Purinergic Signalling in Pancreatic Islet Endothelial Cells - Extracellular ATP and Adenosine as Regulators of Endothelial Cell Function - page 213

Fig. 12.2 ( a ) Isolated islet
attached to a holding pipette
( left ) with an arteriole
cannulated via a perfusion
pipette ( right ). ( b )Structure
of the islet arteriole (diameter
25 m). The endothelial cell

lining clearly demarcates the
vessel lumen. The periphery
of the arteriole wall contains
vascular smooth muscle
(VSM)

μ

vasodilation. In a recent set of experiments a real-time, multidimensional imaging
technique allowed the study of the islet blood flow and its direction in vivo [69].
A flow from pole-to-pole was found in some islets, and centripetal flow in others.
These observations support the idea of a difference in blood flow pattern between
islets in the same animal. It is not known if an islet can change its flow pattern or
not, even if this is likely to be the case.
The drainage of the islets is accomplished directly through veins, as seen in large
islets, but also by an insulo-acinar portal system [8]. Especially the latter is the sub-
ject to large species variations, and the functional importance is unclear. Evidence
has been provided that the portal system allows exposure of peri-insular acini to
high concentrations of islet hormones [25, 38], and hereby resulting in an increase
in their protein synthesis. Taken together available data suggest that precapillary
VSM in the arterioles is the most important site for islet blood flow regulation, and
that postcapillary venulae contribute little, or not.