Role of prostaglandins in spinal transmission of the exercise pressor reflex in decerebrated rats

Previous studies found that prostaglandins in skeletal muscle play a role in evoking the exercise pressor reflex; however the role played by prostaglandins in the spinal transmission of the reflex is not known. We determined, therefore, whether or not spinal blockade of cyclooxygenase (COX) activity and/or spinal blockade of endoperoxide (EP) 2 or 4 receptors attenuated the exercise pressor reflex in decerebrated rats. We first established that intrathecal doses of a non-specific COX inhibitor Ketorolac (100 μg in 10 μl), a COX-2-specific inhibitor Celecoxib (100 μg in 10 μl), an EP2 antagonist PF-04418948 (10 μg in 10 μl), and an EP4 antagonist L-161,982 (4 μg in 10 μl) effectively attenuated the pressor responses to intrathecal injections of arachidonic acid (100 μg in 10 μl), EP2 agonist Butaprost (4 ng in 10 μl), and EP4 agonist TCS 2510 (6.25 μg in 2.5 μl), respectively. Once effective doses were established, we statically contracted the hind limb before and after intrathecal injections of Ketorolac, Celecoxib, the EP2 antagonist and the EP4 antagonist. We found that Ketorolac significantly attenuated the pressor response to static contraction (before Ketorolac: 23 ± 5 mmHg, after Ketorolac 14 ± 5 mmHg; p<0.05) whereas Celecoxib had no effect. We also found that 8 μg of L-161,982, but not 4 μg of L-161,982, significantly attenuated the pressor response to static contraction (before L-161,982: 21 ± 4 mmHg, after L-161,982 12 ± 3 mmHg; p<0.05), whereas PF-04418948 (10 μg) had no effect. We conclude that spinal COX-1, but not COX-2, plays a role in evoking the exercise pressor reflex, and that the spinal prostaglandins produced by this enzyme are most likely activating spinal EP4 receptors, but not EP2 receptors.

ERK1/2 signaling pathway in mast cell activation-induced sensitization of esophageal nodose C-fiber neurons

Sensitization of esophageal nociceptive afferents by inflammatory mediators plays an important role in esophageal inflammatory nociception. Our previous studies demonstrated that esophageal mast cell activation increases the excitability of esophageal nodose C-fibers. But the intracellular mechanism of this sensitization process is still less clear. We hypothesize that extracellular signal-regulated kinases 1 and 2 (ERK1/2) signaling pathway plays an important role in mast cell activation-induced sensitization of esophageal nodose C-fiber neurons. Mast cell activation and in vivo esophageal distension-induced phosphorylations of ERK1/2 were studied by immuno-staining and Western blot in esophageal nodose neurons. Extracellular recordings were performed from nodose neurons using ex vivo esophageal-vagal preparations with intact nerve endings in the esophagus. Nerve excitabilities were compared by action potentials evoked by esophageal distensions before and after mast cell activations with/without pretreatment of mitogen-activated protein kinases (MAPK)/ERK kinase inhibitor U0126. The expressions of phospho-ERK1/2 (p-ERK1/2) in the same nodose ganglia were then studied by Western blot. Mast cell activation enhances in vivo esophageal distension-induced phosphorylation of ERK1/2 in nodose neurons. This can be prevented by pretreatment with mast cell stabilizer cromolyn. In ex vivo esophageal-vagal preparations, both mast cell activation and proteinase-activated receptor 2 (PAR2)-activating peptide perfusion increases esophageal distension-induced mechano-excitability of esophageal nodose C-fibers and phosphorylation of ERK1/2 in nodose neurons. Pretreatment with MAPK/ERK kinase inhibitor U0126 prevents these potentiation effects. Collectively, our data demonstrated that mast cell activation enhances esophageal distension-induced mechano-excitability and phosphorylation of ERK1/2 in esophageal nodose C-fiber neurons. This reveals a new intracellular pathway of esophageal peripheral sensitization and inflammatory nociception.

Barosensory cells in the nucleus tractus solitarius receive convergent input from group III muscle afferents and central command

Some neural mechanism must prevent the full expression of the baroreceptor reflex during static exercise because arterial blood pressure increases even though the baroreceptors are functioning. Two likely candidates are central command and input from the thin fiber muscle afferents evoking the exercise pressor reflex. Recently, activation of the mesencephalic locomotor region, an anatomical locus for central command, was found to inhibit the discharge of nucleus tractus solitarius cells that were stimulated by arterial baroreceptors in decerebrated cats. In contrast, the effect of thin fiber muscle afferent input on the discharge of nucleus tractus solitarius cells stimulated by baroreceptors is not known. Consequently in decerebrated unanesthetized cats, we examined the responses of barosensory nucleus tractus solitarius cells to stimulation of thin fiber muscle afferents and to stimulation of the mesencephalic locomotor region, a maneuver which evoked fictive locomotion. We found that electrical stimulation of either the mesencephalic locomotor region or the gastrocnemius nerve at current intensities that recruited group III afferents inhibited the discharge of nucleus tractus solitarius cells receiving baroreceptor input. We also found that the inhibitory effects of both gastrocnemius nerve stimulation and mesencephalic locomotor region stimulation converged onto the same barosensory nucleus tractus solitarius cells. We conclude that the nucleus tractus solitarius is probably the site whereby input from both central command and thin fiber muscle afferents function to reset the baroreceptor reflex during exercise.

Estrogen attenuates the exercise pressor reflex in female cats

In humans, the pressor and muscle sympathetic nerve responses to static exercise are less in women than in men. The difference has been attributed to the effect of estrogen on the exercise pressor reflex. Estrogen receptors are abundant in areas of the dorsal horn receiving input from group III and IV muscle afferents, which comprise the sensory limb of the exercise pressor reflex arc. These findings prompted us to investigate the effect of estrogen on the spinal pathway of the exercise pressor reflex arc. Previously, we found that the threshold concentration of 17beta-estradiol needed to attenuate the exercise pressor reflex in male decerebrate cats was 10 microg/ml (Schmitt PM and Kaufman MP. J Appl Physiol 94: 1431-1436, 2003). The threshold concentration for female cats, however, is not known. Consequently, we applied 17beta-estradiol to a well covering the L6-S1 spinal cord in decerebrate female cats. The exercise pressor reflex was evoked by electrical stimulation of the L7 or S1 ventral root, a maneuver that caused the hindlimb muscles to contract statically. We found that the pressor response to contraction averaged 38 +/- 7 mmHg before the application of 17beta-estradiol (0.01 microg/ml) to the spinal cord, whereas it averaged only 23 +/- 4 mmHg 30 min after application (P < 0.05). Recovery of the pressor response to contraction was not obtained for 2 h after application of 17beta-estradiol. Application of 17beta-estradiol in a dose of 0.001 microg/ml had no effect on the exercise pressor reflex (n = 5). We conclude that the concentration of 17beta-estradiol required to attenuate the exercise pressor reflex is 1,000 times more dilute in female cats than that needed to attenuate this reflex in male cats.

Bicuculline and strychnine suppress the mesencephalic locomotor region-induced inhibition of group III muscle afferent input to the dorsal horn

We examined the effect of iontophoretic application of bicuculline methiodide and strychnine hydrochloride on the mesencephalic locomotor region (MLR)-induced inhibition of dorsal horn cells in paralyzed cats. The activity of 60 dorsal horn cells was recorded extracellularly in laminae I, II, V-VII of spinal segments L7-S1. Each of the cells was shown to receive group III muscle afferent input as demonstrated by their responses to electrical stimulation of the tibial nerve (mean latency and threshold of activation: 20.1+/-6.4 ms and 15.2+/-1.4 times motor threshold, respectively). Electrical stimulation of the MLR suppressed transmission in group III muscle afferent pathways to dorsal horn cells. Specifically the average number of impulses generated by the dorsal horn neurons in response to a single pulse applied to the tibial nerve was decreased by 78+/-2.8% (n=60) during the MLR stimulation. Iontophoretic application (10-50 nA) of bicuculline and strychnine (5-10 mM) suppressed the MLR-induced inhibition of transmission of group III afferent input to laminae I and II cells by 69+/-5% (n=10) and 29+/-7% (n=7), respectively. Likewise, bicuculline and strychnine suppressed the MLR-induced inhibition of transmission of group III afferent input to lamina V cells by 59+/-13% (n=14) and 39+/-11% (n=10), respectively. Our findings raise the possibility that GABA and glycine release onto dorsal horn neurons in the spinal cord may play an important role in the suppression by central motor command of thin fiber muscle afferent-reflex pathways.

Responses of C fiber afferents of the rabbit airways and lungs to changes in extra-vascular fluid volume

Effects of changes in extra-vascular fluid volume produced by pulmonary lymphatic obstruction and plasmapheresis on the activities of bronchial and pulmonary C fiber receptors and rapidly adapting receptors (RARs) were investigated in New Zealand White rabbits. In intact rabbits, pulmonary lymphatic obstruction either alone or in combination with plasmapheresis did not stimulate pulmonary C fiber receptors. Only the combined stimulus activated the bronchial C fiber receptors. Bronchial C fiber receptors were also stimulated by graded increases in left atrial pressure (+5 and +10 mmHg). In contrast, RARs were activated by lymphatic obstruction either alone or in combination with plasmapheresis. These procedures increase the extra-vascular fluid volume in the carina and bronchi but not in the lungs (alveoli). In rabbits with chronic pulmonary venous congestion secondary to mitral valve damage, bronchial C fiber receptors were not stimulated by these increments in left atrial pressure which were insufficient to increase the extra vascular fluid content of the airways. However, both pulmonary and bronchial C fiber receptors were stimulated when the left atrial pressure was raised to 25 mmHg in these animals to cause pulmonary edema.

Comparison of the exercise pressor reflex between forelimb and hindlimb muscles in cats

In thirteen cats anesthetized with alpha-chloralose, we compared the cardiovascular and ventilatory responses to both static contraction and tendon stretch of a hindlimb muscle group, the triceps surae, with those to contraction and stretch of a forelimb muscle group, the triceps brachii. Static contraction and stretch of both muscle groups increased mean arterial pressure and heart rate, and the responses were directly proportional to the developed tension. The cardiovascular increases, however, were significantly greater (P < 0.05) when the triceps brachii muscles were contracted or stretched than when the triceps surae muscles were contracted or stretched, even when the tension developed by either maneuver was corrected for muscle weight. Likewise, the ventilatory increases were greater when the triceps brachii muscles were stretched than when the triceps surae muscles were stretched. Contraction of either muscle group did not increase ventilation. Our results suggest that in the anesthetized cat the cardiovascular responses to both static contraction and tendon stretch are greater when arising from forelimb muscles than from hindlimb muscles.

Gadolinium attenuates exercise pressor reflex in cats

The exercise pressor reflex, which arises from the contraction-induced stimulation of group III and IV muscle afferents, is widely believed to be evoked by metabolic stimuli signaling a mismatch between blood/oxygen demand and supply in the working muscles. Nevertheless, mechanical stimuli may also play a role in evoking the exercise pressor reflex. To determine this role, we examined the effect of gadolinium, which blocks mechanosensitive channels, on the exercise pressor reflex in both decerebrate and alpha-chloralose-anesthetized cats. We found that gadolinium (10 mM; 1 ml) injected into the femoral artery significantly attenuated the reflex pressor responses to static contraction of the triceps surae muscles and to stretch of the calcaneal (Achilles) tendon. In contrast, gadolinium had no effect on the reflex pressor response to femoral arterial injection of capsaicin (5 microg). In addition, gadolinium significantly attenuated the responses of group III muscle afferents, many of which are mechanically sensitive, to both static contraction and to tendon stretch. Gadolinium, however, had no effect on the responses of group IV muscle afferents, many of which are metabolically sensitive, to either static contraction or to capsaicin injection. We conclude that mechanical stimuli arising in contracting skeletal muscles contribute to the elicitation of the exercise pressor reflex.

Both central command and exercise pressor reflex reset carotid sinus baroreflex

In decerebrate unanesthetized cats, we determined whether either "central command," the exercise pressor reflex, or the muscle mechanoreceptor reflex reset the carotid baroreflex. Both carotid sinuses were vascularly isolated, and the carotid baroreceptors were stimulated with pulsatile pressure. Carotid baroreflex function curves were determined for aortic pressure, heart rate, and renal vascular conductance. Central command was evoked by electrical stimulation of the mesencephalic locomotor region (MLR) in cats that were paralyzed. The exercise pressor reflex was evoked by statically contracting the triceps surae muscles in cats that were not paralyzed. Likewise, the muscle mechanoreceptor reflex was evoked by stretching the calcaneal tendon in cats that were not paralyzed. We found that each of the three maneuvers shifted upward the linear relationship between carotid sinus pressure and aortic pressure and heart rate. Each of the maneuvers, however, had no effect on the slope of these baroreflex function curves. Our findings show that central command arising from the MLR as well as the exercise pressor reflex are capable of resetting the carotid baroreflex.

Stimulation of the mesencephalic locomotor region inhibits the discharge of neurons in the superficial laminae of the dorsal horn of cats

In decerebrate cats, we found that stimulation of the mesencephalic locomotor region (MLR) attenuated the responses of neurons in the superficial laminae of the dorsal horn to thin fiber muscle afferent input. The attenuation appeared to be more effective for group III afferent input than for group IV. These findings may shed light on the interaction between central command, (i.e. the MLR) and the muscle reflex, mechanisms which both contribute to the cardiovascular responses to exercise.

Stimulation of the MLR inhibits the discharge of dorsal horn neurons responsive to muscular contraction

We found that electrical stimulation of the mesencephalic locomotor region (MLR) inhibited the discharge of deep dorsal horn neurons receiving group III afferent input from the triceps surae muscles. In contrast, contraction of these muscles induced by electrical stimulation of the tibial nerve activated these dorsal horn neurons. Our findings show that descending central motor commands can inhibit dorsal horn interneurons receiving input from group III afferents during exercise.

Fictive locomotion and scratching inhibit dorsal horn neurons receiving thin fiber afferent input

In decerebrate paralyzed cats, we examined the effects of two central motor commands (fictive locomotion and scratching) on the discharge of dorsal horn neurons receiving input from group III and IV tibial nerve afferents. We recorded the impulse activity of 74 dorsal horn neurons, each of which received group III input from the tibial nerve. Electrical stimulation of the mesencephalic locomotor region (MLR), which evoked fictive static contraction or fictive locomotion, inhibited the discharge of 44 of the 64 dorsal horn neurons tested. The mean depth from the dorsal surface of the spinal cord of the 44 neurons whose discharge was inhibited by MLR stimulation was 1.77 +/- 0.04 mm. Fictive scratching, evoked by topical application of bicuculline to the cervical spinal cord and irritation of the ear, inhibited the discharge of 22 of the 29 dorsal horn neurons tested. Fourteen of the twenty-two neurons whose discharge was inhibited by fictive scratching were found to be inhibited by MLR stimulation as well. The mean depth from the dorsal surface of the cord of the 22 neurons whose discharge was inhibited by fictive scratching was 1.77 +/- 0.06 mm. Stimulation of the MLR or the elicitation of fictive scratching had no effect on the activity of 22 dorsal horn neurons receiving input from group III and IV tibial nerve afferents. The mean depth from the dorsal surface of the cord was 1.17 +/- 0.07 mm, a value that was significantly (P < 0.05) less than that for the neurons whose discharge was inhibited by either MLR stimulation or fictive scratching. We conclude that centrally evoked motor commands can inhibit the discharge of dorsal horn neurons receiving thin fiber input from the periphery.

Group III muscle afferents evoke reflex depressor responses to repetitive muscle contractions in rabbits

Repetitive-twitch contraction of the hindlimb muscles in anesthetized rabbits consistently evokes a reflex depressor response, whereas this type of contraction in anesthetized cats evokes a reflex pressor response in about one-half of the preparations tested. Rapidly conducting group III fibers appear to comprise the afferent arm of the reflex arc, evoking the depressor response to twitch contraction in rabbits because electrical stimulation of their axons reflexly decreases arterial pressure. In contrast, electrical stimulation of the axons of slowly conducting group III and group IV afferents reflexly increases arterial pressure in rabbits. In the present study, we examined the discharge properties of group III and IV muscle afferents and found that the former (i.e., 13 of 20), but not the latter (i.e., 0 of 10), were stimulated by 5 min of repetitive-twitch contraction (1 Hz) of the rabbit triceps surae muscles. Moreover, most of the group III afferents responding to contraction appeared to be mechanically sensitive, discharging in synchrony with the muscle twitch. On average, rapidly conducting group III afferents responded for the 5-min duration of 1-Hz repetitive-twitch contraction, whereas slowly conducting group III afferents responded only for the first 2 min of contraction. We conclude that rapidly conducting group III afferents, which are mechanically sensitive, are primarily responsible for evoking the reflex depressor response to repetitive-twitch contractions in anesthetized rabbits.

Responses of group III and IV muscle afferents to distension of the peripheral vascular bed

This study was undertaken to test the hypothesis that group III and IV afferents with endings in skeletal muscle signal the distension of the peripheral vascular network. The responses of these slowly conducting afferents to pharmacologically induced vasodilation and to acute obstruction of the venous drainage of the hindlimbs were studied in barbiturate-anesthetized cats. Afferent impulses arising from endings in the triceps surae muscles were recorded from the L(7) and S(1) dorsal roots. Fifteen of the 48 group IV and 3 of the 19 group III afferents tested were stimulated by intra-aortic injections of papaverine (2-2.5 mg/kg). Sixty-two percent of the afferents that responded to papaverine also responded to isoproterenol (50 microg/kg). Seven of the 36 group IV and 2 of the 12 group III afferents tested were excited by acute distension of the hindlimb venous system. Four of the seven group IV afferents responding to venous distension also responded to papaverine (57 vs. 13% for the nonresponding). Finally, we observed that most of the group IV afferents that were excited by dynamic contractions of the triceps surae muscles also responded either to venous distension or to vasodilatory agents. These results are consistent with the histological findings that a large number of group IV endings have their receptive fields close to the venules and suggest that they can be stimulated by the deformation of these vascular structures when peripheral conductance increases. Moreover, such a mechanism offers the possibility of encoding both the effects of muscle contraction through intramuscular pressure changes and the distension of the venular system, thereby monitoring the activity of the veno-muscular pump.

Effect of arterial occlusion on responses of group III and IV afferents to dynamic exercise

Our laboratory has shown previously that a low level of dynamic exercise induced by electrical stimulation of the mesencephalic locomotor region (MLR) stimulated group III and IV muscle afferents in decerebrate unanesthetized cats (C. M. Adreani, J. M. Hill, and M. P. Kaufman. J. Appl. Physiol. 83: 1811-1817, 1997). In the present study, we have extended these findings by examining the effect of occluding the arterial supply to the dynamically exercising muscles on the afferents' responses to MLR stimulation. In decerebrate cats, we found that arterial occlusion increased the responsiveness to a low level of dynamic exercise in 44% of the group III and 47% of the group IV afferents tested. Occlusion, compared with the freely perfused state, did not increase the concentrations of either hydrogen ion or lactate ion in the venous effluent from the exercising muscles. We conclude that arterial occlusion caused some unspecified substance to accumulate in the working muscles to increase the sensitivity of equal percentages of group III and IV afferents to dynamic exercise.

Central command, but not muscle reflex, stimulates cutaneous sympathetic efferents of cats

We determined the effects of stimulation of the mesencephalic locomotor region (MLR) and the muscle reflex, each evoked separately, on the discharge of cutaneous sympathetic fibers innervating the hairy skin of decerebrate cats. Electrical stimulation of the MLR was performed while the cats were paralyzed with vecuronium bromide. The muscle reflex was evoked while the cats were not paralyzed by electrical stimulation of the tibial nerve at current intensities that did not activate directly group III and IV muscle afferents. MLR stimulation increased, on average, the discharge of the 23 cutaneous sympathetic fibers tested (P < 0.05). The muscle reflex, in contrast, had no overall effect on the discharge of 21 sympathetic fibers tested (P > 0.05). Both maneuvers markedly increased mean arterial pressure and heart rate (P < 0.05). Prevention of the baroreceptor reflex with the alpha-adrenergic blocking agent phentolamine did not reveal a stimulatory effect of the muscle reflex on cutaneous sympathetic discharge. We conclude that the MLR is a more important mechanism than is the muscle reflex in controlling sympathetic discharge to hairy skin during dynamic exercise.

Effect on airway caliber of stimulation of the hypothalamic locomotor region

Airway dilation is one of the many autonomic responses to exercise. Two neural mechanisms are believed to evoke these responses: central command and the muscle reflex. Previously, we found that activation of central command, evoked by electrical and chemical stimulation of the mesencephalic locomotor region, constricted the airways rather than dilated them. In the present study we examined in decerebrate paralyzed cats the role played by the hypothalamic locomotor region, the activation of which also evokes central command, in causing the airway dilator response to exercise. We found that activation of the hypothalamic locomotor region by electrical and chemical stimuli evoked fictive locomotion and, for the most part, airway constriction. Fictive locomotion also occurred spontaneously, and this too, for the most part, was accompanied by airway constriction. We conclude that central command plays a minor role in the airway dilator response to exercise.

Substance P analogues potentiate the pressor response to microinjection of L-glutamate into laminas I and II of the cat dorsal horn

Microinjection of a substance P analogue (1 mM; 7 or 10 nl) into laminae I and II of the L7 dorsal horn of decerebrate cats significantly potentiated (P < 0.05) the increase in arterial pressure evoked by microinjection of L-glutamate (109 mM; 7 or 10 nl) into these spinal sites. Microinjection of the substance P analogues (i.e., GR73638 and [Sar9,Met(O2)11]-substance P) which were selective NK-1 receptor agonists, had no impact on the cardioacceleration evoked by microinjection of L-glutamate (P > 0.05). In addition, microinjection of these analogues had no effect on the modest and non-significant increase in phrenic nerve discharge evoked by L-glutamate. We conclude that stimulation of NK-1 receptors in the superficial laminae of the dorsal horn potentiates the pressor responses to microinjection of L-glutamate.

Responses of group III and IV muscle afferents to dynamic exercise

Tetanic contraction of hindlimb skeletal muscle, induced by electrical stimulation of either ventral roots or peripheral nerves, is well known to activate group III and IV afferents. Nevertheless, the effect of dynamic exercise on the discharge of these thin fiber afferents is unknown. To shed some light on this question, we recorded in decerebrate cats the discharge of 24 group III and 10 group IV afferents while the mesencephalic locomotor region (MLR) was stimulated electrically. Each of the 34 afferents had their receptive fields in the triceps surae muscles. Stimulation of the MLR for 1 min caused the triceps surae muscles to contract rhythmically, an effect induced by an alpha-motoneuron discharge pattern and recruitment order almost identical to that occurring during dynamic exercise. Eighteen of the 24 group III and 8 of the 10 group IV muscle afferents were stimulated by MLR stimulation. The oxygen consumption of the dynamically exercising triceps surae muscles was increased by 2.5-fold over their resting levels. We conclude that low levels of dynamic exercise stimulate group III and IV muscle afferents.

Stimulation of the mesencephalic locomotor region constricts the airways of cats

The neural mechanisms causing the airway dilation evoked by exercise are not understood. Three candidates are central command, a reflex arising from contracting skeletal muscle, and the Hering-Breuer reflex. Activation of the latter two mechanisms has been shown to dilate the airways. In contrast, the role played by central command in the control of airway caliber is not known. We, therefore, tested the hypothesis that stimulation of the mesencephalic locomotor region (MLR) in paralyzed decerebrate cats decreased total lung resistance (TLR). Electrical stimulation (20-80 microA) of the MLR increased TLR from 28.2 +/- 1.9 to 38.1 +/- 2.4 cmH2OL-1 sec-1 (24 sites in 19 cats; p < 0.001). Similarly, microinjection of picrotoxin, a GABA antagonist, (8 mM, 100-400 nl) increased TLR from 26.1 +/- 3.3 to 36.3 +/- 5.4 cmH2OL-1 sec-1 (9 sites in 9 cats; p < 0.02). All of the changes evoked by electrical and chemical stimulation of the MLR were accompanied by increases in arterial pressure, heart rate, and ventral root motoneuron discharge. In contrast, electrical stimulation of the tibial nerve at intensities that recruited C-fibers reflexly decreased TLR (12 cats; p < 0.001). Our findings provide little evidence for the central command signal originating from the MLR in causing the airway dilation evoked by exercise.

Muscle reflex stimulates sympathetic postganglionic efferents innervating triceps surae muscles of cats

Two neural mechanisms contribute to the cardiovascular responses to exercise. The first, central command, proposes a parallel activation of central locomotor and brain stem circuits controlling cardiovascular function. The second, the muscle reflex, proposes that contraction-activated group III and IV afferents increase cardiovascular function. In humans, whole nerve recordings of sympathetic discharge suggest that central command increases sympathetic outflow to skin but not to skeletal muscle and that the muscle reflex increases sympathetic outflow to skeletal muscle but not to skin. We therefore tested the hypothesis that the muscle reflex, but not central command, increases the discharge of single sympathetic postganglionic efferents innervating the triceps surae muscles of decerebrate unanesthetized cats. Central command was evoked by electrical stimulation of the mesencephalic locomotor region. The reflex was evoked by electrical stimulation of the tibial nerve, which in turn contracted the triceps surae muscles. Hexamethonium abolished spontaneous and evoked activity, verifying that the recordings were from sympathetic postganglionic fibers. The discharge of 13 efferents was increased by static contraction (from 0.6 +/- 0.2 to 1.0 +/- 0.3 imp/s; P < 0.05) but was not increased by central command (from 0.6 +/- 0.2 to 0.8 +/- 0.2 imp/s; P > 0.05). Nevertheless, the discharge of nine efferents, not increased by central command before alpha-adrenergic blockade (from 0.5 +/- 0.2 to 0.9 +/- 0.4 imp/s; P > 0.05), was increased after blockade (from 1.3 +/- 0.2 to 3.2 +/- 0.8 imp/s; P < 0.05). We conclude that the muscle reflex stimulates sympathetic postganglionic efferents innervating the vasculature of skeletal muscle. Furthermore, baroreceptors appear to buffer the central command-induced increases in the discharge of these efferents.

Blockade of glutamate receptors in CVLM and NTS attenuates airway dilation evoked from parabrachial region

Previous work from our laboratory has shown that stimulation of cell bodies and dendrites in the medial and lateral parabrachial nuclei dilates the airways. The sites participating in the pathway mediating this airway response are not known. Two likely candidates are the caudal ventrolateral medulla (CVLM) and the nucleus tractus solitarii (NTS). Using chloralose-anesthetized cats, we assessed the airway dilation evoked from the parabrachial region before and during bilateral blockade of the NTS or the CVLM. The airway dilation arising from stimulation of the parabrachial region was evoked by microinjection of DL-homocysteic acid (25 nl, 100 mM). Bilateral blockade of the NTS or CVLM, achieved by microinjection of kynurenic acid (50 nl, 100 mM), reversibly attenuated the airway dilation in every cat tested. On average, kynurenic acid-induced blockade of the NTS caused a more complete attenuation of the dilation evoked from the parabrachial region than did blockade of the CVLM. Bilateral microinjection of cobalt chloride (50 nl, 50 mM) into the CVLM gave inconclusive results, attenuating the airway dilation evoked from the parabrachial region in six cats and potentiating it in three others. We conclude that the CVLM and the NTS participate in the airway dilation arising from the parabrachial region.

Intrathecal blockade of both NMDA and non-NMDA receptors attenuates the exercise pressor reflex in cats

In decerebrate unanesthetized cats we tested the hypothesis that glutamatergic-receptor blockade in the lumbosacral spinal cord attenuated the reflex increases in mean arterial pressure, inspired minute ventilation, and renal sympathetic nerve activity (RSNA) evoked by static contraction of the triceps surae muscles. Blockade of N-methyl-D-aspartate (NMDA) receptors by intrathecal injection of DL-2-amino-5-phosphonovaleric acid had no effect on the initial phase of the pressor, ventilatory, and RSNA responses to contraction but did attenuate the secondary phase of these responses. Subsequent blockade of non-NMDA receptors in the lumbosacral spinal cord by intrathecal injection of 6-cyano-7-nitroquinoxaline-2,3-dione attenuated both the initial phase of the pressor, RSNA, and ventilatory responses to contraction and the secondary phase of these responses. In addition, NMDA-receptor blockade had no effect on the pressor or RSNA responses to tendon stretch, whereas non-NMDA-receptor blockade abolished these responses. We confirmed that our results were not related to the order of the antagonists injected by performing a series of experiments in which a non-NMDA-receptor antagonist was injected first. Our findings suggest that non-NMDA receptors mediate the spinal transmission of the initial and secondary phases of the pressor, RSNA, and ventilatory responses to contraction and tendon stretch. Therefore, non-NMDA receptors in the dorsal horn appear to be involved in the spinal processing of input from mechanoreceptors and metaboreceptors. Our findings also suggest that NMDA receptors mediate the spinal transmission of the secondary phase of the pressor, RSNA, and ventilatory responses to contraction but do not mediate the spinal transmission of the responses to tendon stretch. Therefore, NMDA receptors in the dorsal horn appear to be involved in the spinal processing of input from metaboreceptors.

Pressor responses to stimulation of non-NMDA receptors in the superficial laminae of the cat spinal cord

Microinjection of L-glutamate (109 mM; 12-18 nl) or AMPA (150-300 microM; 12 nl) into the superficial laminae of the L7 dorsal horn of decerebrate or chloralose anesthetized cats significantly increased mean arterial pressure. In contrast, microinjection of NMDA (300 microM; 12 nl) had no effect on mean arterial pressure. The pressor response to L-glutamate microinjection was blocked by prior microinjection of CNQX, an antagonist to non-NMDA receptors, but not by AP-5, an antagonist to NMDA receptors. We conclude that stimulation of non-NMDA receptors in the superficial laminae of the lumbar dorsal horn increases arterial blood pressure.

Role of the parabrachial nuclei in the airway dilation evoked by the Hering-Breuer reflex

We tested the hypothesis that blockade of glutamatergic receptors in the parabrachial nucleus (PBN) of chloralose-anesthetized cats attenuated the reflex airway dilation evoked by activation of pulmonary stretch receptors. Unilateral microinjection of kynurenic acid (50 nl, 100 mM) into the PBN reversibly attenuated the reflex relaxation of the trachealis muscle in 7 cats. These findings suggest that the PBN is part of the central pathway mediating the airway dilation component of the Hering-Breuer reflex.

Role of muscle afferents in the inhibition of motoneurons during fatigue

In conscious humans, fatiguing muscular contractions are accompanied by a decrease in the discharge rate of alpha motoneurons. The association between alpha motoneuron discharge rate and the generation of force by skeletal muscle has been called "muscle wisdom" (Marsden et al., 1983). Its purpose is believed to ensure that central neural drive to skeletal muscle, which is fatigued, matches that needed to generate the required force. In addition, muscle wisdom may be one mechanism that functions either to decrease or to postpone central neural fatigue (Enoka & Stuart, 1992). Bigland-Ritchie and colleagues (1986) have suggested that a reflex arising from fatigued skeletal muscle is responsible, at least in part, for muscle wisdom. This chapter has two purposes. The first is to evaluate the evidence that a reflex arising from fatigued skeletal muscle causes muscle wisdom, and the second is to examine the discharge properties of muscle afferents to determine which ones are most likely to initiate reflexly this phenomenon.

NMDA receptors in caudal ventrolateral medulla mediate reflex airway dilation arising from the hindlimb

The caudal ventrolateral medulla (CVLM) has been shown to participate in the reflex airway dilation evoked by stimulation of thin fiber afferents innervating the hindlimb of anesthetized dogs. Nevertheless, the pharmacological mechanism in the CVLM by which hindlimb afferents evoke this reflex airway dilation is not known. Therefore, we examined the role played by excitatory amino acid receptors in the CVLM in the reflex airway dilation arising from the hindlimb. Using chloralose-anesthetized dogs, we found that bilateral microinjections into the CVLM of either (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (25 mM, 50 nl) or (+/-)-2-amino-5-phosphonovaleric acid (50 mM, 50 nl), both of which block N-methyl-D-aspartate (NMDA) receptors, reversibly attenuated the decrease in total lung resistance that was evoked by either electrical stimulation of C-fibers in the sciatic nerve or by static contraction of both gastrocnemius muscles. In contrast, bilateral microinjection into the CVLM of 6-cyano-7-nitroquinoxaline-2,3-dione (39 microM, 50 nl), which blocks non-NMDA receptors, augmented the reflex decrease in total lung resistance that was evoked by either sciatic nerve stimulation or contraction of the gastrocnemius muscles. Bilateral microinjections of xanthurenic acid (100 mM, 50 nl) into the CVLM had no effect on the decrease in total lung resistance that was evoked by sciatic nerve stimulation. We conclude that NMDA, but not non-NMDA, receptors in the CVLM play an important role in the reflex arc that dilates the airways when hindlimb afferents are stimulated by either muscular contraction or electrical stimulation.

Blockade of non-NMDA receptors attenuates reflex pressor response to static contraction

Considerable evidence suggests that both substance P and glutamate play a role in the spinal transmission of the exercise pressor reflex. We tested two hypotheses. First, after a lumbosacral intrathecal injection of a glutamatergic receptor antagonist, the reflex cardiovascular and ventilatory responses to static contraction are attenuated. Second, after a lumbosacral intrathecal injection of a substance P receptor antagonist and a glutamatergic receptor antagonist, the reflex cardiovascular and ventilatory responses to static contraction are abolished. We found that 1) the reflex cardiovascular responses to static contraction were unaffected (P > 0.05) after the intrathecal injection of the N-methyl-D-aspartate (NMDA) receptor antagonists, dl-2-amino-5-phosphonopentanoate (+/- AP-5) or 3-[(+-)-2-carboxypiperazin-4-yl]propyl-1-phosphonic acid (+/- CPP); 2) the reflex pressor response to static muscular contraction was attenuated by > 50% after the intrathecal injection of the non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX); and 3) the reflex pressor response to static contraction was almost abolished after the intrathecal injection of the substance P receptor antagonist, CP-96,345, and CNQX. Our results suggest that substance P and glutamate are two neurotransmitters involved in the spinal transmission of the exercise pressor reflex and that substance P and glutamate exert their effects via neurokinin-1 (NK-1) and non-NMDA receptors, respectively.

Stimulation of parabrachial nuclei dilates airways in cats

Stimulation of the parabrachial nuclei has been shown to increase mean arterial pressure as well as to terminate inspiration. Nevertheless, the effect on airway caliber evoked by stimulation of the parabrachial nuclei is not known. Therefore, in chloralose-anesthetized cats, we microinjected DL-homocysteic acid (25 nl; 100 mM) into 44 sites in or near the lateral and medial parabrachial nuclei while calculating breath-by-breath total lung resistance and dynamic compliance. We found that, in 43 of these sites, microinjection of this excitatory amino acid consistently decreased total lung resistance but had no effect on dynamic compliance. The decrease in lung resistance was caused by a withdrawal of cholinergic tone to the airways. We could find no evidence that the decrease in total lung resistance evoked by stimulation of the parabrachial nuclei was caused by activation of either beta-adrenergic or nonadrenergic noncholinergic pathways. The decrease in total lung resistance evoked by stimulation of the parabrachial nuclei was not secondary to the baroreceptor reflex even though microinjection frequently increased mean arterial pressure. In addition, microinjection did not have consistent effects on phrenic nerve activity, although in individual circumstances the effect on this activity was quite large. We conclude that stimulation of cell bodies and dendrites in the parabrachial nuclei dilates the airways of anesthetized cats and that the effect is not secondary to the baroreceptor reflex.

Dynamic exercise stimulates group III muscle afferents

1. In decerebrate cats, we investigated the responses of group III muscle afferents to dynamic exercise. The cats performed low intensity dynamic exercise on a treadmill. Group III afferent activity from the dynamically exercising triceps surae muscles was recorded from L7-S1 dorsal root filaments. 2. Single-unit recordings were obtained from 15 group III afferent fibers whose receptive fields were in the triceps surae muscles and from one group III afferent whose receptive field was in the flexor digitorum longus muscle. Conduction velocities for the 16 group III afferents ranged from 3.0 to 27.9 m/s (15.6 +/- 1.9 m/s, mean +/- SE). 3. Ten of 16 group III muscle afferents were stimulated by dynamic exercise. Of the 10, 7 were strongly responsive and 3 were mildly responsive to dynamic exercise. Each of the 10 afferents displayed at least some activity that was synchronized to the contraction phase of the step cycle. The mean developed tensions for strongly responsive afferents, mildly responsive afferents, and afferents that did not respond were 0.8 +/- 0.3, 1.3 +/- 0.5, and 0.7 +/- 0.3 Kg, respectively (P > 0.05). Thus differences in the responsiveness of the afferents to exercise were not attributable to differences in developed tensions. 4. The group III afferents that were strongly responsive to dynamic exercise were also mechanically sensitive. Each strongly responsive afferent (n = 7) was stimulated by nonnoxious pressure applied to its receptive field. Most strongly responsive afferents (n = 5) were stimulated by stretch of the triceps surae muscles.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of contraction and lactic acid on the discharge of group III muscle afferents in cats

1. In barbiturate-anesthetized cats we examined the interaction of lactic acid and static contraction on the discharge of group III muscle afferents. Only afferents whose receptive fields were located in the triceps surae muscles were studied. 2. Twelve of 20 afferents were stimulated by a 60-s static contraction. The majority of firing occurred within the first few seconds of contraction. Thirteen of 20 afferents were stimulated by femoral arterial injections of 24 mM lactic acid (1-4 ml) with the muscle at rest. Repeated injections of lactic acid with the muscle at rest led to tachyphylaxis. Lactic acid was then injected (24 mM; 4 ml) during the last 15 s of static contraction. In eight of nine afferents that were tachyphylactic to lactic acid with the muscle at rest, we noted a restored sensitivity to lactic acid during contraction. 3. In separate experiments we examined the effects of dichloroacetate (DCA) on the responses of group III muscle afferents to static contraction. DCA reduces the production of lactic acid by increasing levels of the active form of the enzyme pyruvate dehydrogenase. 4. DCA lowered arterial and venous lactate concentrations at rest and during contraction. DCA significantly decreased (31%; P < 0.05) the responses of the afferents to contraction. This effect was most prominent within the first 10 s of contraction and was not due to a reduced level of mechanical stimulation after DCA, because peak tension levels were the same during the two bouts of contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of vagal afferents inhibits locomotion in mesencephalic cats

Using electrical stimulation of the mesencephalic locomotor region, we made decerebrate unanesthetized cats walk on a treadmill. The locomotion induced by stimulation of this midbrain area was assessed before and during activation of vagal afferents by either intravenous injection of phenylbiguanide or inflation of a balloon placed in the left atrium. Inflation of a balloon, which increased left atrial pressure by 7-25 mmHg, abolished locomotion in 9 of 10 cats tested. Bilateral cervical vagotomy prevented the abolition of locomotion by balloon inflation in each of two cats tested. Intravenous phenylbiguanide (50 or 100 micrograms/kg) or serotonin (40 micrograms/kg) injections abolished or attenuated walking induced by midbrain stimulation in 11 of 13 cats tested. In addition, intravenous phenylbiguanide injections abolished or attenuated locomotion with a shorter onset time than did systemic injections of this substance in five of six cats tested. Bilateral cervical vagotomy prevented the abolition of locomotion by phenylbiguanide injection in each of five cats tested. We conclude that locomotion can be prevented by a viscerosomatic reflex arising from the lungs and heart. The afferent arm of this reflex arc is the vagus nerve. Afferents such as slowly and rapidly adapting pulmonary stretch receptors, atrial receptors, and lung C-fibers may have had a role in preventing locomotion during the increase in left atrial pressure in our experiments. On the other hand, pulmonary C-fibers had a crucial role in preventing locomotion during intravenous injection of phenyl-biguanide. We speculate that this viscerosomatic reflex may help to explain in part the intolerance for exercise displayed by patients with congestive heart failure.

Effects of hypoxia on the discharge of group III and IV muscle afferents in cats

The reflex pressor response evoked by static muscular contraction is widely believed to be caused by the stimulation of group III and IV afferents. Although the specific nature of the contraction-induced stimulus to these thin-fiber afferents is unknown, they are thought to be stimulated in part by a condition arising from a mismatch between blood supply and demand in the exercising muscle. Hypoxia, a condition found in skeletal muscle during such a mismatch, may stimulate these afferents. We have therefore tested the hypothesis that perfusion of the triceps surae muscles with hypoxic blood stimulates group III and IV afferents in barbiturate-anesthetized cats. We found that 3-3.5 min of hypoxia with the triceps surae muscles at rest significantly (P < 0.05) increased the average discharge rate of contraction-sensitive group IV afferents but had no effect on the average discharge rate of contraction-sensitive group III afferents. Hypoxia had only trivial effects on the discharge of contraction-insensitive group III and IV afferents. Hypoxia stimulated 4 of 11 contraction-sensitive group IV afferents and 2 of 13 contraction-sensitive group III afferents. The responses of the afferents stimulated by hypoxia were small in magnitude. Hypoxia with the muscles at rest appeared to have no effect on either hydrogen or lactate ion concentrations in the femoral venous blood. In addition, hypoxia increased the responses to contraction in only 3 of 22 group III and 4 of 21 group IV afferents tested. We conclude that muscle tissue hypoxia is a minor stimulus to afferents that sense a mismatch between blood supply and demand during static contraction.

Attenuation of reflex pressor and ventilatory responses to static contraction by an NK-1 receptor antagonist

The chemical messengers released onto second-order dorsal horn neurons from the spinal terminals of contraction-activated group III and IV muscle afferents have not been identified. One candidate is the tachykinin substance P. Related to substance P are two other tachykinins, neurokinin A (NKA) and neurokinin B (NKB), which, like substance P, have been isolated in the dorsal horn of the spinal cord and have receptors there. Whether NKA or NKB plays a transmitter/modulator role in the spinal processing of the exercise pressor reflex is unknown. Therefore, we tested the following hypotheses. After the intrathecal injection of a highly selective NK-1 (substance P) receptor antagonist onto the lumbosacral spinal cord, the reflex pressor and ventilatory responses to static muscular contraction will be attenuated. Likewise, after the intrathecal injection either of an NK-2 (NKA) receptor antagonist or an NK-3 (NKB) receptor antagonist onto the lumbrosacral spinal cord, the reflex pressor and ventilatory responses to static contraction will be attenuated. We found that, 10 min after the intrathecal injection of 100 micrograms of the NK-1 receptor antagonist, the pressor and ventilatory responses to contraction were significantly (P < 0.05) attenuated. Mean arterial pressure was attenuated by 13 +/- 3 mmHg (48%) and minute volume of ventilation by 120 +/- 38 ml/min (34%). The cardiovascular and ventilatory responses to contraction before either 100 micrograms of the NK-2 receptor antagonist or 100 micrograms of the NK-3 receptor antagonist were not different (P > 0.05) from those after the NK-2 or the NK-3 receptor antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Bronchomotor vagal preganglionic cell bodies in the dog: an anatomic and functional study

A previous study in our laboratory demonstrated that the stimulation with microinjection of DL-homocysteic acid of cell bodies in the rostral portion of the external formation of the nucleus ambiguus (Aext) increased total lung resistance in dogs. In the present study anatomic experiments were conducted in dogs to determine if the rostral Aext contains vagal preganglionic cell bodies that give rise to axons in the pulmonary branches of the vagus nerve. The application of horseradish peroxidase (HRP) to either the pulmonary branches or the vagus at a point between the pulmonary branches and the cardiac branches resulted in retrograde labeling of cell bodies in both rostral Aext and the dorsal motor nucleus of the vagus (DMN). On the other hand, application of HRP to the vagus at a point below the pulmonary branches did not result in any retrogradely labeled cell bodies in rostral Aext but did result in labeled cell bodies in DMN. In another series of experiments DL-homocysteic acid (2.5 nmol in 25 nl) was microinjected at sites in rostral Aext and DMN. As we previously reported the injection of DL-homocysteic acid in rostral Aext increased total lung resistance. In contrast, in the same animals, the injection of DL-homocysteic acid in DMN did not change total lung resistance. We conclude that bronchomotor vagal preganglionic cell bodies are located in rostral Aext but not in DMN. The functional significance of vagal preganglionic cell bodies in DMN whose axons contribute to the pulmonary branches of the vagus nerve remains to be determined.

Role of caudal ventrolateral medulla in reflex and central control of airway caliber

We investigated the role played by the caudal ventrolateral (CVL) medulla in the reflex and central neural control of airway caliber in chloralose-anesthetized dogs. Changes in total lung resistance were evoked by four different stimuli. These changes were compared before and after bilateral injection of either ibotenic acid (75 nl; 100 mM) or cobalt chloride (75 nl; 50 mM) into the CVL medulla. The four stimuli used to change lung resistance were static muscular contraction, electrical stimulation of thin fiber afferents in the sciatic nerve, electrical stimulation of the posterior diencephalon, and hypoxia. The first three stimuli have been shown to decrease total lung resistance, whereas the latter stimulus has been shown to increase resistance. We found that injection of both ibotenic acid, which destroys cell bodies but not fibers of passage, and cobalt, which prevents synaptic transmission, either abolished or greatly attenuated the decrease in total lung resistance evoked by static contraction, by sciatic nerve stimulation, and by posterior diencephalic stimulation. We also found that injection of ibotenic acid and cobalt attenuated the reflex increase in lung resistance evoked by hypoxia. In control experiments, we found that bilateral injection of ibotenic acid into the dorsal medulla had no effect on the changes in total lung resistance evoked by these four stimuli. We conclude that the CVL medulla plays an important role in the reflex and central control of airway caliber.

Ischemically sensitive abdominal visceral afferents: response to cyclooxygenase blockade

Ischemically sensitive abdominal visceral afferents are known to reflexly stimulate the cardiovascular system. These nerve endings respond to severe hypoxia as well as to exogenously administered bradykinin and prostaglandins such as PGI2, PGE, and PGF2 alpha. We have shown previously that these prostaglandins can sensitize some previously unresponsive afferents to respond to ischemia. To determine if endogenously produced prostaglandins contribute to the observed increase in activity during ischemia, we recorded activity of 6 A delta- and 23 C-fiber sympathetic afferents in anesthetized cats during 5 min of ischemia before and 15-30 min after intravenous administration of either indomethacin (5 mg/kg) or aspirin (50 mg/kg). Before cyclooxygenase inhibition, we noted repeatable increases of 1.44 +/- 0.22 and 1.44 +/- 0.36 impulses/s in the A delta- and C-fibers, respectively, in response to ischemia. After indomethacin or aspirin, these increases were significantly reduced (P less than 0.05) in both thinly myelinated and unmyelinated afferents (0.69 +/- 0.36 and 0.46 +/- 0.21 impulses/s, respectively). In a second protocol, we observed that the activity of six A delta- and seven C-fibers was significantly reduced by aspirin or indomethacin when a single period of ischemia preceded cyclooxygenase blockade. These data, in conjunction with our previous observations, indicate that prostaglandins significantly contribute to the increased afferent discharge activity associated with ischemia of the abdominal visceral region.

Cardiovascular and respiratory response to static exercise in the newborn kitten

Muscle contraction produces a reflex increase in blood pressure, heart rate, and minute ventilation in adults. To evaluate the role of this reflex in newborns, we compared the blood pressure, heart rate, and ventilatory responses to static contraction of the hindlimb muscles in sedated newborn and adult felines. The reflex response to muscle contraction was compared with the baroreflex, the chemoreceptor reflex, and the response to maximal stimulation of sciatic nerve afferents. With muscle contraction, newborn systolic blood pressure increased by 8.5 +/- 2.6%, which was significantly less than the adult response of 15.9 +/- 1.8% (p less than 0.025). Heart rate response to muscle contraction was less in newborns compared with adults, increasing by 1.4 +/- 0.5 and 8.3 +/- 1.3%, respectively (p less than 0.025). In contrast to heart-rate and blood-pressure responses, ventilatory responses to muscle contraction were similar in both age groups, increasing by 34 +/- 20 and 34 +/- 10% in newborns and adults, respectively. With stimulation of sciatic nerve afferents and with hypoxemia, blood pressure and heart rate increased similarly in both newborns and adults. When the baroreflex was elicited, heart rate decreased similarly in both age groups. We conclude that newborn cats have a reduced heart rate and blood pressure response to muscle contraction compared with the adult. We speculate that the postnatal development of this reflex is due to maturation of integrative and modulatory mechanisms in the CNS.

Activation of neurons in the rostral ventrolateral medulla increases bronchomotor tone in dogs

Previous work from this laboratory has demonstrated that the chemical activation of cell bodies in the caudal ventrolateral medulla of chloralose-anesthetized dogs decreased bronchomotor tone by withdrawing cholinergic input to airway smooth muscle. In the present study we determined the bronchomotor responses to microinjection of DL-homocysteic acid (100 mM; 25-50 nl) into the rostral ventrolateral (RVL) medulla of chloralose-anesthetized dogs. Total lung resistance was used as a functional index of bronchomotor tone. Microinjection of DL-homocysteic acid into the 20 sites located in the lateral aspect of the RVL medulla increased both total lung resistance [from 6.5 +/- 0.4 to 9.1 +/- 0.8 (SE) cmH2O.l-1.s; P less than 0.05] and mean arterial pressure (from 125 +/- 5 to 148 +/- 8 mmHg; P less than 0.05). Microinjection of this amino acid into nine sites located in the medial aspect of the RVL medulla increased mean arterial pressure (from 130 +/- 6 to 153 +/- 6 mmHg; P less than 0.05) but had no effect on total lung resistance. We confirmed in three sites that the increase in total lung resistance evoked by microinjection of DL-homocysteic acid was accompanied by an increase in tracheal smooth muscle tension. The increase in total lung resistance evoked by DL-homocysteic acid was not affected by beta-adrenergic blockade but was abolished by muscarinic blockade.

Intrathecal serotonin attenuates the pressor response to static contraction

We tested the hypothesis that intrathecal injection of serotonin onto the lumbosacral spinal cord of chloralose-anesthetized cats attenuates the pressor response to static contraction of the triceps surae muscles. Serotonin (10 micrograms) significantly attenuated the contraction-induced reflex increases in mean arterial pressure but not in ventilation. This attenuation of the reflex pressor response to static contraction by serotonin was prevented by prior intrathecal injection of mianserin hydrochloride, a serotonergic receptor antagonist. In addition, the reflex pressor response to contraction was significantly attenuated by intrathecal injection of carboxamidotryptamine maleate, a 5-HT1 agonist but not by DOI-hydrochloride, a 5-HT2 agonist. We conclude that stimulation of 5-HT1 receptors in the lumbosacral spinal cord attenuates the reflex pressor response to static muscular contraction.

Attenuating effects of intrathecal clonidine on the exercise pressor reflex

We tested the hypothesis that intrathecal injection of clonidine, an alpha 2-adrenergic agonist, attenuated the reflex cardiovascular and ventilatory responses to static muscular contraction in cats. Before clonidine (1 microgram in 0.2 ml), contraction-induced reflex increases (n = 10) in mean arterial pressure and ventilation averaged 25 +/- 3 mmHg and 359 +/- 105 ml/min, respectively, whereas after clonidine these increases averaged 8 +/- 4 mmHg and 200 +/- 114 ml/min, respectively (P less than 0.05). Clonidine had no effect on the heart rate response to contraction. Intrathecal injection of yohimbine (10 micrograms; n = 5), an alpha 2-adrenergic antagonist, but not prazosin (10 micrograms; n = 3), an alpha 1-adrenergic antagonist, prevented the attenuating effects of clonidine on the reflex pressor and ventilatory responses to contraction. Our findings were not due to the spread of clonidine to the medulla, because the reflex pressor and ventilatory responses to contraction were not attenuated by injection of clonidine (1 microgram) onto the medulla (n = 3). In addition, our findings were not due to a clonidine-induced withdrawal of sympathetic outflow, because intrathecal injection of clonidine (1 microgram) did not attenuate increases in arterial pressure and ventilation evoked by high-intensity electrical stimulation of the cut central end of the sciatic nerve (n = 5). Furthermore, our findings were not due to a local anesthetic action of clonidine, because application of this agent to the dorsal roots had no effect on the discharge of group IV muscle afferents. We conclude that stimulation of alpha 2-adrenergic receptors in the spinal cord attenuates the reflex pressor and ventilatory responses to static contraction.

Cyclooxygenase blockade attenuates responses of group IV muscle afferents to static contraction

Cyclooxygenase products of arachidonic acid might be some of the substances that accumulate in contracting muscle to cause the reflex increases in arterial pressure and ventilation that are evoked by exercise. Recently, cyclooxygenase blockade has been shown to attenuate the reflex cardiovascular responses to static muscular contraction in anesthetized cats. Group IV afferents are believed to comprise part of the afferent arm of the reflex arc, the activation by which static muscular contraction causes these cardiovascular effects. We therefore examined the effects of indomethacin and aspirin, two cyclooxygenase-blocking agents, on the responses to static contraction of group IV afferents with endings in the triceps surae muscles of anesthetized cats. We found that indomethacin (5 mg/kg iv) decreased the responses to contraction of each of eight group IV afferents tested. Likewise, aspirin (50 mg/kg iv) decreased the responses to contraction of each of four group IV afferents tested. On the other hand, we found that arachidonic acid (2 mg) injected into the femoral artery did not increase the responses to contraction of four group IV afferents that were stimulated by this maneuver. In addition, arachidonic acid injection did not cause any of seven group IV afferents not stimulated by static contraction to become responsive to this maneuver. Nevertheless, arachidonic acid injection with the muscle at rest stimulated five of seven contraction-insensitive and two of four contraction-sensitive group IV afferents. Our data suggest that cyclooxygenase metabolites of arachidonic acid are needed for the full expression of the responses of group IV muscle afferents to static contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Ischemia potentiates the reflex bronchodilation evoked by static muscular contraction in dogs

In eleven anesthetized dogs, we found that static contraction of hindlimb muscles that were freely perfused decreased total lung resistance by 0.7 +/- 0.1 cm H2O.L-1.sec, whereas static contraction of the same muscles rendered ischemic decreased total lung resistance by 1.5 +/- 0.4 cm H2O.L-1.sec (P less than 0.025). In ten other dogs, we found that static contraction of freely perfused hindlimb muscles decreased total lung resistance by 0.9 +/- 0.2 cm H2O.L-1.sec, whereas dynamic contraction of the same freely perfused muscles decreased total lung resistance by 1.1 +/- 0.3 cm H2O.L-1.sec. The difference in the magnitudes of the bronchodilator responses to the two modes of contraction was not significant (P greater than 0.05). We conclude that a mismatch between blood supply and demand in working skeletal muscle increases the reflex bronchodilator response to static contraction. We also conclude that dynamic contraction evokes a reflex bronchodilation equivalent to that evoked by static contraction provided that the tension produced by the two modes of contraction are equal.

Attenuation of reflex pressor and ventilatory responses to static muscular contraction by intrathecal opioids

We have tested the hypothesis that intrathecal injections of opioid peptides attenuate the reflex pressor and ventilatory responses to static contraction of the triceps surae muscles of chloralose-anesthetized cats. We found that before intrathecal injections of [D-Ala2]Met-enkephalinamide (100 micrograms in 0.2 ml), static contraction increased mean arterial pressure and ventilation by 32 +/- 5 (SE) mmHg and 227 +/- 61 (SE) ml/min, whereas after injection of this opioid peptide, static contraction increased mean arterial pressure and ventilation by only 15 +/- 5 mmHg and 37 +/- 33 ml/min, respectively. The attenuation of both the pressor and ventilatory responses to static contraction by [D-Ala2]Met-enkephalinamide were statistically significant (P less than 0.05). Moreover, the attenuation was probably not caused by an opioid-induced withdrawal of sympathetic outflow because [D-Ala2]Met-enkephalinamide had no effect on the pressor and ventilatory responses evoked by high-intensity electrical stimulation of the central cut end of the sciatic nerve. In addition, intrathecal injection of peptides that were highly selective agonists for either the opioid mu- or delta-receptor attenuated the reflex responses to static contraction. Naloxone (1,000 micrograms), injected intrathecally, prevented the attenuation of the reflex responses to contraction by opioid peptides. We speculate that the opioid-induced attenuation of the reflex pressor and ventilatory responses to static contraction may have been due to suppression of substance P release from group III and IV muscle afferents.

Pulmonary afferent control of breathing as end-expiratory lung volume decreases

We studied reflex changes in breathing elicited by graded reductions in end-expiratory lung volume (EEVL) and the vagal nerves responsible. The chests of nine dogs anesthetized with alpha-chloralose were opened, and the lungs were ventilated by a phrenic nerve-driven servo-respirator. The immediate effects of a 50% reduction in end-expiratory transpulmonary pressure (EEPtp) from control (EEVL equivalent to functional residual capacity) were to significantly increase both tidal volume (VT) and breathing frequency (f) from 0.402 +/- 0.101 to 0.453 +/- 0.091 liter (mean +/- SD) and 11.8 +/- 5.4 to 15.7 +/- 6.4 breaths/min, respectively (P less than 0.05). Further reductions in EEPtp to 0 cmH2O did not change VT but augmented f to 19.6 +/- 6.6 breaths/min (P less than 0.05). The increase in f as EEVL decreased was due entirely to a reduction in expiratory time. Vagotomy abolished these reflexes. By 90 s after reduction in EEVL, arterial PCO2 fell significantly and VT returned to or below control values. We therefore repeated these experiments in five dogs whose blood gases were controlled by cardiopulmonary bypass. There were no secondary changes in VT and by 90 s breathing pattern could be characterized as rapid and deep. In another eight dogs submitted to the same collapse protocol, we recorded action potentials from all known categories of pulmonary vagal afferents. These studies demonstrated that the changes in breathing pattern induced by a 50% reduction in EEPtp were due to a withdrawal of slowly adapting stretch receptor activity; however, continued increases in f as EEVL was reduced further were due to increases in rapidly adapting stretch receptor activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Sensitization of group III muscle afferents to static contraction by arachidonic acid

The afferent arm of the reflex are responsible for the pressor response to static contraction is comprised of group III and IV fibers. The nature of the contraction-induced stimulus activating these fibers remains unclear. Evidence suggests that most group III afferents are sensitive to mechanical stimuli, whereas most group IV afferents are sensitive to metabolic stimuli. Recently, in anesthetized cats, stimulation of group III mechanoreceptors has been shown to have a role in the reflex pressor response to static contraction. In skin, the sensitivity of thin fiber mechanoreceptors to distortion of their receptive fields has been shown to be increased by both cyclooxygenase and lipoxygenase products of arachidonic acid metabolism. Therefore, in barbiturate-anesthetized cats we recorded the responses of group III muscle afferents to static contraction before and after arachidonic acid (1-2 mg ia) and/or indomethacin (5 mg/kg iv). Arachidonic acid increased the responses of group III afferents (n = 11) to contraction by 265% (from 0.17 +/- 0.07 to 0.62 +/- 0.24 impulses/s; P less than 0.025). Indomethacin decreased the responses of group III afferents (n = 9) to contraction by 61% (from 1.00 +/- 0.37 to 0.39 +/- 0.16 impulses/s; P less than 0.025). Arachidonic acid given after indomethacin increased the responses of two of four group III afferents to contraction. We conclude that both cyclooxygenase and lipoxygenase products of arachidonic acid metabolism sensitize group III muscle afferents to static contraction.

Reflex cardiovascular and ventilatory responses to increasing H+ activity in cat hindlimb muscle

Static exercise increases arterial pressure, heart rate, and ventilation, effects which are believed in part to arise reflexly from a metabolic stimulus in the working muscle. In anesthetized cats, we tested the hypothesis that intra-arterial injections of lactic and hydrochloric acid, which created levels of these substances in muscle similar to those seen during contraction, reflexly increased cardiovascular and ventilatory function. Hydrochloric acid (32 and 57 mM; 1 ml) injected into the arterial supply of the triceps surae decreased intramuscular pH from 7.26 +/- 0.05 to 7.17 +/- 0.05 (P less than 0.01) and reflexly increased arterial pressure (23 +/- 7 mmHg; P less than 0.01), heart rate (11 +/- 2 beats/min; P less than 0.001), and ventilation (187 +/- 72 ml/min; P less than 0.05). Static contraction of the triceps surae decreased intramuscular pH from 7.28 +/- 0.06 to 7.13 +/- 0.06 (P less than 0.01). Lactic acid was more potent in causing reflexes than was equimolar HCl. For example, lactic acid containing 4 mM lactate and 0.87 mM H+ reflexly increased arterial pressure, heart rate, and ventilation, whereas 0.87 mM HCl did not. Intra-arterial sodium lactate (13 and 33 mM) at a neutral pH had no effect on these variables. We conclude that contraction-induced accumulation of H+, especially that arising from lactic acid, might provide a metabolic stimulus to evoke reflex autonomic effects.

Static contraction increases arachidonic acid levels in gastrocnemius muscles of cats

Static contraction of hind-limb muscles is well known to increase reflexly cardiovascular function. Recently, blockade of cyclooxygenase activity has been reported to attenuate the reflex pressor response to contraction, a finding which suggests that working skeletal muscle releases arachidonic acid metabolites. Therefore, we measured the effects of static contraction and ischemia on arachidonic acid levels in the gastrocnemius muscles of barbiturate-anesthetized cats treated with indomethacin. Unesterified arachidonic acid levels were measured by high-pressure liquid chromatography. We found that static contraction of freely perfused gastrocnemius muscles increased arachidonic acid levels from 4.4 +/- 1.0 to 10.3 +/- 2.2 nmol/g wet wt (n = 12; P less than 0.005). Likewise, static contraction of gastrocnemius muscles made ischemic for 2 min before the onset of the contraction period increased arachidonic acid levels from 12.6 +/- 2.3 to 21.0 +/- 2.0 nmol/g wet wt (n = 12; P less than 0.01). Lastly, 2 min of ischemia with the gastrocnemius muscles at rest increased arachidonic acid levels from 5.9 +/- 1.1 to 10.5 +/- 3.0 nmol/g wet wt (n = 18; P less than 0.02). We conclude that both static contraction and ischemia increase arachidonic acid levels in working hindlimb muscle.