Primary hyperparathyroidism: review and recommendations on evaluation, diagnosis, and management. A Canadian and international consensus.

The purpose of this review is to assess the most recent evidence in the management of primary hyperparathyroidism (PHPT) and provide updated recommendations for its evaluation, diagnosis and treatment. A Medline search of "Hyperparathyroidism. Primary" was conducted and the literature with the highest levels of evidence were reviewed and used to formulate recommendations. PHPT is a common endocrine disorder usually discovered by routine biochemical screening. PHPT is defined as hypercalcemia with increased or inappropriately normal plasma parathyroid hormone (PTH). It is most commonly seen after the age of 50 years, with women predominating by three to fourfold. In countries with routine multichannel screening, PHPT is identified earlier and may be asymptomatic. Where biochemical testing is not routine, PHPT is more likely to present with skeletal complications, or nephrolithiasis. Parathyroidectomy (PTx) is indicated for those with symptomatic disease. For asymptomatic patients, recent guidelines have recommended criteria for surgery, however PTx can also be considered in those who do not meet criteria, and prefer surgery. Non-surgical therapies are available when surgery is not appropriate. This review presents the current state of the art in the diagnosis and management of PHPT and updates the Canadian Position paper on PHPT. An overview of the impact of PHPT on the skeleton and other target organs is presented with international consensus. Differences in the international presentation of this condition are also summarized.

Targeted deletion of neurokinin-1 receptor expressing nucleus tractus solitarii neurons precludes somatosensory depression of arterial baroreceptor-heart rate reflex.

Neurokinin-1 receptor (NK1-R) expressing neurons are densely distributed throughout the nucleus tractus solitarii (NTS). However, their fundamental role in arterial baroreflex function remains debated. Previously, our group has shown that activation of contraction-sensitive somatic afferents evoke substance P (SP) release in the NTS and resets the arterial baroreflex via activation of a GABAergic NTS circuit. Based on these findings, we hypothesized that modulation of arterial baroreflex function by somatic afferents is mediated by NK1-R dependent inhibition of barosensitive NTS circuits. In the present study, SP-conjugated saporin toxin (SP-SAP) was used to ablate NK1-R expressing NTS neurons. Contraction-sensitive somatic afferents were activated by electrically-evoked muscle contraction and the arterial baroreceptor-heart rate reflex was assessed by constructing reflex curves using a decerebrate, arterially-perfused preparation. Baseline baroreflex sensitivity was significantly attenuated in SP-SAP-treated rats compared with control rats receiving either unconjugated SAP or vehicle. Muscle contraction significantly attenuated baroslope in SAP and vehicle-treated animals and shifted the baroreflex curves to higher systemic pressure. In contrast, somatic afferent stimulation failed to alter baroslope or shift the baroreflex curves in SP-SAP-treated animals. Moreover, when reflex sensitivity was partially restored in SP-SAP animals, somatic stimulation failed to attenuate baroreflex bradycardia. In contrast, SP-SAP and somatic stimulation failed to blunt the reflex bradycardia evoked by the peripheral chemoreflex. Immunohistochemistry revealed that pretreatment with SP-SAP significantly reduced the number of NK1-R expressing neurons in the caudal NTS, while sparing NK1-R expressing neurons rostral to the injection site. This was accompanied by a significant reduction in the number of glutamic acid decarboxylase (GAD67) expressing neurons at equivalent levels of the NTS. These findings indicate that immunolesioning of NK1-R expressing NTS neurons selectively abolishes the depressive effect of somatosensory input on arterial baroreceptor-heart rate reflex function.

Discharge patterns of somatosensitive neurons in the nucleus tractus solitarius of the cat.

Encoding of sensory information by nucleus of the solitary tract (NTS) neurons is incompletely understood. Using extracellular single-unit recording in alpha-chloralose-urethane anesthetized cats, we have examined the discharge characteristics of NTS neurons to activation of somatic Adelta and C fiber afferents by skeletal muscle contraction evoked by electrical stimulation of lower lumbar/upper sacral ventral roots. Generally, somatic afferent stimulation evoked two distinct firing patterns. The first population (36/43 cells) increased their firing rate to brief somatic stimuli. A subset (21/27 cells) exhibited a rapid decay of their firing rate during sustained somatic stimulation. Peak instantaneous firing frequency (F(p)) increased proportionally with the intensity of somatic stimulation (105+/-4 vs. 119+/-4 vs. 139+/-4 Hz, 10, 20 and 40 Hz, respectively, P<0.0001), whereas steady-state firing frequency (F(ss)) was not altered (25+/-2 vs. 27+/-2 vs. 27+/-2 Hz, 10, 20 and 40 Hz, respectively, P=0.72). Two indices were derived to quantify the decay properties. The decay rate constant (obtained from exponential curve fitting) was not altered by stimulation frequency (461+/-10 vs. 442+/-14 vs. 429+/-26 ms, 10, 20 and 40 Hz, respectively, P=0.415), nor was the decay index (derived to express the percent reduction in firing rate with respect to the initial peak firing rate; 76+/-2 vs. 77+/-2 vs. 81+/-2%, 10, 20 and 40 Hz, respectively, P=0.187). In contrast, the second population (seven of 43 cells) decreased their firing rate to stimulation. Of the NTS neurons tested for barosensitivity (29/36), none responded to pressure stimulation. These results have identified a population of somatosensitive NTS neurons that exhibit rapid firing rate decay properties during sustained stimulation. However, this population could faithfully encode phasic excitation during rhythmic somatosensory input. These results are discussed in relation to the role of somatosensory input on baroreflex function.

Contraction-sensitive skeletal muscle afferents inhibit arterial baroreceptor signalling in the nucleus of the solitary tract: role of intrinsic GABA interneurons.

Arterial baroreceptor and skeletal muscle receptor afferents relay sensory information to the nucleus of the solitary tract (NTS) during exercise. Previous studies have suggested that skeletal muscle afferent input inhibits baroreflex function; however, detailed information on the role of muscle afferents and GABAergic mechanisms in the NTS is limited. Furthermore, identification of specific afferent modalities that activate GABAergic neurons in the NTS remains unknown. In the present study, we examined the neuroanatomical and physiological interactions between spinal dorsal horn cells that transmit contraction-sensitive input from skeletal muscle and GABAergic interneurons in the NTS. Biotinylated dextran amine (BDA, 10%, 25-100 nL) microinjection into dorsal horn of the cervical spinal cord was combined with glutamate decarboxylase (GAD) immunohistochemistry to visualize the nature of the relationship of BDA-labeled fibers in the NTS with GAD immunoreactivity (GAD-ir). BDA-labeled axons and terminal processes were localized in the medial, commissural, dorsomedial and dorsolateral subdivisions of the caudal NTS. Moreover, BDA-labeled fibers were observed in close proximity to GAD-ir structures throughout these regions of the NTS. The physiological interaction between skeletal muscle receptor and arterial baroreceptor afferents was investigated using an arterially perfused, decerebrate rat preparation. Activation of skeletal muscle afferents by electrically evoked twitch contraction of the forelimb attenuated baroreflex responsiveness (BR, calculated as the ratio of changes in heart rate to systemic pressure) from -1.5+/-0.3 bpm.mm Hg(-1) to -0.1+/-0.1 bpm.mm Hg(-1) (control versus contraction, P<0.05, n=15). However, forelimb contraction failed to inhibit the reflex bradycardia evoked by activation of peripheral chemoreceptor afferents, indicating a reflex-specific action. Bilateral microinjection of bicuculline methiodide (BIC, 10 microM, 40-60 nL) into the caudal NTS restored baroreflex responsiveness during contraction (-1.6+/-0.2 versus -0.1+/-0.1 versus -1.5+/-0.2 bpm.mmHg(-1), control versus contraction versus contraction+BIC P<0.05, n=8). We conclude that activation of ascending spinal neurons from the cervical dorsal horn by contraction-sensitive skeletal muscle afferents selectively inhibits arterial baroreceptor signaling in the NTS via activation of a GABAergic mechanism.

Exercise and sensory integration. Role of the nucleus tractus solitarius.

Since NTS neurons receive synaptic input from many sensory modalities, it is crucial to understand the neuronal mechanisms involved in synaptic processing. We have proposed that GABA-containing neurons in the NTS are the primary target for somatic afferent fibers activated by skeletal muscle contraction. In our model, local inhibition of baroreceptor signaling is necessary to counteract the increase in baroreceptor input such that NTS output is normalized and baroreflex sensitivity is maintained during exercise. This GABAergic mechanism, in conjunction with sympathoexcitation evoked by somatic afferents, preserves reflex sensitivity and resets the baroreflex, respectively. Unfortunately, there is insufficient data to date to support or refute the proposed role for GABA on baroreflex function during exercise. However, we feel that this model will be useful in formulating future experiments to explore these synaptic interactions.

Enhanced activity in parathyroid hormone-(1-14) and -(1-11): novel peptides for probing ligand-receptor interactions.

The amino-terminal portion of PTH is critical for PTH-1 receptor (P1Rc) activation. In exploring this component of the ligand receptor interaction, we recently showed that the agonist potency of the weakly active PTH-(1-14)NH(2) peptide can be enhanced by natural amino acid substitutions at several positions, including position 11 (normally leucine). Here we show that the potency of PTH-(1-14)NH(2) can be enhanced by using nonnatural amino acids that increase the length and polarizability of the position 11 side-chain. Thus, in LLC-PK(1) cells stably expressing high levels of the human P1Rc, [homoarginine([Har)(11)]PTH-(1-14)NH(2) was 30-fold more potent for cAMP production than was native PTH-(1-14)NH(2). Combining the homoarginine-11 substitution with other recently identified activity-enhancing substitutions yielded [Ala(3,12),Gln(10),Har(11),Trp(14)]PTH-(1-14)NH(2), which was 1500-fold more potent than PTH-(1-14)NH(2) (EC(50) = 0.12 +/- 0.04 and 190 +/- 20 microM, respectively) and only 63-fold less potent than PTH-(1-34) (EC(50) = 1.9 +/- 0.5 nM). The even shorter analog [Ala(3),Gln(10),Har(11)]PTH-(1-11)NH(2) was also a full cAMP agonist (EC(50) = 3.1 +/- 1.5 microM). Receptor mutations at Phe(184) and Leu(187) located near the boundary of the amino-terminal domain and transmembrane domain-1 severely impaired responsiveness to the PTH-(1-11) analog. Overall, these studies demonstrate that PTH analogs of only 11 amino acids are sufficient for activation of the PTH-1 receptor through interaction with its juxtamembrane region.

NO formation in nucleus tractus solitarii attenuates pressor response evoked by skeletal muscle afferents.

We have previously shown that static muscle contraction induces the expression of c-Fos protein in neurons of the nucleus tractus solitarii (NTS) and that some of these cells were codistributed with neuronal NADPH-diaphorase [nitric oxide (NO) synthase]-positive fibers. In the present study, we sought to determine the role of NO in the NTS in mediating the cardiovascular responses elicited by skeletal muscle afferent fibers. Static contraction of the triceps surae muscle was induced by electrical stimulation of the L7 and S1 ventral roots in anesthetized cats. Muscle contraction during microdialysis of artificial extracellular fluid increased mean arterial pressure (MAP) and heart rate (HR) 51 +/- 9 mmHg and 18 +/- 3 beats/min, respectively. Microdialysis of L-arginine (10 mM) into the NTS to locally increase NO formation attenuated the increases in MAP (30 +/- 7 mmHg, P < 0.05) and HR (14 +/- 2 beats/min, P > 0.05) during contraction. Microdialysis of D-arginine (10 mM) did not alter the cardiovascular responses evoked by muscle contraction. Microdialysis of N(G)-nitro-L-arginine methyl ester (2 mM) during contraction attenuated the effects of L-arginine on the reflex cardiovascular responses. These findings demonstrate that an increase in NO formation in the NTS attenuates the pressor response to static muscle contraction, indicating that the NO system plays a role in mediating the cardiovascular responses to static muscle contraction in the NTS.

Activation of skeletal muscle afferents evokes release of glutamate in the subretrofacial nucleus (SRF) of cats.

The subretrofacial nucleus (SRF) is a region of the rostral ventrolateral medulla known to play a crucial role in sympathoexcitation. SRF neurons send direct projections to the intermediolateral cell columns of the spinal cord where they form synaptic contact with preganglionic sympathetic motor neurons. Activation of this neural pathway increases sympathetic outflow to the heart and blood vessels affecting cardiac function and vasomotor tone. Previous studies utilizing electrophysiological recording techniques and c-Fos expression have established that the activity of SRF neurons is increased during skeletal muscle contraction. However, the excitatory neurotransmitter mediating this increased activity remains in question. In the present study, static contraction of the triceps surae was induced by electrical stimulation of L7 and S1 ventral roots in anesthetized cats (n=12). Endogenous release of glutamate (Glu) from the SRF was recovered by microdialysis and measured by HPLC. Static muscle contraction for 4 min increased mean arterial pressure (MAP) 38+/-4 mmHg from a control level of 102+/-12 mmHg (P< 0.05). During muscle contraction the extracellular concentration of Glu recovered from the SRF increased from 623+/-117 to 1078+/-187 nM (P<0.05). To determine the effect of muscle contraction on Glu release in the absence of synaptic input from other reflexogenic areas, contraction was repeated following acute sinoaortic denervation and vagotomy. Following this denervation, muscle contraction increased MAP 41+/- 4 mmHg (P < 0.05) and Glu concentration from 635+/-246 to 1106+/-389 nM (P < 0.05). Muscle paralysis prevented the increases in MAP and Glu concentration during ventral root stimulation. These results suggest that: (i) Glu is released in the SRF during activation of contraction-sensitive skeletal muscle afferent fibers in the cat; and (ii) synaptic input from other reflexogenic areas appears to be ineffective in modulating the release of Glu in the SRF during static muscle contraction.

Naturalistic activation of barosensitive afferents release substance P in the nucleus tractus solitarius of the cat.

The role for substance P (SP) in baroreceptor transmission in the nucleus tractus solitarius (NTS) remains an area of active research. The purpose of the present study was to determine whether naturalistic activation of barosensitive afferent fibers in the glossopharygneal and vagus nerves release SP in the caudal NTS. Experiments were performed on chloralose anesthetized, artificially ventilated and paralyzed cats. A microdialysis probe was stereotaxically positioned unilaterally in the NTS. Dialysate samples were collected and SP-like immunoreactivity (SP-LI) was measured by radioimmunoassay. Barosensitive afferents were mechanically activated by inflation of a balloon catheter positioned in the thoracic aorta at heart level. Graded balloon inflation produced increases in mean arterial pressure (MAP) of 33+/-5 mmHg and 60+/-3 mmHg (P<0.05) and evoked proportional baroreflex decreases in heart rate of 8+/-3 b.p.m. and 19+/-3 b.p.m. (P<0.05). This was accompanied by increases in SP-LI of 16+/-3% and 39+/-8%, respectively (P<0.05). A positive linear relationship was found between changes in MAP and SP-LI (slope=1.73 fmol/microl/mmHg, r(2)=0.62) that was completely abolished following barodenervation. These findings provide evidence that naturalistic activation of pressure-sensitive afferents in the glossopharygneal and vagus nerves release SP in a region of the NTS that receives primary afferent projections from aortic, carotid sinus and cardiac receptors in the cat.

Reflex cardiovascular responses evoked by selective activation of skeletal muscle ergoreceptors.

It is well known that the exercise pressor reflex (EPR) is mediated by group III and IV skeletal muscle afferent fibers, which exhibit unique discharge responses to mechanical and chemical stimuli. Based on the difference in discharge patterns of group III and IV muscle afferents, we hypothesized that activation of mechanically sensitive (MS) fibers would evoke a different pattern of cardiovascular responses compared with activation of both MS and chemosensitive (CS) fibers. Experiments were conducted in chloralose-urethane-anesthetized cats (n = 10). Passive muscle stretch was used to activate MS afferents, and electrically evoked contraction of the triceps surae was used to activate both MS and CS muscle afferents. No significant differences were shown in reflex heart rate and mean arterial pressure (MAP) responses between passive muscle stretch and evoked muscle contraction. However, when the reflex responses were matched according to tension-time index (TTI), the peak MAP response (67 +/- 4 vs. 56 +/- 4 mmHg, P < 0.05) was significantly greater at higher TTI (427 +/- 18 vs. 304 +/- 13 kg. s, high vs. low TTI, P < 0.05), despite different modes of afferent fiber activation. When the same mode of afferent fiber activation was compared, the peak MAP response (65 +/- 7 vs. 55 +/- 5 mmHg, P < 0.05) was again predicted by the magnitude of TTI (422 +/- 24 vs. 298 +/- 19 kg. s, high vs. low TTI, P < 0.05). Total sensory input from skeletal muscle ergoreceptors, as predicted by TTI and not the modality of afferent fiber activation (muscle contraction vs. passive stretch), is suggested to be the primary determinant of the magnitude of the EPR-evoked cardiovascular response.

Somatosympathetic reflex in a working heart-brainstem preparation of the rat.

The purpose of the present study was to examine the cardiorespiratory responses (CR) evoked by a somatosympathetic reflex (SSR) in the working heart-brainstem preparation (WHBP). Sprague-Dawley rats (75-100 g) were anesthetized with halothane, bisected sub-diaphramatically and decerebrated pre-collicularly (n = 15). The preparation was transferred to a recording chamber and perfused via the thoracic aorta with Ringer's solution containing an oncotic agent (Ficoll, 1.25%). SSR was activated by electrical stimulation (5 s) of the brachial nerve (0.5-40 Hz, 1-20 V, 0.1 ms) or the forelimb (0.5-40 Hz, 5-60 V, 2 ms). Stimulation at 40 Hz significantly increased heart rate (HR, 366 +/- 10 to 374 +/- 9 beats/min), systemic perfusion pressure (PP, 83 +/- 5 to 89 +/- 6 mmHg) and phrenic nerve discharge (PND, 0.4 +/- 0.1 to 1.4 +/- 0.3 Hz). Ganglionic blockade with hexamethonium (300 microM) eliminated the tachycardia and pressor response but did not alter the tachypnea to forelimb stimulation (n = 3). Transection of the brachial nerve plexus abolished the increase in PP and PND (n = 4). This indicates that a neural reflex mediated these responses. Spinal transection (C1-C2) completely abolished all responses indicating that they were mediated via a supraspinal pathway (n = 2). Based upon these findings, we conclude that activation of somatosensory afferent fibers in the WHBP evokes a programmed pattern of autonomic responses altering the activity-state of both the cardiovascular and respiratory systems. The WHBP provides a unique opportunity to investigate the medullary circuits and neuronal mechanisms that may be involved in coupling cardiorespiratory and somatomotor activity during locomotion/exercise.

Minimization of parathyroid hormone. Novel amino-terminal parathyroid hormone fragments with enhanced potency in activating the type-1 parathyroid hormone receptor.

The amino-terminal and carboxyl-terminal portions of the 1-34 fragment of parathyroid hormone (PTH) contain the major determinants of receptor activation and receptor binding, respectively. We investigated how the amino-terminal signaling portion of PTH interacts with the receptor by utilizing analogs of the weakly active fragment, rat (r) PTH(1-14)NH(2), and cells transfected with the wild-type human PTH-1 receptor (hP1R-WT) or a truncated PTH-1 receptor which lacked most of the amino-terminal extracellular domain (hP1R-delNt). Of 132 mono-substituted PTH(1-14) analogs, most having substitutions in the (1-9) region were inactive in assays of cAMP formation in LLC-PK1 cells stably expressing hP1R-WT, whereas most having substitutions in the (10-14) region were active. Several substitutions (e.g. Ser(3) --> Ala, Asn(10) --> Ala or Gln, Leu(11) --> Arg, Gly(12) --> Ala, His(14) --> Trp) enhanced activity 2-10-fold. These effects were additive, as [Ala(3),(10,12),Arg(11), Trp(14)] rPTH(1-14)NH(2) was 220-fold more potent than rPTH(1-14)NH(2) (EC(50) = 0.6 +/- 0.1 and 133 +/- 16 micrometer, respectively). Native rPTH(1-11) was inactive, but [Ala(3,10), Arg(11)]rPTH(1-11)NH(2) achieved maximal cAMP stimulation (EC(50) = 17 micrometer). The modified PTH fragments induced cAMP formation with hP1R-delNt in COS-7 cells as potently as they did with hP1R-WT; PTH(1-34) was 6,000-fold weaker with hP1R-delNt than with hP1R-WT. The most potent analog, [Ala(3,10,12),Arg(11), Trp(14)]rPTH(1-14)NH(2), stimulated inositol phosphate production with hP1R-WT. The results show that short NH(2)-terminal peptides of PTH can be optimized for considerable gains in signaling potency through modification of interactions involving the regions of the receptor containing the transmembrane domains and extracellular loops.

Amino-terminal modifications of human parathyroid hormone (PTH) selectively alter phospholipase C signaling via the type 1 PTH receptor: implications for design of signal-specific PTH ligands.

Parathyroid hormone (PTH) and PTH-related peptide (PTHrP) activate the PTH/PTHrP receptor to trigger parallel increases in adenylyl cyclase (AC) and phospholipase C (PLC). The amino (N)-terminal region of PTH-(1-34) is essential for AC activation. Ligand domains required for activation of PLC, PKC, and other effectors have been less well-defined, although some studies in rodent systems have identified a core region [hPTH-(29-32)] involved in PKC activation. To determine the critical ligand domain(s) for PLC activation, a series of truncated hPTH-(1-34) analogues were assessed using LLC-PK1 cells that stably express abundant transfected human or rat PTH/PTHrP receptors. Phospholipase C signaling and ligand-binding affinity were reduced by carboxyl (C)-terminal truncation of hPTH-(1-34) but were coordinately restored when a binding-enhancing substitution (Glu(19) --> Arg(19)) was placed within hPTH-(1-28), the shortest hPTH peptide that could fully activate both AC and PLC. Phospholipase C, but not AC, activity was reduced by substituting Gly(1) for Ser(1) in hPTH-(1-34) and was eliminated entirely by removing either residue 1 or the alpha-amino group alone. These changes did not alter binding affinity. These findings led to design of an analogue, [Gly(1),Arg(19)]hPTH-(1-28), that was markedly signal-selective, with full AC but no PLC activity. Thus, the extreme N-terminus of hPTH constitutes a critical activation domain for coupling to PLC. The C-terminal region, especially hPTH-(28-31), contributes to PLC activation through effects upon receptor binding but is not required for full PLC activation. The N-terminal determinants of AC and PLC activation in hPTH-(1-34) overlap but are not identical, as subtle modifications in this region may dissociate activation of these two effectors. The [Gly(1),Arg(19)]hPTH-(1-28) analogue, in particular, should prove useful in dissociating AC- from PLC-dependent actions of PTH.

Skeletal muscle afferent fibres release substance P in the nucleus tractus solitarii of anaesthetized cats.

1. The tachykinin substance P was recovered from the commissural subdivision of the nucleus tractus solitarii (cNTS) using in vivo microdialysis during activation of cardiorespiratory and skeletal muscle receptors in thirteen chloralose-anaesthetized cats. 2. Tetanic muscle contraction was evoked by stimulating L7-S1 ventral roots (n = 7). Electrically induced muscle contraction increased mean arterial pressure (MAP) by 55 +/- 10 mmHg and heart rate by 29 +/- 6 beats min-1. During contraction the dialysate concentration increased 154 % above resting control levels (from 0.217 +/- 0.009 to 0.546 +/- 0.023 fmol (100 microl)-1, control vs. contraction, P < 0.05). 3. Loss of cardiorespiratory input following disruption of the carotid sinus and vagus nerves significantly blunted, but did not abolish, the increase in substance P during muscle contraction (from 0.247 +/- 0.022 to 0.351 +/- 0.021 fmol (100 microl)-1, control vs. contraction, P < 0.05). Approximately 44 % of the substance P release during contraction was independent of cardiorespiratory input transmitted by carotid sinus and vagus nerves. 4. To determine the contribution of cardiorespiratory related neural input on substance P release, an intravascular balloon positioned in the thoracic aorta was inflated to increase arterial pressure (n = 6). Balloon inflation increased MAP by 50 +/- 5 mmHg and substance P increased from 0.251 +/- 0.025 to 0.343 +/- 0. 028 fmol (100 microl)-1 (control vs. balloon inflation, P < 0.05). This increase was completely abolished following interruption of vagal and carotid sinus nerves (from 0.301 +/- 0.012 to 0.311 +/- 0. 014 fmol (100 microl)-1, control vs. balloon inflation). This finding shows that neural input from cardiorespiratory receptors (primarily arterial baroreceptors) accounted for 37 % of the total substance P release during muscle contraction. 5. The findings from this study demonstrate that activation of skeletal muscle receptors and cardiorespiratory receptors (predominantly arterial baroreceptors) increases the extraneuronal concentration of substance P in the cNTS. Because substance P release was not completely abolished during muscle contraction following disruption of carotid sinus and vagus nerves it is proposed that: (1) afferent projections from contraction-sensitive skeletal muscle receptors may release substance P in the NTS; (2) neural input from muscle receptors activates substance P-containing neurones within the NTS; and (3) convergence of afferent input from skeletal muscle receptors and arterial baroreceptors onto substance P-containing neurones in the cNTS facilitates the release of substance P. The role of tachykininergic modulation of cardiorespiratory input is discussed.

Rapid resetting of carotid baroreceptor reflex by afferent input from skeletal muscle receptors.

Resetting of the arterial baroreflex is mediated by central (central command) or peripheral (exercise pressor reflex) mechanisms. The purpose of this study was to determine the effect of somatosensory input from skeletal muscle receptors on resetting of the carotid baroreceptor reflex. Resetting of the baroreflex was determined by measuring carotid sinus threshold pressure (Pth) during a ramp protocol that consisted of a linear increase in sinus pressure from 50 to 250 mmHg at approximately 3 mmHg/s. Experiments were performed in seven alpha-chloralose-anesthetized and vagotomized dogs. To determine the effect of skeletal muscle afferent input on resetting, electrically induced muscle contraction was used to activate mechanically and metabolically senstive afferent fibers, whereas passive stretch of the hindlimb was used to activate predominantly mechanically sensitive afferent fibers. Pth for heart rate (HR) and arterial blood pressure (BP) during the control ramp protocol was 110 +/- 4 and 118 +/- 7 mmHg, respectively. Electrically induced muscle contraction increased hindlimb tension (5.7 +/- 0.4 kg) and significantly increased Pth-HR and Pth-BP above control (135 +/- 6 and 141 +/- 5 mmHg, respectively; P < 0.05). Muscle paralysis prevented the increase in Pth-HR and Pth-BP during ventral root stimulation (104 +/- 7 and 116 +/- 5 mmHg, respectively; P = not significant). Passive muscle stretch (n = 3) increased hindlimb tension (5.9 +/- 0.9 kg) and significantly increased Pth-BP (125 +/- 21 vs. 159 +/- 16 mmHg, control vs. contraction; P < 0.05). There was no difference in the magnitude of Pth resetting between muscle contraction or stretch. The present study demonstrates that activation of skeletal muscle afferent fibers, by either muscle contraction or stretch, increases Pth of the carotid baroreflex. Therefore, neural input from skeletal muscle receptors resets the carotid baroreflex in a manner similar to that ascribed by central command.

Interaction between carotid baroreflex and exercise pressor reflex depends on baroreceptor afferent input.

Because arterial baroreceptor and skeletal muscle receptor afferents project to cardiovascular regions in the lower brain stem such as the nucleus tractus solitarii (NTS), it is likely that the level of baroreceptor afferent input will modify the excitatory cardiovascular responses evoked by contraction-sensitive skeletal muscle afferents. The purpose of this study was to determine the effect of carotid sinus baroreceptor afferent input (CSA) on reflex heart rate (HR) and mean arterial pressure (MAP) responses evoked by activation of skeletal muscle receptor afferents (SMA). CSA input was servo controlled at three levels of carotid sinus pressure using the isolated carotid sinus preparation, and SMA input was varied by induced muscle contraction (L7-S1 ventral root stimulation) or passive muscle stretch. Experiments were performed in alpha-chloralose-anesthetized and vagotomized dogs (n = 9). When CSA input was low (106 +/- 35 mmHg), electrically induced muscle contraction increased HR and MAP (30 +/- 8 beats/min and 42 +/- 12 mmHg, respectively, P < 0.05). However, when CSA input was high (221 +/- 9 mmHg), the reflex changes in HR and MAP during muscle contraction were attenuated (6 +/- 4 beats/min and 18 +/- 4 mmHg, respectively, P < 0.05). Similarly, the sympathoexcitatory responses evoked by passive muscle stretch were attenuated in a baroreceptor-dependent manner. These results suggest that changing CSA input from low (106 mmHg) to high (221 mmHg) shifts the interaction from facilitation to inhibition. Therefore, it is concluded that the nature of the interaction (i.e., facilitation or inhibition) between the baroreflex and the exercise pressor reflex is dependent on the level of baroreceptor input. Moreover, our findings substantiate early studies showing that the level of afferent input from arterial baroreceptors is a powerful modulator of sympatho-excitation evoked by mechanically and metabolically sensitive skeletal muscle receptors.

Central interaction between carotid baroreceptors and skeletal muscle receptors inhibits sympathoexcitation.

To determine the potential of an inhibitory interaction between the carotid sinus baroreflex (CSB) and the exercise pressor reflex (EPR), both pathways were activated to produce sympathoexcitation. It was hypothesized that, under conditions when the baroreflex increased sympathetic outflow, the interaction between CSB and EPR would be inhibitory. Bilateral carotid occlusion (BCO), electrically induced muscle contraction (EMC), and passive muscle stretch (PMS) were used to evoke sympathoexcitation. BCO decreased sinus pressure 50 +/- 5 mmHg, and the levels of muscle tension generated by EMC and PMS were 7 +/- 2 and 8 +/- 1 kg, respectively. This resulted in significant increases in mean arterial pressure (MAP) of 55 +/- 9, 50 +/- 7, and 50 +/- 6 mmHg (P = not significant, BCO vs. EMC vs. PMS) and in heart rate (HR) of 7 +/- 2, 19 +/- 4, and 17 +/- 2 beats/min (P < 0. 05, BCO vs. EMC and PMS). When BCO was combined with EMC or PMS, the reflex increase in MAP was augmented (80 +/- 8 and 79 +/- 10 mmHg; BCO+EMC and BCO+PMS, respectively; P < 0.05). However, summation of the individual MAP responses was greater than the response evoked during coactivation (106 +/- 11 and 103 +/- 12 mmHg, respectively, P < 0.05). Because summing the individual blood pressure responses exceeded the response during coactivation, the net effect was that the CSB and EPR interacted in an occlusive manner. In contrast, summation of the individual chronotropic responses was the same as the response evoked during coactivation. Moreover, there was no difference in summation of the individual MAP or HR responses when muscle afferents were activated by either EMC or PMS. In conclusion, the interaction between the CSB and the EPR in control of MAP was occlusive when both reflexes were stimulated to evoke sympathoexcitation. However, summation of the reflex changes in HR was simply additive.

Reduction in arterial compliance alters carotid baroreflex control of cardiac output in a model of hypertension.

Baroreflex regulation of cardiac output is determined by the performance of the heart as well as the available blood flow returning to the heart (i.e., venous return). We hypothesized that a decrease in arterial compliance (C(a)) would affect carotid baroreflex control of cardiac output by altering the slope of the venous return curve (VR curve). Baroreflex control of systemic arterial pressure (Pa), central venous pressure (Pv), heart rate, cardiac output (CO), and peripheral vascular resistance (R) were determined during bilateral carotid occlusion (BCO) in spontaneously hypertensive (hypertensive, HT) and Sprague-Dawley (normotensive, NT) rats. C(a) was determined from the rate of arterial pressure decay when CO was transiently stopped, and the VR curve was obtained during graded inflation of a vascular balloon positioned in the right atrium. The inverse slope of the VR curve was used as an index of the resistance to venous return (RVR). The baseline slope of the VR curve was -50.5 +/- 3.3 vs. -35.5 +/- 2.6 ml.kg-1.min-1.mmHg-1 in NT vs. HT, respectively (P < 0.05). Control values of Pa (96 +/- 5 vs. 124 +/- 8 mmHg) and R [0.43 +/- 0.04 vs. 0.80 +/- 0.07 peripheral resistance units (PRU)] were reduced in NT, whereas Ca (0.062 +/- 0010 vs. 0.036 +/- 0.003 ml.kg-1.mmHg-1) was elevated in NT vs. HT, respectively (P < 0.05). Analysis of the pressure dependence of C(a) demonstrated that C(a) was a nonlinear function of Pa, and the exponential decay constant for the C(a)-Pa relationship was reduced in HT (0.0055 +/- 0.0012 vs. 0.0012 +/- 0.0002 min, NT vs. HT, P < 0.05). Baroreflex activation by BCO significantly increased Pa (delta Pa, 20 +/- 4 vs. 28 +/- 3 mmHg) and R (delta R, 0.16 +/- 0.04 vs. 0.24 +/- 0.06 PRU) in NT vs. HT, respectively. However, BCO significantly decreased CO in NT but not HT (delta CO, -24 +/- 5 vs. -4 +/- 6 ml.kg-1.min-1, P < 0.05). In NT, RVR was increased 39 +/- 9% during BCO (P < 0.05), whereas RVR increased 8 +/- 3% in HT (P = NS). From these findings, we conclude that the difference in baroreflex control of CO is mediated, in part, by the reduction in C(a), which minimized the baroreflex-evoked increase in RVR.