Evidence that somatic and sympathetic nervous system neurotransmitters, not circulating epinephrine, mediate the immediate responses to a sudden threat in the American alligator (Alligator mississippiensis)

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AbstractThe qualitative and quantitative temporal effects of a 2‐min handling stress, both with and without pretreatment with tubocurarine, and the intraaortic infusion of epinephrine, both with and without pretreatment with a combination of phentolamine and propranolol, on plasma lactate, glucose, and hematocrit levels were compared in the American alligator (Alligator mississippiensis). The drugs and epinephrine were administered, and serial blood samples were collected, via a nonocclusive cannula chronically placed in the left aortic arch. Plasma lactate and glucose were measured by standard enzymatic procedures. Plasma lactate rose approximately 92% by the end of the 2‐min handling period and to maxima of 1,091 ± 476% greater than the time‐zero levels by 15 min. The hematocrit also increased rapidly to maxima of 11 ± 3.1% greater than the time‐zero levels by 5 min. Plasma glucose levels did not change significantly. The handling‐induced lactate and hematocrit increases were prevented by pretreatment with tubocurarine that blocks acetylcholine receptors at the skeletal muscle neuroeffector junction. Epinephrine infusion resulted in delayed, comparatively slow increases in plasma lactate, glucose, and the hematocrit. The peak levels for lactate were 334 ± 92% by 6 hr, for glucose were 135 ± 8% by 90 min, and for hematocrit were 10.9 ± 5.6% by 90 min. Pretreatment with the combination of phentolamine and propranolol that blocks mammalian “alpha” and “beta” adrenergic receptors, respectively, prevented the epinephrine‐induced increases in glucose and hematocrit levels, but not the plasma lactate increases. The results support the conclusion that the immediate responses of the alligator to a sudden threat are mediated by the synergistic effects of the somatic nervous system neurotransmitter acetylcholine, released at the skeletal muscle neuroeffector junction, and the sympathetic nervous system neurotransmitter (presumably norepinephrine), released at the neuroeffector junctions of the relevant visceral tissues and organs. In contrast, the participation of adrenal chromaffin tissue epinephrine secretion in the immediate responses is precluded by the long circulation and response times.