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

Responses to intra-arterial injections of the α-agonist, norepinephrine, the β-agonist, isoproterenol, and to selected blockers were investigated in vivo by intravital microscopy. Luminal diameters (D) of pulpal microvessels and red cell velocities (V_rbc) were measured simultaneously to facilitate calculation of volumetric blood flow (Q). Norepinephrine caused a decrease in D, mean red cell velocity (V_m) and Q within individual microvessels. These responses were blocked by the α-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 β-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.