The progressive onset of cholinergic and adrenergic control of heart rate during development in the green iguana, Iguana iguana

Made available in DSpace on 2018-11-26T16:16:38Z (GMT). No. of bitstreams: 0 Previous issue date: 2015-10-01INCT in Comparative PhysiologyFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)National Science FoundationThe autonomic control of heart rate was studied throughout development in embryos of the green iguana, Iguana iguana by applying receptor agonists and antagonists of the parasympathetic and sympathetic systems. Acetylcholine (Ach) slowed or stopped the heart and atropine antagonized the response to Ach indicating the presence of muscarinic cholinoceptors on the heart of early embryos. However, atropine injections had no impact on heart rate until immediately before hatching, when it increased heart rate by 15%. This cholinergic tonus increased to 34% in hatchlings and dropped to 24% in adult iguanas. Although epinephrine was without effect, injection of propranolol slowed the heart throughout development, indicating the presence of beta-adrenergic receptors on the heart of early embryos, possibly stimulated by high levels of circulating catecholamines. The calculated excitatory tonus varied between 33% and 68% until immediately before hatching when it fell to 25% and 29%, a level retained in hatchlings and adults. Hypoxia caused a bradycardia in early embryos that was unaffected by injection of atropine indicating that hypoxia has a direct effect upon the heart. In later embryos and hatchlings hypoxia caused a tachycardia that was unaffected by injection of atropine. Subsequent injection of propranolol reduced heart rate both uncovering a hypoxic bradycardia in late embryos and abolishing tachycardia in hatchlings. Hypercapnia was without effect on heart rate in late stage embryos and in hatchlings. (C) 2015 Elsevier Inc All rights reserved.Univ Estadual Paulista, Inst Biociencias, Dept Zool, Rio Claro, SP, BrazilUniv Fed Sao Carlos, Dept Ciencias Fisiol, BR-13560 Sao Carlos, SP, BrazilUniv N Texas, Dept Biol Sci, Dev Integrat Biol Cluster, Denton, TX 76203 USAUniv Birmingham, Sch Biosci, Birmingham B15 2TT, W Midlands, EnglandUniv Estadual Paulista, Inst Biociencias, Dept Zool, Rio Claro, SP, BrazilINCT in Comparative Physiology: CNPq 573921/2008-3INCT in Comparative Physiology: FAPESP 2008/57712-4FAPESP: 2012/06938-8FAPESP: 2012/16537-0National Science Foundation: IBN-IOS 084574

Seasonal reproductive endothermy in tegu lizards

With some notable exceptions, small ectothermic vertebrates are incapable of endogenously sustaining a body temperature substantially above ambient temperature. This view was challenged by our observations of nighttime body temperatures sustained well above ambient (up to 10°C) during the reproductive season in tegu lizards (~2 kg). This led us to hypothesize that tegus have an enhanced capacity to augment heat production and heat conservation. Increased metabolic rates and decreased thermal conductance are the same mechanisms involved in body temperature regulation in those vertebrates traditionally acknowledged as “true endotherms” : the birds and mammals. The appreciation that a modern ectotherm the size of the earliest mammals can sustain an elevated body temperature through metabolic rates approaching that of endotherms enlightens the debate over endothermy origins, providing support for the parental care model of endothermy, but not for the assimilation capacity model of endothermy. It also indicates that, contrary to prevailing notions, ectotherms can engage in facultative endothermy, providing a physiological analog in the evolutionary transition to true endothermy