Modulation by Thyroid Status of the Glucagon Receptor-Adenyl Cyclase System in Rat Liver Plasma Membranes*

Abstract

The in vivo and in vitro cAMP responses of several tissues after stimulation with glucagon are known to be modified by thyroidal status. To examine whether this thyroidal modulation is mediated in the liver via changes in the number or affinity of hepatic glucagon receptors, as recently reported for myocardial β-adrenergic receptors, we investigated the characteristics of the glucagon receptor-adenyl cyclase system in liver plasma membranes prepared from hyperthyroid (hyper.), euthyroid (euth.) or hypothyroid (hypo.) rats. The plasma T₄ level of 9.1 ± 0.7 μg/dl (mean ± SEM) in hyper, animals was greater than that in euth. rats (5.9 ± 0.5; P < 0.01); both were greater than that in hypo, rats (0.3 ± 0.1; P < 0.001). Liver and heart weights were higher in hyper, animals, while body, liver, and heart weights were lower in the hypo, than in the euth. group (P 125I]glucagon per 50 μg liver plasma membrane protein was similar in euth. and hyper, rats (24 ±4.2% vs. 29 ± 3.2%), but both were greater than that in hypo, rats (12 ± 2.7%; P< 0.01). Scatchard analysis revealed curvilinear plots compatible with either negative cooperativity or two orders of binding sites; when plotted according to average affinity profiles, the data were also compatible with the negative cooperativity model. The binding capacities of high affinity receptor sites were similar in hyper, and euth. rats (19.6 ± 1.9 and 22.7 ± 2.0 M × 10¹¹), but both were greater than that in hypo, animals (9.3 ± 1.8 M × 10¹¹). A similar pattern was present for the low affinity sites. Affinity constants for each order of binding sites were similar (high, 1.2-1.6 × 10⁹ M^-1; low, 3.4-5.3 × 10⁸ M^-1) regardless of thyroidal status. These differences in receptor number were associated with comparable differences in cAMP production in response to glucagon (10^-9-10^-6 M) stimulation. In hyper, and euth. rats, cAMP production increased by 163 ± 72 to 546 ± 110 pM/ mg protein · 10 min, whereas in hypo, animals no response above basal was apparent until the glucagon concentration was 10^-7 M, while maximal response was only 262 ± 54 (P < 0.05). Basal and sodium fluoride-stimulated cAMP production were similar in all groups, suggesting that the glucagon-responsive hepatic receptor-adenylate cyclase system was impaired by hypothyroidism. The ratio of receptor number in hyper, or euth. rats to that hypo, animals correlated with the cAMP production for these states. We conclude that in the rat, hypothyroidism results in a decreased number of functional liver glucagon receptors without a change in affinity. The absence of a difference between hyper, and euth. states in glucagon receptor characteristics or cAMP response indicates that the maximal effect on these parameters is established at relatively low thyroid hormone concentrations.