The direct effect of insulin on barosensitive neurones in the nucleus tractus solitarii of rats.

The present investigation was designed to determine the direct effect of insulin on the spontaneous discharge of barosensitive neurones in the nucleus tractus solitarii (NTS) of rats anaesthetized with urethane. Microinjection of 20 nl insulin (10 IU/ml) into NTS decreased the spontaneous discharge of 38 of the 52 units studied (73.1%), and this decrease was augmented by increasing the concentration to 40 IU/ml. Microinjections of insulin vehicle, glucose, hydralazine or phenylephrine did not elicit significant changes in the spontaneous discharge of NTS barosensitive neurones. These results demonstrate that insulin inhibits the spontaneous discharge of barosensitive NTS neurones. They suggest that insulin increases sympathetic nervous activity via a central neural mechanism and may play a role in the modulation of cardiovascular information within the NTS.

Increase of cerebral phosphocreatine in normal rats after intracerebroventricular administration of creatine.

Intracerebroventricular (ICV) administration of creatine increased cerebral phosphocreatine in normal rats by 67%, the highest increase so far reported in an in vivo model. We used osmotic minipumps (Alzet, Palo Alto, CA, USA) to administer creatine, 0.5 mM, to the lateral ventricle at the rate of 10 microl/h for 3 days. Brain phosphocreatine in saline-treated controls was 33 +/- 17 microM/g protein (mean +/- SD, N = 9). In creatine-treated rats (0.5 mM for 3 days) such content was 55 +/- 17 microM/g protein (mean +/- SD, N = 7). This difference is statistically significant (p = 0.02, t-test). The increase we found in cerebral phosphocreatine is of an order of magnitude comparable to the increase previously found in in vitro experiments, and may be effective in protecting brain tissue from ischemic damage.

Reflex cardiovascular responses to somatic stimulation in spontaneously hypertensive rats.

OBJECTIVES: This study aimed to test whether the cardiovascular responses to somatic stimulation in spontaneously hypertensive rats (SHR) were enhanced compared with those in normotensive Wistar-Kyoto (WKY) rats, and to examine any role of the impaired baroreflex function in the hypertensive rats. METHODS: Experiments were done in anaesthetized SHR (n = 34) and WKY (n = 31). Baroreceptor reflexes were assessed by continuous infusion of incremental doses (5-30 microg/kg per min) of phenylephrine over a 3 min infusion period. Cardiovascular responses to sciatic nerve stimulation (5 s trains, 1 ms pulse duration, 400 microA intensity) were studied before and after baroreceptor deactivation. The latter was achieved either by carotid occlusion and cutting the vagi and aortic nerves (SHR, n = 28 and WKY rats, n = 27), or by complete baroreceptor denervation (SHR, n = 6 and WKY rats, n = 4). RESULTS: We confirmed that baroreceptor sensitivity was significantly lower in SHR (0.40 +/- 0.05 ms/mmHg) than in WKY rats (0.90 +/- 0.04 ms/mmHg). Sciatic nerve stimulation elicited significantly greater increases in mean arterial pressure (MAP) and in heart rate in SHR than in WKY rats (+32.5 +/- 1.9 mmHg versus +20.2 +/- 1.1 mmHg and +13.5 +/- 1.5 bpm versus +8.0 +/- 1.1 bpm, respectively). Following baroreceptor deactivation, the responses to the same sciatic nerve stimulation of MAP and heart rate in SHR (+38.5 +/- 2.4 mmHg and +15.5 +/- 1.5 bpm) were still significantly greater than those in WKY rats (+29.5 +/- 1.3 mmHg and +11.6 +/- 1.2 bpm). CONCLUSIONS: These results show that cardiovascular responses to sciatic nerve stimulation are increased in SHR compared to WKY rats, and that this increased reactivity to somatic stimuli in hypertensive rats does not depend upon the impairment in baroreflex function demonstrated in this strain.

Insulin sensitivity and glucose effectiveness estimated by the minimal model technique in spontaneously hypertensive and normal rats.

This study was performed to compare glucose metabolism in anaesthetised spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY) in an attempt to clarify whether this animal model of hypertension approximates the insulin-resistant state seen in human hypertension. With this aim the minimal model of glucose kinetics was applied to glucose and insulin data derived from a 12-sample, 120 min intravenous glucose tolerance test (IVGTT) performed in ten SHR and nine WKY rats under pentobarbital anaesthesia. This method provided two metabolic indices: the glucose effectiveness, S(G), which quantifies the ability of glucose per se to enhance its rate of disappearance and to inhibit hepatic glucose production, and the insulin sensitivity, S(I), which measures the ability of insulin to enhance plasma glucose disappearance and to inhibit hepatic glucose production. Systolic and diastolic arterial pressures in the SHR group were significantly higher (P < 0.0005) than in the WKY group. Mean S(G) and S(I) estimates from the SHR group (S(G) = 16.2 (+/- 2.0) x 10(-2) dl x min(-1) x kg(-1) and S(I) = 12.5 (+/- 1.9) x 10(-4) dl x min(-1) x kg(-1) (microU ml(-1))(-1)) were not significantly different (P > 0.05) from mean estimates that characterised the WKY group (S(G) = 13.1 (+/- 1.5) x 10(-2) dl x min(-1) x kg(-1) and S(I) = 15.8 (+/- 4.3) x 10(-4) dl x min(-1) x kg(-1) (microU ml(-1))(-1)). This result is in contrast with reported findings from humans in which insulin sensitivity is significantly reduced in the presence of hypertension.

Influence of GABAergic mechanisms on baroreceptor inputs to nucleus tractus solitarii of rats.

The firing frequency of baroreceptive neurons in the nucleus tractus solitarii (NTS) during microiontophoretic application of muscimol, a gamma-aminobutyric acid (GABA)A agonist, or baclofen, a GABAB agonist, was monitored in anesthetized rats. Muscimol decreased the spontaneous discharge of 69 of 73 (94.5%) NTS baroreceptive neurons without affecting the remaining four neurons (5.5%). The statistical comparison on a bin-by-bin basis of the peri-R wave interval histograms of the discharge of each NTS neuron showed that the inhibitory action of muscimol was always exerted on the whole neuronal discharge independently of its correlation to the cardiac cycle. Baclofen inhibited 60 of 73 (82.2%) NTS neurons without affecting the remaining 13 neurons (17.8%). In 31 of 60 (51.7%) neurons inhibited by baclofen, this substance significantly affected only pulse-synchronous peaks of neuronal discharge without significant inhibition of the neuronal firing between cardiac cycle-related peaks. Fifty-eight of 73 (79.5%) NTS neurons studied were inhibited by both muscimol and baclofen, 11 neurons (15%) only by muscimol, 2 neurons (2.7%) only by baclofen, and 2 neurons (2.7%) were unaffected by both substances. These results demonstrate that both GABAA and GABAB receptors mediate inhibition of the spontaneous discharge in the great majority of the NTS baroreceptive neurons studied and suggest different functions of the two types of GABA receptors in influencing baroreceptor inputs to the NTS.

Cardiovascular effects of microinjection of atrial natriuretic factor (ANF) in the nucleus tractus solitarii of spontaneously hypertensive rats.

The effects on mean arterial pressure (MAP) and heart rate (HR) of unilateral microinjections of atrial natriuretic factor (ANF) into discrete sites of the nucleus tractus solitarii (NTS) of spontaneously hypertensive rats (SHR) were compared with those observed in normotensive Wistar-Kyoto rats (WKY). NTS sites were identified to be involved in cardiovascular control on the basis of the bradycardia and hypotension elicited by microinjections of 20 nl of 0.1 M L-glutamate. Microinjection of 20 nl of 10(-7) M ANF into 38 NTS 'cardiovascular sites' in rats of the SHR strain decreased MAP (-8.7 +/- 1.8 mmHg) and HR (-7.8 +/- 1.9 bpm) in 9 sites (24%), but caused no changes in the remaining 29 sites (76%). In WKY rats 35 cardiovascular sites within the NTS were studied. In 18 sites (51%) ANF microinjections induced a decrease in MAP (-15.1 +/- 1.9 mmHg) and in HR (-18.1 +/- 3.9 bpm), whereas the remaining 17 sites (49%) were unaffected. The decreased responsiveness of the NTS to ANF in the SHR animals could play a role in the development and/or maintenance of the elevated arterial blood pressure in genetically hypertensive rats.