized matrix.

Wnt ligands influence tumour initiation by controlling the number of intestinal stem cells

Many epithelial stem cell populations follow a pattern of stochastic stem cell divisions called 'neutral drift'. It is hypothesised that neutral competition between stem cells protects against the acquisition of deleterious mutations. Here we use a Porcupine inhibitor to reduce Wnt secretion at a dose where intestinal homoeostasis is maintained despite a reduction of Lgr5+ stem cells. Functionally, there is a marked acceleration in monoclonal conversion, so that crypts become rapidly derived from a single stem cell. Stem cells located further from the base are lost and the pool of competing stem cells is reduced. We tested whether this loss of stem cell competition would modify tumorigenesis. Reduction of Wnt ligand secretion accelerates fixation of Apc-deficient cells within the crypt leading to accelerated tumorigenesis. Therefore, ligand-based Wnt signalling influences the number of stem cells, fixation speed of Apc mutations and the speed and likelihood of adenoma formation.

Treatment with a growth hormone secretagogue in a model of developing heart failure: effects on ventricular and myocyte function

BACKGROUND

Exogenous administration of growth hormone (GH) and subsequently increased production of insulin-like growth factor-1 can influence left ventricular (LV) myocardial growth and geometry in the setting of congestive heart failure (CHF). This study determined the effects of an orally active GH secretagogue (GHS) treatment that causes a release of endogenous GH on LV function and myocyte contractility in a model of developing CHF.

METHODS AND RESULTS

Pigs were randomly assigned to the following treatment groups: (1) chronic rapid pacing at 240 bpm for 3 weeks (n=11); (2) chronic rapid pacing and GHS (CP-424,391 at 10 mg x kg(-1) x d(-1), n=9); and (3) sham controls (n=8). In the untreated pacing CHF group, LV fractional shortening was reduced (21+/-2% versus 47+/-2%) and peak wall stress increased (364+/-21 versus 141+/-5 g/cm(2)) from normal control values (P:<0.05). In the GHS group, LV fractional shortening was higher (29+/-2%) and LV peak wall stress lower (187+/-126 g/cm(2)) than untreated CHF values (P:<0.05). With GHS treatment, the ratio of LV mass to body weight increased by 44% from untreated values. Steady-state myocyte velocity of shortening was reduced with pacing CHF compared with controls (38+/-1 versus 78+/-1 microm/s, P:<0.05) and was increased from pacing CHF values with GHS treatment (55+/-7 microm/s, P:<0.05).

CONCLUSIONS

The improved LV pump function that occurred with GHS treatment in this model of CHF was most likely a result of favorable effects on LV myocardial remodeling and contractile processes. On the basis of these results, further studies are warranted to determine the potential role of GH secretagogues in the treatment of CHF.

Metabolic influences of fiber size in aerobic and anaerobic locomotor muscles of the blue crab, Callinectes sapidus

Diameters of some white locomotor muscle fibers in the adult blue crab, Callinectes sapidus, exceed 500 μm whereas juvenile white fibers are &lt;100 μm. It was hypothesized that aerobically dependent processes,such as metabolic recovery following burst contractions, will be significantly impeded in the large white fibers. In addition, dark aerobic fibers of adults,which rely on aerobic metabolism for both contraction and recovery, grow as large as the white fibers. These large aerobic fibers are subdivided, however,thus decreasing the effective diameter of each metabolic functional unit and enabling aerobic contraction. The two goals of this study were: (1) to characterize the development of subdivisions in the dark levator muscle fibers and (2) to monitor post-contractile metabolism as a function of fiber size in aerobic and anaerobic levator muscles. Dark levator muscle fibers from crabs ranging from &lt;0.1 g to &gt;190 g were examined with transmission electron microscopy to determine the density of mitochondria and subdivision diameters. Across all size classes, there was a constant mitochondrial fractional area(25% of the total subdivision area) and subdivision size (mean diameter of 36.5±2.7 μm). Thus, blue crab dark levator fibers are unusual in having metabolic functional units (subdivisions) that do not increase in size during development while the contractile functional units (fibers) grow hypertrophically. The body mass scaling of post-contractile lactate dynamics was monitored during recovery from anaerobic, burst exercise in white and dark muscle, and in hemolymph. There were no differences among size classes in lactate accumulation during exercise in either muscle. However, in white fibers from large crabs, lactate continued to increase after exercise, and lactate removal from tissues required a much longer period of time relative to smaller crabs. Differences in lactate removal among size classes were less pronounced in dark fibers, and post-contractile lactate accumulation was significantly higher in white than in dark fibers from large animals. These data suggest that the large white fibers invoke anaerobic metabolism following contraction to accelerate certain phases of metabolic recovery that otherwise would be overly slow. This implies that, in addition to the typical mass-specific decrease in oxidative capacity that accompanies increases in animal mass, aerobic metabolic processes become increasingly limited by surface area to volume and intracellular diffusion constraints in developing white muscle fibers.

Poly(L-alanine) as a universal reference material for understanding protein energies and structures

We present a proposition, the "poly(L-alanine) hypothesis," which asserts that the native backbone geometry for any polypeptide or protein of M residues has a closely mimicking, mechanically stable, image in poly(L-alanine) of the same number of residues. Using a molecular mechanics force field to represent the relevant potential energy hypersurfaces, we have carried out calculations over a wide range of M values to show that poly(L-alanine) possesses the structural versatility necessary to satisfy the proposition. These include poly(L-alanine) representatives of minima corresponding to secondary and supersecondary structures, as well as poly(L-alanine) images for tertiary structures of the naturally occurring proteins bovine pancreatic trypsin inhibitor, crambin, ribonuclease A, and superoxide dismutase. The successful validation of the hypothesis presented in this paper indicates that poly(L-alanine) will serve as a good reference material in thermodynamic perturbation theory and calculations aimed at evaluating relative free energies for competing candidate tertiary structures in real polypeptides and proteins.

A serum substitute promotes osteoblast-like phenotypic expression in cultured cells from chick calvariae

The effects of medium supplements were tested on embryonic chick calvarial cells in culture. Isolates were divided among four treatment groups: Nu-Serum, chicken serum, fetal bovine serum, or calf serum. Expression of the osteoblastic phenotype was assessed by cell morphology, DNA content, [3H]thymidine incorporation, lactate production, cellular and medium alkaline phosphatase activities, and collagen synthesis. Cells grown in Nu-Serum demonstrated increased alkaline phosphatase activity and a six-fold higher rate of collagen synthesis compared to chicken serum. These cells displayed a polygonal profile, abundant rough endoplasmic reticulum, Golgi apparati, and elaborated an extensive matrix of banded collagen which was well mineralized by day 10 of culture. Although highly mitogenic, chicken serum promoted a more fibroblastoid morphology. Compared to the sera tested, Nu-Serum preferentially promoted the osteoblast-like phenotype in chick calvarial cells in culture.

Formation of the arthrodial membrane in the blue crab, Callinectes sapidus

In this study the pattern of arthrodial membrane deposition in Callinectes sapidus was determined by histological and ultrastructural examination of tissues from the carpus joint of the cheliped collected during premolt, ecdysis, postmolt, and intermolt. Apolysis in the arthrodial membrane occurs at stage D(0) and is synchronous with apolysis of the calcified cuticle. Epicuticle formation begins at early stage D(1) and is completed in late stage D(1). Procuticle deposition starts at D(2) and continues until ecdysis. Numerous cytoplasmic extensions occur throughout the lamellae. Component fibers of the arthrodial membrane are intimately associated with dense plaques on the apical membrane of the underlying hypodermal cells, suggesting a site for fiber polymerization. Deposition of the arthrodial membrane continues after ecdysis, with most of the cuticle thickening occurring during stage C. When stained with PAS and counterstained with hematoxylin, a difference can be discerned between preecdysial and postecdysial procuticle of the arthrodial membrane, a distinction not made in previous studies. The boundary between the arthrodial membrane and calcified cuticle is thicker than either of the two layers and the layers overlap rather than butting up against one another. This pattern suggests that underlying hypodermal cells have to produce multiple types of cuticle over the molt cycle. A summary of the various molting patterns in C. sapidus suggests that the control of these diverse events may prove to be complex.

A RAC-GEF network critical for early intestinal tumourigenesis

RAC1 activity is critical for intestinal homeostasis, and is required for hyperproliferation driven by loss of the tumour suppressor gene Apc in the murine intestine. To avoid the impact of direct targeting upon homeostasis, we reasoned that indirect targeting of RAC1 via RAC-GEFs might be effective. Transcriptional profiling of Apc deficient intestinal tissue identified Vav3 and Tiam1 as key targets. Deletion of these indicated that while TIAM1 deficiency could suppress Apc-driven hyperproliferation, it had no impact upon tumourigenesis, while VAV3 deficiency had no effect. Intriguingly, deletion of either gene resulted in upregulation of Vav2, with subsequent targeting of all three (Vav2-/- Vav3-/- Tiam1-/-), profoundly suppressing hyperproliferation, tumourigenesis and RAC1 activity, without impacting normal homeostasis. Critically, the observed RAC-GEF dependency was negated by oncogenic KRAS mutation. Together, these data demonstrate that while targeting RAC-GEF molecules may have therapeutic impact at early stages, this benefit may be lost in late stage disease.

Comparison of amlodipine or nifedipine treatment with developing congestive heart failure: effects on myocyte contractility

BACKGROUND

Past studies have suggested that amlodipine, a dihydropyridine L-type Ca(2+) channel antagonist, may exert useful effects in congestive heart failure (CHF). The present study examined the effects of amlodipine or nifedipine treatment in a model of developing CHF on left ventricular (LV) pump function and myocyte contractility.

METHODS AND RESULTS

Pigs (25 kg) were randomly assigned to 1 of 4 groups: 1) pacing-induced CHF (rapid atrial pacing at 240 bpm) for 3 weeks (n = 9), 2) concomitant Ca(2+) channel blockade with amlodipine (1.5 mg/kg/day) and rapid pacing (n = 7), 3) concomitant Ca(2+) channel blockade with nifedipine (0.7 mg/kg twice daily) and rapid pacing (n = 7), and 4) sham controls (n = 7). LV fractional shortening fell with pacing CHF from baseline values (17% +/- 1% v 42% +/- 1%, P <.05). With rapid pacing and concomitant amlodipine treatment, LV fractional shortening increased from pacing CHF values (24% +/- 1%, P <.05) but was unchanged with concomitant nifedipine treatment (20% +/- 2%, P =.2). LV myocyte velocity of shortening, as measured by high speed videomicroscopy, was reduced with pacing CHF compared with controls (42 +/- 2 microm/s v 87 +/- 9 microm/s, P <.05), and increased from pacing CHF values with amlodipine or nifedipine treatment (62 +/- 8 microm/s, 64 +/- 4 microm/s, respectively; P <.05). Inotropic response to extracellular Ca(2+) (8 mmol/L) was reduced with pacing CHF (94 +/- 5 microm/s v 160 +/- 15 microm/s, P <.05) and increased from CHF values with amlodipine or nifedipine treatment (132 +/- 14 microm/s and 133 +/- 7 microm/s, respectively, P <.05) CONCLUSIONS: These results suggest that the primary mechanism for the effects of amlodipine on myocyte contractility in developing CHF is because of direct Ca(2+) channel blockade.

Bradykinin degradation and relation to myocyte contractility

BACKGROUND

Past studies have demonstrated that exogenous bradykinin (BK) causes vasodilation and increases coronary blood flow, effects that may be beneficial in the setting of cardiac disease states. An important pathway for BK degradation is through angiotensin-converting enzyme (ACE), which results in the formation of a degradative peptide, BK((1-7)). The goal of this study was to examine the effects of BK, BK((1-7)), and the potential modulation of BK by ACE inhibition on myocyte contractility.

METHODS AND RESULTS

Contractile function was examined in isolated adult porcine (n = 15) left ventricular (LV) myocyte preparations in the presence or absence of BK (10(-8) mol/L), BK((1-7)) (10(-8) mol/L), and with pretreatment by ACE inhibition (benazaprilat). Myocyte velocity of shortening fell by over 15% in the presence of BK and by 8% with BK((1-7)) (P <.05 vs basal). ACE inhibition blunted the negative effect of BK on myocyte velocity of shortening by over 60% (P <.05). Furthermore, robust ACE activity coupled with significant BK degradation was demonstrated in LV-isolated myocyte preparations, and BK proteolysis was influenced by ACE inhibition.

CONCLUSION

These results suggest that BK has a direct effect on LV myocyte contractility, and that this effect may be mediated by proteolysis of BK at the level of the LV myocyte sarcolemma.

Madura Foot Due to Monosporium Apiospermum in a Native American

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Selective vasopressin, angiotensin II, or dual receptor blockade with developing congestive heart failure

With developing congestive heart failure (CHF), activation of the vasopressin V(1a) and angiotensin II type 1 (AT(1)) receptors can occur. In the present study, we examined the direct effects of V(1a) receptor blockade (V(1a) block), selective AT(1) receptor blockade (AT(1) block), and dual V(1a)/AT(1) receptor blockade (dual block) with respect to left ventricular (LV) function and contractility during the progression of CHF. LV and myocyte functions were examined in pigs with pacing CHF (rapid pacing, 240 beats/min, 3 weeks, n = 10), pacing CHF with concomitant V(1a) block (SR49059, 60 mg/kg, n = 8), pacing CHF with concomitant AT(1) block (irbesartan, 30 mg/kg, n = 7), or pacing CHF with dual block (n = 7). LV end-diastolic dimension and peak wall stress were reduced in all receptor blockade groups compared with CHF values. However, LV fractional shortening was increased only in the dual block group compared with CHF values (29 +/- 3 versus 21 +/- 2, P <.05). Basal LV myocyte percent shortening increased in the dual block group compared with CHF values (3.44 +/- 0.23 versus 2.88 +/- 0.11, P <. 05). Although V(1a) or AT(1) block reduced LV loading conditions, only dual block resulted in improved LV and myocyte shortening.