Influence of temperature on the responses to serotonin (5-HT) in perfused rat lung

The effects of serotonin (5-HT) on the perfusion pressure of the isolated rat lung was investigated at 2 different temperatures (37 degrees C and 15 degrees C). 5-HT (1-30 micrograms) increased the perfusion pressure dose-dependently and a biphasic vasoconstrictor response appeared with the doses of 3-30 micrograms at 37 degrees C. Both phases were only blocked by methysergide (5 X 10(-10) M) among the 5-HT antagonists tested. At 15 degrees C, 5-HT (0.3-30 micrograms) elicited a similar response but the second phase appeared with the higher doses of the agonist compared to that at 37 degrees C. The initial phase, which was significantly higher than that at 37 degrees C, was antagonized by ketanserin, mianserin, pizotifen and methysergide in a noncompetitive manner. The order of potencies of the 4 serotonergic antagonists was compared with their affinity for central 5-HT receptors. The results led us to conclude that receptors mediating the initial phase of the vasoconstrictor response to 5-HT at 15 degrees C are neither of the 5-HT1 nor of the 5-HT2 type, but are 5-HT1 ones at 37 degrees C in the rat pulmonary vascular bed.