Biomedical subjects
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The clearance and metabolism of synthetic and tritiated (3H) neurotensin (NT) were studied following its intravenous injection in a pharmacologic dose (500 pmol/kg) into anesthesized rats. Immunoreactive NT (iNT), measured in a radioimmunoassay (RIA) with use of a carboxyl-(C)-terminal directed antiserum, displayed an apparent half-life (t 1/2) of 0.55 min, while that measured by an amino-(N)-terminal directed antiserum had a t 1/2 of 5 min. The radiolabel from injected 3H-NT (3H on Tyr3,11) had a t 1/2 of 6.5 min. High-pressure liquid chromatography of extracts of plasma obtained from the circulation 0.5-3 min after injection of NT and 3H-NT showed the presence of NT and the generation mainly of the fragments NT1-8, NT1-11, and NT9-13, as well as free 3H-labeled tyrosine. The apparent half-lives of intravenously injected synthetic NT1-8, NT1-11 and NT1-12 measured with the N-terminal RIA were 9, 5 and 5 min, respectively, while that for NT9-13 was less than 0.5 min. These results indicate that exogenously injected NT is rapidly metabolized to form N-terminal fragments which are cleared more slowly than NT. These findings suggest that use of N-terminal antisera to detect the release of endogenous NT into the circulation is likely to yield measurements of the fragments NT1-8 and NT1-11 which thus far have been found to be biologically inactive.
Vasoactive intestinal peptide (VIP) has been shown to increase chloride secretion from the rectal gland of the spiny dogfish, Squalus acanthias. Immunohistochemistry was used to localize the distribution of immunoreactive VIP (iVIP). Rectal glands were perfused with either buffered acrolein or paraformaldehyde/glutaraldehyde, sectioned (20 micron) and processed by either avidin-biotin complex (ABC) or peroxidase anti-peroxidase (PAP) methods. At the light microscopic level, iVIP was observed in thick fibers which traversed the fibromembranous capsule of the rectal gland. In the parenchyma, smaller iVIP-containing fibers were noted within connective tissue and in close approximation to tubule cells. At the ultrastructural level, iVIP axons in the fibromembranous capsule were unmyelinated. Immunoreactive fibers within the parenchyma frequently terminated on the basal side of tubule cells. Within the glands, iVIP bouton terminals were observed and contained vesicles of different sizes, with reaction product in dense core vesicles (60-120 nm). We conclude that iVIP is distributed in nerve fibers throughout the dogfish rectal gland. The anatomic distribution suggests that VIP may act as a neurotransmitter in this model of chloride ion transport.