ABSTRACT
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 α-adrenergic blocker phentolamine and the β-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 β-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. Compared with voluntary dives, heart rate was lower before forced dives in unrestrained animals and fell to lower values during diving. Underwater endurance in forced dives was markedly reduced after atropine and phentolamine treatment (either individually or combined) compared with that of control animals. Hence, bradycardia and peripheral vasoconstriction are necessary for maximum underwater endurance. A puzzling finding was that animals without bradycardia and/or peripheral vasoconstriction still dived voluntarily for periods as long as the calculated aerobic dive limit of muskrats and survived 5 min of forcible submergence. We hypothesize that anaerobic metabolism may play an important role during diving in muskrats.
