Regional femoral bone blood flow rates in laying and non-laying chickens estimated with fluorescent microspheres

Abstract

The metabolic rate of vertebrate bone tissue is related to bone growth, repair and homeostasis, which are all dependent on life stage. Bone metabolic rate is difficult to measure directly, but absolute blood flow rate (Q̇) should reflect local tissue oxygen requirements. A recent ‘foramen technique’ has derived an index of blood flow rate (Qi) by measuring nutrient foramen sizes of long bones. Qi is assumed to be proportional to Q̇, however, the assumption has never been tested. This study used fluorescent microsphere infusion to measure femoral bone Q̇ in anaesthetised non-laying hens, laying hens and roosters. Mean cardiac output was 338±38 ml min−1 kg−1, and the two femora received 0.63±0.10 % of this. Laying hens had higher wet bone mass-specific Q̇ to femora (0.23±0.09 ml min−1 g−1) than the non-laying hens (0.12±0.06 ml min−1 g−1) and roosters (0.14±0.04 ml min−1 g−1), presumably associated with higher bone calcium mobilization during eggshell production. Estimated metabolic rate of femoral bone was 0.019 ml O2 min−1 g−1. Femoral Q̇ increased significantly with body mass, but was not correlated with nutrient foramen radius (r), probably due to a narrow range in foramen radius. Over all 18 chickens, femoral shaft Q̇/r was 1.07±0.30 ml min−1 mm−1. Mean Qi in chickens was significantly higher than predicted by an allometric relationship for adult cursorial bird species, possibly because the birds were still growing.

Morphology of the nutrient artery and its foramen in relation to femoral bone perfusion rates of laying and non‐laying hens

“…If a femur has more than one nutrient foramen, we cannot determine artery radius or estimate blood flow rates from foramen size directly. However, the summed foramen areas of paired‐femora with one and two foramina are not significantly different (Figure 9a,b), which aligns with no significant difference in total blood flow rates into left and right femora measured by microsphere infusion (Hu et al, 2021). This suggests that nutrient artery blood flow rates to the left and right femora should be similar, even when they have different foramen numbers.…”

“…The scaling exponent is 1.22 ± 0.84, which is not significantly different from the 1.32 ± 0.93 scaling exponent reported in our study of femur perfusion measured with microspheres (Hu et al, 2021). The value of 1.3 used in Hu et al's (2021) study was selected to compare mass‐independent blood flow rates between the two studies. Mass‐independent blood flow rates of nutrient arteries, therefore, have a unit of ml min −1 kg −1.3 .…”

“…This is done by dividing the blood flow rate by body mass raised to the exponent evident in the regression across all three chicken groups (Figure 12). The scaling exponent is 1.22 ± 0.84, which is not significantly different from the 1.32 ± 0.93 scaling exponent reported in our study of femur perfusion measured with microspheres (Hu et al, 2021). The value of 1.3 used in Hu et al's (2021) study was selected to compare mass‐independent blood flow rates between the two studies.…”

“…Although the mean for laying hens is greater than that for non‐laying hens, the difference is not significant ( p = 0.67). However, the difference is significant when measured with microsphere infusion ( p = 0.04; Hu et al, 2021). The relationships between blood flow rates calculated from imaging and microspheres are shown in the Supporting Information (Figure ).…”

“…This study shows a trend for greater bone perfusion that probably relates to intense calcium turnover in laying hens. The relationship between chicken femoral bone blood flow and bone metabolism has been described in more detail in Hu et al (2021).…”