Lymph is transported along collecting lymphatic vessels by intrinsic and extrinsic pumping. The walls have muscle of a type intermediate between blood-vascular smooth muscle and myocardium; a contracting segment between two valves (a lymphangion) constitutes a pump. This intrinsic mechanism is investigated ex vivo in isolated, spontaneously contracting, perfused segments subjected to controlled external pressures. The reaction to varying afterload is probed by slowly ramping up the outlet pressure until pumping fails. Often the failure occurs when the contraction raises intralymphangion pressure insufficiently to overcome the outlet pressure, open the outlet valve and cause ejection, but many segments fail by other means, the mechanisms of which are not clear. We here elucidate those mechanisms by resort to a numerical model. Experimental observations are paired with comparable findings from computer simulations, using a lumped-parameter model that incorporates previously measured valve properties, plus new measurements of active contractile and passive elastic properties, and the dependence of contraction frequency on transmural pressure, all taken from isobaric twitch contraction experiments in the same vessel. Surprisingly, the model predicts seven different possible modes of pump failure, each defined by a different sequence of valve events, with their occurrence depending on the parameter values and boundary conditions. Some, but not all, modes were found experimentally. Further model investigation reveals routes by which a vessel exhibiting one mode of failure might under altered circumstances exhibit another.

This study was conducted to determine the relative contribution of feeding behavior, DMI, apparent DM digestibility (DMD), and passage rate on variation in BW gain. One hundred forty-three crossbred steers were used in this study to determine the factors that contribute to variation in BW gain. Steers were 304 ± 1 d of age and had an initial BW of 338 ± 3 kg. Steers had ad libitum access to feed, and fresh feed was offered twice daily at 0800 and 1500 h. Individual feed intake and BW gains were determined for 106 d. Titanium dioxide was used as an external marker to estimate apparent DMD and passage rate. A multiple-regression analysis was used to determine the relative contribution of initial BW, feed intake, feeding behaviors, digestibility, and passage rate to variance in BW gain, and a path analysis was conducted to determine direct and indirect relationships of the variables. In the regression analysis, initial BW and DMI accounted for 33% of the variation in BW gain. Substituting meal events and meal size for DMI did not increase the amount of variance in BW gain (27%) accounted for by the regression model. There was a slight increase in the variance in BW gain (35%) when apparent DMD was added to initial BW and DMI. When meal events and meal size were substituted for DMI, there was a decrease (30%) in variance accounted for in BW gain. The regression coefficient for the passage rate did not differ from zero (P = 0.63) when the passage rate was substituted for apparent DMD in the regression model that also included initial BW and DMI, nor did it differ from zero (P = 0.39) in the model that substitutes meal events and meal size for DMI. Three models were used in the path analysis. For all 3 models there was a positive correlation (P < 0.001) between DMI and BW gain but not between apparent DMD and BW gain. In all 3 models there was a significant correlation between meal size, meal events, and initial BW on DMI (P < 0.001). Results of both the regression and path analyses suggest that both apparent DMD and passage rate accounted for small proportions of the variance, suggesting that they are minor contributors toward variance in BW gain among cattle fed the same concentrate diet.

Rate of oxygen uptake by muscle mitochondria and respiratory chain protein concentrations differed between high- and low-residual feed intake (RFI) animals. The hypothesis of this research was that complex I (CI), II (CII), and III (CIII) mitochondria protein concentrations in lymphocyte (blood) mitochondria were related to the RFI phenotype of beef steers. Daily feed intake (ADFI) was individually recorded for 92 Hereford-crossbreed steers over 63 d using GrowSafe individual feed intake system. Predicted ADFI was calculated as the regression of ADFI on ADG and midtest BW. Difference between ADFI and predicted ADFI was RFI. Lymphocytes were isolated from low-RFI (-1.32 ± 0.11 kg/d; = 10) and high-RFI (1.34 ± 0.18 kg/d; = 8) steers. Immunocapture of CI, CII, and CIII proteins from the lymphocyte was done using MitoProfile CI, CII, and CIII immunocapture kits (MitoSciences Inc., Eugene, OR). Protein concentrations of CI, CII, and CIII and total protein were quantified using bicinchoninic acid colorimetric procedures. Low-RFI steers consumed 30% less ( = 0.0004) feed and had a 40% improvement ( < 0.0001) in feed efficiency compared with high-RFI steers with similar growth ( = 0.78) and weight measurements ( > 0.65). High- and low-RFI steers did not differ in CI ( = 0.22), CII ( = 0.69), and CIII ( = 0.59) protein concentrations. The protein concentration ratios for CI to CII ( = 0.03) were 20% higher and the ratios of CI to CIII ( = 0.01) were 30% higher, but the ratios of CII to CIII ( = 0.89) did not differ when comparing low-RFI steers with high-RFI steers. The similar magnitude difference in feed intake, feed efficiency measurements, and CI-to-CIII ratio between RFI phenotypes provides a plausible explanation for differences between the phenotypes. We also concluded that mitochondria isolated from lymphocytes could be used to study respiratory chain differences among differing RFI phenotypes. Further research is needed to determine if lymphocyte mitochondrial complex proteins can be used for identification of RFI phenotype.

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Gestating Angus, nonlactating, spring-calving cows were used to determine variation in maintenance energy requirements (MR); to evaluate the relationship among MR and cow and calf performance, plasma concentrations of IGF-I, T4, glucose, insulin, and ruminal temperature; and to describe the LM proteome and evaluate protein abundance in cows with different MR. Cows (4 to 7 yr of age) with a BCS of 5.0 ± 0.2 and BW of 582 ± 37 kg in the second to third trimester of gestation were studied in 3 trials (trial 1, n = 23; trial 2, n = 32; trial 3, n = 38). Cows were individually fed a complete diet in amounts to meet predicted MR (Level 1 Model of NRC), and feed intake was adjusted weekly until constant BW was achieved for at least 21 d (maintenance). Cows were classified on the basis of MR as low (>0.5 SD less than mean, LMR), moderate (±0.5 SD of mean, MMR), or high (>0.5 SD more than mean, HMR) MR. Blood samples were taken at maintenance and at 2 mo postpartum in trial 2. Muscle biopsies were taken from LMR and HMR after cows consumed actual MR for 28 d (trial 2) or 21 d (trial 3). Proteins from LM were separated by 2-dimensional difference gel electrophoresis and were identified, and abundance was quantified and compared. The greatest differences in MR between cows were 29%, 24%, and 25% in trials 1, 2, and 3, respectively. Daily MR (NEm, kcal·BW(-0.75)·d(-1)) averaged 89.2 ± 6.3, 93.0 ± 4.9, and 90.4 ± 4.6 in trials 1, 2, and 3, respectively. Postpartum BW and BCS, calf birth and weaning weights, postpartum luteal activity, and ruminal temperature were not influenced by MR of the cows. Concentrations of IGF-I were greater (P = 0.001) in plasma of MMR compared with LMR cows consuming predicted MR diets, and MR was negatively correlated with concentrations of IGF-I in plasma (r = -0.38; P = 0.05) at 2 mo postpartum. A total of 103 proteins were isolated from LM; 52 gene products were identified. Abundance of specific proteins in the LM was not influenced (P > 0.11) by MR. Variation in MR of cows will make it possible to improve feed efficiency by selection. Identification of biomarkers for MR will allow selection of more efficient cows, which consume less feed and produce calves with similar weaning weights. Productive cows that require less feed for maintenance will improve efficiency of production and enhance sustainability of the environment.