Desensitization of β3-adrenergic receptor-stimulated adenylyl cyclase activity and lipolysis in rats

The beta(3)-adrenergic receptor is an integral membrane protein consisting of seven transmembrane domains. Unlike the beta(1) and beta(2) receptors, this subtype lacks the consensus phosphorylation sites required for desensitization by serine kinases. Using the rodent specific beta(3) agonist BRL, 35135, our initial data indicated that beta(3) 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 beta(3) 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 beta(3) 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 (similar to 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, beta(3)-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 beta(3) 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.