Regulation of beta 1- and beta 3-adrenergic agonist-stimulated lipolytic response in hyperthyroid and hypothyroid rat white adipocytes

Abstract

1. This study examined the effects of thyroid status on the lipolytic responses of rat white adipocytes to beta-adrenoceptor (beta-AR) stimulation. The beta 1- and beta 3-AR mRNAs and proteins were measured by Northern and saturation analyses, respectively. Glycerol production and adenyl cyclase (AC) activity induced by various non-selective and selective beta 1/beta 3-AR agonists and drugs which act distal to the receptor in the signalling cascade were measured in cells from untreated, triiodothyronine (T3)-treated and thyroidectomized rats. 2. The beta 3-AR density was enhanced (72%) by T3-treatment and reduced (50%) by introduction of a hypothyroid state while beta 1-AR number remained unaffected. The beta 1- and beta 3-AR density was correlated with the specific mRNA level in all thyroid status. 3. The lipolytic responses to isoprenaline, noradrenaline (beta 1/beta 3/beta 3-AR agonists) and BRL 37344 (beta 3-AR agonist) were potentiated by 48, 58 and 48%, respectively in hyperthyroidism and reduced by about 80% in hypothyroidism. 4. T3-treatment increased the maximal lipolytic response to the partial beta 3-AR (CGP 12177) and beta 1-AR (xamoterol) agonists by 234 and 260%, respectively, increasing their efficacy (intrinsic activity: 0.95 versus 0.43 and 1.02 versus 0.42). The maximal AC response to these agonists was increased by 84 and 58%, respectively, without changing their efficacy. 5. In the hypothyroid state, the maximal lipolytic and AC responses were decreased with CGP (0.17 +/- 0.03 versus 0.41 +/- 0.08 mumol glycerol/10(6) adipocytes; 0.048 +/- 0.005 versus 0.114 +/- 0.006 pmol cyclic AMP min-1 mg-1) but not changed with xamoterol. 6. The changes in lipolytic responses to postreceptor-acting agents (forskolin, enprofylline and dibutenyl cyclic AMP, (Bu)2cAMP) suggest the modifications on receptor coupling and phosphodiesterase levels in both thyroid states. 7. Thyroid status affects lipolysis by modifying beta 3-AR density and postreceptor events without changes in the beta 1-AR functionality.

Figures

Figure 1

Northern analysis of β1 and β3-AR mRNA levels (a) in adipocytes from hyperthyroid (□), euthyroid control (Δ) and hypothyroid (○) rats. Blots were hybridized with 32P-labelled DNA probes (3×106 d.p.m. ml−1) for individual rat β-AR subtypes or for the ubiquitous cyclophilin to correct for fluctuations in the amount of RNA applied to the gel. The results from four independent observations per group are represented in (b). *, Significantly different from euthyroid control mRNA (P<0.05). (Hyper: hyperthroid, eut: euthyroid control and hypo: hypothyroid).

Figure 2

Concentration-response curves for stimulation of glycerol release from hyperthyroid (□), euthyroid control (Δ) and hypothyroid (○) rats, elicited by the full β-agonists, noradrenaline (a), isoprenaline (b) and the β3-selective agonist BRL 37344 (c). Each curve is a representative experiment performed in triplicate. Each point is the mean±s.e.mean over basal lipolysis value. Standard deviations not shown are within the symbol.

Figure 3

Concentration-response curves for stimulation of glycerol release from hyperthyroid (□), euthyroid control (Δ) and hypothyroid (○) rats, elicited by the partial agonists, the β3-selective agonist CGP 12177 (a) and the β1-selective agonist xamoterol (b). Each curve is a representative experiment performed in triplicate. Each point is the mean±s.e.mean over basal lipolysis value. Standard deviations not shown are within the symbol.

Figure 4

Concentration-response curves for stimulation of glycerol release from hyperthyroid (□), euthyroid control (Δ) and hypothyroid (○) rats, elicited by postreceptor-acting agents, the adenyl cyclase activator, forskolin (a), the phosphodiesterase inhibitor enprofylline (b) and the protein kinase A activator (Bu)2cAMP (c). Each curve is a representative experiment performed in triplicate. Each point is the mean±s.e.mean over basal lipolysis value. Standard deviations not shown are within the symbol.

Figure 5

Concentration-response curves for adenyl cyclase activity in purified membranes from hyperthyroid (□), euthyroid control (Δ) and hypothyroid (○) rats, elicited by isoprenaline (a), CGP 12177 (b), xamoterol (c) and forskolin (d). The results illustrated are means over basal value (vertical lines show standard deviation) of 3–4 experiments performed in duplicate. Standard deviations not shown are within the symbol.