Effects of central administration of insulin or l-NMMA on rat skeletal muscle microvascular perfusion.

AIM: Intracerebroventricular (ICV) administration of a nitric oxide synthase (NOS) inhibitor to rats has been reported to raise blood pressure (BP) and cause insulin resistance, suggestive of a central effect of insulin that is NO dependent. Herein we test whether ICV insulin has peripheral haemodynamic and metabolic effects and whether peripheral effects of systemic insulin are affected by the ICV administration of the NOS inhibitor N(G) -methyl-l-arginine (l-NMMA). METHODS: Anaesthetized rats were fitted with an ICV cannula for insulin, artificial cerebrospinal fluid (aCSF) or l-NMMA infusion. Rats receiving ICV l-NMMA (500 µg) underwent systemic insulin clamp (10 mU/min/kg) or saline treatment for 70 min and were compared with animals receiving an equal amount of l-NMMA infused systemically. RESULTS: ICV aCSF or insulin (135 mU/min/kg brain) for 70 min or systemic l-NMMA (500 µg) had no effect on BP, heart rate (HR), femoral blood flow (FBF), glucose infusion rate, muscle 2-deoxyglucose uptake, microvascular perfusion or plasma insulin. However, ICV l-NMMA reduced systemic insulin-mediated increases in FBF (2.05 ± 0.08 to 1.55 ± 0.15 ml/min), 2-deoxyglucose uptake (17.7 ± 0.15 to 10.0 ± 0.03 µg/g/min) and microvascular perfusion (10.5 ± 0.5 to 6.6 ± 1.1 mol/min) (each mean ± SE, p < 0.05); plasma insulin, HR and BP were unaffected. CONCLUSIONS: Central insulin administration had no effect on skeletal muscle haemodynamics or glucose metabolism. However, systemic insulin-mediated increases in limb blood flow, muscle microvascular perfusion and glucose uptake may be regulated by a central pathway that is NO dependent.

Insulin-like action of catecholamines and Ca2+ to stimulate glucose transport and GLUT4 translocation in perfused rat heart.

The uptake of 2-deoxyglucose by perfused rat hearts was compared to the distribution of the insulin-regulatable glucose transporter (GLUT4) in membrane preparations from the same hearts. The hearts were treated with the alpha-adrenergic combination of epinephrine + propranolol, the beta-adrenergic agonist isoproterenol, high (8 mM) Ca2+ concentrations, insulin and the alpha adrenergic combination or insulin alone. Epinephrine (1 microM) + propranolol (10 microM), isoproterenol (10 microM), high Ca2+, insulin (1 microM) + epinephrine (1 microM) + propranolol (10 microM) and insulin (1 microM) each led to an increase in 2-deoxyglucose uptake and a shift in the recovery of the GLUT4 from a high-speed pellet membrane fraction (putatively intracellular) to a low-speed pellet membrane fraction (putatively sarcolemmal). There were significant correlations (r = -0.673, P less than 0.001) between the stimulation of 2-deoxyglucose uptake and the loss of GLUT4 from the intracellular membrane fraction, or the increase in the sarcolemmal fraction. The data provide evidence that the GLUT4 is translocated by agents that stimulate glucose transport in heart, and therefore this mechanism is not restricted to insulin.

The effects of alpha- and beta-adrenergic agents, Ca2+ and insulin on 2-deoxyglucose uptake and phosphorylation in perfused rat heart.

Insulin (0.1 microM) and 1 microM epinephrine each increased the uptake and phosphorylation of 2-deoxyglucose by the perfused rat heart by increasing the apparent Vmax without altering the Km. Isoproterenol (10 microM), 50 microM methoxamine and 10 mM CaCl2 also increased uptake. Lowering of the perfusate Ca2+ concentration from 1.27 to 0.1 mM Ca2+, addition of the Ca2+ channel blocker nifedipine (1 microM) or addition of 1.7 mM EGTA decreased the basal rate of uptake of 2-deoxyglucose and prevented the stimulation due to 1 microM epinephrine. Stimulation of 2-deoxyglucose uptake by 0.1 microM insulin was only partly inhibited by Ca2+ omission, nifedipine or 1 mM EGTA. Half-maximal stimulation of 2-deoxyglucose uptake by insulin occurred at 2 nM and 0.4 nM for medium containing 1.27 and 0.1 mM Ca2+, respectively. Maximal concentrations of insulin (0.1 microM) and epinephrine (1 microM) were additive for glucose uptake and lactate output but were not additive for uptake of 2-deoxyglucose. Half-maximal stimulation of 2-deoxyglucose uptake by epinephrine occurred at 0.2 microM but maximal concentrations of epinephrine (e.g., 1 microM) gave lower rates of 2-deoxyglucose uptake than that attained by maximal concentrations of insulin. The addition of insulin increased uptake of 2-deoxyglucose at all concentrations of epinephrine but epinephrine only increased uptake at sub-maximal concentrations of insulin. The role of Ca2+ in signal reversal was also studied. Removal of 1 microM epinephrine after a 10 min exposure period resulted in a rapid return of contractility to basal values but the rate of 2-deoxyglucose uptake increased further and remained elevated at 20 min unless the Ca2+ concentration was lowered to 0.1 mM or nifedipine (1 microM) was added. Similarly, removal of 0.1 microM insulin after a 10 min exposure period did not affect the rate of 2-deoxyglucose uptake, which did not return to basal values within 20 min unless the concentration of Ca2+ was decreased to 0.1 mM. Insulin-mediated increase in 2-deoxyglucose uptake at 0.1 mM Ca2+ reversed upon hormone removal. It is concluded that catecholamines mediate a Ca2+-dependent increase in 2-deoxyglucose transport from either alpha or beta receptors. Insulin has both a Ca2+-dependent and a Ca2+-independent component. Reversal studies suggest an additional role for Ca2+ in maintaining the activated transport state when activated by either epinephrine or insulin.

alpha-Adrenergic stimulation of trans-sarcolemma electron efflux in perfused rat heart. Possible regulation of Ca2+-channels by a sarcolemma redox system.

The role of trans-sarcolemma membrane electron efflux in the alpha-adrenergic control of Ca2+ influx in perfused rat heart was examined. Electron efflux was measured by monitoring the rate of reduction of extracellular ferricyanide and compared with changes in contractility, as an indirect assessment of changes in cytoplasmic Ca2+ concentration. Methoxamine and phenylephrine each increased the rate of ferricyanide reduction from 80 to approx. 114 nmol/min per g wet wt. of heart, with half-maximal activation occurring at 10 microM for each agonist. Activation of the rate of ferricyanide reduction by both 10 microM methoxamine and 10 microM phenylephrine was blocked by the alpha-adrenergic antagonist, phenoxybenzamine, but not by the beta-antagonist, propranolol. Stimulation of the rate of ferricyanide reduction by the alpha-agonist coincided with the increase in contractility, each reaching maximum values at approx. 80 s. Removal of the alpha-agonists led to parallel decreases in contractility and the rate of reduction, each returning to pre-stimulation values in approx. 400 s. In addition, the relationship between Ca2+ and ferricyanide reduction was examined. Perfusion of the heart with medium containing 6 mM CaCl2 significantly increased contractility and the rate of ferricyanide reduction. Perfusion of the heart with low Ca2+ diminished contractility, did not affect the rate of ferricyanide reduction, but amplified the stimulatory effect of methoxamine on this rate. The increase in ferricyanide reduction by alpha-adrenergic agonists resulted from a change in the apparent Vmax, indicative of an increase in electron efflux sites in the plasma membrane. It is concluded that alpha-adrenergic control of electron efflux closely parallels changes in contractility and therefore changes in the cytoplasmic concentration of Ca2+. The data suggest that alpha-agonist-mediated changes in electron efflux may lead to Ca2+ influx.

Properties and regulation of a trans-plasma membrane redox system of perfused rat heart.

Ferricyanide was reduced to ferrocyanide by the perfused rat heart at a linear rate of 78 nmol/min per g of heart (non-recirculating mode). Ferricyanide was not taken up by the heart and ferrocyanide oxidation was minimal (3 nmol/min per g of heart). Perfusate samples from hearts perfused without ferricyanide did not reduce ferricyanide. A single high-affinity site (apparent Km = 22 microM) appeared to be responsible for the reduction. Perfusion of the heart with physiological medium containing 0.5 mM ferricyanide did not alter contractility, biochemical parameters or energy status of the heart. Perfusate flow rate and perfusate oxygen concentration exerted opposing effects on the rate of ferricyanide reduction. A net decreased reduction rate resulted from a decreased perfusion flow rate. Thus, the rate of supply of ferricyanide dominated over the stimulatory effect of oxygen restriction; the latter effect only becoming apparent when the oxygen concentration was lowered at a high perfusate flow rate. Whereas glucose (5 mM) increased the rate of ferricyanide reduction, pyruvate (2 mM), acetate (2 mM), lactate (2 mM) and 3-hydroxybutyrate (2 mM) each had no effect. Insulin (3 nM), glucagon (0.5 microM), dibutyryl cyclic AMP (0.1 mM) and the beta-adrenergic agonist ritodrine (10 microM) also had no effect, however, the alpha 1-adrenergic agonist, methoxamine (10 microM), produced a net increase in the rate of ferricyanide reduction. It is concluded that a trans-plasma membrane electron efflux occurs in perfused rat heart that is sensitive to oxygen supply, glucose, perfusion flow rate, and the alpha-adrenergic agonist methoxamine.

Acute impairment of insulin-mediated capillary recruitment and glucose uptake in rat skeletal muscle in vivo by TNF-alpha.

The vascular actions of insulin may contribute to the increase in glucose uptake by skeletal muscle. We have recently shown that when capillary recruitment by insulin is blocked in vivo, an acute state of insulin resistance is induced. Another agent that may have vascular effects is the inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha), which has been reported to play an important role in the insulin resistance of obesity, type 2 diabetes, and sepsis in both animals and humans. Thus, in the present study, we have investigated the effect of an intravenous 3-h TNF treatment (0.5 microg x h(1) x kg(-1)) in control and euglycemic-hyperinsulinemic-clamped (10 mU x min(-1) x kg(-1) for 2 h) anesthetized rats. Hind-leg glucose uptake, muscle uptake of 2-deoxyglucose (2-DG), femoral blood flow (FBF), vascular resistance (VR), and capillary recruitment as measured by metabolism of infused 1-methylxanthine (1-MX) were assessed. Insulin alone caused a significant (P < 0.05) increase in FBF (1.7-fold) and capillary recruitment (2.5-fold), with a significant decrease in VR. In addition, hind-leg glucose uptake was increased (fourfold), as was 2-DG uptake in the soleus and plantaris muscles. TNF completely prevented the insulin-mediated changes in FBF, VR, and capillary recruitment and significantly reduced (P < 0.05) the insulin-mediated increase in total hind-leg glucose uptake (by 61%) and muscle 2-DG uptake (by at least 50%). TNF alone had no significant effect on any of these variables. It is concluded that acute administration in vivo of TNF completely blocks the hemodynamic actions of insulin on rat skeletal muscle vasculature and blocks approximately half of the glucose uptake by muscle. It remains to be determined whether these two effects are interdependent.

Hemodynamic actions of insulin in rat skeletal muscle: evidence for capillary recruitment.

Insulin-induced increases in blood flow are hypothesized to enhance overall glucose uptake by skeletal muscle. Whether the insulin-mediated changes in blood flow are associated with altered blood flow distribution and increased capillary recruitment in skeletal muscle is not known. In the present study, the effects of insulin on hemodynamic parameters in rat skeletal muscle in vivo were investigated. Mean arterial blood pressure, heart rate, femoral blood flow, hind leg vascular resistance, and glucose uptake were measured in control and euglycemic insulin-clamped (10 mU x min(-1) x kg[-1]) anesthetized rats. Blood flow distribution within the hind leg muscles was assessed by measuring the metabolism of 1-methylxanthine (1-MX), an exogenously added substrate for capillary xanthine oxidase. Insulin treatment had no effect on heart rate but significantly increased arterial blood pressure (12 mmHg) and femoral blood flow (80%) and decreased hind leg vascular resistance (31%). Changes were similar in magnitude and in time of onset to those reported in humans. Insulin treatment increased hind leg glucose uptake approximately fourfold and also increased hind leg 1-MX metabolism by 50%, suggesting increased exposure to endothelial xanthine oxidase. To ascertain whether the increased 1-MX metabolism was simply due to increased bulk femoral blood flow, epinephrine was infused at a dose (0.125 microg x min(-) x kg[-1]) chosen to match the insulin-induced increase in femoral blood flow. This dose of epinephrine had no significant effects on arterial blood pressure or heart rate but increased femoral blood flow and lowered hind leg vascular resistance to a similar extent as insulin. Epinephrine did not significantly alter 1-MX metabolism as compared with control animals. These results demonstrate that insulin increases total hind leg blood flow and metabolism of 1-MX, suggesting a recruitment of capillary blood flow in rat hind leg not mimicked by epinephrine.

Acute vasoconstriction-induced insulin resistance in rat muscle in vivo.

Insulin-mediated changes in blood flow are associated with altered blood flow distribution and increased capillary recruitment in skeletal muscle. Studies in perfused rat hindlimb have shown that muscle metabolism can be regulated by vasoactive agents that control blood flow distribution within the hindlimb. In the present study, the effects of a vasoconstrictive agent that has no direct effect on skeletal muscle metabolism but that alters perfusion distribution in rat hindlimb was investigated in vivo to determine its effects on insulin-mediated vascular action and glucose uptake. We measured the effects of alpha-methylserotonin (alpha-met5HT) on mean arterial blood pressure, heart rate, femoral blood flow, hindlimb vascular resistance, and glucose uptake in control and euglycemic insulin-clamped (10 mU x min(-1) x kg(-1)) anesthetized rats. Blood flow distribution within the hindlimb muscles was assessed by measuring the metabolism of 1-methylxanthine (1-MX), an exogenously added substrate for capillary xanthine oxidase. Alpha-met5HT (20 microg x min(-1) x kg(-1)) infusion alone increased mean arterial blood pressure by 25% and increased hindlimb vascular resistance but caused no change in femoral blood flow. These changes were associated with decreased hindlimb 1-MX metabolism indicating less capillary flow. Insulin infusion caused decreased hindlimb vascular resistance that was associated with increased femoral blood flow and 1-MX metabolism. Treatment with alpha-met5HT infusion commenced before insulin infusion prevented the increase in femoral blood flow and inhibited the stimulation of 1-MX metabolism. Alpha-met5HT infusion had no effect on hindlimb glucose uptake but markedly inhibited the insulin stimulation of glucose uptake (P < 0.05) and was associated with decreased glucose infusion rates to maintain euglycemia (P < 0.05). A significant correlation (P < 0.05) was observed between 1-MX metabolism and hindlimb glucose uptake but not between femoral blood flow and glucose uptake. The results indicate that in vivo, certain types of vasoconstriction in muscle such as elicited by 5HT2 agonists, which prevent normal insulin recruitment of capillary flow, cause impaired muscle glucose uptake. Moreover, if vasoconstriction of this kind results from stress-induced increase in sympathetic outflow, then this may provide a clue as to the link between hypertension and insulin resistance that is often observed in humans.

Exercise training improves insulin-mediated capillary recruitment in association with glucose uptake in rat hindlimb.

Exercise training is considered to be beneficial in the treatment and prevention of insulin insensitivity, and much of the effect occurs in muscle. We have recently shown that capillary recruitment by insulin in vivo is associated with and may facilitate insulin action to increase muscle glucose uptake. In the present study, we examined the effect of 14 days of voluntary exercise training on euglycemic-hyperinsulinemic clamped (10 mU. min(-1). kg(-1) for 2 h), anesthetized rats. Whole-body glucose infusion rate (GIR), hindleg glucose uptake, femoral blood flow (FBF), vascular resistance, and capillary recruitment, as measured by metabolism of infused 1-methylxanthine (1-MX), were assessed. In sedentary animals, insulin caused a significant (P < 0.05) increase in FBF (1.6-fold) and capillary recruitment (1.7-fold) but a significant decrease in vascular resistance. In addition, hindleg glucose uptake was increased (4.3-fold). Exercise training increased insulin-mediated GIR (24%), hindleg glucose uptake (93%), and capillary recruitment (62%) relative to sedentary animals. Neither capillary density nor total xanthine-oxidase activity in skeletal muscle were increased as a result of the training regimen used. We concluded that exercise training improves insulin-mediated increases in capillary recruitment in combination with augmented muscle glucose uptake. Increased insulin-mediated glucose uptake may in part result from the improved hemodynamic control attributable to exercise training.

Skeletal muscle microvascular recruitment by physiological hyperinsulinemia precedes increases in total blood flow.

Supraphysiological doses of insulin enhance total limb blood flow and recruit capillaries in skeletal muscle. Whether these processes change in response to physiological hyperinsulinemia is uncertain. To examine this, we infused either saline (n = 6) or insulin (euglycemic clamp, 3.0 mU x min(-1) x kg(-1), n = 9) into anesthetized rats for 120 min. Femoral artery flow was monitored continuously using a Doppler flow probe, and muscle microvascular recruitment was assessed by metabolism of infused 1-methylxanthine (1-MX) and by contrast-enhanced ultrasound (CEU). Insulin infusion raised plasma insulin concentrations by approximately 10-fold. Compared with saline, physiological hyperinsulinemia increased femoral artery flow (1.02 +/- 0.10 vs. 0.68 +/- 0.09 ml/min; P < 0.05), microvascular recruitment (measured by 1-MX metabolism [6.6 +/- 0.5 vs. 4.5 +/- 0.48 nmol/min; P < 0.05] as well as by CEU [167.0 +/- 39.8 vs. 28.2 +/- 13.8%; P < 0.01]), and microvascular flow velocity (beta, 0.14 +/- 0.02 vs. 0.09 +/- 0.02 s(-1)). Subsequently, we studied the time dependency of insulin's vascular action in a second group (n = 5) of animals. Using CEU, microvascular volume was measured at 0, 30, and 90 min of insulin infusion. Insulin augmented microvascular perfusion within 30 min (52.8 +/- 14.8%), and this persisted at 90 min (64.6 +/- 9.9%). Microvascular recruitment occurred without changes to femoral artery flow or beta. We conclude that insulin increases tissue perfusion by recruiting microvascular beds, and at physiological concentrations this precedes increases in total muscle blood flow by 60-90 min.

Contraction-associated translocation of protein kinase C in rat skeletal muscle.

Electrical stimulation of the sciatic nerve of the anaesthetized rat in vivo led to a time-dependent translocation of protein kinase C from the muscle cytosol to the particulate fraction. Maximum activity of protein kinase C in the particulate fraction occurred after 2 min of intermittent short tetanic contractions of the gastrocnemius-plantaris-soleus muscle group and coincided with the loss of activity from the cytosol. Translocation of protein kinase C may imply a role for this kinase in contraction-initiated changes in muscle metabolism.

Co-ordinated regulation of muscle glycolysis and hepatic glucose output in exercise by catecholamines acting via alpha-receptors.

Recent findings indicate that glucose uptake by contracting hindlimb #Acta Physiol. Scand. (1982) 116, 215-222 #and heart #Biochem. Biophys. Res. Commun. (1982) 108, 124-131 # of the rat is stimulated by epinephrine acting through alpha-adrenergic mechanisms. Since in exercise hepatic glucose output may be increased markedly by activation of alpha-adrenergic receptors and matched by the increase in muscle glucose uptake (maintaining blood glucose levels relatively constant), it is now proposed that a general coordination of glucose metabolism may operate via alpha-adrenergic receptor mechanisms. The basis for this proposal is discussed.

Retrograde axonal transport of horseradish peroxidase in peripheral autonomic nerves.

An exogeneous marker protein, horseradish peroxidase (HRP) was used to race peripheral autonomic pathways in adult guinea pigs and cats. Small doses of HRP were injected into various organs and after a brief survival period, HRP activity appeared in the perikarya of autonomic neurons that supplied each injection site. After injection of HRP into the anterior chamber of the eye, reaction product was detected in the postganglionic sympathetic neurons of the superior cervical sympathetic ganglion. In another experiment, HRP reaction product was found in the cell bodies of the preganglionic sympathetic neurons that supply the adrenal medulla. These were located in the lateral gray column of the spinal cord at T6 and T7 segmental levels. Reaction product appeared in intramural postganglionic parasympathetic neurons close to an injection site in the wall of the urinary bladder and in a similiar situation in Meissner's ganglia of the ileum. Following injection into the walls of the stomach and ileum, HRP labelled cells were detected in the nodose ganglion of the vagus and in preganglionic parasympathetic neurons in the dorsal motor nucleus of this nerve. After injection into the subepicardial tissue of the heart, reaction product appeared in the stellate ganglion and also in an upper thoracic dorsal root ganglion. These data suggest that HRP is taken up by peripheral autonomic nerves of all types, and then undergoes rapid retrograde axonal transport to the perikaryon. It appears, therefore, that HRP may be useful in tracing both motor and sensory peripheral autonomic pathways.

Obesity and the regulation of phosphofructokinase in heart: an apparent insensitivity to adrenergic activation in mature-age genetically obese rats.

The activity ratio of phosphofructokinase in perfused rat heart and its activation by epinephrine was examined in non-obese, fat-fed obese, and genetically obese rats. For non-obese colony rats there was an age-dependent increase in the activity ratio of phosphofructokinase from 0.2 at 40 days to 0.4 at mature age (greater than 200 days). Epinephrine (10 microM) treatment of the heart for 5 min increased the ratio at all ages but the proportional increase diminished with age. For mature-age lean Zucker rats carrying the genetic determinant for obesity the results were similar to those obtained for comparable non-obese colony rats. For fat-fed obese rats the activity ratio of phosphofructokinase at 200 days of age was 0.2 and was increased to 0.6 by epinephrine treatment. For mature-age obese Zucker rats the activity ratio was 0.2 and no significant response to epinephrine occurred. The activity ratio of glycogen phosphorylase and its response to epinephrine (beta-adrenergic receptor mediated) in heart was unaffected by age, diet or the gene for obesity. The present findings indicate a specific defect in the adrenergic regulatory mechanism for phosphofructokinase in genetically obese rats.

Perceptual auditory gap detection deficits in male BXSB mice with cerebrocortical ectopias.

Underlying impairments in rapid auditory processing may contribute to disrupted phonological processing, which in turn characterizes developmental language impairment (LI). Identification of a neurobiological feature of LI that is associated with auditory deficits would further support this model. Accordingly, we found that adult male rats with induced cortical malformations were impaired in rapid auditory processing. Since 40-60% of BXSB mice exhibit spontaneous focal cerebrocortical ectopias (as seen in dyslexics brains), we assessed auditory gap detection in adult male BXSB mice. Ectopic mice were significantly worse than non-ectopics in detecting a 5 ms silent gap, but were not significantly impaired at longer gap durations (10-100 ms). Our results confirm that focal cortical malformations are associated with impairments in rapid auditory processing.

Aerobic muscle contraction impaired by serotonin-mediated vasoconstriction.

Vasoconstriction mediated by serotonin (5-HT) inhibits muscle metabolism in resting constant-flow-perfused rat hindlimb and may do so by vascular shunting. In the present study, the effects of 5-HT on tension development and contraction-induced oxygen uptake by the sciatic nerve-stimulated gastrocnemius-plantaris-soleus muscle group of the perfused rat hindlimb and tension development by electrically stimulated isolated incubated soleus and extensor digitorum longus muscles were examined. In both erythrocyte and erythrocyte-free perfusions, 0.25 microM 5-HT increased perfusion pressure and markedly decreased contraction-induced tension, oxygen uptake, and lactate release. The release of metabolic vasodilators from exercising skeletal muscle did not appear to affect 5-HT-mediated vasoconstriction; rather, vascular resistance increased during the period of muscle contraction. In contrast, vasoconstriction during muscle contraction mediated by alpha-adrenoceptor stimulation did not impair tension and was partially overcome by metabolic vasodilators. In addition, contraction of isolated incubated soleus and extensor digitorum longus muscles was not affected by 5-HT addition to the incubation medium. We conclude that 5-HT impairs contractility of working muscle during the aerobic phase by limiting oxygen delivery through redistributing perfusate flow. The results are consistent with a vasoconstrictor action of 5-HT on larger vessels, perhaps at feed arteries external to the working muscle. When constricted by 5-HT, these vessels are apparently insensitive to metabolic vasodilatation.

Vasoconstrictor-mediated release of lactate from the perfused rat hindlimb.

The effects of different vasomodulators on lactate release by the constant-flow-perfused rat hindlimb were examined and compared with that by perfused mesenteric artery, incubated preparations of aortas, soleus and epitrochlearis muscles, and perifused soleus muscles. Infusion of vasopressin (0.5 nM), angiotensin II (5 nM), norepinephrine (50 nM), and methoxamine (10 microM) into the hindlimbs of 180- to 200-g rats increased the perfusion pressure by 112-167% from 30.4 +/- 0.8 mmHg, O2 consumption by 26-68% from 6.4 +/- 0.2 mumol.g-1 x h-1, and lactate efflux by 148-380% from 5.41 +/- 0.25 mumol.g-1 x h-1. Hindlimbs of 100- to 120-g rats responded similarly to angiotensin II. Isoproterenol (1 microM) had no effect on O2 uptake or perfusion pressure but increased lactate release by 118%. Nitroprusside (0.5 mM) markedly inhibited the vasoconstrictor-mediated increases in lactate release, perfusion pressure, and O2 consumption by the hindlimb but had no effect on isoproterenol-mediated lactate efflux. Serotonin (6.7 microM) increased lactate release from the perfused mesenteric artery by 120% from 5.48 mol.g-1 x h-1. Lactate release by incubated aorta was increased by angiotensin II (50 nM), isoproterenol (1 microM), and mechanical stretch. The increase mediated by angiotensin II was blocked by glycerol trinitrate (2.2 microM), which had no effect on lactate release by isoproterenol. Neither angiotensin II (5 nM) nor vasopressin (0.5 nM) increased lactate release from incubated soleus and epitrochlearis muscles; however, lactate release was increased by isoproterenol, and this increase was unaffected by glycerol trinitrate (2.2 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired two-tone processing at rapid rates in male rats with induced microgyria.

We previously reported that adult male rats with bilateral induced microgyria exhibit deficits in rapid auditory processing, which appear similar to auditory processing deficits seen in individuals with developmental language disabilities. The current study was designed to further elaborate that finding using an improved paradigm in which stimulus duration was uncoupled from testing experience and learning effects. Specifically, two-tone stimuli with durations of 540, 390, 332 and 249 ms were all presented within a single test session in a modified operant conditioning paradigm. Subjects were tested over a period of 12 days using this variable-stimulus format. Results confirmed microgyric male rats were impaired only in processing two-tone stimuli presented at rapid rates (i.e., 249 ms duration). Thus the current results support the previously observed link between focal malformations and deficits in rapid auditory processing.

Hypertension in obesity may reflect a homeostatic thermogenic response.

Systemic hypertension of mild to moderate degree is often associated with obesity. The hypothesis is that over-eating leads to increased sympathetic activity targeted at the peripheral vasculature as well as other tissues in an attempt (that in many cases may be futile) to stimulate facultative thermogenesis and burn-off the excess energy. This hypothesis represents an important modification of one proposed by Landsberg and is supported by: 1) recent observations that carbohydrate feeding to humans specifically increases muscle sympathetic vasoconstrictor activity in the peroneal nerve, and 2) studies with animal models in which active vasoconstriction in the limbs and elsewhere is associated with marked increases in oxygen consumption (energy expenditure).

Serotonin-mediated acute insulin resistance in the perfused rat hindlimb but not in incubated muscle: a role for the vascular system.

We have recently shown that the vasoconstrictor serotonin (5-HT) inhibits oxygen uptake in perfused hindlimb possibly due to vascular shunting. Thus in the present study the effect of 5-HT on insulin-mediated glucose uptake was assessed. Rat hindlimbs were perfused at constant flow with medium containing 8.3 mM glucose and a tracer amount of 2-deoxy-D-[1-3]glucose (2DG) with and without 10 microM 5-HT, 15 nM insulin and a combination of the two. 5-HT inhibited insulin-mediated stimulation of glucose uptake by 30.4% when added after insulin and 34.4% when added before insulin. In addition, 5-HT inhibited insulin-mediated 2DG uptake by perfused muscles with inhibition ranging from 32% (soleus) to 80% (extensor digitorum longus). The effects of 5-HT on insulin-mediated glucose uptake were partially reversed by vasodilation with carbachol. In contrast to the results for the hindlimb, 10 microM 5-HT had no significant effect on either basal glucose uptake or the stimulation of glucose uptake mediated by 15 nM insulin by isolated incubated soleus or extensor digitorum longus muscles. It is concluded that 5-HT impairs insulin-mediated glucose uptake in the perfused rat hindlimb that may derive from vascular shunting not apparent when muscles are incubated with 5-HT in vitro. These findings may have implications for the link between hypertension and diabetes.