Analysis of 5-hydroxytryptamine- 14C uptake and metabolism in intestinal smooth muscle

Abstract

The distribution and metabolism of 5-hydroxytryptamine- 14C (5-HT- 14C) in both guinea pig ileal longitudinal smooth muscle and taenia coli preparations were measured. The uptake of 5-HT- 14C increased rapidly over 2 hr to tissue space values of f11.58 ± 0.57 (ileum) and 44.77 ± 0.59 (taenia) times the extracellular 5-HT- 14C concentration. Incubation with 13.3 μM β-phenylisopropylhydrazine (PIH) reduced 5-HT- 14C uptake by about half. Chemical separation of 14C-labeled compounds in the tissue indicated that 87–91% of the radioactivity present was 5-hydroxy-3-indoleacetic acid (5-HIAA) and 7–10% was 5-HT. In both paired ileum and taenia, PIH reduced 5-HIAA levels by 71–72% and doublet 5-HT levels. Addition of 2.67 mM non-radioactive 5-HT to incubation solutions reduced 5-HIAA- 14C levels and either did not increase 5-HT- 14C levels (taenia) or reduced them (ileum). Efflux studies showed that most of the 14C labeled material was lost at a slow rate; this rate was not altered by addition of 2.67 mM non-radioactive 5-HT or 5-HIAA during the washout. Uptake of 5-HIAA- 14C did not exceed an amount which would occupy only the extracellular space. Negligible amounts of 5-HIAA- 14C were formed when muscles were exposed to the 5-HT precursors tryptophan- 14C or 5-hydroxytryptophan- 14C. Thus, uptake of 5-HT- 14C into intestinal muscles occurs rapidly and most of the 5-HT- 14C is then metabolized to 5-HIAA- 14C which cannot readily cross cellular membranes. Synthesis of 5-HT does not occur in significant amounts in these smooth muscle tissues; they are systems in which changes in 5-HT uptake and metabolism can be measured with precision.