Julio Moratinos

Ras-GRF1 signaling is required for normal β-cell development and glucose homeostasis

Development of diabetes generally reflects an inadequate mass of insulin‐producing β‐cells. β‐cell proliferation and differentiation are regulated by a variety of growth factors and hormones, including insulin‐like growth factor I (IGF‐I). GRF1 is a Ras‐guanine nucleotide exchange factor known previously for its restricted expression in brain and its role in learning and memory. Here we demonstrate that GRF1 is also expressed in pancreatic islets. Interest ingly, our GRF1‐deficient mice exhibit reduced body weight, hypoinsulinemia and glucose intolerance owing to a reduction of β‐cells. Whereas insulin resistance is not detected in peripheral tissues, GRF1 knockout mice are leaner due to increased lipid catabolism. The reduction in circulating insulin does not reflect defective glucose sensing or insulin production but results from impaired β‐cell proliferation and reduced neogenesis. IGF‐I treatment of isolated islets from GRF1 knockouts fails to activate critical downstream signals such as Akt and Erk. The observed phenotype is similar to manifestations of preclinical type 2 diabetes. Thus, our observations demonstrate a novel and specific role for Ras‐GRF1 pathways in the development and maintenance of normal β‐cell number and function.

Effects of catecholamines on plasma potassium: the role of alpha‐ and beta‐adrenoceptors

Summary— The sympathetic nervous system plays an important role in the control of plasma potassium levels. Administration of adrenaline or noradrenaline evokes, in the majority of mammal species a dual response: first a short transient hyperkalaemia, followed by a maintained hypokalaemia. Alpha1‐ and alpha2‐adrenoceptors mediate the initial hyperkalaemia through the activation of hepatic Ca2+‐dependent‐K+‐ channels. Stimulation of beta1‐ and beta2‐adrenoceptors induces the late hypokalaemia by stimulation of skeletal muscle Na+‐K+‐ATPase. Beta3‐adrenoceptor stimulation may also have an effect on plasma potassium control since administration of selective beta3‐adrenoceptor agonists induces a decrease in plasma potassium. The simultaneous infusion of phenyleprine (alpha‐adrenoceptor agonist) and isoprenaline (beta‐adrenoceptor agonist) increases plasma potassium levels: this effect is several times larger than the algebric summation of the changes in plasma potassium when each agent is infused separately, thus suggesting potentiation. The physiological (changes in cell volume and function secondary to changes in ion fluxes) and clinical implications (pathophysiological conditions with hypo or hyperkalaemia, hyperkalaemic periodic paralysis, ventricular arrythmias) of these findings are discussed.

Changes in plasma glucose and lactate evoked by α and β2‐adrenoceptor stimulation in conscious fasted rabbits

Summary— In conscious fasted rabbits, the iv infusion of salbutamol (3 μg/kg per min) and clonidine (2 μg/kg per min) induced a blood glucose increase amenable to blockade, respectively by ICI 118551 (1 μg/kg per min) and idazoxan (20 μg/kg per min). Amidephrine (10 μg/kg per min) and salbutamol mediated an increase in plasma lactate which was attenuated by prazosin (50 μg/kg, sc) and ICI 118551 respectively. Clonidine did not alter basal plasma lactate. The iv infusion of adrenaline (0.3 μg/kg per min) evoked an increase in plasma lactate more sensitive to blockade by ICI 118551 than by prazosin. ICI 118551 also shortened the hyperglycaemic response to adrenaline, 3‐Mercaptopicolinic acid (25 mg/kg) reduced salbutamol‐ and adrenaline‐mediated hyperglycaemia and increased at the same time the lactate/glucose ratio. Our data show that plasma lactate levels may be regulated by α1 ‐ and β2‐excitatory adrenoceptor stimulation. However, only the increase in blood lactate derived from β2‐adrenergic stimulation seems to contribute to the overall catecholamine‐mediated hyperglycaemia.

Role of μ-opioid receptors in insulin release in the presence of inhibitory and excitatory secretagogues

In mouse pancreatic islets incubated under static conditions, the inhibitory effects on glucose-evoked insulin release induced by adrenaline (1 microM), clonidine (2 microM) and UK 14,304 (brimonidine, 0.001-1 microM) were abolished by naloxone (30 nM). Only CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Phe-Thr-NH(2), 0.1 microM), a very selective mu-opioid receptor antagonist, blocked the response to UK 14,304. Glucose-induced insulin secretion was attenuated by both beta-endorphin (0.01 microM) and endomorphin-1 (0.1 microM). Naloxone and CTOP prevented these inhibitory responses. The stimulatory effect of glibenclamide (1 microM) was also reduced by endomorphin-1. However, when islets were incubated in the presence of K(+) (30 mM), carbachol (100 microM) or forskolin (0.1 microM), neither the inhibitory effect induced by UK 14,304 was reversed by naloxone, nor endomorphin-1 altered the responses promoted by the excitatory agents. Thus, alpha(2)-adrenoceptor stimulation might inhibit glucose-induced insulin secretion by releasing endogenous opioids. Mu-Opioid receptor activation and opening of K(ATP) channels could be involved in the response.

Differential sensitivity to adrenergic stimulation underlies the sexual dimorphism in the development of diabetes caused by Irs-2 deficiency

The diabetic phenotype caused by the deletion of insulin receptor substrate-2 (Irs-2) in mice displays a sexual dimorphism. Whereas the majority of male Irs-2(-/-) mice are overtly diabetic by 12 weeks of age, female Irs-2(-/-) animals develop mild obesity and progress less rapidly to diabetes. Here we investigated β-cell function and lipolysis as potential explanations for the gender-related differences in this model. Glucose-stimulated insulin secretion was enhanced in islets from male null mice as compared to male WT whereas this response in female Irs-2(-/-) islets was identical to that of female controls. The ability of α(2)-adrenoceptor (α(2)-AR) agonists to inhibit insulin secretion was attenuated in male Irs2 null mice. Consistent with this, the expression of the α(2A)-AR was reduced in male Irs-2(-/-) islets. The response of male Irs-2(-/-) islets to forskolin was enhanced, owing to increased production of cAMP. Basal lipolysis was increased in male Irs-2(-/-) but decreased in female Irs-2(-/-) mice, concordant with the observation that adipose tissue is sparse in males whereas female Irs2 null mice are mildly obese. Adipocytes from both male and female Irs-2(-/-) were resistant to the anti-lipolytic effects of insulin but female Irs-2(-/-) fat cells were additionally resistant to the catabolic effects of beta-adrenergic agonists. This catecholamine resistance was associated with impaired generation of cAMP. Consequently, targets of cAMP-dependent protein kinase (PKA) which mediate lipolysis were not phosphorylated in adipose tissue of female Irs-2(-/-) mice. Our findings suggest that IRS-2 deficiency in mice alters the expression and/or sensitivity of components of adrenergic signaling.

Effects of verapamil and elgodipine on isoprenaline-induced metabolic responses in rabbits

Verapamil (0.17 microg kg(-1) min(-1) intravenous, i.v.) but not elgodipine (35 ng kg(-1) min(-1)) modestly enhanced the weak blood glucose increase induced by the i.v. infusion of isoprenaline (0.3 microg kg(-1) min(-1)) in conscious rabbits. However, elgodipine but not verapamil suppressed the increase in circulating insulin evoked by the agonist. Both drugs enhanced the rise in plasma lactate mediated by isoprenaline but only elgodipine potentiated the lipolytic effect of the agonist. In isolated islets elgodipine (10(-6) M) blocked forskolin (10(-6) M)-induced insulin release. However, in rabbit adipocytes elgodipine potentiated both glycerol release and cAMP accumulation induced by isoprenaline (10(-8)-10(-6) M). Excess K(+) (40-60 mM) did not alter basal lipolysis or the response to isoprenaline in either rabbit or mouse adipocytes. Therefore, Ca2+ influx through L-type Ca2+ channels does not seem to play a significant role in the lipolytic effect of isoprenaline. Metabolic alterations found with Ca2+ channel antagonists were of minor intensity and probably devoid of pathological implications.

Adipocitocinas: implicaciones en el pronóstico y tratamiento farmacológico de patología cardiovascular

Las adipocitocinas, factores derivados del tejido graso con propiedades reguladoras, estan implicadas en la fisiopatologia de procesos ateromatosos y metabolicos tales como: cardiopatia isquemica, resistencia a la insulina, obesidad, dislipidemia y diabetes mellitus. El aumento de los depositos de tejido adiposo visceral determina una peor evolucion para esas afecciones. Farmacos como los inhibidores de la enzima convertidora de la angiotensina (IECA), las tiazolidindionas (glitazonas) o los antagonistas del receptor de angiotensina-II, generalmente asociados con la adecuada medicacion hipolipidemiante (estatinas, fibratos) o antiobesidad (orlistat, sibutramina, rimonabant), incrementarian las adipocitocinas con propiedades anti-inflamatorias e insulino-sensibilizadoras (es decir, adiponectina o visfatina), mientras que reducirian aquellas pro-inflamatorias y trombogenicas (como leptina, factor de necrosis tumoral [TNF]-a, inhibidor del activador del plasminogeno tipo 1 [PAI-1]). Por tanto, estas medidas terapeuticas farmacologicas podrian tener un efecto beneficioso al disminuir la morbimortalidad y mejorar el pronostico de los pacientes que padecen dichas enfermedades, todas ellas relacionadas con un elevado riesgo cardiovascular.

Role of Ca2+ channel blockers in insulin secretion resulting from α1- and β-adrenoceptor stimulation in the rabbit

In conscious fasted rabbits the i.v. infusion of amidephrine (alpha 1-agonist) or isoprenaline (beta-agonist) induced an increase in plasma levels of immunoreactive insulin. The alpha 1-mediated response was suppressed in animals pretreated with calcium channel blockers (verapamil and elgodipine). The potentiated insulin secretory response in rabbits exposed to dual (alpha 1 + beta) adrenoceptor stimulation was also prevented by verapamil and elgodipine. These results suggest that extracellular calcium is required to mediate the effects of amidephrine on insulin secretion and to support potentiation.

Effects of salbutamol and BRL 37344 on diastolic arterial blood pressure, plasma glucose and plasma lactate in rabbits.

Summary— The aim of this study was to investigate in rabbits the diastolic arterial blood pressure, plasma glucose and plasma lactate responses to salbutamol (a selective beta‐2 adrenoceptor agonist) and BRL 37344 (a selective beta‐3 adrenoceptor agonist) in comparison with CGP 12177 (a potent beta‐1 and beta‐2 adrenoceptor antagonist which also acts as a partial beta‐3 agonist), isoprenaline (a non‐selective beta‐1, beta‐2 and beta‐3 adrenoceptor agonist) and adrenaline (a non‐selective beta and alpha adrenoceptor agonist). All drugs were iv infused at the same dose: 0.3 μg/kg/min (30 min). In sodium pentobarbitone (40 mg/kg)‐anasthetized animals none of these compounds altered diastolic arterial blood pressure. BRL 37344 (0.1, 0.3, 1 μg/kg/min) did not modify this parameter either. In conscious 24‐h fasted rabbits, only adrenaline was able to increase plasma glucose levels. By contrast, under the same experimental conditions, salbutamol, isoprenaline and adrenaline, but not BRL 37344 or CGP 12177, induced a significant increase in plasma lactate levels. Finally, the salbutamol‐mediated plasma lactate response was inhibited in the presence of clonidine (2 μg/kg/min, an alpha‐2 adrenoceptor agonist), a drug considered to have opposite effects (stimulatory and inhibitory) on the adenylate cyclase system. In conclusion, these data suggest that only beta‐2 adrenoceptor stimulation is able to increase plasma lactate levels, a response which is inhibited by alpha‐2 adrenoceptor stimulation.