Responsiveness of microcirculation and local cold vasodilation to capsaicin in the intact and chronically denervated canine tongue

Lingual blood flow and its distribution were determined at rest and in response to local cooling of the tongue (32 degrees C) in 6 anaesthetized, paralyzed and artificially ventilated dogs before and after two intraarterial (i.a.) injections of capsaicin (2.5 mg) at an interval of about 40 min. In 3 dogs, the same protocol was performed after degeneration of the chorda-lingual and glossopharyngeal nerves due to prior transection. In general the first i.a. injection of capsaicin resulted in a marked and the second injection in a smaller decrease of lingual blood flow. Local cooling of the tongue induced significant increases in lingual blood flow before as well as after capsaicin treatment, regardless of whether sensory innervation was intact or degenerated. In both the untreated and capsaicin treated dogs the increase in lingual blood flow during local cooling of the tongue was solely due to an increase in blood flow through the arteriovenous anastomoses, while blood flow through the capillaries of the mucosa and muscles even decreased. The findings suggest that capsaicin-induced vasoconstriction of the tongue vessels is due to a direct effect on vascular receptors. It is further suggested that cold vasodilatation of the canine tongue is not mediated by axon collaterals releasing substance P. Direct thermal effects on the intramural ganglia and the postganglionic vasomotor efferents innervating the AVAs, or on AVAs basal tone itself are suggested as the underlying mechanism.