Baroreceptor control of heart rate in the awake toad: peripheral autonomiceffectors and arterial baroreceptor areas

Systemic injection of sodium nitroprusside (30 mu g/kg, i.v.) in the awake Bufo paracnemis toad induced a fall in arterial blood pressure and tachycar dia. This tachycardia, but not the hypotension, was significantly reduced i n toads with bilateral electrolytic lesion of the caudal and commissural re gions of the solitary tract nucleus and in animals with transection of the spinal cord, 2 mm below the obex. This indicates that the tachycardia is re flex, depends on the integrity of the solitary tract nucleus and is due to descending spinal autonomic activation. Pretreatment with propranolol (4 mg /kg, i.v.) significantly reduced the tachycardia but did not block it compl etely, showing the importance of beta-adrenoceptors in its genesis. The ref lex increase in heart rate induced by nitroprusside was not statistically s ignificant in animals with bilateral section of the laryngeal nerve, whose baroreceptor fibers originate from the pulmocutaneous artery or in animals in which the bilateral section of the laryngeal nerve was performed togethe r with section of the glossopharyngeal nerves, which incorporate fibers ori ginating from the carotid labyrinth. The reduction of the reflex tachycardi a was significant in toads with aortic arch denervation alone or combined w ith section of the laryngeal nerves or in animals with complete denervation of the three baroreceptors areas. These results suggest that the region of the aortic arch, when submitted to unloading, is the most important barore ceptor zone for cardiac compensation in toads. (C) 2000 Elsevier Science B. V. All rights reserved.