Normal mice, as well as knockout mice for platelet endothelial cell adhesion molecule (PECAM-1 -/-) and endothelial NO synthase (eNOS-/-) mice were tested to see how their vessels responded to histamine. Histamine increased vessel dilation and then vessel constriction in the normal and PECAM-1 -/- mice, but the eNOS -/- mice had only constriction. Several agonists and antagonists were tested in each different mice. The results show that the response of vessels to histamine is regulated through the H1 receptor and smooth muscle.
This study aims to see if histamine can increase blood flow through the muscle. This was done by increasing the release of nitric oxide from microvascular endothelium.
Histamine is released in injury and inflammation conditions, and has been studied due to its involvement on the permeability and dilation/constriction of various vessels. This study chose to look at PECAM-1 -/- and eNOS -/- mice in comparison to normal control mice to see if PECAM or eNOS are involved in the effects of histamine. The H1 and H2 receptors regulate histamine, but the mechanism has not been well determined. Their hypothesis was that histamine would increase blood flow by increasing the nitric oxide release from the microvascular endothelium.
The diameters of the vessels in each condition were measured in micrometers. There was no difference between normal, PACAM -/-, and eNOS -/- mice in the resting condition, maxim diameter, and after oxygen stimulated contraction.
The normal and PECAM -/- mice both had a similar response to the histamine, where they experienced dilation until a peak, and proceeded to constrict as the histamine concentration increased. In the eNOS -/-, only constriction was seen and there was no dilation at all. The histamine receptor antagonists used were pyrilamine and cimetidine. These had no effect on the vessel diameter at resting conditions. The pyrilamine inhibited both constriction and dilation in the PECAM -/- and eNOS -/- mice, whereas the cimetidine did not have much of an effect.
The N-nitro-L-arginine (L-NNA) caused the vessels to constrict. The condition where the L-Arginine (L-Arg) was also added to the L-NNA reversed the constricting effects of the L-NNA. Nifedipine restored the activity of the vessels when added to histamine, but this effect was reversed when the velles were washed with histamine
This study shows that histamine increased vasodilation at low concentrations in the normal and PECAM -/- mice, but vasoconstriction of eNOS -/- mice. This shows that histamine dependent vasodilation requires nitric oxide to be present.
The nifedipine condition showed that the constriction of the vessel due to histamine exposure was due to the smooth muscle L-type Ca2+ channels. Since H1 receptors are known to activate L-type Ca2+ channels and H2 receptors are known to use a different pathway, this study shows that only H1 receptors are responsible for the effects of histamine.
After isolating the tissue samples from the mice, they were tested with different reagents and visualized with brightfield light microscopy. Histamine, pyrilamine, cimetidine, N-nitro-L-arginine (L-NNA), L-arginine (L-Arg), sodium nitroprusside, and nifedipine were all used in this experiment to generate the different conditions.
Vessels of normal mice will either constrict or relax based on the concentration of histamine they are exposed to, but this is not the sace in a vessel that is depleted of nitric oxide. This mechanism is regulated by the H1 receptor and the actility of L-type Ca2+ channels in the smooth muscle. Histamine is a very important regulator of blood flow in injury and inflammation conditions.