Prevention by nicotinamide of desensitization to thyrotropin stimulation in cultured human thyroid cells.

The presence of 50 mM nicotinamide together with 100 milliunits/ml of TSH in the incubation medium prevented the decline in human thyroid cell cAMP from maximum, stimulated levels (15-30 min) that occurs when the cells are exposed to TSH alone. Nicotinamide in the absence of TSH did not increase thyroid cell cAMP content. TSH desensitization, and its prevention by nicotinamide, occurred in the presence or absence of 3-isobutyl-methylxanthine. 1-Methyl nicotinamide and N'-methyl nicotinamide similarly prevented TSH desensitization. Recovery from TSH desensitization was prolonged and incomplete after 72 h. The presence of 50 mM nicotinamide hastened recovery from desensitization. Desensitization of the cAMP response to 10(6) M prostaglandin E1 and 1 mM adenosine was unaffected by nicotinamide. Other inhibitors of poly(ADP-ribose) polymerase activity, 5-bromouridine, 5-bromo-2'-deoxyuridine, and thymidine (all at 50 mM) completely or partially prevented TSH desensitization. Pyridoxine (50 mM) similarly prevented this phenomenon. As with dog thyroid cells, 10(-4) M cycloheximide blocked TSH desensitization. The combination of 10(-4) M cycloheximide and 50 mM nicotinamide had a synergistic effect in augmenting the thyroid cell cAMP response to TSH stimulation.