In humans, insulin resistance is associated with a shift towards a higher proportion of the relatively insulin-resistant type 2 fast-twitch fibres at the expense of the insulin-sensitive type 1 slow-twitch fibres, along with a reduction in capillary density (Marin et al. 1994). Previously we have shown in sheep that an early or late gestation maternal nutrition restriction reduces myofibre number and alters fibre type composition in the fetal triceps brachii (Costello et al. 2006) but not in the slow-twitch soleus muscle (unpublished observations). The aim of the study was therefore to investigate the effect of an early or late gestation nutrient restriction on muscle capillary density in the late gestation fetal sheep.
Pregnant Welsh Mountain ewes of uniform body weight were housed individually, and received either 100% of total nutrient requirements throughout gestation (C, n=6), 40% from 1-31 days gestation (dGA) (ER, n=8) or 50% from 104 dGA until post mortem (LR, n=7), with 100% requirements at all other times. All fetuses were singletons and groups contained equal numbers of males and females. At 127 ± 1 dGA (term ~147 dGA) ewes were killed by an overdose of barbiturate (i.v., 145 mg/kg), and the fetal triceps brachii and soleus muscles were removed and immersed in freezing isopentane. 10 μm sections of muscle were cut, stained with anti-human von Willebrand factor and five random fields (magnification x40) were captured. From these fields the capillary density and capillary:muscle fibre ratio were measured, averaged and analysed by ANOVA with Bonferroni post-hoc tests. Data are expressed as mean ± SEM.
The density of capillaries in the triceps brachii was reduced in both the ER (p<0.01) and LR (p<0.05) fetuses as compared to C (C, 1874 ± 57; ER, 1480 ± 65; LR, 1389 ± 98 capillaries/mm²). Capillary:muscle fibre ratios were reduced in both the ER (p<0.01) and LR (p<0.01) as compared to C fetuses (C, 1.80 ± 0.09; ER, 1.48 ± 0.06; LR, 1.44 ± 0.12 capillaries:muscle fibre). No differences were seen in the soleus muscle.
Our findings of decreased capillary density following reduced maternal nutrition in either the peri-implantation period or in late gestation is muscle bed dependent and parallels the effect on myofibre density previously observed (Costello et al. 2006). Moreover, the reduction in capillary:muscle fibre ratio in the triceps brachii indicates that the reduction in capillary density was greater than that in the myofibres, i.e. that each fibre would be supplied by fewer vessels. These data suggest a link between blood flow and skeletal muscle growth, and the reductions in both capillary and myofibre density may have long-term implications for skeletal muscle function. Acknowledgements: This work was supported by the BBSRC, BHF and Gerald Kerkut Trust.Reference 1 : Costello P et al. (2006) Proc Physiol Soc 3, C116.Reference 2 : Marin P et al. (1994) Diabetes Care 17, 382-86.