Proceedings of The Physiological Society
University of Cambridge (2004) J Physiol 555P, C81
Unilateral vagal control of pulmonary blood flow in the rattlesnake, Crotalus durissus terrificus
E.W. (Ted) Taylor*, Denis Andrade*, Augusto Abe* and Tobias Wang*
*Departamento de Zoologica, UNESP, Rio Claro, Brazil, School of Biosciences, University of Birmingham, Birmingham B15 2TT, UK and Zoophysiology, Aarhus University, 8000 Aarhus C, Denmark
Complete vagotomy in the rattlesnake caused heart rate to rise and become unvarying, blood flow to increase in both the systemic and more markedly in the pulmonary circuit (a left to right shunt) and the breathing rhythm to slow, with greatly increased lung volumes (Wang et al. 2001). These data can be interpreted as loss of vagal tone on the heart and on the pulmonary sphincter, plus denervation of lung stretch receptors. As the rattlesnake only possesses a single functional lung, developed from the left side of the body, vagal efferent innervation of the sphincter on the pulmonary artery may be similarly unilateral. This interesting possibility was tested by peripheral stimulation of the left and right cervical vagi.
Rattlesnakes, 8 of either sex, mass 260 to 850 g, were fasted for more than one week prior to experiments. Snakes were terminally anaesthetised with 20 mg Nembutal kg-1 injected into a caudal blood vessel. A ventrolateral incision above the heart enabled non-occlusive cannulation of the right aortic arch, for measurements of blood pressure. Blood flow probes (Transonic Systems Inc.) were placed around the pulmonary artery and the left aortic arch. The cervical vagi was exposed and lifted onto platinum hooks for electrical stimulation (2 msec pulses at 2-10volts). A range of frequency/response determinations was then made for each nerve. Each stimulation was followed by a period of recovery that constituted the control conditions for the subsequent stimulation.
The haemodynamic changes elicited by peripheral electrical stimulation differed substantially between the right and left vagi. Thus, while stimulation of either side led to pronounced reductions in heart rate and a drop in blood pressure, stimulation of the left vagus also led to large reductions in pulmonary blood flow. A recording from one experiment, shown in Fig. 1, highlights these differences.
This demonstration of vagal control of the heart and pulmonary artery in Crotalus is consistent with previous studies on turtles and other reptiles, but the unilateral influence of the left vagus on the pulmonary artery is novel.
Where applicable, experiments conform with Society ethical requirements