Presynaptic inhibitory effects of sotalol, propranolol, and acebutolol on noradrenaline release upon cardiac sympathetic nerve stimulation in anesthetized dogs

Effect of sotalol (STL) was compared with that of (+/-)-propranolol, (+)-propranolol (PPL), and acebutolol (ABL) on noradrenaline (NA) release as measured in coronary sinus (CS) blood during postganglionic stimulation (2 Hz, 30 s) of the left cardiac sympathetic nerves in anesthetized dogs. In control dogs receiving saline, increasing responses of CS-NA concentration, mean CS blood flow, and CS-NA output to repetitive stimulation were relatively stable throughout a given experimental period. Both STL (1,2.5, and 5 mg/kg, i.v.) and (+/-)-PPL (0.5 and 2.5 mg/kg, i.v.) diminished the increased CS-NA concentration by approximately 35 (P less than 0.05) to 60% (P less than 0.01) in a dose-dependent fashion. However, (+)-PPL (0.02-2.5 mg/kg, i.v.) and ABL (0.5-5 mg/kg, i.v.) did not significantly alter the increasing response of CS-NA concentration upon stimulation. STL, (+/-)-PPL, and ABL markedly inhibited the CS blood flow response to stimulation at all doses tested, while (+)-PPL did not significantly diminish the flow response even at the highest dose tested. Consequently, CS-NA output decreased significantly (p less than 0.01) in the presence of STL, (+/-)-PPL, and ABL at all doses tested but not with (+)-PPL at any dose tested. The inhibitory effect of STL and (+/-)-PPL on the increasing response of CS-NA concentration upon stimulation could be related to their beta-blocking effect, which exerts presumably on postulated presynaptic beta-adrenoceptors, as (+)-PPL did not at all diminish the response.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence for increased hepatic sympathetic nerve activity resulting in hyperglycemia in response to hemorrhage-induced reflex stimulation in anesthetized dogs

To investigate the role of the sympathoadrenal system in glucose mobilization by the liver during hemorrhage, catecholamine (CA) output from both adrenal glands was determined in anesthetized dogs. Venous blood draining from both adrenal glands was combined in a Y-tube that was connected to an electromagnetic flow probe to measure total adrenal venous blood flow. Plasma concentrations of norepinephrine (NE), epinephrine (E), dopamine (DA), and glucose (GL) were determined in various vascular regions. Adrenal CA output (nanograms per minute) under basal conditions was 50.2 +/- 13.6, 181.4 +/- 41.9, and 13.7 +/- 4.8 for NE, E, and DA, respectively. These values were found to increase significantly (P less than 0.05) in response to 5 min of hemorrhage, reaching a maximum output (nanograms per minute) of 663.6 +/- 160.6 (NE), 2503.4 +/- 607.8 (E), and 141.7 +/- 43.7 (DA). Aortic CAs (nanograms per millilitre) increased significantly with a predominant increase in E (0.33 +/- 0.08 to 3.75 +/- 1.03, P less than 0.05). In contrast, increases in portal and hepatic venous CAs (nanograms per millilitre) were characterized by a predominant increase in NE (0.30 +/- 0.06 to 0.64 +/- 0.11 and 0.17 +/- 0.02 to 0.31 +/- 0.07, respectively, P less than 0.05). Hepatic venous and aortic GL concentrations also increased significantly during hemorrhage. Among the various correlations between plasma CA and GL concentrations, the strongest correlation was found between hepatic venous NE and hepatic venous GL (r = 0.804, P less than 0.001). Correlation coefficients obtained with aortic NE and E were weaker but significant (r = 0.603 and r = 0.608, respectively, P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)     

Ventilatory responses to intravenous and airway CO2 administration in anesthetized dogs

The ventilatory responses to steady-state venous CO2 loading (iv CO2) and CO2 inhalation have been observed in chloralose-urethan-anesthetized dogs. Intravenous CO2 was administered by increasing the CO2 fraction of gas ventilating a membrane gas exchanger in an arteriovenous bypass; blood flow rate was fixed at 30 ml/min. During the study, we identified a time-dependent hyperventilation in all 14 experimentally treated dogs and in 4 additional sham-treated dogs. When we tested 8 of these animals with a protocol having small progressive increments in iv CO2 loading rate, we observed a response approaching isocapnia during iv CO2 and a large hypocapnia when we returned to control conditions. The use of a randomized protocol in 6 animals demonstrated the necessity of accounting for this systematic base-line shift, because before doing so the response depended more on the passage of time than on the nature of the CO2 load. After this analytical adjustment was made, there was no significant difference between the respiratory controller gains (delta nu E/delta Paco2) for inhaled and iv CO2.     

Neonatal maternal separation enhances phrenic responses to hypoxia and carotid sinus nerve stimulation in the adult anesthetized rat.

In awake animals, our laboratory recently showed that the hypoxic ventilatory response of adult male (but not female) rats previously subjected to neonatal maternal separation (NMS) is 25% greater than controls (Genest SE, Gulemetova R, Laforest S, Drolet G, and Kinkead R. J Physiol 554: 543-557, 2004). To begin mechanistic investigations of the effects of this neonatal stress on respiratory control development, we tested the hypothesis that, in male rats, NMS enhances central integration of carotid body chemoafferent signals. Experiments were performed on two groups of adult male rats. Pups subjected to NMS were placed in a temperature-controlled incubator 3 h/day from postnatal day 3 to postnatal day 12. Control pups were undisturbed. At adulthood (8-10 wk), rats were anesthetized (urethane; 1.6 g/kg), paralyzed, and ventilated with a hyperoxic gas mixture [inspired O2 fraction (Fi(O2)) = 0.5], and phrenic nerve activity was recorded. The first series of experiments aimed to demonstrate that NMS-related enhancement of the inspiratory motor output (phrenic) response to hypoxia occurs in anesthetized animals also. In this series, rats were exposed to moderate, followed by severe, isocapnic hypoxia (Fi(O2) = 0.12 and 0.08, respectively, 5 min each). NMS enhanced both the frequency and amplitude components of the phrenic response to hypoxia relative to controls, thereby validating the use of this approach. In a second series of experiments, NMS increased the amplitude (but not the frequency) response to unilateral carotid sinus nerve stimulation (stimulation frequency range: 0.5-33 Hz). We conclude that enhancement of central integration of carotid body afferent signal contributes to the larger hypoxic ventilatory response observed in NMS rats.     

Correlation between endogenous noradrenaline and glucose released from the liver upon hepatic sympathetic nerve stimulation in anesthetized dogs

The metabolic role of neurally released noradrenaline (NA) was studied in the liver of anesthetized dogs. Sustained stimulation with various frequencies was directly applied on the anterior plexus of hepatic nerves. Stimulation-induced changes in plasma concentrations of endogenous catecholamines in hepatic venous blood were determined in correlation with concomitant changes in those of glucose (GL). Mean basal values for hepatic venous NA, adrenaline, dopamine, and GL were 0.062, 0.022, 0.032 ng/mL, and 97.9 mg%, respectively. Among these catecholamines, NA was the only one being released significantly during stimulation. While hepatic venous NA increased rapidly during stimulation, being maximum within 3 min, hepatic venous GL increased gradually, reaching a maximum value 5 min after the onset of stimulation. A highly significant correlation (r = 0.90, P less than 0.001) was found between changes in hepatic venous NA and GL concentrations observed during stimulation at various frequencies (2-16 Hz). However, hepatic vasoconstricting responses to stimulation were not correlated with increased hepatic venous GL. An alpha-blockade with phentolamine (2 mg/kg, iv) resulted in diminished release of GL by approximately 50% (P less than 0.05) and reduced hepatic arterial vasoconstriction by approximately 47% (P less than 0.01) upon stimulation (8 Hz, 5 min), even though NA release was markedly enhanced. We conclude that in the dog, NA is the sole catecholamine released within the liver in response to direct hepatic nerve stimulation, and NA thus released mediates the hepatic glycogenolysis via alpha-adrenoceptors.     

Submandibular responses to stimulation of the parasympathetic innervation in anesthetized sheep.

Submandibular secretory and vascular responses to stimulation of the parasympathetic innervation and the output of vasoactive intestinal peptide (VIP) were investigated in anaesthetized sheep in the presence and absence of atropine (>/=0.5 mg/kg). In the absence of atropine, parasympathetic stimulation caused an increase in the flow of saliva and a decrease in submandibular vascular resistance; the latter response persisted after the administration of atropine and was then significantly reduced at the lowest but not at the higher frequencies tested. The output of VIP from the gland was frequency dependent over the range of 10-20 Hz (continuously) and significantly increased after atropine (P < 0.02). Furthermore, the fall in vascular resistance was linearly related to log VIP output after total muscarinic blockade. Intracarotid infusions of synthetic VIP produced dose-dependent falls in submandibular vascular resistance, together with a corresponding increase in submandibular blood flow. It is concluded that the atropine-resistant vasodilatation that occurs in this gland during parasympathetic stimulation is likely to be due largely, if not entirely, to the release of VIP.     

Mentalis muscle responses to median nerve stimulation

Electrical stimulation may produce excitation or inhibition of the motor neurons, as represented the blink reflex and masseter silent period in response to trigeminal nerve stimulation. Clinically, a light touch on the palm may evoke a mentalis muscle response (MMR), i.e. a palmomental reflex. In this study, we attempted to characterize the MMR to median nerve stimulation. Electrical stimulation was applied at the median nerve with recordings at the mentalis muscles. An inhibition study was done with continuous stimuli during muscle contraction (I1 and I2 of MMRaverage). Excitation was done with a single shot during muscle relaxation (MMRsingle) or by continuous stimuli during muscle contraction (E1 and E2 of MMRaverage). The characteristic differences between MMRaverage and MMRsingle were as follows: earlier onset latencies of MMRaverage (MMRaverage < 45 ms; MMRsingle > 60 ms), and a lower amplitude of MMRaverage (MMRaverage < 50 microV; MMRsingle > 150 microV). The receptive field of MMRsingle was widespread over the body surface and that of MMRaverage was limited to the trigeminal, median and index digital nerves. Series of stimuli usually significantly decreased the amplitude of MMRsingle, as a phenomenon of habituation. On the other hand, it was difficult to evoke the earlier response (i.e. MMRaverage) without continuous stimuli and an average technique. MMRaverage had the components of both excitation (E) and inhibition (I); for example, E1-I1-E2-I2 or I1-E2-I2. E2 was the most consistent component. In patients with dorsal column dysfunction, median nerve stimulation could successfully elicit MMRsingle, but not MMRaverage. Contrarily, in patients with pain sensory loss, it was more difficult to reproduce MMRsingle than MMRaverage. It seemed that MMRaverage and MMRsingle did not have equivalents across the different modalities of stimulation.     

Ovarian blood flow responses to electroacupuncture stimulation depend on estrous cycle and on site and frequency of stimulation in anesthetized rats.

Electroacupuncture (EA) applied to the abdomen and hindlimb modulates the ovarian blood flow (OBF) response. The present study aimed to further elucidate the role of the site and the frequency of short-term EA stimulation and the influence of the estrous cycle on the OBF response using anesthetized rats. EA stimulation was applied to the abdominal or the hindlimb muscles at three different frequencies (2, 10, and 80 Hz) during the estrus or diestrus phase. Involvement of spinal and supraspinal reflexes in OBF responses to EA stimulation was investigated by spinal cord transection. Abdominal EA stimulation at 10 Hz increased the OBF response, whereas hindlimb EA stimulation at 10 Hz and abdominal and hindlimb stimulation at 80 Hz decreased the OBF response; 2-Hz EA caused no OBF response. The OBF response to abdominal EA was more pronounced in the estrus than the diestrus phase. The OBF response to abdominal and hindlimb EA stimulation at both 10 and 80 Hz was almost abolished, both after severance of the sympathetic nerves and after spinal cord transection. In conclusion, the OBF response to both abdominal and hindlimb EA stimulation was mediated as a reflex response via the ovarian sympathetic nerves, and the response was controlled via supraspinal pathways. Furthermore, the OBF response to segmental abdominal EA stimulation was frequency dependent and amplified in the estrous phase.     

Corelease of neuropeptide Y like immunoreactivity with catecholamines from the adrenal gland during splanchnic nerve stimulation in anesthetized dogs

The release of neuropeptide Y like immunoreactivity (NPY-li) from the adrenal gland was studied in relation to the secretion of catecholamines (CA: NE, norepinephrine; E, epinephrine) during the left splanchnic nerve stimulation in thiopental-chloralose anesthetized dogs (n = 16). Plasma concentrations of NE, E, and NPY-li were determined in the left adrenal venous and aortic blood. Adrenal outputs of NPY-li, NE, and E were 2.4 +/- 0.4, 1.4 +/- 0.2, and 7.3 +/- 1.7 ng/min, under basal conditions, respectively. These values increased significantly (p less than 0.05; n = 8) in response to a continuous stepwise stimulation at frequencies of 1, 3, and 10 Hz given at 3-min intervals during 9 min, reaching a maximum output of 4.6 +/- 0.9 (NPY-li), 240.2 +/- 50.2 (NE), and 1412.5 +/- 309.7 ng/min (E) at a frequency of 10 Hz. Burst electrical stimulation at 40 Hz for 1 s at 10-s intervals for a period of 10 min produced similar increases (p less than 0.05) in the release of NPY-li (4.8 +/- 1.0 ng/min, n = 8), NE (283.5 +/- 144.3 ng/min, n = 8), and E (1133.5 +/- 430.6 ng/min, n = 8). Adrenal NPY-li output was significantly correlated with adrenal NE output (r = 0.606; n = 24; p less than 0.05) and adrenal E output (r = 0.640; n = 24; p less than 0.05) in dogs receiving the burst stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Determination of cardiac output and other reference physiology parameters in lightly anesthetized dogs

1. Cardiac output; arterial, pulmonary artery, central venous and pulmonary wedge pressures; heart rate, hematocrit, and plasma sodium and potassium; arterial and mixed venous blood gases; and respiratory rates were measured in 45 mixed sex, non-pregnant, clinically normal mongrel dogs of 8-30 kilograms body weight following light anesthesia with halothane/50% N20-02. 2. Arithmetic means and standard deviations were calculated to develop tables of reference values. 3. Mean measured cardiac outputs were found to be 31-59% higher in these dogs than the values indicated by published standards; cardiac indices were 30-44% higher; heart rates were 16-30% lower; calculated stroke volumes were 60-112% greater; and total peripheral resistances were 35-57% less than the standard published values. 4. All other measured or calculated parameters fell within previously published canine or human reference limits.     

Regional hemodynamic responses to nicotine in conscious and anesthetized dogs: comparative effects of pentobarbital and chloralose

This study was conducted in 12 dogs to evaluate regional hemodynamic responses during intravenous infusion of nicotine (36 micrograms/kg/min) in the conscious state and compare them with those in the same dogs following either pentobarbital (n = 6) or chloralose anesthesia (n = 6). Values for regional blood flow were obtained with 15-microns radioactive microspheres and used to calculate regional vascular conductance. In the conscious state, nicotine increased aortic pressure (+70%) and caused hyperventilation that reduced arterial PCO2 (-44%). These systemic effects were associated with decreases in vascular conductance in the renal cortex (-48%), pancreas (-81%), duodenum (-58%), and cerebral cortex (-55%), whereas no significant change in vascular conductance was evident in spleen, liver, or myocardium. Pentobarbital anesthesia blunted the increases in aortic pressure and respiratory activity and the reductions in vascular conductance in the renal cortex, pancreas, duodenum, and cerebral cortex during nicotine infusion. In contrast, chloralose anesthesia accentuated the increase in aortic pressure and the decrease in vascular conductance in the renal cortex during nicotine infusion, while it converted no change in vascular conductance in the spleen into a decrease and no change in vascular conductance in the myocardium into an increase. Chloralose anesthesia blunted nicotine-induced hyperventilation. These findings demonstrate that general anesthetic agents may have markedly different effects on cardiovascular reflex pathways. They emphasize the importance of considering the particular characteristics of the anesthetic agent used in interpreting results from studies of cardiovascular pharmacology and physiology in anesthetized animals.     

Two-component responses to sympathetic nerve stimulation in the rat tail artery

Membrane potential and tension were recorded simultaneously from the smooth muscle of the rat tail artery. A single stimulus to the perivascular nerves caused a tension transient. The tension transient had two components, one due to a muscle action potential and one due to alpha-adrenoceptor activation. During trains of stimuli most of the tension was due to alpha-receptor activation, even when every stimulus caused a smooth muscle action potential.