If you are going for skin blood flow I have no experience, but if it is systemic administration through the skin, I suggest intraperitoneal administration of metacholine instead (it has slower metabolism). Faster absorption. 1.5; 3 and 6 micrograms per injection are a proper doses in (normal) mice, in rats probaly 10-20-30 microg.

Also, if it is systemic administration, systolic blood pressure is a simple readout.

Isolated vessels preparation using either pressure myography or wire myography are also common methods to evaluate endothelial dysfunction

Isolating the aorta and measuring something like acetylcholine-mediated nitric oxide-dependent relaxation after a suitable contractile stimulus (we use phenylephrine) is one of the best ways to determine endothelial cell function in rats, though obviously this is a one time point measurement. We have also looked at adhesion molecules both release through ELISA kits with no difference being found between companies and kits in terms of sensitivity, as well as mRNA expression of the molecules using RT-PCR. You can also look at inflammatory cytokine release such as IL-8 or maybe release of endothelin from endothelial cells. The only whole animal technique I have used is measuring blood pressure using the tail vein which can work but we never found it gave us reproducible results when trying to measure endothelial cell/cardiac dysfunction in various disease models in rats.