The cardiovascular responses of the freshwater turtle Trachemys scripta to warming and cooling

Seven freshwater turtles Trachemys scripta were instrumented with flow probes and cannulated for blood pressure measurements. The turtles were warmed from 24 to 34 degrees C, and cooled down to 24 degrees C, with and without atropine. Animals exhibited a hysteresis of heart rate and blood flow to both the pulmonary and systemic circulations, which was not cholinergically mediated. Blood pressure remained constant during both warming and cooling, while systemic resistance decreased during heating and increased during cooling, indicating a barostatic response. There was a large right-to-left (R-L) shunt during warming and cooling in untreated animals, which remained relatively constant. Atropinisation resulted in a large L-R shunt, which decreased during warming and increased during cooling. Nevertheless, heating rates were the same in untreated and atropinised animals, and cooling rates were significantly longer in atropinised animals, indicating that shunt patterns contribute little to heat exchange.