A selective human beta3 adrenergic receptor agonist increases metabolic rate in rhesus monkeys.

Activation of beta3 adrenergic receptors on the surface of adipocytes leads to increases in intracellular cAMP and stimulation of lipolysis. In brown adipose tissue, this serves to up-regulate and activate the mitochondrial uncoupling protein 1, which mediates a proton conductance pathway that uncouples oxidative phosphorylation, leading to a net increase in energy expenditure. While chronic treatment with beta3 agonists in nonprimate species leads to uncoupling protein 1 up-regulation and weight loss, the relevance of this mechanism to energy metabolism in primates, which have much lower levels of brown adipose tissue, has been questioned. With the discovery of L-755,507, a potent and selective partial agonist for both human and rhesus beta3 receptors, we now demonstrate that acute exposure of rhesus monkeys to a beta3 agonist elicits lipolysis and metabolic rate elevation, and that chronic exposure increases uncoupling protein 1 expression in rhesus brown adipose tissue. These data suggest a role for beta3 agonists in the treatment of human obesity.

Desensitization of beta3-adrenergic receptor-stimulated adenylyl cyclase activity and lipolysis in rats.

The beta3-adrenergic receptor is an integral membrane protein consisting of seven transmembrane domains. Unlike the beta1 and beta2 receptors, this subtype lacks the consensus phosphorylation sites required for desensitization by serine kinases. Using the rodent specific beta3 agonist BRL 35135, our initial data indicated that beta3 receptor-mediated glycerol levels progressively decreased following daily oral doses of 5 mg/kg. Therefore, we initiated studies designed to delineate the possible mechanism(s) for this decreased response. Within 3 hours following a single oral dose of BRL 35135, serum glycerol levels and UCP (uncoupling protein) RNA levels were significantly increased whereas beta3 RNA levels were significantly decreased. Rats were dosed daily for 5 days with either vehicle or BRL 35135 (5 mg/kg, p.o.) and blood samples were collected for glycerol analysis. Adipose tissue was excised for lipolysis and adenyl cyclase measurements. In addition, UCP and beta3 receptor RNA levels were assessed. No effect on adipocyte BRL 37344-stimulated adenylyl cyclase activity was observed 3 hours following the initial dose of BRL 35135. Although a slight decrease (approximately 25%) in adenylyl cyclase activity could be observed 24 hours following the initial dose, it wasn't until day 4 of dosing that a significant decrease (50%) was observed. In contrast, beta3- stimulated lipolysis in adipocytes from BRL 35135-treated rats was decreased 85% within 24 hours and this decrease persisted through four days of treatment. These data indicate that the lipolytic response to beta3 receptor activation is decreased after only a single oral dose of BRL 35135, whereas receptor-mediated adenylyl cyclase activation, although initially unaffected, also desensitizes by day four of treatment.

A novel insulin secretagogue is a phosphodiesterase inhibitor.

The arylpiperazine L-686,398 was described as an oral hypoglycemic agent and is shown to be an insulin secretagogue in vitro. The characteristics of its activity were similar to those of the incretin glucagon-like peptide I (GLP-I). We demonstrate that both the peptide and L-686,398 increase the accumulation of cAMP in isolated ob/ob mouse pancreatic islet cells, but by different mechanisms. Although GLP-I activates adenylate cyclase, the arylpiperazine has no effect on this enzyme or on the binding of 125I-labeled GLP-I to its receptor on RINm5F rat insulinoma cell membranes. However, L-686,398 inhibits the total cAMP phosphodiesterase (PDE) activity in homogenates of ob/ob mouse pancreatic islets with an EC50 of approximately 50 mumol/l. To determine the mechanism of PDE inhibition by the arylpiperazine and to examine its specificity, we studied the kinetics of arylpiperazine inhibition of two recombinant PDEs. The arylpiperazine is a competitive inhibitor of both a human heart type III PDE and a rat type IV-D PDE. Inhibition of the type III and IV isozymes are characterized by Ki values of 27 and 5 mumol/l, respectively. Although not extremely potent, the arylpiperazine does exhibit modest selectivity between these PDEs. The observation that L-686,398 acts as a PDE inhibitor suggests that exploration for beta-cell-specific PDE isoforms may reveal novel PDEs as targets for the development of therapeutically useful glucose-dependent insulin secretagogues.

Glucose-dependent alterations of intracellular free calcium by glucagon-like peptide-1(7-36amide) in individual ob/ob mouse beta-cells.

Depolarizing concentrations of glucose produce characteristic alterations of intracellular free Ca2+ ([Ca2+]i) in pancreatic beta-cells. The effects of the proposed incretin, glucagon-like peptide-1(7-36amide) (GLP-1a) on [Ca2+]i were determined from Fura-2 fluorescence ratio imaging of cultured ob/ob mouse pancreatic beta-cells. In control cells, [Ca2+]i is low in 3 mM glucose; increasing [glucose] to 8-12 mM results in an initial dip in [Ca2+]i followed by slow oscillating increases in [Ca2+]i. GLP-1a (0.03-10,000 pM) does not alter [Ca2+]i in 3 mM glucose, but does change the response to elevated glucose (8-12 mM). The time integral of the initial dip is reduced ([GLP-1a] 10-100 pM), and the integral of the [Ca2+]i signal is increased ([GLP-1a] > or = 1 pM). GLP-1a increases the frequency of sustained, stable plateau responses to elevated glucose, and the frequency of large, rapid spikes of increased [Ca2+]i associated with either plateaus, or oscillations. Application of a cAMP analog mimics most of the actions of GLP-1a. Activation of the GLP-1a receptor, or application of cAMP alters pancreatic beta-cell [Ca2+]i only when [glucose] is high.

Vanadate treatment of streptozotocin diabetic rats restores expression of the insulin-responsive glucose transporter in skeletal muscle.

Streptozotocin-treated rats were diabetic, as assessed by blood glucose and plasma insulin values, while vanadate treatment restored blood glucose values to normal. Immunoblot analysis using a monoclonal antibody to the insulin-responsive glucose transporter demonstrated a 70% decline in transporter expression in skeletal muscle of diabetic rats. Subsequent treatment of diabetic animals with vanadate resulted in renewed expression of the transporter to 87% of control levels. Northern blot analysis of total skeletal muscle RNA from diabetic animals revealed a 55% decline in the steady state level of muscle glucose transporter mRNA, while vanadate treatment led to a 187% increase in transporter mRNA over normal levels. These results support the conclusion that vanadate acts to relieve diabetic hyperglycemia by inducing expression of the insulin-responsive glucose transporter at the pretranslational level.

Effects of an alpha 2-adrenoceptor antagonist on glucose tolerance in the genetically obese mouse (C57BL/6J ob/ob).

This study examines the effects of a relatively selective alpha 2-adrenoceptor antagonist, 8-(L-piperazinyl)imado-[1,2-alpha] pyrazine (compound A), and the preferential alpha 2-agonist clonidine on blood glucose, glucose tolerance, and plasma insulin levels in the C57BL/6J ob/ob mouse and its lean littermate. While clonidine raised blood glucose levels and impaired glucose tolerance, oral administration of compound A resulted in decreased blood glucose levels, as well as improved glucose tolerance in ob/ob mice. Insulin levels in ob/ob mice treated with clonidine were significantly reduced, while compound A raised insulin levels threefold and blocked the effects of clonidine when co-administered to the same animals. Clonidine-induced hyperglycemia in lean littermates was not accompanied by a decrease in insulin levels, while a small but significant increase in insulin levels was observed by compound A administration. Glycogen synthesis in diaphragm of ob/ob mice was enhanced after oral administration of compound A and was accompanied by an increase in plasma insulin levels. Concomitant treatment with a potent somatostatin analog to inhibit insulin release blocked the effects of the alpha 2-adrenoceptor antagonist, compound A. These observations suggest that the alpha 2-antagonist studied, increased plasma insulin levels with an accompanying reduction in blood glucose and an improvement in glucose tolerance in a genetic model of insulin resistance. Differential sensitivity to alpha 2-agonist in these genetically obese mice, ob/ob, was demonstrated by decreased insulin levels due to clonidine administration.

Insulin receptor tyrosine kinase activity is unaltered in ob/ob and db/db mouse skeletal muscle membranes.

Insulin binding and insulin receptor tyrosine kinase activity were examined in two rodent models with genetic insulin resistance using partially-purified skeletal muscle membrane preparations. Insulin binding activity was decreased about 50% in both 12-week (219 +/- 184 vs 1255 +/- 158 fmoles/mg, p less than 0.01) and 24-week old (2120 +/- 60 vs 1081 +/- 60 fmoles/mg, p less than 0.01) ob/ob mice. In contrast, insulin binding to membrane derived from 24-week old db/db mice was not significantly different from lean controls (1371 +/- 212 vs 1253 +/- 247 fmoles/mg). Insulin-associated tyrosine kinase activity of membranes from ob/ob skeletal muscle was decreased, compared to its normal lean littermate, when compared on a per mg of protein basis in both 12-week (37 +/- 3 vs 21 +/- 3 pmoles/min/mg, p less than 0.05) and 24-week old (71 +/- 5 vs 37 +/- 6 pmoles/min/mg, p less than 0.01) mice. However, no significant differences in kinase activities were observed when the data were normalized and compared on a per fmole of insulin-binding activity basis for the 12-week (12 +/- 1 vs 11 +/- 2) and 24-week (27 +/- 2 vs 20 +/- 3) age groups. Insulin receptor tyrosine kinase activity of db/db skeletal muscle membranes was not different than its normal lean littermate whether expressed on a protein (34 +/- 7 vs 30 +/- 3) or fmole of insulin-binding activity (21 +/- 4 vs 18 +/- 4) basis. These data suggest that insulin receptor tyrosine kinase is not associated with the insulin resistance observed in ob/ob and db/db mice and demonstrate differences in receptor regulation between both animal models.

Inhibition of luteinizing hormone (LH)-releasing hormone-induced secretion of LH in rat anterior pituitary cell culture by testosterone without conversion to 5 alpha-dihydrotestosterone.

The role of 5 alpha-reduction of testosterone in the inhibition of LH secretion was investigated in rat anterior pituitary cell cultures. Pituitary cells were preincubated with testosterone or dihydrotestosterone (17 beta-hydroxy-5 alpha-androstan-3-one) for 17 h and then with LHRH for an additional 4 h. Dihydrotestosterone was 6-fold more potent than testosterone in the inhibition of LHRH-induced LH release. Basal LH secretion was not affected by either androgen. The inhibition curves of testosterone and dihydrotestosterone were not shifted by the presence of the 5 alpha-reductase inhibitors 17 beta-N,N-diethylcarbamoyl-4-methyl-4-aza-5 alpha-androstan-3-one (4-MA) and 17 beta-N,N-diisopropylcarbamoyl-4-aza-androstan-3-one (DIPA). Neither 4-MA nor DIPA alone had an effect on either basal or LHRH-induced LH release. When pituitary cells were incubated with [3H]testosterone for 17 h, the radioactivities were found to be unmetabolized testosterone (66.9 +/- 2.4%), dihydrotestosterone (13.3 +/- 0.5%), androstenedione (15.9 +/- 1.3%), 5 alpha-androstane-3,17-dione (2.8 +/- 0.3%), and 3 alpha (beta), 17 beta-androstanediol (less than 1%). In the presence of 4-MA or DIPA, 5 alpha-reduction of testosterone was completely inhibited; androstenedione was the only metabolite. Androstenedione was only 12% as potent as testosterone in the inhibition of LHRH stimulation of LH release, and conversion of [3H]androstenedione to testosterone and dihydrotestosterone did occur in these cells. When [3H]dihydrotestosterone was incubated with pituitary cells, the radioactivities were dihydrotestosterone (64.4 +/- 0%), 5 alpha-androstanedione (19.3 +/- 1%), 3 alpha (beta), 17 beta-androstanediol (7.7 +/- 1.7%), and unknown polar metabolites. 4-MA and DIPA had no effect on the metabolism of dihydrotestosterone. These results indicate that both testosterone and dihydrotestosterone inhibit LHRH-induced LH release and that this activity of testosterone does not depend on its 5 alpha-reduction.