RESUMO
In squamate reptiles, extensive innervation of the heart and vascular beds allows for continuous modulation of the cardiovascular system by the autonomic nervous system. The systemic vasculature is the main target of excitatory sympathetic adrenergic fibers, while the pulmonary circulation has been described as less responsive to both nervous and humoral modulators. However, histochemical evidence has demonstrated the presence of adrenergic fibers in pulmonary circulation. Besides, reduced responsiveness is intriguing since the balance of regulation between systemic and pulmonary vascular circuits has critical hemodynamic implications in animals with an undivided ventricle and consequent cardiovascular shunts. The present study investigated the role and functional relevance of α and ß-adrenergic stimulation in regulating systemic and mainly the pulmonary circulations in a decerebrate, autonomically responsive rattlesnake preparation. The use of the decerebrate preparation allowed us to observe a new diverse functional modulation of vascular beds and the heart. In resting snakes, the pulmonary vasculature is less reactive to adrenergic agonists at 25 °C. However, the ß-adrenergic tone is relevant for modulating resting peripheral pulmonary conductance, while both α- and ß-adrenergic tones are relevant for the systemic circuit. Active dynamic modulation of both pulmonary compliance and conductance effectively counterbalances alterations in the systemic circulation to maintain the R-L shunt pattern. Furthermore, we suggest that despite the great attention given to cardiac adjustments, vascular modulation is sufficient to support the hemodynamic adjustments needed to control blood pressure.
Assuntos
Adrenérgicos , Crotalus , Animais , Adrenérgicos/farmacologia , Ventrículos do Coração , América do SulRESUMO
AbstractUnderstanding the basis of vascular tonus regulation is fundamental to comprehending cardiovascular physiology. In the present study, we used the recently developed decerebrate rattlesnake preparation to investigate the role of nitric oxide (NO) in the control of vascular tonus in a squamate reptile. This preparation allowed multiple concomitant cardiovascular parameters to be monitored, while avoiding the deleterious effect of anesthetic drugs on autonomic modulation. We observed that both systemic and pulmonary circuits were clearly responsive to NO signaling. NO increased vascular conductance in the systemic and pulmonary systems. Vasodilation by NO of the systemic circulation was compensated by cardiovascular alterations involving venous return, cardiac output, and cardiac shunt adjustments. The cardiac shunt seemed to be actively used for hemodynamic adjustments via modulation of the pulmonary artery constriction. N(ω)-nitro-L-arginine methyl ester injection demonstrated that NO contributes to modulating resting vasodilation in the systemic circuit. In contrast, NO-mediated vasodilation did not have an important role in the pulmonary circulation in inactive decerebrated snakes at 25°C. These responses vary importantly from those described for anesthetized snakes.