title: Autonomic control of heart rate during orthostasis and the importance of orthostatic-tachycardia in the snake Python molurus.
abstract: Orthostasis dramatically influences the hemodynamics of terrestrial vertebrates, especially large and elongated animals such as snakes. When these animals assume a vertical orientation, gravity tends to reduce venous return, cardiac filling, cardiac output and blood pressure to the anterior regions of the body. The hypotension triggers physiological responses, which generally include vasomotor adjustments and tachycardia to normalize blood pressure. While some studies have focused on understanding the regulation of these vasomotor adjustments in ectothermic vertebrates, little is known about regulation and the importance of heart rate in these animals during orthostasis. We acquired heart rate and carotid pulse pressure (P PC) in pythons in their horizontal position, and during 30 and 60° inclinations while the animals were either untreated (control) or upon muscarinic cholinoceptor blockade and a double autonomic blockade. Double autonomic blockade completely eradicated the orthostatic-tachycardia, and without this adjustment, the P PC reduction caused by the tilts became higher than that which was observed in untreated animals. On the other hand, post-inclinatory vasomotor adjustments appeared to be of negligible importance in counterbalancing the hemodynamic effects of gravity. Finally, calculations of cardiac autonomic tones at each position revealed that the orthostatic-tachycardia is almost completely elicited by a withdrawal of vagal drive.
authors.affiliation: Department of Physiological Sciences, Federal University of São Carlos (UFSCar), Via Washington Luiz, km 235, São Carlos, SP, 13565-905, Brazil.
authors.forename: Vinicius Araújo
authors.initials: VA
authors.lastname: Armelin
authors.forename: Victor Hugo
authors.initials: VH
authors.lastname: da Silva Braga
authors.forename: Augusto Shinya
authors.initials: AS
authors.lastname: Abe
authors.forename: Francisco Tadeu
authors.initials: FT
authors.lastname: Rantin
authors.forename: Luiz Henrique
authors.initials: LH
authors.lastname: Florindo
mesh_terms: D000818:Animals; D001794:Blood Pressure; D017821:Boidae / Q000502:physiology*; D004244:Dizziness / Q000503:physiopathology*; D006339:Heart Rate / Q000502:physiology*; D013610:Tachycardia / Q000503:physiopathology*