The directionality of conducted vasodilations in the microvasculature of skeletal muscle
In skeletal muscle tissue blood flow matches metabolism such that increases in blood flow are directed to active skeletal muscle fibers via dilations. Skeletal muscle fibers can overlap multiple branch levels of the microvasculature and the products of muscle activation and metabolism can induce dilations at all microvascular levels. It was hypothesized that (i) the dilators involved in muscle contraction could induce the directional dilations necessary for directing blood flow to specific muscle fibers; and (ii) the branch level stimulated will influence the directionality of the conducted dilations. Products of muscle activation and metabolism such as acetylcholine (ACh, muscarinic receptor agonist), adenosine (ADO, purinergic receptor agonist), opening of KATP channels (via pinacidil, PIN), potassium (K +), and nitric oxide (NO) demonstrated predominantly bidirectional dilations along a transverse arteriole (TA). Bidirectional dilations along the TA also occurred in the presence of multiple dilators when applied in combination or when produced by muscle contraction. In response to PIN stimulation of the TA, the dilations spread into all upstream and downstream branch and MI arterioles associated with this TA. PIN stimulation of the branch arteriole induced dilations of the upstream TA and upstream branch arterioles, but not the downstream TA and downstream branch arterioles. PIN application over the capillaries induced upstream dilations of the arterioles which directly supplied the stimulated capillaries only. The differences in the conducted dilations between individual dilators and between stimulation of different branch orders could be made possible by the existence of multiple conduction pathways. The presence of multiple pathways was demonstrated at the capillaries, as the conducted dilations initiated via ACh were attenuated by non-specific gap junction uncouplers, halothane (HAL) and 18-beta-glycyrrhetinic acid ([beta]GA) while PIN induced conducted dilations were attenuated by HAL but not by PGA. At the TA, [beta]GA attenuated the conducted dilations initiated by ACh, but not the conducted dilations initiated by ADO, NO, PIN and K+ . Overall, the predominant bidirectional dilations along the TA may not provide the mechanisms in directing dilations to specific capillaries. Instead, capillary stimulation induced the directional dilations which could provide a mechanism to direct blood flow to specific muscle fibers.