Effects of the alpha-adrenoceptor antagonists phentolamine, phenoxybenzamine, and idazoxan on sympathetic blood flow control in the periodontal ligament of the cat.

Blood flow changes in the periodontal ligament (PDL) were measured indirectly by monitoring the local clearance of 125I- during electric sympathetic nerve stimulation or close intra-arterial infusions of either noradrenaline (NA) or adrenaline (ADR) before and after administration of phentolamine (PA), phenoxybenzamine (PBZ), or Idazoxan (RX). At the doses used in the present study, PA was the only antagonist that significantly reduced the blood flow decrease seen on activation of sympathetic fibers, although PBZ also reduced this response. Idazoxan, however, did not induce the consistent effect on blood flow decreases seen on sympathetic activation. All three alpha-adrenoceptor antagonists almost abolished the effects of exogenously administered NA and ADR. The results suggest the presence of functional post-junctional adrenoceptors of both the alpha 1 and alpha 2 subtypes in the sympathetic regulation of blood flow in the PDL of the cat. A component of the response elicited by electrical sympathetic stimulation appeared to be resistant to alpha-adrenoceptor blockade. Administration of guanethidine (which inhibits further release of NA and neuropeptide Y) after PA abolished this residual sympathetic response.