“…Electrical stimulation of the vagus nerve results in an increase in pulmonary vascular resistance in turtles, lizards and snakes, promoting a net R->L shunt. The R-»L shunt is abolished by the addition of atropine, which is a competitive antagonist of muscarinic cholinergic receptors and blocks cholinergic vasoconstriction of the pulmonary circulation (Berger, 1972;Burggren, 1977;Milsom et al, 1977;Smith and Mclntyre, 1979;Donald etal, 1990;Comeau and Hicks, 1994;Hicks and Comeau, 1994). In contrast, the L->R shunt is adrenergically mediated.…”