Ionic mechanisms regulating thyroidal secretion: effects of ouabain and medium sodium concentration on radioiodine release from cultured porcine thyroid cells.

Thyrotrophin stimulated release of radioiodine from the organic iodine pool of cultured porcine thyroid cells. The response was well developed within 2-4 h of incubation. Inhibition of the Na+/K+ pump with ouabain or incubation in sodium-free medium inhibited the response to TSH. The magnesium content of cultures was reduced by ouabain, and increasing the magnesium concentration of the medium to 10 mmol/l reversed the inhibition of the TSH response by ouabain. After prolonged incubation (4-6 h), ouabain in magnesium-enriched medium stimulated release of radioiodine. Its effects were not additive with those of TSH. Incubation for 4-6 h in media of reduced sodium concentration (34 mmol/l) also stimulated release. Sodium-free medium alone did not alter basal release rates, but magnesium enrichment of sodium-free medium promoted release after 4 h of incubation. It was concluded that the previously reported inhibition of the TSH response in thyroid tissue by ouabain or sodium-free medium was due to secondary derangements of cellular function rather than to a specific blockade of the secretomotor signal. The data are consistent with the hypothesis that a reduction in the sodium ion electrochemical gradient across the cell membrane mediates the secretomotor effect of TSH.