Role for endothelial cell conduction in ascending vasodilatation and exercise hyperaemia in hamster skeletal muscle.

1. Vasodilatation initiated by contracting skeletal muscle 'ascends' from the arteriolar network to encompass feed arteries. Acetylcholine delivery from a micropipette onto a feed artery evokes hyperpolarisation at the site of application; this signal can conduct through gap junctions along the endothelium to produce vasodilatation. We tested whether conduction along the endothelium contributes to the ascending vasodilatation that occurs in response to muscular exercise. 2. In anaesthetised hamsters, a feed artery (resting diameter 64 +/- 4 microm) supplying the retractor muscle was either stimulated by local microiontophoretic application of acetylcholine or the muscle was contracted rhythmically (once per 2 s, 1-2 min), before and after light-dye treatment (LDT) to disrupt the endothelial cells within a 300 microm-long segment located midway along the vessel. Endothelial cell damage with LDT was confirmed by the local loss of vasodilatation in response to acetylcholine and labelling with propidium iodide. Local vasodilatation in response to acetylcholine applied 500 microm proximal (upstream) or distal (downstream) to the central segment with LDT remained intact. 3. Before LDT, vessel diameter increased by more than 30 % along the entire feed artery (observed 1000 microm upstream from the retractor muscle) in response to distal acetylcholine or muscle contractions. Following LDT, vasodilatation in response to acetylcholine and to muscle contractions encompassed the distal segment but did not travel through the region of endothelial cell damage. At the upstream site, wall shear rate (and luminal shear stress) increased more than 3-fold, with no change in vessel diameter. Thus, flow-induced vasodilatation did not occur. 4. In response to muscle contractions, feed artery blood flow increased nearly 6-fold; this hyperaemic response was reduced by half following the loss of ascending vasodilatation. 5. These findings indicate that rhythmic contractions of skeletal muscle can initiate the conduction of a signal along the endothelium. We propose that this signalling pathway underlies ascending vasodilatation and promotes the full expression of exercise hyperaemia.

Motor nerve topology reflects myocyte morphology in hamster retractor and epitrochlearis muscles.

Neuromuscular activation is a primary determinant of metabolic demand and oxygen transport. The m. retractor and m. epitrochlearis are model systems for studying metabolic control and oxygen transport; however, the organization of muscle fibers and motor nerves in these muscles is unknown. We tested whether the topology of motor innervation was related to the morphology of muscle fibers in m. retractor and m. epitrochlearis of male hamsters ( approximately 100 g). Respective muscles averaged 47 and 12 mm in length 100 and 35 mg in mass. Staining for acetylcholinesterase revealed neuromuscular junctions arranged in clusters throughout m. retractor and as a central band across m. epitrochlearis, suggesting differences in fiber morphology. For both muscles, complete cross-sections contained approximately 1,700 fibers. Fiber cross-sectional areas were distributed nearly normal in m. epitrochlearis (mean = 1,559 +/- 17 microm(2)) and skewed left (P < 0.05) in m. retractor (mean = 973 +/- 15 microm(2)). Single fiber length (Lf) spanned muscle length (Lm) in m. epitrochlearis, while fibers tapered to terminate within m. retractor (Lf/Lm = 0.43 +/- 0. 02). With myelin staining, a single branch of ulnar nerve projected axons across the midregion of m. epitrochlearis. For m. retractor, the spinal accessory nerve branched to give rise to proximal and distal regions of innervation, with intermingling of axons between nerve branches. Nerve bundle cross-sections stained for acetylcholinesterase indicate that each motor axon projects to an average of 65 muscle fibers in m. epitrochlearis and 100 in m. retractor. Differences in fiber morphology, innervation topology, and neuromuscular organization may contribute to the heterogeneity of metabolic demand and oxygen supply in skeletal muscle.

Attenuation of vasodilatation with skeletal muscle fatigue in hamster retractor.

We tested the hypothesis that muscle fatigue would attenuate vasodilatory responsiveness throughout the resistance network. The retractor muscle of anaesthetized hamsters was contracted (once per 2 s for 1 min) at duty cycles of 2.5, 10 and 20 % before and after fatiguing contractions that diminished peak tension and muscle glycogen by >50 %. Arterioles and feed arteries (FA) dilated maximally during fatiguing contractions. Resting vasomotor tone consistently recovered following contractions. Peak blood flow was proportional to integrated tension (tension x time, expressed in mN mm-2 s); both increased with duty cycle and decreased with fatigue. Total integrated vasodilatory responses (diameter x time, expressed in microm s) increased with duty cycle and decreased with fatigue. Vasodilatation during contractions plateaued at approximately 50 % of peak integrated tension. Post-contraction vasodilatation increased with integrated tension and both were attenuated with fatigue. As integrated tension increased, distal arterioles dilated first and to the greatest extent relative to proximal arterioles and FA. Fatigue had little effect on dilatation of distal arterioles whereas dilatation of proximal arterioles and FA was suppressed. Latency of onset for vasodilatation decreased as duty cycle increased and was unaffected by fatigue. Vasodilatation and blood flow increase in proportion to integrated tension, with an ascending locus of vasomotor control and prolongation of post-contraction vasodilatation. With muscle fatigue, the locus of flow control resides in distal arterioles; both ascending and post-contraction vasodilatations are attenuated despite normal vasomotor tone.

Evaluation of health risks for contaminated aquifers.

This review focuses on progress in the development of transport models for heterogeneous contaminated aquifers, the use of predicted contaminant concentrations in groundwater for risk assessment for heterogeneous human populations, and the evaluation of aquifer remediation technologies. Major limitations and areas for continuing research for all methods presented in this review are identified.

Second moment method for evaluating human health risks from groundwater contaminated by trichloroethylene.

Pollutants in groundwater aquifers may constitute a significant human health risk. A large variation in response may result among human populations experiencing the same level and duration of exposure to pollutants. Variability in response, as a result of exposure to a carcinogenic contaminant such as trichloroethylene (TCE), can be represented by a distribution function of safe doses. Spatial variability in aquifer characteristics and contaminant transport parameters requires the use of stochastic transport models to quantify variability in exposure concentrations. A second moment method is used to evaluate the probability of exceeding safe dose levels for a contaminated aquifer. The name of this method stems from the fact that the formulation is based on the first and second moments of the random variables. With this method, the probability is a function of the variability of contaminant concentration (which incorporates variability in hydrogeologic parameters such as hydraulic conductivity) and the variability in response in the human population. In this manner, the severity of the health risk posed by a contaminated aquifer and the evaluation of appropriate strategies and technologies for aquifer remediation are a function of contaminant concentrations and human health risks. The applicability and limitations of this method are demonstrated with data on groundwater contaminated by TCE at Hill Air Force Base, Utah.

Artifacts associated with quick-freezing and freeze-drying.

We have studied the structures produced when nonbiological samples were subjected to quick-freezing and freeze-drying with a liquid helium cooled freeze-slamming device. Samples examined in this way included sodium chloride, sucrose, and Tris buffer. A variety of filamentlike and trabeculumlike structures were formed in these preparations. These structures may represent eutectic mixtures formed during the growth of small ice crystals during the freezing process, and exposed during the rapid sublimation of pure ice during the etching process. Samples of biological membranes (isolated chloroplast membranes) were prepared in various buffers by means of this technique. In distilled water, excellent replicas of membrane surfaces were obtained. In salt solutions, however, the membranes appeared to be embedded in a network of thin filaments appearing very much like a cytoskeletal lattice. It is concluded that extreme caution must be used when employing this preparation technique for studies of cell architecture, and that extensive washing of cell components in distilled water may be necessary to obtain faithful representations of cell structure.