Release of Prostanoids from Paired Venules Regulates Functional Vasodilation of 3rd Order But NOT 4th Order ARterioles.
Previous studies from our laboratory suggest that functional arteriolar dilation of larger diameter “feed” arterioles paired with venules is mediated by arachidonic acid metabolites released from the venular endothelium. The aim of the present study was to test the hypothesis that the release of prostanoids from venules is important in the functional arteriolar dilation of 3rd and 4th order arterioles. The microcirculation of the hamster cremaster muscle was visualized using in vivo video microscopy. We measured the diameter responses of arterioles paired and unpaired with venules in response to 2 minutes of field stimulation (40 μs, 10V, 1 Hz). Indomethacin (28 μM) superfused on the preparation was used to inhibit prostanoid release. For the unpaired vessels field stimulation resulted in an increase in diameter of 3rd order vessels from 14 ± 1 to 26 ± 2 μm (n=6) and 4th order vessels from 11 ± 2 to 21 ± 2 μm (n=6). This response was not affected by indomethacin. 3rd order arterioles dilated 14 ± 2 to 26 ± 3 μm, 4th order arterioles dilated 10 ± 1 to 19 ± 2 μm. For the paired vessels there were differences in the responses between 3rd and 4th order arterioles. The diameter of 3rd-order arterioles averaged 32 ± 3 μm (n=6) and increased to 57 ± 5 μm following 2 minutes of field stimulation. During indomethacin treatment an inhibition of the functional vasodilation was observed as the arteriolar diameter increased from 29 ± 4 to only 38 ± 5 μm (p<0.05). The paired 4th order arterioles averaged 18 ± 4 μm and increased to 35 ± 5 μm with field stimulation (n=4). This response was not affected by indomethacin treatment (17 ± 5 to 32 ± 7 μm following stimulation). In conjunction with previous studies from our laboratory, these results support the hypothesis that cyclooxygenase products are released from the venular endothelium and are an important regulator of functional vasodilation of 3rd order arterioles but not 4th order arterioles.