THE REGULATION OF BRANCHIAL BLOOD-FLOW IN THE BLACKTIP REEF SHARK, CARCHARHINUS-MELANOPTERUS (CARCHARHINIDAE, ELASMOBRANCHII)

The innervation of the branchial vasculature of the blacktip reef shar k, Carcharhinus melanopterus, appears similar to that described for ot her elasmobranchs, Isolated perfused gill arch preparations were used in a study of vascular responses to branchial nerve stimulation. The p re-addition of 10(-5) M concentrations of pancuronium to the perfusate prevented skeletal muscle contractions and significantly reduced (P < 0.001) changes in perfusion pressure in response to nerve stimulation , The adductor muscles received an extensive innervation by nerve fibr es ramifying from the gill arch, In pharmacological studies, pancuroni um completely antagonized the normal vasoconstrictive response to acet ylcholine, At low concentrations, noradrenaline produced a vasoconstri ction, but this changed to a vasodilatation at concentrations above 10 (-5) M, The use of antagonists demonstrated the presence of both alpha -, and beta-adrenoceptors in the branchial vasculature, Anatomical stu dies failed to show adrenergic nerves in the gills, suggesting al lack of a spinal autonomic ('sympathetic') innervation, Adrenergic regulat ion of gill blood flow may involve the release of catecholamines into the blood stream from chromaffin tissue stores.