Effect of pharmacological blockade on cardiovascular responses to voluntary and forced diving in muskrats.

Neural control of free and forced diving bradycardia and peripheral resistance was studied in the muskrat (Ondatra zibethicus) by means of acute pharmacological blockade with the muscarinic blocker atropine, the alpha-adrenergic blocker phentolamine and the beta-adrenergic blockers nadolol and propranolol. Saline injection was used as a control. Heart rate in control animals increased before voluntary dives and dropped markedly as soon as the animals submerged. Heart rate started increasing towards the end of voluntary dives and reached pre-dive values within the first 5 s of recovery. Pre-dive and post-dive tachycardia were reduced in beta-blocked animals, emphasizing the role of the sympathetic system during the preparatory and recovery periods of voluntary dives. Diving bradycardia and the acceleration in heart rate before surfacing were abolished by atropine and unaffected by nadolol, demonstrating the importance of vagal efferent activity during diving. The results after blockade with nadolol suggest that there is an accentuated antagonism between the two branches of the autonomic nervous system during diving, so that parasympathetic influences on the heart predominate. Propranolol-treated muskrats had a higher diving heart rate than saline- and nadolol-treated animals, which may be due to a sedative effect caused by propranolol crossing the blood-brain barrier, a blockade of central catecholaminergic pathways or a peripheral neural effect, due to the anaesthetic properties of propranolol. Phentolamine did not affect diving bradycardia, indicating that diving bradycardia occurs independently of peripheral vasoconstriction.(ABSTRACT TRUNCATED AT 250 WORDS)