Fat cell adenylate cyclase and β-adrenergic receptors in altered thyroid states

The lipolytic sensitivity of fat cells from hypothyroid rats to epinephrine is severely blunted although 100 μM epinephrine stimulates lipolysis to the same level demonstrated by fat cells from euthyroid rats. Hypothyroidism shifts the lipolytic log-dose response curve to epinephrine to the right by a factor of 10. Epinephrine produced no detectable increase in adenosine 3':5'-monophosphate (cyclic AMP) accumulation in fat cells from hypothyroid rats. In contrast, fat cells from hyperthyroid rats accumulated more cyclic AMP in response to epinephrine than did those from euthyroid rats. Fat cell ghosts prepared from hypothyroid rats display reduced catecholamine-stimulated adenylate cyclase activity, although the maximal catalytic activity of fat cell ghost adenylate cyclase as measured in the presence of 10 mM sodium fluoride is the same as that of the euthyroid. Fat cell ghosts from hypothyroid rats were incubated with the guanine nucleotide analog, guanyl-5'-yl imidodiphosphate in an attempt to restore the reduced catecholamine-stimulated adenylate cyclase activity. This preincubation with guanyl-5'-yl imidodiphosphate activated the adenylate cyclase and increased catecholamine-stimulated activity of ghosts prepared from fat cells of both euthyroid and hypothyroid rats but did so to a much greater extent in the fat cell ghosts from euthyroid rats. Fat cell ghosts from hypothyroid rats incubated in the presence of a maximal concentration of guanyl-5'-yl imidodiphosphate displayed 37 ± 3% less adenylate cyclase activity than the fat cell ghosts obtained from euthyroid animals. Cyclic AMP phosphodiesterase activity of fat cell ghosts preincubated with or without guanyl-5'-yl imidophosphate, when measured at 0.125 μM cyclic AMP, was the same in ghosts from hypothyroid rats as in those from normal rats. Similarly, 1-methyl-3-isobutyl xanthine (1mM) did not rectify the reduced response of adenylate cyclase of the fat cell ghosts from hypothyroid rats to catecholamine stimulation. β-Adrenergic receptors were examined in membrane prepared from isolated fat cells obtained from hyperthyroid, hypothyroid, and euthyroid rats. Putative β-adrenergic receptors were identified with the use of the potent, β-adrenergic antagonist, ( - )-[³H]dihydroalprenolol. Specific binding of ( - )-[³H]dihydroalprenolol to fat cell membranes was rapid, reversible, and saturated at 80 nM. Scatchard plots of specific binding were curvilinear with upward concavity; Hill plots yield coefficients of 0.7. Competition studies display stereospecificity and a potency order of ( - )agonists (isoproterenol>> epinephrine ≃ norepinephrine) indicative of a β₁-adrenergic receptor. Calculating from membrane binding data, there appear to be approximately 10⁵ β-adrenergic receptors/fat cell. Maximum specific binding of ( -)-[³H]dihydroalprenolol/mg of membrane protein was the same for fat cell membranes prepared from hypothyroid, hyperthyroid, and euthyroid rats. Scatchard plots of binding studies performed with 1 to 100 nM ( - )-[³H]dihydroalprenolol were nearly identical for fat cell membranes prepared from the three different groups, each displaying curvilinearity with an upward concavity. β-Adrenergic agonists compete for ( - )-[³H]dihydroalprenolol binding with the same potency when measured in fat cell membranes of euthyroid or hypothyroid rats. The total number of β-adrenergic receptors per fat cell was the same for fat cells obtained from hyperthyroid, hypothyroid, and control rats. These data suggest that thyroid hormones alter neither the maximum catalytic activity of adenylate cyclase nor the number and affinity of putative β-adrenergic receptors of the fat cell. Thyroid hormones, thus, may exert their influence on fat cells by regulating the transduction of information between hormone receptors and adenylate cyclase.