The widely scattered cardiovascular chromaffin cells of Petromyzon marinus appear to form an intrinsic control system of circulatory function. In response to blood-borne stimuli, a checkpoint-like accumulation of epinephrine cells in the heart releases its hormone; epinephrine, in turn, stimulates the release of norepinephrine, and probably also of dopamine, from other cardiovascular chromaffin cells. The myocardium seems to be a major target of norepinephrine. On the other hand, high disappearance rates of epinephrine and dopamine in the gills point to these organs as possible major targets of the latter two secretions. Carbon dioxide and hypovolemia are strong stimuli of catecholamine release.

Radioimmunoassays revealed the presence of the following steroids (or immunoreactive steroid fractions) in the plasma of larval sea lampreys: progesterone, corticosterone, cortisol, androstenedione, testosterone, dihydrotestosterone, estrone, and estradiol. Several types of stress served to provoke the release of these substances. A statistically significant effect of the treatment was noted under only one condition: squeezing of the body of decapitated ammocoetes during blood collection caused an increase in plasma dihydrotestosterone and a decrease in plasma estradiol. The discovery of all eight steroids (or immunoreactivities) assayed for suggests that the plasma of the ammocoete may contain a considerable number of additional steroids yet to be identified.

Presence, sex differences, and stress response of 6 circulating steroids were studied with specific radioimmunoassays in anadromous Petromyzon marinus. There were no detectable plasma levels of 5α-dihydrotestosterone. Testosterone was absent in 75 of 77 blood samples, and progesterone was present in 33 of 77 blood samples from 27 lampreys of both sexes. No statistically significant sex differences in the plasma levels of androstenedione, estrone, and estradiol were found. The plasma levels of these steroids were studied under the following experimental conditions: (1) chronic surgical stress (heart cannulation); (2) acute short-term stress (agitation); (3) decapitation without anesthesia within 1 min following capture; (4) decapitation after 10 min of submersion in a 0.05% solution of the anesthetic tricaine methanesulfonate. The androstenedione titers increased in response to all four experimental conditions, while estrone showed no statistically significant changes. Progesterone increased only during chronic stress. The estradiol levels dropped after heart cannulation and also 24 hr after agitation. The appearance of testosterone in 2 animals during chronic stress cannot be explained. Our data suggest that in adult lampreys (1) the roles of “sex” steroids differ from those typical for many gnathostomes; (2) androstenedione is a stress-related substance; (3) circulating androstenedione and estradiol may be—mainly or exclusively—released from different sources.

The clearance kinetics of T3 and T4 were studied in anadromous sea lampreys, 6–7 days after heart cannulation. Continuous sampling from 0.5 to 48 hr after injection of labeled hormones revealed clearance curves characterized by an initial very fast component, disappearing within less than 7 hr, and a slow component that persisted for the duration of the experiment. Production of radiolabeled ∗T3 from ∗T4 was detected only within the first 2 hr following injection. The disappearance rates of plasma T3 and T4 calculated for cannulated sea lampreys are comparable to those reported in trout.

The circulating levels of dopamine, norepinephrine, and epinephrine (about 1300, 1300, and 3600 pg/ml, respectively) of the adult sea lamprey are very high when compared with the levels in teleosts and mammals, but similar to the levels found in the shark. Contrary to the situation in the shark, teleosts, and mammals, however, epinephrine has the highest plasma levels of the three catecholamines studied, and dopamine seems to be released mainly outside the brain. We find two types of responses: agitation causes a decrease of circulating dopamine, whereas decapitation causes an enormous increase of norepinephrine release. Details of a technique of heart cannulation for repeated blood sampling from individual lampreys are also given.