Effects of thyrotropin, carbachol, and protein kinase-C stimulators on glucose transport and glucose oxidation by primary cultures of dog thyroid cells

Abstract

Thyroid glucose metabolism can provide NADPH and H2O2 for thyroid hormone synthesis. Several agents stimulate glucose oxidation in thyroid slices, but little is known about glucose transport in this tissue. In the present study, various thyroid stimulators were tested on glucose transport and oxidation using primary cultures of dog thyroid cells. After preincubating the cells with the agonists, glucose uptake was measured by adding 2-deoxy-D-[1-3H]glucose [( 3H]2-DOG) for 5 min, and glucose oxidation was assessed during a single 60-min incubation with agonist and D-[1-14C]glucose. TSH (0.1-10 mU/ml), 12-O-tetradecanoyl phorbol-13-acetate (TPA; 10(-8)-10(-6) M), and carbachol 10(-6)-10(-2) M) stimulated [3H]2-DOG transport and glucose oxidation in a dose-dependent manner. The effect of TSH appears to be mediated by cAMP, since N6,2'-O-dibutyryl cAMP, 8-bromo-cAMP, cholera toxin, and isobutylmethylxanthine also stimulated [3H]2-DOG transport. Norepinephrine, which had no effect by itself on either transport or oxidation, inhibited TSH stimulation of [3H]2-DOG transport via an alpha 2-adrenergic receptor. The mechanism of the TPA and carbachol effect does not involve cAMP. A combination of maximal amounts of TSH or bromo-cAMP and carbachol or TPA produced additive effects on transport, while addition of TPA with carbachol produced no such additive effect. Kinetic analysis of 2-DOG transport indicated that all three agonists reduced the Km and increased the maximum velocity. Basal 2-DOG transport was increased in Ca2+-free medium, with or without EGTA, or in the presence of calcium channel blockers such as La3+ or Mn2+. In the presence of such increased basal glucose transport, TSH further stimulated it when Mn2+ was used, had no effect in Ca2+-free buffer plus EGTA, and caused an inhibition with La3+. Such inhibition was not caused when N6,2'-O-dibutyryl cAMP was used in the presence of La3+. Carbachol and TPA did not stimulate transport when the Ca2+ channel blockers were used, but a small increase was seen in Ca2+-free buffer containing EGTA. TSH stimulation of cAMP production was also diminished in the presence of La3+, but enhanced in the presence of Mn2+. The calmodulin inhibitor W-7 and the intracellular Ca2+ blocker 8-N,N-diethylamino octyl-3,4,5-trimethoxybenzoate hydrochloride diminished the stimulation of [3H]2-DOG transport and glucose oxidation induced by TSH, carbachol, and TPA. These data indicate that thyroid glucose transport and glucose oxidation are regulated by both cAMP-dependent and cAMP-independent systems.