Alessandra L. Gasparetti

Cold Exposure Induces Tissue‐Specific Modulation of the Insulin‐Signalling Pathway in Rattus Norvegicus

Cold exposure provides a reproducible model of improved glucose turnover accompanied by reduced steady state and glucose‐induced insulin levels. In the present report we performed immunoprecipitation and immunoblot studies to evaluate the initial and intermediate steps of the insulin‐signalling pathway in white and brown adipose tissues, liver and skeletal muscle of rats exposed to cold. Basal and glucose‐induced insulin secretion were significantly impaired, while glucose clearance rates during a glucose tolerance test and the constant for glucose decay during a 15 min insulin tolerance test were increased, indicating a significantly improved glucose turnover and insulin sensitivity in rats exposed to cold. Evaluation of protein levels and insulin‐induced tyrosine (insulin receptor, insulin receptor substrates (IRS)‐1 and −2, ERK (extracellular signal‐related kinase)) or serine (Akt; protein kinase B) phosphorylation of proteins of the insulin signalling cascade revealed a tissue‐specific pattern of regulation of the molecular events triggered by insulin such that in white adipose tissue and skeletal muscle an impaired molecular response to insulin was detected, while in brown adipose tissue an enhanced response to insulin was evident. In muscle and white and brown adipose tissues, increased 2‐deoxy‐D‐glucose (2‐DG) uptake was detected. Thus, during cold exposure there is a tissue‐specific regulation of the insulin‐signalling pathway, which seems to favour heat‐producing brown adipose tissue. Nevertheless, muscle and white adipose tissue are able to take up large amounts of glucose, even in the face of an apparent molecular resistance to insulin.

Peroxisome proliferator-activated receptor γ coactivator-1-dependent uncoupling protein-2 expression in pancreatic islets of rats: a novel pathway for neural control of insulin secretion

Aims/hypothesisSympathetic inputs inhibit insulin secretion through α2-adrenergic receptors coupled with Gi protein. High adrenergic tonus generated by exposure of homeothermic animals to cold reduces insulin secretion. In this study we evaluate the participation of UCP-2 in cold-induced regulation of insulin secretion.MethodsStatic insulin secretion studies, western blotting and immunohistochemistry were used in this investigation.ResultsExposure of rats to cold during 8 days promoted 60% (n=15, p<0.05) reduction of basal serum insulin levels concentration accompanied by reduction of the area under insulin curve during i.p. GTT (50%, n=15, p<0.05). Isolated islets from cold-exposed rats secreted 57% (n=6, p<0.05) less insulin following a glucose challenge. Previous sympathectomy, partially prevented the effect of cold exposure upon insulin secretion. Islets isolated from cold-exposed rats expressed 51% (n=6, p<0.5) more UCP-2 than islets from control rats, while the inhibition of UCP-2 expression by antisense oligonucleotide treatment partially restored insulin secretion of islets obtained from cold-exposed rats. Cold exposure also induced an increase of 69% (n=6, p<0.05) in PGC-1 protein content in pancreatic islets. Inhibition of islet PGC-1 expression by antisense oligonucleotide abrogated cold-induced UCP-2 expression and partially restored insulin secretion in islets exposed to cold.Conclusion/interpreatationOur data indicate that sympathetic tonus generated by exposure of rats to cold induces the expression of PGC-1, which participates in the control of UCP-2 expression in pancreatic islets. Increased UCP-2 expression under these conditions could reduce the beta-cell ATP/ADP ratio and negatively regulate insulin secretion.

β3-Adrenergic-dependent and -independent mechanisms participate in cold-induced modulation of insulin signal transduction in brown adipose tissue of rats

During cold exposure, homeothermic animals mobilize glucose with higher efficiency than at thermoneutrality. An interaction between the insulin signal transduction machinery and high sympathetic tonus is thought to play an important role in this phenomenon. In the present study, rats were exposed to cold during 8 days and treated, or not, with a β3-adrenergic agonist, BRL37344 sodium 4-2-2-(3-chlorophenyl)-2-hydroxyethyl amino propyl phenoxy-acetic acid sodium (BRL37344), or antagonist, SR59230A 3-(2-ethylphenoxy)-[(1S)-1,2,3,4-tetrahydronaphth-1-ylamino]-(2S)-2-propanol oxalate (SR59230A), to evaluate the cross-talk between insulin and β3-adrenergic intracellular signaling in brown adipose tissue. The drugs did not modify food ingestion, body temperature, and body weight in control and cold-exposed rats. Treatment of control rats with BRL37344 led to higher insulin-induced tyrosine phosphorylation of the insulin receptors, insulin receptor substrate (IRS)-1 and ERK, higher insulin-induced IRS-1/PI3-kinase association, and higher [Ser473] phosphorylation of Akt. Cold exposure alone promoted higher insulin-induced tyrosine phosphorylation of the insulin receptors, IRS-1, IRS-2, and ERK, and higher insulin-induced IRS-1 and IRS-2/PI3-kinase association. Except for the regulation of ERK, SR59230A abolished all the cold-induced effects upon the insulin signal transduction pathway. However, this antagonist only partially inhibited the cold-induced increase of glucose uptake. Thus, the sympathetic tonus generated during cold-exposure acts, in brown adipose tissue, through the β3-adrenergic receptor and modulates insulin signal transduction, with the exception of ERK. However, insulin-independent mechanisms other than β3-adrenergic activation participate in cold-induced glucose uptake in brown adipose tissue of rats.

Chronotropic response of β-adrenergic-, muscarinic-, and calcitonin gene-related peptide-receptor agonists in right atria from neonatal capsaicin-treated rats

We evaluated the potency of isoproterenol, carbachol, pilocarpine and calcitonin gene-related peptide (CGRP) in the rat right atria at 30, 60 and 90 days after neonatal capsaicin treatment. Neonatal rats were pretreated on the second day of life with capsaicin (50 mg/kg). The capsaicin pretreatment caused a five-fold rightward shift at the pEC(50) level on the concentration-response curves to isoproterenol in 30-day-old rats. Propranolol (10 mg/kg, given 15 min prior to capsaicin treatment) prevented this subsensitivity. No changes in the potency of isoproterenol were observed at 60 and 90 days after capsaicin pretreatment. The potency and maximal responses of CGRP in the right atria in 30-day-old rats were significantly higher than in 60- and 90-day-old rats; however, no differences were found between control and capsaicin groups. The potency and maximal responses to carbachol and pilocarpine were not changed in all groups. The neonatal capsaicin treatment reduced by about 74% the CGRP content in the heart in all groups. In summary, capsaicin treatment in newborn rats produces a desensitization of chronotropic response mediated by beta-adrenoceptors in isolated right atria from 30-day-old rats possibly due to a massive release of catecholamines.