SUMMARY1. The effects of reserpine and exposure to a stressing atmosphere containing 20 % C02 for 5 hr on catecholamine (CA) levels and dopamine fl-hydroxylase (DBH) activity of adrenal, spleen and plasma of cat, rabbit, rat and guinea-pig were studied.2. Twenty-four hr after an i.P. injection of reserpine, the CA contents of the adrenals and spleens were markedly reduced in all the four species studied. Adrenal and splenic DBH activity was unchanged in reserpine treated cats and rats. The enzyme activity decreased by 28 and 36 % in adrenals of rabbits and guinea-pigs, and a 33 % diminution was observed in the spleens of both species.3. A marked rise in guinea-pig plasma DBH activity was obtained 24 hr after reserpine, while plasma DBH activity was unchanged in the rat.4. Exposure of rats to a stressing atmosphere containing 20 % C02 for 5 hr increased the circulating noradrenaline (NA) levels by 600 %, but plasma DBH activity remained unchanged. C02 exposure caused both, an increase in plasma NA levels and DBH activity in the guinea-pig (950 % and 1000 %, respectively).5. The combined treatment with reserpine plus immediate exposure to C02 produced a 230 % rise in rat plasma NA with no concomitant change in DBH activity. A similar treatment caused a 75 % fall of NA levels and an increase in DBH activity of 600 % in the guinea-pig.6. The ratios of total DBH activity to CA in the spleen and adrenal were much higher in the guinea-pig than in the rat. The fraction of the total DBH activity that can be solubilized by osmotic shock of purified adrenomedullary chromaffin granules was 28 % in the rat and 71 % in the guinea-pig. If one makes the assumption that 'soluble' and 'releasable' DBH may be equated, the amount of 'releasable' DBH into the circulation is much greater in the guinea-pig than in the rat.7. The results suggest that the guinea-pig is a better model than the rat to study circulating DBH activity as an index of exocytotic CA release from adrenergic

Exposure of guinea-pigs to a C02-enriched atmosphere (20% C02, 25% 02, 55% N2) for I to 5 h caused a marked, progressive increase of plasma dopamine f3-hydroxylase (DBH) activity which reached its peak after 2 h of CO2 exposure and then gradually decreased. The increase was abolished by mecamylamine administration before exposure to CO2. Plasma levels of noradrenaline (NA) also increased after CO2 exposure. 2 Guanethidine administration, before exposure to C02, abolished the increase of plasma NA but potentiated the increase of circulating DBH. Phenoxybenzamine injection, before exposure to CO2, also potentiated the increase of plasma enzyme activity. In both cases, DBH activity was increased to almost 10 times the basal circulating enzyme levels. 3 Injection of 6-hydroxydopamine (6-OHDA) caused a pronounced decrease of DBH activity in the right atrium, thoracic aorta and spleen; the adrenal enzyme activity was unchanged. Exposure to CO2 of 6-OHDA-treated animals still evoked a dramatic increase of plasma DBH activity comparable to that found in control animals. 4 The increase of plasma DBH activity evoked by exposure to CO2 of adrenalectomized animals was considerably diminished. 5 These data suggest that in the guinea-pig, the adrenal is the main source of the increase of circulating DBH activity evoked by exposure of the animals to a CO2-enriched gas mixture.