Quantitative assessment of microcirculation in the rat dental pulp in response to alpha- and beta-adrenergic agonists

Responses to intra-arterial injections of the alpha-agonist, norepinephrine, the beta-agonist, isoproterenol, and to selected blockers were investigated in vivo by intravital microscopy. Luminal diameters (D) of pulpal microvessels and red cell velocities (Vrbc) were measured simultaneously to facilitate calculation of volumetric blood flow (Q). Norepinephrine caused a decrease in D, mean red cell velocity (Vm) and Q within individual microvessels. These responses were blocked by the alpha-antagonist, phenoxybenzamine. In response to the arteriolar D increase and venular D and Q decrease with isoproterenol, a transient increase in arteriolar Q was followed by a decrease within the first 60 s. No such effects were observed when the beta-antagonist, propranolol, was administered before the isoproterenol injection. The biphasic response in Q in response to isoproterenol is most likely related to the low compliance of the pulp. In the low-compliance environment a passive compression of venules could result from an active dilation of arterioles with an attendant rise in extravascular tissue pressure.