Differential effects from parapyramidal region and rostral ventrolateral medulla mediated by substance P.

Rostral ventrolateral medulla (rVLM) and parapyramidal region (PPr) serve as important medullary control sites for sympathoexcitation. rVLM and PPr have direct projections to the intermediolateral cell column (IML) that are thought to be important in maintaining mean arterial blood pressure (MAP). Substance P (SP) is found in PPr neurons and in and near the subretrofacial area of the rVLM. At least some of these cells project to the IML. We investigated the involvement of SP at the IML in mediating rVLM- and PPr-evoked pressor responses in the chloralose-anesthetized cat. Pressor responses to electrical and chemical PPr and rVLM stimulation were altered after intrathecal injection, at the level of the T1-T3 spinal cord, of either SP antagonist [D-Pro(2), D-Phe(7), D-Trp(9)]-SP, SP antagonist CP 96,345, or SP antiserum. Although MAP and heart rate responses to PPr stimulation were attenuated by intrathecal SP antagonists or antiserum, MAP responses to rVLM stimulation were augmented. Previous studies have revealed differences in transmitters associated with these two areas, even though the general response of both areas is sympathoexcitatory. The present study implies that the identical substance may increase or decrease the MAP response depending on the pathway activated.

Mobilization of glucoregulatory hormones and glucose by hypothalamic locomotor centers.

Recent studies suggest that, in addition to classical humoral metabolic feedback mechanisms, the mobilization of glucoregulatory hormones and glucose in exercise may be regulated by motor centers in the brain. We, therefore, studied the effect of electrically stimulating the posterior hypothalamic locomotor region (PHLR) for 10 min in decorticated (n = 6) and alpha-chloralose-anesthetized (n = 8) cats. Blood pressure and heart rate were measured, and blood samples were drawn for analysis of hormones and metabolites before, during, and after 10 min of PHLR stimulation. Feedback from contracting muscles was prevented by neuromuscular blockade in decorticated cats and by the anesthesia in anesthetized cats. In decorticated cats, PHLR stimulation elicited increases (2 P less than 0.05) in glucose production (delta 54 +/- 16 mumol.min-1.kg-1), plasma glucose (delta 2.2 +/- 0.7 mmol/l), epinephrine (delta 4.9 +/- 1.8 pmol/l), norepinephrine (delta 2.2 +/- 0.9 pmol/l), glucagon (delta 16 +/- 5 pmol/l), decreases (2 P less than 0.05) in plasma insulin (delta 27 +/- 7 pmol/l), and increases (2 P less than 0.05) in blood pressure (delta 48 +/- 9 mmHg) and heart rate (delta 26 +/- 7 beats/min). In anesthetized cats, PHLR stimulation elicited increases (2 P less than 0.05) in glucose production (delta 12 +/- 4 mumol.min-1.kg-1), plasma glucose (delta 0.4 +/- 0.1 mmol/l), blood pressure (delta 39 +/- 7 mmHg), and heart rate (delta 28 +/- 7 beats/min), whereas changes in catecholamine and insulin concentrations did not reach statistical significance.(ABSTRACT TRUNCATED AT 250 WORDS)

Occlusion of pressor responses to posterior diencephalic stimulation and muscular contraction.

Although neural occlusion has been suggested to occur between the central and reflex mechanisms increasing arterial pressure, evidence consistent with this phenomenon is lacking. To assess the possibility of neural occlusion we recorded, in chloralose-anesthetized cats, the pressor responses to statically contracting the hindlimb muscles and to electrically stimulating histologically confirmed sites in the posterior hypothalamus and subthalamus. We also recorded the pressor responses to topical application of capsaicin onto the intestine and to stimulation of these diencephalic sites. The pressor responses to simultaneous static contraction and diencephalic stimulation were significantly smaller than the algebraic sum of the pressor responses to contraction and diencephalic stimulation evoked separately. Likewise, the pressor responses to simultaneous capsaicin application and diencephalic stimulation were significantly smaller than the algebraic sum of the responses evoked separately. High intensity stimulation of the L7 dorsal root or the diencephalic sites evoked pressor responses similar in magnitude to the algebraic sum of the two responses evoked separately; thus, the inability of the simultaneous maneuvers to evoke pressor responses that summed algebraically was not due to the fact that they caused a maximal effect. Our findings are consistent with the hypothesis that neural occlusion occurs during stimulation of the posterior diencephalon and static muscular contraction.

Pressor reflex response to static muscular contraction: its afferent arm and possible neurotransmitters.

Static muscular contraction has been shown to increase cardiovascular and ventilatory function in reflex manner. The sensory arm of this reflex arc is comprised of group III and IV muscle afferents. The discharge properties of these muscle afferents whose activation causes the pressor reflex response to contraction were investigated. Group III afferents were more responsive to mechanical stimuli, such as tendon stretch and probing their receptive fields than were group IV afferents. In contrast, group III afferents were less responsive to ischemic contraction than were group IV afferents. Equal percentages of group III and IV afferents were stimulated by potassium, lactic acid and arachidonic acid, each of which are metabolic products of contraction. Adenosine, phosphate and lactate, however, had no effect on the discharge of the afferents. Intrathecal injection of antagonists or antibodies to substance P and somatostatin attenuated the pressor response to contraction by about half, a finding that suggests a role for these 2 peptides in the spinal transmission of the reflex.

Attenuation of the reflex pressor response to muscular contraction by an antagonist to somatostatin.

Although group III and IV fibers are known to compose the afferent pathway of the reflex arc causing the pressor response to static muscular contraction, little is known about the neurotransmitters released by these muscle afferents. Somatostatin might be one of these neurotransmitters because this peptide is found in the terminals of fine afferent fibers ending in the dorsal horn of the lumbar spinal cord. Therefore, in chloralose-anesthetized cats, the reflex pressor response to static contraction was examined before and after subarachnoid injections onto the lumbosacral cord of a peptide antagonist to somatostatin. We found that before giving the antagonist, the pressor response to contraction of the triceps surae muscles in 12 cats averaged 33 +/- 4 mm Hg, while 37 +/- 7 minutes after giving the antagonist, the pressor response averaged only 18 +/- 3 mm Hg (p less than 0.001). In contrast, the antagonist to somatostatin had no effect on either the pressor response to electrical stimulation of the cut central end of the sciatic nerve or the pressor response to stimulation of the posterior diencephalon. Furthermore, subarachnoid injection of a peptide antagonist to luteinizing hormone-releasing hormone had no effect on the reflex pressor response to static contraction. Our findings are consistent with the hypothesis that somatostatin plays a role in the spinal transmission of the contraction-induced pressor reflex arising from hind limb skeletal muscle.

Discharge properties of group III and IV muscle afferents: their responses to mechanical and metabolic stimuli.

Static contraction of the hind limb muscles induced by electrical stimulation of the ventral roots has been firmly established to reflexly increase cardiovascular and ventilatory function. Moreover, Group III and IV afferents are known to compose the afferent arm of this reflex arc. The present experiments investigated the discharge properties of Group III and IV afferents whose activation is responsible for the pressor reflex response to static contraction. In general, we found that Group III afferents were more responsive to mechanical stimuli, such as distortion of their receptive fields and tendon stretch, than were Group IV afferents. In contrast, Group IV afferents were more responsive to ischemic contraction than were Group III afferents. In addition, equal percents of Group III and IV afferents were found to be stimulated by increasing interstitial potassium to levels that were similar to those found during muscular contraction. The afferents' response to potassium adapted within seconds, while the interstitial concentration of this ion remained elevated for several minutes. This rapidly adapting response casts doubt on the effectiveness of potassium as the metabolite that signals blood supply and demand in a working muscle are improperly matched.

Acetylstrophanthidin does not enhance the reflex pressor response to static muscular contraction.

Cardiac glycosides have been shown to enhance the sensitivity of the reflex cardiovascular responses to stimulation of mechanoreceptors in the heart, carotid sinus and aorta. Little is known, however, about the effect of glycosides on the reflex cardiovascular responses to the contraction-induced stimulation of afferent endings in hindlimb skeletal muscle. We therefore examined the reflex heart rate and arterial pressure responses to static contraction of the hindlimb muscles before and after femoral arterial injection of two doses of acetylstrophanthidin (20 and 80 micrograms/kg). Neither of the two doses enhanced the reflex cardiovascular responses to contraction, although the larger of the two significantly increased femoral venous potassium concentrations from 3.4 +/- 0.2 to 3.8 +/- 0.1 mM. Although injection of the two doses as well as injection of a very large dose of acetylstrophanthidin (400 micrograms/kg) increased baseline mean arterial pressure, these effects were probably caused by the vasoconstrictor action of this agent and not by a chemoreflex, because the increase was not attenuated by denervation of the hindlimb.

Immunoneutralization of substance P attenuates the reflex pressor response to muscular contraction.

We previously reported that subarachnoid injection of a peptide antagonist to substance P attenuated by half the reflex pressor response to static muscular contraction. Subsequently, some of the peptide antagonists to substance P have been found to possess local anesthetic effects. Therefore, we have repeated our experiments using a substance P antiserum, which was shown to be without local anesthetic effect. We found that intrathecal injection of the antiserum attenuated by more than half the reflex pressor response to static contraction of the triceps surae muscles of cats.

Hindlimb muscular contraction reflexly decreases total pulmonary resistance in dogs.

We have previously shown that contraction of the gracilis muscles of anesthetized dogs reflexly relaxes tracheal smooth muscle. We have also found that electrical stimulation of these afferents decreases total pulmonary resistance (TPR), a calculation that provides a functional index of airway caliber. Despite these findings, we have yet to show that muscular contraction reflexly decreases TPR. Therefore, in 11 alpha-chloralose-anesthetized dogs, we contracted the hindlimb muscles by electrically stimulating the L6-L7 ventral roots while measuring TPR breath by breath. We found that static contraction decreased TPR from 12.6 +/- 1.1 to 10.4 +/- 0.9 cmH2O X l-1 X s (P less than 0.05). This decrease was reflex in origin because it was prevented by section of the spinal roots innervating the working hindlimb. Repetitive twitch contractions (5 Hz) also reflexly decreased TPR, but the effect was smaller than that evoked by static contraction. The reflex decreases in TPR evoked by contraction were unaffected by propranolol but were abolished by atropine. We conclude that muscular contraction dilates the airways by a reflex mechanism whose efferent arm consists of a withdrawal of cholinergic input to airway smooth muscle.

Pressor reflex evoked by muscular contraction: contributions by neuraxis levels.

The pressor reflex evoked by muscular contraction (exercise pressor reflex) is one important model of cardiovascular adjustments during static exercise. The central nervous system (CNS) structures mediating this reflex have remained largely obscure. Therefore, we examined the contribution of selected levels of the neuraxis in mediating the pressor reflex evoked by muscular contraction from stimulation of ventral roots. Decerebrate cats exhibited larger pressor reflexes than those found in intact alpha-chloralose-anesthetized cats, a difference more apparent at low (5 Hz or repeated twitch) rather than at high (50 Hz or tetanic) stimulus frequencies. Although a depressor response to 5-Hz stimulation was observed in the intact anesthetized cats, it appeared to be primarily due to anesthetic level, since a depressor response was not observed in decerebrate animals (nonanesthetized). Cerebellectomy produced no changes in the reflexes of the decerebrate animal. Further transection of the neuraxis (caudal to the midcollicular level) attenuated the exercise pressor reflex. The spinal cat demonstrated slight evidence of exercise pressor reflex activity. These results provide clarification as to representation of this pressor reflex within the CNS and establish the reflex's characteristics at several levels of neuraxis integration.

Attenuation of the reflex pressor response to muscular contraction by a substance P antagonist.

In chloralose-anesthetized cats, we found that D-Pro2-D-Phe7-D-Trp9-substance P (40-100 micrograms), injected intrathecally, reduced the reflex pressor response to static muscular contraction by more than half.

Increasing gracilis muscle interstitial potassium concentrations stimulate group III and IV afferents.

Static muscular contraction reflexly increases arterial blood pressure and heart rate. One possible mechanism evoking this reflex is that potassium accumulates in the interstitial space of a working muscle to stimulate group III and IV afferents whose activation in turn evokes a pressor response. The responses of group III and IV muscle afferents to increases in interstitial potassium concentrations within the range evoked by static contraction are unknown. Thus we injected potassium chloride into the gracilis artery of anesthetized dogs while we measured both gracilis muscle interstitial potassium concentrations with potassium-selective electrodes and the impulse activity of afferents in the gracilis nerve. We found that increasing interstitial potassium concentrations to levels similar to those seen during static contraction stimulated 14 of 16 group III and 29 of 31 group IV afferents. The responses of the afferents to potassium were concentration dependent. The typical response to potassium consisted of a burst of impulses, an effect that returned to control firing rates within 26 s, even though interstitial potassium concentrations remained elevated for several minutes. Although our results suggest that potassium may play a role in initiating the reflex cardiovascular responses to static muscular contraction, the accumulation of this ion does not appear to be solely responsible for maintaining the pressor response for the duration of the contraction.

Stimulation of group III and IV muscle afferents reflexly decreases total pulmonary resistance in dogs.

Although previous investigations have shown that stimulating group III and IV afferents reflexly decreased transverse tension from the trachealis muscle, these measurements provided no functional information about airway caliber. We therefore electrically stimulated gracilis muscle afferents in paralyzed, chloralose anesthetized dogs while recording total pulmonary resistance breath by breath. In addition, we recorded compound action potentials to determine which afferents were stimulated by current intensities of 3, 5, 20, 70 and 200 times motor threshold. We found that stimulating (20 Hz) the nerves at 3 times threshold, a current intensity which activated only group I and II afferents, had no effect on total pulmonary resistance, whereas stimulating the nerves at 5, 20 and 70 times threshold, current intensities which activated group I, II and III afferents, significantly decreased this variable. Stimulating the nerves at 200 times threshold, a current intensity which activated group IV as well as group I, II and III afferents, decreased total pulmonary resistance significantly more than did stimulating the nerves at 5, 20 or 70 times threshold. In addition stimulating the nerves at 200 times threshold but at frequencies of 2 and 5 Hz significantly decreased total pulmonary resistance. The decrease in total pulmonary resistance evoked by electrically stimulating the nerves at 200 times threshold was unaffected by propranolol but was abolished by atropine methylnitrate. We conclude that stimulating group III and IV gracilis muscle afferents in dogs reflexly decreases total pulmonary resistance, an effect due to the withdrawal of a tonic cholinergic input to the airways.

Effects of static and rhythmic twitch contractions on the discharge of group III and IV muscle afferents.

Although both static and rhythmic twitch contractions of the hindlimb muscles of anaesthetised cats have been shown to reflexly evoke pressor responses, the increase in arterial pressure evoked by the former type of contraction has been shown to be substantially larger than that evoked by the latter. We have therefore recorded the impulse activity of single group III and IV muscle afferents, whose activation reflexly increases arterial pressure, while we both statically and rhythmically twitch-contracted the triceps surae muscles of anaesthetised cats. We found that group III afferents (n = 17) discharged significantly more impulses in response to static contraction than in response to rhythmic contraction. By contrast, group IV afferents (n = 18) fired approximately the same number of impulses in response to the two types of contraction. In addition, we found that many of the group III but only a few of the group IV afferents displayed discharge properties suggestive that these afferents were mechanoreceptors. We conclude that the discharge of group III afferents are likely to be responsible for the difference in the magnitudes of the reflex pressor responses evoked by static and rhythmic contraction.

Activation of visceral thin-fiber afferents increases respiratory output in cats.

Respiratory responses to chemical activation of thin-fiber afferents from the stomach and the gallbladder were measured in anesthetized cats. Capsaicin or bradykinin applied to the serosal surface of either the stomach or the gallbladder elicited increases in breathing and phrenic nerve activity. Transection of the cervical vagi or the carotid sinus nerves had no effect on these responses. However, the respiratory responses to visceral stimulation were abolished by bilateral transection of the splanchnic nerves. We conclude that activation of thin-fiber afferents from the stomach and gallbladder causes a reflex increase in respiratory output. The initial afferent limb of this reflex is via the splanchnic nerves.

Arterial pressure responses to increasing interstitial potassium in hindlimb muscle of dogs.

Static contraction of hindlimb skeletal muscle is known to increase reflexly arterial pressure and heart rate. Potassium is known to be released by the working muscle and is thought to activate the afferents responsible for the reflex cardiovascular responses to muscular contraction. However, it is not known whether potassium, at interstitial concentrations within the range observed during static contraction, is capable of stimulating these afferents. Thus we injected potassium into the gracilis artery of chloralose-anesthetized dogs while we measured interstitial potassium concentrations in the gracilis muscle with potassium-selective electrodes. In 16 dogs, we found that potassium injections, which increased interstitial potassium concentrations by 4.7 +/- 0.3 mM, increased mean arterial pressure by 18 +/- 3 mmHg and heart rate by 12 +/- 8 beats/min; cutting the obturator nerve abolished these increases. These heart rate and blood pressure responses were of short duration (20 +/- 7 s), even though interstitial potassium remained elevated for a period of several minutes. In 5 of the 16 dogs, static contraction of the gracilis muscle for 60 s increased interstitial potassium concentration by 4.3 +/- 0.3 mM. Our data are consistent with the hypothesis that potassium plays a role in causing the reflex cardiovascular responses to static muscular contraction.

Effect of ischemia on responses of group III and IV afferents to contraction.

Static contraction of the hindlimb muscles of cats reflexly increases cardiovascular function, an effect that is potentiated by occlusion of the arterial supply to the working muscles. Although group III and IV afferents are known to be stimulated by and to cause the reflex cardiovascular responses to static muscular contraction, little is known about the responses of these afferents to static contraction when the arterial supply to a working muscle is occluded. We therefore recorded the impulse activity of 24 group III afferents and 30 group IV afferents with endings in the triceps surae while we statically contracted this muscle group, both when the abdominal aorta was occluded and when it was patent. A chi 2 analysis revealed that ischemia increased the responses to static contractions of a significantly higher percentage of group IV afferents than group III afferents (46.7% vs. 12.5%, respectively; P less than 0.02). In addition, two patterns of responses to ischemic contraction were observed. The first pattern was displayed by afferents (n = 10) that were stimulated by nonischemic contraction but were stimulated more by ischemic contraction. The second pattern was displayed by afferents (n = 7) that were not stimulated by nonischemic contraction but were stimulated by ischemic contraction. We conclude that afferents displaying both patterns are likely to contribute to the reflex cardiovascular responses to ischemic contraction.

Effect on arterial pressure of rhythmically contracting the hindlimb muscles of cats.

Although static contraction of the hindlimb muscles of anesthetized cats is known to reflexly increase arterial pressure and heart rate, the cardiovascular effects of rhythmic contractions of these muscles is unclear. To help clarify this issue, we determined, in chloralose-anesthetized cats, the effects on arterial pressure and heart rate of rhythmically contracting the hindlimb muscles at a frequency of 5 Hz. In addition, we determined the effect of rhythmic contractions on the impulse activity of group III and IV muscle afferents whose activation is known to increase cardiovascular function. We found that rhythmic contractions increased arterial pressure (from 108 +/- 8 to 134 +/- 9 mmHg; P less than 0.05) and heart rate (from 192 +/- 13 to 208 +/- 10 beats/min; P less than 0.05) in 10 cats and decreased arterial pressure (from 107 +/- 8 to 93 +/- 9 mmHg; P less than 0.05) but did not change heart rate in 9 other cats. The increases were reflex, because they were prevented by cutting the spinal roots innervating the contracting hindlimb. The decreases, however, were not reflex, because they persisted after spinal root section. The differences in the arterial pressure responses to rhythmic contractions may have been partly due to individual differences in the level of anesthesia, because in three cats the pressor responses to this maneuver were converted to depressor responses after giving the cats additional chloralose. Rhythmic contractions of the triceps surae muscles stimulated 8 of 10 group III afferents and 9 of 16 group IV afferents. We conclude that rhythmic contraction is capable of reflexly increasing cardiovascular function in cats provided that the effect is not depressed by anesthesia.

Chemical activation of group I and II muscle afferents has no cardiorespiratory effects.

Although stimulation of group III and IV muscle afferents is known to cause reflex changes in cardiorespiratory function, it has not been resolved whether group I and II afferents contribute to this reflex activation. Therefore, we measured the effects of intra-arterial nonparalyzing doses of succinylcholine (50-100 micrograms/kg) on the firing of muscle afferents from the gastrocnemius muscle in one group of cats, and heart rate, blood pressure, and integrated phrenic nerve activity in a second group of cats. In nonparalyzed cats, succinylcholine injections caused muscular fasciculations and firing of all four groups of muscle afferents. However, succinylcholine stimulated only group I and II afferents after paralysis with gallamine triethiodide. Succinylcholine caused increases in blood pressure, heart rate, and phrenic nerve activity before paralysis. After paralysis, succinylcholine had no effects on any of the cardiorespiratory measures. We conclude that activation of only group I and II afferent fibers from the gastrocnemius muscle has no reflex effects on blood pressure, heart rate, or phrenic nerve activity. These afferents, therefore, are unlikely to play a role in increasing cardiorespiratory function during exercise.

Muscular contraction reflexly relaxes tracheal smooth muscle in dogs.

Although contraction of hindlimb skeletal muscle is well known to reflexly increase ventilation, heart rate and arterial pressure, little is known about the reflex effect of this maneuver on airway smooth muscle tone. Therefore, in chloralose-anesthetized dogs, we recorded transverse tension from the trachealis muscle while we contracted both gracilis muscles by electrically stimulating the gracilis nerves at 5 and 40 Hz. In 11 of the 13 dogs studied, static (40 Hz) contraction decreased tracheal tension, whereas in the remaining 2 dogs, static contraction increased tension. In 9 of 11 dogs, rhythmic (5 Hz) contraction decreased tracheal tension, whereas in the remaining 2, this maneuver increased tension. The changes in tracheal tension induced by stimulating the gracilis nerves were abolished by paralyzing the dogs and were restored, for the most part, after paralysis had dissipated. In addition, the contraction-induced changes in tension were not present when the gracilis muscles were contracted by stimulating the cut peripheral ends of the gracilis nerves. We conclude that muscular contraction reflexly relaxes tracheal smooth muscle in most dogs.