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Calcitonin (CT) and neurotensin (NT) secreting cell lines (6-23,44-2) were established from a transplantable rat medullary thyroid carcinoma (rMTC). The 44-2 line was used to obtain two colony clones 44-2 B and C secreting 20-30-fold greater NT than CT. These cells were used to study the regulation of Ca2+-modulated NT secretion and to ascertain the role of K+ and NE in the rapid release of NT. Medium NT was measured by a specific RIA with the antiserum N-1-11. Secretion experiments were in replicate 35 mm dishes in Krebs-Ringer-bicarbonate buffer supplemented with 90 mg% glucose (KRBG). Ca2+ (0.5-4.0 mM) stimulated NT release in a dose-dependent manner with an ED50 of 2.0 mM. Ca2+ was required for NT release induced by the Ca2+ ionophore, ionomycin, also K+ (50 mM) and norepinephrine (NE). To determine the mode of NT release in the presence of control (1.0 mM Ca2+) or experimental conditions (Ca2+ 1.0 mM plus 10(-6) NE and/or 50 mM K+), 44-2 cells were incubated in KRBG using an experimental paradigm wherein medium was changed at 10-min intervals, and NT release was quantitated. NE stimulated release of NT by these cells and the amount of NT released with each repetitive pulse of NE remained constant throughout the experiment. K+ (50 mM) elicited a rapid release of NT in the first 0-10 min incubation and the amount of NT released into the buffer was greater than that measured with NE; however, in these experiments, the response to K+ declined progressively and reached basal values at 20-30 min. Our results show neither pulse stimulation nor continuous incubation of cells with NE affected the response of the cells to a subsequent challenge with K+. These results suggest the presence of differentially stimulated, releasable pools of NT in these cells. We conclude that these newly established 44-2 B and C cells provide a useful model to study the regulation of NT release.