Baroreflex sensitivity in an amphibian, Rana catesbeiana, and a reptilian, Pseudemys scripta elegans

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Abstract
An inverse relationship between heart rate and systematic arterial pressure is demonstrated in unanesthetized bullfrogs (Rana catesbeiana) and red‐eared turtles (Pseudemys scripta elegans). Drug‐induced elevation of systemic blood pressure resulted in proportional decreases in heart rate and lengthening of the pulse interval. Conversely, nitroglycerin‐induced decreases in blood pressure produced compensatory increases in heart rate. This relationship in frogs is defined by y = 25.4x + 1684, and in turtles by y = 64.8x + 1686, where y is pulse interval in msec and x is systolic blood pressure in mmHg. Resting and drug‐induced systolic pressure limits were not different in the two species. However, both resting and reflexly altered heart rates were significantly lower in turtles than in frogs (p<.05). The heart rate responses were abolished by atropinization, without effect on the blood pressure responses. While anatomic identification of the baroreceptor site sensing the pressure changes remains to be made, the translation of pressure information via reflex pathways analogous to mammalian baroreceptor reflex arc system is evidenced in these lower vertebrates.