The concentrations of plasma epinephrine (E) and norepinephrin (N) measured at rest in bullfrogs (Rana catesbeiana) were 12.0 and 8.2 nmol liter -1 respectively; the ratio of [E]/[N] was 1.33 (ยฑ SE 0.35). Adrenal glands contained high concentrations of epinephrine (2,923 nmole g wet weight -1 ) and norepinephrine (6,194), at a ratio of 0.46 (ยฑ SE 0.04) [E]/[N]. This differs from the measured plasma ratio and endogenous release ratios of about 2 for [E]/[N] reported for other Rana species, although the 95% confidence interval of our plasma ratio (0.97) spans the range of values from 0.36 to 2.3, including the observed plasma ratio of 0.46. Therefore, resting plasma catecholamine levels generally reflect the proportional adrenal content of catecholamines.
Plasma epinephrine and norepinephrine concentrations significantly increased after activity to 50.4 and 18.1 nmol liter -1 , respectively. The ratio of epinephrine to norepinephrine ([E]/[N]) also increased (but not significantly) to 8.53 (ยฑSE 4.23), suggesting a shift away from some adrenal tone at rest to sympathetic nerve dominance with activity. Graded hemorrhage led to further increases in plasma epinephrine concentration and [E]/[N] but not norepinephrine, indicating sympathetic but not adrenal involvement.
The in vitro epinephrine sensitivity of vascular beds indicates recruitment of the dorsal aorta vascular beds before the pulmocutaneous vascular bed. The minimum sensitivity of vascular beds to perfused epinephrine (10 4 nmol liter -1 ) was at higher concentrations than maximal plasma concentrations measured during hemorrhage.
The bullfrog is less tolerant of hemorrhage than the cane toad Bufo marinus. The major difference in the catecholamine response of these two species was the massive contribution of adrenal catecholamines with severe hemorrhage in toads, which is absent in bullfrogs. This suggests that the enhanced hemorrhage and dehydration tolerance of toads may in part be the result of their greater adrenal gland development and activity.