Changes in R-R and Q-T Intervals Following Cardiac Vagotomy in Neonatal Swine

Asymmetric innervation of the myocardium, especially a predominance of sympathetic innervation, may establish conditions whereby electrical instability could result. Using a swine animal model, we studied the effect of right cardiac vagal denervation on the variability of R-R and Q-T intervals. Newborn pigs were assigned randomly to two groups: sham-operated controls (C), or denervation of the right cardiac vagus nerve (RCVX). EKGs were recorded weekly until the two groups exhibited significant heart rate differences. Analysis of the EKG included measurements of R-R and Q-T intervals and corrected Q-T intervals (QTc). Poincaré plots were used to display age-related differences in R-R and Q-T intervals. For RCVX animals, decreased QTc and R-R intervals were noted at 6 and 7 weeks after denervation, respectively. Unexpectedly, one RCVX animal exhibited marked sudden pauses in sinus rhythm. These data indicated that reduced vagal cardiac modulation during development might alter cardiac electrical stability in conscious swine.

Autonomic regulation of cardiovascular function in neonates

The degree of participation of the vascular bed supplied by the superior mesenteric artery in autonomic regulation of cardiovascular function during postnatal maturation was evaluated in piglets, ranging in age from birth to two months. Animals were anaesthetized with 0.25-0.5% halothane in a 50% mixture of N2O and O2, paralysed with decamethonium bromide and artificially ventilated to maintain normal arterial blood gases and pH. Mesenteric arterial flow was recorded continuously with an electromagnetic flow transducer; simultaneously aortic pressure, heart rate, and femoral, renal and carotid flows were monitored. Resistance was calculated as the ratio of mean aortic pressure to mean flow. The central neural regulation of the cardiovascular system was altered: (1) by electrical stimulation of medullary vasoactive sites, (2) by changing the input from visceral or somatic afferents and (3) by subjecting the animals to stress, such as haemorrhage or hypoxia. Postnatal development of adrenergic mechanisms was studied with the aid of adrenergic agonist both in vivo and in vitro (helical strips cut from superior mesenteric artery were placed in an organ bath for isometric tension recordings). Recordings of spontaneous efferent splanchnic discharge permitted a more direct examination of autonomic regulation of the mesenteric vascular bed. The results indicate that the vascular bed supplied by the superior mesenteric artery and innervated by the efferent splanchnic nerve is actively involved in the cardiovascular responses to alterations in the cardiovascular regulatory system. Furthermore, this vascular bed may have an inappropriate response to both feeding and stress such that pathological changes in the gastrointestinal tract may result.

Rhythmicities in sympathetic discharge: a signal of cardiorespiratory integration in developing animals

We have been pursuing various avenues of investigation to elucidate the postnatal maturation of neural regulation of cardiovascular and respiratory integration. In this paper we present our results from a systematic analysis of age-related modulations of sympathetic (SYMP) activity with respect to experimental alterations in baroreceptor afferent inputs. The three age groups of piglets were chosen based on different responses to a complex stimulus, i.e., the Valsalva maneuver. Postnatal maturation of SYMP activity was examined by spectral analysis of SYMP discharge using cross-power, full and partial coherence. Three general oscillations were observed in spontaneous SYMP discharges in the 0-30 Hz range. We divided that range into five frequency bands (0-2, 2-6, 6-12, 12-20, 20-30 Hz), which included periodicities in phase with both central respiratory activity and the cardiac cycle. Spectral analyses of SYMP activity after either baroreceptor activation (phenylephrine) or deactivation (nitroprusside) revealed that respiratory modulation was age-related across all frequencies while baroreceptor modulation was usually age-related within three of the five frequency bands. These results lead to questions concerning the possible role of the autonomic nervous system and/or central interactions between the respiratory and SYMP rhythm generators in the etiology of sudden infant death syndrome (SIDS).

Electrocardiographic changes during postnatal development in conscious swine with cardiac autonomic imbalance

Using a conscious swine model, we studied the effects of different patterns of cardiac autonomic denervation on alterations of R-R and Q T intervals for 8 postnatal weeks. Newborn pigs were assigned randomly to four different groups: sham-operated controls (C), stellate ganglion ablation (SGX), either left (LSGX) or right (RSGX), and the right cardiac vagus nerve (RCVX) transection. The ECGs were recorded by telemetry while animals rested quietly or were judged behaviorally to be asleep. Analyses of the ECG included measurements of R-R and Q-T intervals, as well as corrected Q-T intervals (QTc). Poincaré plots were used to display age-related differences in R-R and Q-T intervals. For stellectomized animals, significantly prolonged R-R intervals were first observed at post-surgical week 3 in the RSGX group and at week 5 in the LSGX group. Significantly prolonged QTc was found only in the RSGX group. In the RCVX group, shortened QTc and R-R intervals were noted at 6 and 7 weeks after denervation. Furthermore, three of six RSGX animals (50%) and one of four RCVX animals (25%) exhibited marked pauses in sinus rhythm that were unrelated to changes in heart rate or to sinus arrhythmia. These results in conscious animals support our hypothesis that abnormal autonomic innervation of the heart during maturation, e.g., withdrawal of vagal cardiac modulation or asymmetry of sympathetic innervation, impairs cardiac electrical stability.

CO(2)-induced expression of c-fos in the nucleus of the solitary tract and the area postrema of developing swine

This investigation was performed to determine whether hypercapnic exposure elicited expression of the c-fos protooncogene product, FOS, in nucleus of the solitary tract (NTS) and area postrema (AP) neurons of developing swine. Mean arterial blood pressure (MAP) and heart rate (HR) were also monitored to evaluate whether numbers of neurons containing FOS were related to changes of MAP and HR. In each experiment, two litter-matched piglets were prepared simultaneously, i.e., Saffan anesthesia, paralysis, and artificial ventilation (100% O(2)). One animal was exposed to hypercapnia (1 h of 10% CO(2), balance oxygen), while the other continued to breathe 100% O(2). Animals were studied at three different ages: 5-8 days, 13-15 days, and 26-34 days old. In the NTS, FOS expression was prominent in regions corresponding to the general visceral afferent subdivision; the AP showed no such topographic distribution. The number of NTS and AP neurons with FOS in hypercapnic-exposed animals was significantly greater than those of unexposed animals. However, an age-related increase of FOS was observed only for NTS neurons, with the greatest number observed in 13- to 15-day-old animals. Increases of MAP, not HR, were noted during the early part of hypercapnia in the 5- to 8-day-old group; older animals exhibited no change of MAP. Our findings demonstrated that prolonged hypercapnic stimulation elicited FOS expression in AP and NTS neurons of developing animals, and that such expression was non-uniform, depending upon the region studied.

The area postrema of newborn swine is activated by hypercapnia: relevance to sudden infant death syndrome?

This study was performed to investigate a role of the neonatal area postrema (AP) in the chemoreceptor response to hypercapnia which is defective in sudden infant death syndrome (SIDS). AP responses to CO2 inhalation were monitored in 1 to 5 week old piglets by mapping neurons that were induced to express the c-fos gene product, Fos--a marker of functional activation. Interpretive confounds were minimized by controlling for hypoxia, the effects of surgical procedures and ambient environmental stressors on neuronal activity (c-fos expression). The AP demonstrated a powerful and reproducible response in neonatal swine breathing 10% CO2 for 1 h. Intensely immunolabeled nuclei were detected throughout the longitudinal extent of the circumventricular organ, and were especially heavily concentrated at rostral levels proximal to obex. Quantitative analysis verified statistically significant increases in numbers of cells that were induced to express Fos-like immunoreactivity (FLI) in the AP of CO2- stimulated piglets as compared to control groups. No detectable age-related differences were observed in AP response patterns. Conclusions. The AP responds to hypercapnic stress in the newborn piglet. A mature circumventricular organ response in the neonate may be crucial in defending against common environmental stressors, such as nicotine exposure--an emetic agent acting via the AP and a major risk factor in SIDS. Hence, a defect of the AP or its network may underlie a loss of state-dependent controls over cardiopulmonary reflex function in SIDS.

Presence of a non-NMDA glutamate receptor subtype in the sympathetic nervous system of neonatal swine

For the first time, the GluR-1 subtype of AMPA receptor was identified in the sympathetic nervous system of neonatal swine, an animal model of human development and heart disease. The rationale was to seek evidence of a role ascribed to glutamate in cardiorespiratory regulation in the laboratory rat. The receptor was demonstrated with the avidin-biotin immunoperoxidase technique by using an affinity-purified polyclonal antibody judged to be specific to Glu-R1 in several species. Glu-R1 immunoreactivity was regionally distributed in the thoracic spinal gray, and present intracellularly in neurons and within the surrounding neuropil. Sympathetic preganglionic neurons in the intermediolateral cell column of upper and lower thoracic spinal segments were intensely labeled and surrounded by labeled neuropil. High concentrations of Glu-R1 distinguished laminae II: substantia gelatinosa and the outer region of lamina III. Laminae I and V of the dorsal horn but not IV contained immunolabeled neurons. Arrays of moderately immunoreactive perikarya extended from an intermediate zone of laminae VII to the central gray. Glia and perivascular processes were not labeled, confirming previous observations [Tachibana, M., Wenthold, R.J., Morioka, H., Petralia, R.S., 1994. Light and electron microscopic immunocytochemical localization of AMPA-selective glutamate receptors in the rat spinal cord. J. Comp. Neurol. 344, 431-454]. Neuronal staining patterns corroborated evidence in rats indicating a postsynaptic localization of Glu-R1 associated with plasma membranes and cytoplasmic organelles [Martin, L.J., Blackstone, C.D., Levey, A.I., Huganir, R.L., Price, D.L., 1993. AMPA glutamate receptor subunits are differentially distributed in rat brain. Neuroscience 53, 327-358.; Rubio, M.E., Wenthold, R.J., 1997. Glutamate receptors are selectively targeted to postsynaptic sites in neurons. Neuron 18, 939-950]. Our data predict a role for L-glutamate in postnatal development of cardiorespiratory reflexes in swine.

Emergence of lung-inflation-related sympathetic nerve activity in spinal cord transected neonatal swine

Sympathetic (SYMP) nerve activity in spinal intact neonatal swine is comprised of prominent bursts reflecting modulation by supraspinal structures involved in shaping central respiratory and baroreceptor activity. After spinal cord transection (SCT), we found no evidence of such modulation. SYMP activity was now related to the ventilatory cycle, exhibiting bursts only during lung inflation. Such activity suggests the emergence of latent spinal circuits which may have the capacity to regulate cardiovascular activity.

Induction of c-fos gene expression by spinal cord transection in the rat

Sympathetic nerve activity is maintained after high spinal injury through circuits that remain in question. We evaluated patterns of c-fos gene induction as a monitor of spinal neurons responding to high spinal cord transection in the rat. Rats were anesthetized with isofluorane. Lower cervical or upper thoracic spinal segments were exposed, immersed in warm mineral oil and transected. Spinal cords were exposed but not transected in anesthetized controls. After 2.5 h, spinalized and control rats were perfused for immunocytochemistry. Cervical and thoracolumbar spinal segments and dorsal root ganglia were sectioned coronally. Tissues were incubated in primary, polyclonal antisera raised in rabbit or sheep against a peptide sequence unique to the N-terminal domain of Fos, and processed immunocytochemically. Neurons were induced to express Fos-like immunoreactivity (FLI), bilaterally, in the spinal gray, but not in primary sensory ganglia. Spinal cord transection induced neurons to express FLI in thoracic laminae I, IIo (outer substantia gelatinosa), Vre (lateral reticulated division), VII (lamina intermedia) and X, and the intermediolateral cell column. Lamina VIII was also labeled in spinal-injured but not in control animals. Immunolabeled nuclei were prominent in lumbar segments and were concentrated in the medial third of laminae I and IIo, and in laminae VII and X. Few cells were labeled in upper cervical or sacral segments. FLI was sparse in the spinal gray of controls and expressed mainly within the dorsal root entry zone of upper thoracic segments. Patterns of c-fos gene expression were site-specific and correlated with laminae that respond predominantly to noxious stimulation and that contain sympathetic interneurons. Laminae that are responsive to non-noxious stimuli and activated by walking, IIi, nucleus proprius, medial V and layer VI were not induced to express FLI. We conclude that neurons in specific spinal laminae that process high threshold afferents and that harbor neurons with sympathetic nerve-related activity are activated selectively by spinal cord transections. We hypothesize that peripheral afferents processed by spinal-sympathetic circuit neurons may regulate sympathetic discharge in the absence of supraspinal drive.

Preliminary evidence for the evolution in complexity of heart rate dynamics during autonomic maturation in neonatal swine

Previous studies suggest that the autonomic nervous system plays an important role in the generation of complex heart rate dynamics. Therefore, we hypothesized that the complexity (irregularity) of cardiac interbeat intervals would evolve with the maturation of autonomic innervation to the heart. Twelve healthy newborn piglets were implanted with ECG transmitters and studied at one or more different ages up to 33 days of age, the period during which pigs develop functional sympathetic innervation of the heart from the stellate ganglia. Three animals underwent right stellate ganglionectomy, two a left stellate ganglionectomy, two a right cardiac vagotomy and five a sham procedure. The statistic, approximate entropy (ApEn), was used to quantify the regularity of interbeat interval fluctuations. Sham-operated animals showed an increase in the standard deviation (SD) and irregularity (ApEn) of cardiac interval fluctuations with increasing age. Right stellate ganglionectomized piglets had lower interbeat interval ApEn values, but similar SD's by 26-27 days of age compared to sham-operated animals. Left stellate ganglionectomy, which affects cardiac inotropy rather than chronotropy, had no effect on cardiac interval irregularity, while vagotomy had an indeterminant effect. The increasing irregularity of interbeat interval dynamics during autonomic maturation and the apparent attenuation of heartbeat irregularity when right stellate ganglion innervation is interrupted, provides empirical support for the notion that complex heartbeat dynamics in the mature animal are the result of a network of autonomic neural pathways that enables an organism to adapt to stress.

Induction of c-fos gene expression by spinal cord transection in Sus scrofa

Functional responses of primary sensory afferents and spinal cord were monitored in swine subjected to a high cervical (C1) spinal transection. Two and a half hours after transection, dorsal root ganglia and cervical and thoracolumbar spinal segments were processed immunocytochemically for the c-fos gene product, Fos and related antigens. In spinal-transected animals, Fos-like immunoreactivity (FLI) was induced in spinal laminae I, V, VII and X and the intermediolateral cell column but not in sensory ganglia as compared to controls: spinal-intact age-matched littermates. Spinal laminae expressing FLI harbor sympathetic and somatic interneurons and may aid in maintaining sympathetic outflow.

Anatomy of medullary and peripheral autonomic neurons innervating the neonatal porcine heart

Gross and microscopic anatomical investigations were carried out in 14 piglets aged from 4 to 66 days. True Blue (7-50 microliters) and Diamidino Yellow (7-50 microliters) were injected individually into 2 different cardiac sites (the right atrial ganglionated plexus, the inferior vena cava, inferior atrial ganglionated plexus, the right atrium or the right ventricle). Gross anatomy: Globular superior cervical and nodose ganglia, elongated stellate ganglia, multiple small middle cervical ganglia and multiple small mediastinal ganglia along the course of cardiopulmonary nerves were identified. Microscopic anatomy: Neurons innervating specific cardiac regions or intrinsic cardiac ganglionated plexuses were distributed relatively evenly among stellate (primarily in their cranial poles) and middle cervical ganglia bilaterally, fewer labeled neurons being located in the superior cervical and mediastinal ganglia bilaterally. Parasympathetic efferent preganglionic neurons associated with either intrinsic cardiac ganglionated plexus studied were identified primarily throughout the ventrolateral region (the external formation) of the nucleus ambiguus bilaterally. Labeled neurons were also identified throughout the right and left nodose ganglia. Individual neurons did not project axons to different cardiac regions, as no double-labeled neurons were identified. No correlation between age and the numbers and locations of labeled neurons was apparent. Thus, porcine sympathetic efferent neurons which innervate individual cardiac regions, including intrinsic cardiac ganglionated plexuses, lie scattered primarily throughout the right and left mediastinal and middle cervical ganglia as well as the cranial poles of stellate ganglia at birth, apparently changing little during the first 2 months of age. Porcine cardiac parasympathetic efferent preganglionic neurons are located primarily in the external formation of the nucleus ambiguus bilaterally at birth. The numbers of afferent cardiac neurons distributed throughout the nodose ganglia bilaterally also change little during that time. It is concluded that most of the autonomic neurons which innervate the heart are in place at birth.

Effects of cardiac autonomic imbalance on postnatal changes in the electrocardiographic Q-T interval in conscious swine

The effects of partial autonomic denervation of the heart rate and Q--T interval were examined during maturation in swine. Four groups of newborns were prepared: right stellate ganglionectomy (RSG), left stellate ganglionectomy (LSG), right cardiac vagotomy (RCV) and sham-operated. Swine were studied postsurgically for eight weeks. Unexpected deaths of unknown cause occurred in five of the 20 denervated swine but in none of the 10 sham-operated controls. Prolongation of the Q--T interval was greatest in RSG animals and least in RCV as compared to controls. The results indicate that we have successfully developed an animal model of a long Q--T syndrome in swine.

Age-related effects of cardiac sympathetic denervation on the responses to cardiopulmonary receptor stimulation in piglets

The effects of right stellate ganglionectomy (RSG) and bilateral stellate ganglionectomy (BSG) on cardiovascular responses to phenyl biguanide (PBG, 80 micrograms/kg) were studied in 1- and 8-wk-old piglets. Animals were anesthetized with Saffan, paralyzed, thoractomized, and ventilated with 100% O2. Recordings of the ECG (lead II) and aortic pressure (AoP) were used to compute the maximum R-R interval, heart rate (HR), and mean AoP, and to determine the occurrence of atrioventricular conduction block (AVB). Right atrial injections of PBG in 1-wk-old piglets elicited AVB as well as decreases in AoP and HR in all animals; this response pattern was not altered by either RSG or BSG. The PBG response of neurally intact 8-wk-old animals was comprised of a decrease of HR without change in AoP; AVB occurred in three of six animals. After RSG or BSG, AoP decreased along with decreased HR, and now AVB occurred in all animals; changes of AoP and maximum R-R interval were greater after BSG than after RSG. These results suggest that the stellate ganglia exert a neuroprotective influence on cardiovascular function, requiring some degree of maturation for expression. Our findings support the hypothesis that an imbalance of cardiac autonomic innervation favoring parasympathetic activity may produce immature responses to cardiopulmonary afferent stimulation in older maturing animals.

Hypotension-induced expression of the c-fos gene in the medulla oblongata of piglets

Neural networks that mediate the reflex response to baroreceptor withdrawal were explored in Sus scrofa. Induction of c-fos was used as a monitor of synaptic activity in response to hypotension sustained by systemic administration of a peripheral vasodilator, sodium nitroprusside. Patterns of c-fos gene expression were compared between Saffan-anesthetized experimental animals and age-matched normotensive controls administered vehicle. Effects of other variables were controlled including 1 h preoperative accommodation to the novel environment, anesthesia, blood gases and pH. Identical post-stimulus survival periods were allowed for accumulation of transcript. The c-fos protein, Fos, was identified immunocytochemically with two rabbit antisera raised against amino acids 1-131 of Fos or residues 4-17 of synthetic human transcript. Fos was identified in catecholaminergic neurons labeled with an antiserum to tyrosine hydroxylase (TH). Fos was induced in the nucleus tractus solitarii (NTS) of hypotensive piglets. Neurons encoding Fos matched projection patterns of first order visceral afferents. Induction was prominent in the dorsolateral nucleus coinciding with the baroreceptor field. Indices of increased neuronal activity were evident in other baroreceptor terminal sites, e.g., medial subnucleus, the medial commissural field, the intermediate subnucleus and a ventral A2 noradrenergic area. In reticular formation c-fos protein was induced in circumscribed columns in the lateral tegmental field (LTF) extending from facial nucleus to calamus scriptorius. Catecholaminergic (TH-positive) neurons expressed Fos in the porcine C1 and A1 areas of ventrolateral medulla. Fos was also induced in a dorsal intermediate reticular zone of LTF. Minor or inconsistent differences between experimental and control were observed in nucleus raphe pallidus, rostral paramedian reticular formation, upper thoracic intermediolateral cell column, and stellate ganglia. In conclusion, baroreceptor withdrawal in young animals induced patterns of neuronal response along established cardiovascular reflex pathways.

The presence of coherence in sympathetic and phrenic activities in a developing mammal

To determine whether are development changes in the baroreceptor and central respiratory modulation of sympathetic activity, we used ordinary and partial coherence spectral analyses on cervical and splanchnic sympathetic activity in swine 1-36 days old. Removal of baroreceptor influences from cervical sympathetic and splanchnic spectra using partialization shows that 3-6 Hz peaks are due to baroreceptors since coherence decreased in > 19 days old while remaining unchanged in < 2 weeks old piglets. The 8-12 Hz band (present in normal coherence after 21 days) was revealed in piglets < 14 days old after removal of respiratory modulation by partialization; similarly increased coherence was also observed in the 16-18 Hz band through 3 weeks. Thus, use of partial power and coherency is a useful tool for unmasking the complex relationship found in developing SYMP outflows. These results suggest that there is a period of reorganization within the SYMP rhythm generating circuits, which may be essential for normal development.

Postganglionic sympathetic discharge in neonatal swine

We hypothesized that cardiac and respiratory modulation of postganglionic peroneal activity appeared in an age-related manner. In anesthetized, paralyzed and artificially ventilated piglets, simultaneous recordings of efferent phrenic and peroneal discharges were obtained during hyperoxia (fraction of inspired oxygen, FiO2 = 1.0) and hypoxia (FiO2 = 0.1). Spectral analyses of peroneal and aortic blood pressure signals revealed peaks at the cardiac frequency (3.25-5.0 Hz). Coherence analysis showed that these two signals were highly correlated at those frequencies, providing evidence for baroreceptor entrainment. Statistically significant (p < 0.05) increases of coherence values were observed during hypoxic stimulation. Such results were observed in most animals despite age, and provided evidence of a potent mechanism for insuring vasomotor tone even in newborn animals. In contrast, spontaneous respiration-related peroneal discharges were observed only in animals > or = 20 d old. In animals < 20 d old, hypoxic stimulation elicited respiration-related discharges in peroneal activity. In many cases, peroneal hypoxic discharges exhibited an immature biphasic response pattern despite the presence of a mature response pattern of phrenic activity. Such findings suggest a developmental lag in the linkages of respiratory and sympathetic controlling networks.

Development of a swine animal model for the study of sudden infant death syndrome

An animal model with selective cardiac autonomic denervation was developed in neonatal swine for the purpose of future studies concerning the possible role of abnormal cardiac innervation in the etiology of sudden infant death syndrome. Twenty-six 4- to 8-day old piglets were randomly assigned to one of the following treatment groups: right stellate ganglionectomy (RSG), left stellate ganglionectomy (LSG), right cardiac vagotomy (RCV), and sham-operated control group. Piglets were anesthetized with Saffan and mechanically ventilated on room air. The thoracic wall was opened, and the sternum was divided, permitting exposure of pleura, heart, and lungs. After completion of one of these operative procedures, the thoracic wall was closed in layers. On successful recovery from anesthesia and extubation, piglets were returned to their sows. Postoperative administration of antibiotics was performed during the first week. Electrocardiograms were recorded once a week over an 8-week period. The sham-operated control group had a postnatal increase in QTc and decrease in heart rate. The RSG group had significantly longer QTc than the sham-operated control group QTc, whereas the RCV group had significantly shorter QTc interval than sham-operated controls. Surgical complications caused the death of two piglets (hemorrhage in one and aspiration in the other), infection caused the death of one piglet 2 weeks after surgery, and three died of unknown causes (one from the RSG group and two from the RCV group). The cause of death for these three animals could not be determined (necropsy was not revealing) because they were not monitored at the time of their demise.(ABSTRACT TRUNCATED AT 250 WORDS)

Spectral Analysis: A Tool for Study of Neonatal Sympathetic Systems

Preference for statistical information from frequency domain rather than time domain analysis becomes apparent when applied to complex neural signals. Further, we demonstrate how power spectral methods reveal significant changes in neonatal sympathetic activity.

Neuronal activity of the stellate ganglia in neonatal swine

In anesthetized, paralyzed, and artificially ventilated neonatal pigs, neuronal activity was recorded extracellularly from the intact right stellate ganglion. Of 301 investigated neurons, only a few had spontaneous activity: 27 neurons generated activity either during lung inflation or deflation, and 1 neuron generated activity related to the cardiac cycle. The remaining 273 neurons were activated by stroking of body hair or by light pressure applied to various somatic sites. These data suggest that much of the spontaneous activity in the stellate ganglia appear later during development.

Effects of GABAA receptor antagonism on inspiratory activities in kittens

In anesthetized kittens (< 1 to 24 days old), the effects of GABAA receptor antagonism on phrenic, hypoglossal and cervical sympathetic discharges were examined by i.v. bicuculline infusions during hyperoxia and hypoxia. Administration of bicuculline during hyperoxia produced marked increases in the amplitudes of inspiratory nerve discharges. During hypoxic stimulation (10% O2), the amplitudes of inspiratory activities decreased towards or below those observed during hyperoxia; bicuculline reversed this depression and restored inspiratory discharges. Our results indicated that GABAA receptors were functional shortly after birth, acting to mediate influences shaping inspiratory activity during hyperoxic breathing and during conditions of increased chemical drive.

Presumptive adrenergic neurons containing phenylethanolamine N-methyltransferase immunoreactivity in the medulla oblongata of neonatal swine

Given the importance of the swine (Sus scrofa) as an animal model for human development, physiology and disease, neurons containing the epinephrine-synthesizing enzyme, phenylethanolamine N-methyltransferase (PNMT), were mapped in the medulla oblongata of neonatal swine as a first step in identifying their roles in central autonomic control. Neurons were labeled immunocytochemically by using an antiserum to PNMT raised in rabbits against trypsin-treated enzyme purified from the bovine adrenal gland. The general regional organization of neurons expressing PNMT (-like) immunoreactivity (ir) in the neonatal swine was similar to data obtained in other species and, in some aspects, more closely resembled the pattern observed in the primate brain. Immunolabeled cells appeared to be more abundant and caudally more extensive than observed in other adult animals. PNMT-immunoreactive (ir) neuronal somata, however, were largely confined to the reticular formation in the ventrolateral quadrant and the nucleus tractus solitarii (NTS) and more restricted in distribution than those expressing tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (D beta H)-ir on serial transverse sections. A close correspondence was observed between the distributions of TH- and PNMT-ir neurons and processes throughout the C1 and C2 areas. However, in the C1 and C3 regions TH-ir neurons outnumbered those containing D beta H and PNMT-ir. In contrast, cell groups enriched in PNMT-ir neurons and processes were characterized by relatively weak D beta H-ir. In the ventrolateral medulla (VLM), PNMT-ir cell bodies were concentrated rostrally and extended from the caudal pole of the facial nucleus to a level posterior to the calamus scriptorius. The rostral VLM was characterized by an admixture of bipolar and multipolar primarily medium-diameter immunostained neurons. A prominent cell column (condensation) organized ventromedially to the nucleus ambiguus pars compactus (NAc). A loosely organized cluster bordered the lateral aspect of the special visceral efferent column; another smaller aggregate was located in the ventromedial reticular formation adjacent to the inferior olive. At middle medullary levels, PNMT-ir neurons formed two distinct subgroups (dorsal and ventral) interrupted by a band of precerebellar relay neurons that extended between the medial and lateral limbs of the lateral reticular nucleus of Walberg. At obex, the dorsal cell group formed a diagonal array and assumed a position dorsal and dorsolateral to the medial limb of LRN. This group was distinguished by bipolar neurons with axes of orientation directed perpendicularly to the majority of neurons in the rostal VLM or those lying near the caudal ventromedullary surface.(ABSTRACT TRUNCATED AT 400 WORDS)

Cardiac responses elicited by stimulation of loci within stellate ganglia of developing swine

Stimulation with bipolar electrodes of specific loci in stellate ganglia elicited in anesthetized piglets, 1-4 weeks of age, alterations in cardiac function and aortic pressure. Responses were also elicited by chemical stimulation in specific loci of these ganglia. The probability of eliciting a cardiovascular response by stimulating loci in a stellate ganglion increased with increasing postnatal age. For instance, no responses were elicited when loci in the left stellate ganglia of 1-week-old piglets were stimulated. Significant heart rate responses were obtained only when loci in right stellate ganglia were stimulated. The number of ganglionic loci from which cardiovascular responses were obtained increased with increasing postnatal age. It is concluded that the capacity of stellate ganglion neurons to modulate the cardiovascular system matures during the first four weeks of life, heart rate being modulated primarily by neurons in the right stellate ganglion and inotropism by neurons in both stellate ganglia.

Respiratory modulation of sympathetic activity in neonatal swine

Spontaneous efferent sympathetic activity (cervical sympathetic and splanchnic nerves) was recorded simultaneously with efferent phrenic discharge, with aortic pressure, and with the electrocardiogram in less than 1- to 51-day-old neonatal swine anesthetized with Saffan (alfaxalone). Power spectral analyses of sympathetic discharge revealed a frequency range of 4-36 Hz, with peaks distributed in four regions of the spectrum (4-6 Hz, 8-15 Hz, 16-22 Hz, and 28-36 Hz). Inspiratory modulation was readily apparent since sympathetic power spectral energy was greatest during the time of phrenic activity. Alterations in pulmonary afferent inputs (e.g., lung inflation tests and/or vagotomy) altered both phrenic and sympathetic activity. Use of the respiratory-cardiac modulation ratio indicated that respiratory modulation of sympathetic activity was greater than cardiac modulation. Complex afferent inputs, i.e., fictive Valsalva maneuver, indicated an immaturity of integration within the cardiovascular regulatory system. The adult patterns of blood pressure changes and heart rate responses were delayed until approximately 1 mo of age. Calculations of Valsalva ratios (ratio of maximal R-R interval to minimal R-R interval) indicated delayed onset of heart rate responses, reflecting the maturation of cardiovascular reflexes during the neonatal period.

Power spectral analysis of the baroreflex in neonatal swine

The baroreflex was observed in neonatal swine as young as 4 h of age. Bolus injections of Na nitroprusside (NP) and phenylephrine (PE), induced changes in blood pressure and elicited changes in both heart rate and in cervical sympathetic and splanchnic discharge; changes in sympathetic discharge were reflected in altered power spectral magnitude. Measures of heart rate showed that the magnitude of the PE-induced decreases was positively correlated with increasing postnatal age. The results indicate that the baroreflex, as indicated by changes in sympathetic discharge and heart rate, is present in early neonatal swine.

Effects of elevated plasma magnesium concentration on cerebrospinal fluid levels of magnesium in neonatal swine

To determine whether magnesium (Mg) can cross the blood brain barrier in developing swine, simultaneous measurements of [Mg] in plasma and cerebrospinal fluid (CSF) were made during experimental elevation of plasma [Mg] in 12 swine of differing postnatal age. All were anesthetized with Saffan and maintained at normal arterial blood gas composition. Aortic pressure and heart rate were monitored. Plasma and CSF samples, drawn at the beginning and end of a 60-min intravenous infusion of MgCl2 in all animals and every 10 min during the infusion in three, were analyzed for [Mg] and osmolality. CSF [Mg] increased in all animals as plasma [Mg] increased. There were no changes in CSF osmolality. The differences between plasma and CSF [Mg] was smallest in the youngest animals. These results indicate that Mg crosses the blood brain barrier in neonatal swine and suggest that the blood brain barrier is still maturing within the first postnatal month.

Effects of anesthesia on efferent phrenic activity in neonatal swine

The effects of two levels of Saffan anesthesia (standard recording level: 2-4 mg/kg/min, and 10X recording level) and a single level of pentobarbital (5 mg/kg) on the power spectral density of efferent phrenic discharge were investigated in piglets aged from less than 1 day to 50 days. The phrenic high frequency oscillation (HFO) was present in decerebrate, unanesthetized piglets and in piglets anesthetized with Saffan, albeit reduced at 10 times recording level, but was absent under pentobarbital. The results indicate that Saffan does not have a significantly depressant effect on the phrenic HFO in developing swine.

Age-related changes in power spectra of efferent phrenic activity in the piglet

Power spectral analysis of phrenic nerve discharge in neonatal swine revealed the presence of both high-frequency oscillations (HFO) (95-150 Hz) and medium-frequency oscillations (MFO) (15-35 Hz). The HFO was shown to be age-related; the MFO was not. The data indicated that at least one manifestation of maturation of the respiratory rhythm generator is the increase with age of the frequency of the HFO.

Autoregulatory escape from vasoconstriction of intestinal circulation in developing swine

The capability of the developing intestinal circulation to maintain a vasoconstrictor response during postganglionic adrenergic nerve stimulation or norepinephrine infusion was examined in 34 swine aged 6 h to 2 mo anesthetized with pentobarbital sodium. Aortic and portal venous pressures, electrocardiogram (ECG), and blood flow (F) through the superior mesenteric artery were recorded, and intestinal vascular resistance (MVR) was calculated as mean pressure difference per mean F. Baroreceptor reflex inhibition by bilateral occlusion of the carotid arteries increased MVR, section of the splanchnic nerve and postganglionic fibers decreased MVR, and short-latency F decreases were obtained during mesenteric nerve stimulation (MNS). Latencies for the decreases in F shortened with age and with increasing MNS frequency (5-17 Hz) at any age. Prolonging MNS for 60 s at 10 or 12 Hz led to sustained high MVR in 6-h to 7-day-old animals; however, MVR decreased toward control before the end of the 60-s MNS period in animals 1 to 2 mo old. Intra-arterial infusion of norepinephrine (0.5 microgram X kg-1 X min-1) decreased F and increased MVR in all animals; but by 5 min of infusion, F was returning toward control level in all but the youngest. This demonstration that the least mature intestinal circulation was least capable of autoregulatory escape from vasoconstriction provides further evidence of its functional immaturity.

Age-related superior mesenteric arterial flow changes in piglets: effects of feeding and hemorrhage

Cardiovascular (CV) responses to feeding before and after 15% blood volume hemorrhage (H) were studied in lightly anesthetized piglets less than or equal to 2-days and greater than 2-wk-old. Superior mesenteric (Mes) and renal (Ren) arterial flows (F) were registered by electromagnetic probes before and continuously for 2 h after milk feeding (26 ml/kg) by gavage. Postprandially, in piglets less than or equal to 2-days-old MesF tended to increase (maximum change, mean +/- SE, 16.0 +/- 4.7%) at 30 min, whereas Mes vascular resistance (R) significantly decreased at 30, 90, and 120 min (17.3 +/- 6.2%). In piglets greater than 2-wk-old, MesF significantly increased by 30 min, which lasted 120 min (37.7 +/- 11.7%); MesR decreased by 20.0 +/- 5.8% at 90-120 min. Compared with the less than or equal to 2 days olds, the older piglets demonstrated greater and more sustained postprandial MesF increase. After H, regional F and pulse pressure (PP) decreased, heart rate and R increased in both groups. After stabilization, feeding induced insignificant CV changes in less than or equal to 2-day-old piglets. In contrast, increased MesF (44 +/- 14.4%) and PP (24 +/- 8.5%) and decreased MesR (31 +/- 8.9%) were observed in the greater than 2-wk-olds. MesF changes differed significantly between the age groups. The Ren vascular bed showed no consistent response in both age groups. Thus Mes vascular responses to feeding with or without preceding H in developing piglets were age related.

Age-related responses to stimulation of cardiopulmonary receptors in swine

Cardiovascular responses to stimulation of cardiopulmonary receptors were evaluated in standard breed developing swine (1-2 days, 1-2 wk, and 2 mo of age) and sexually mature miniswine anesthetized with 0.25-0.50% halothane in O2. Cryptenamine, a mixture of veratrum viride alkaloids (VVA), was administered as right atrial or left ventricular bolus injections in doses of 5-20 micrograms/kg. In developing swine, mean aortic pressure (AoP) decreased after 10-20 micrograms/kg VVA in 1- to 2-wk olds and after 5-20 micrograms/kg VVA in 2-mo olds. Bradycardia was always elicited. Renal (Ren) and femoral (Fem) vasodilation occurred in the 1- to 2-wk-old and 2-mo-old groups after 5-20 micrograms/kg VVA. Mesenteric (Mes) vasodilation was elicited with 10-20 micrograms/kg VVA in the 1- to 2-wk-old group and with 5-20 micrograms/kg VVA in 2-mo olds. In the 1- to 2-day-old group, VVA did not significantly alter Ren, Fem, and Mes vascular resistance. In mature miniswine, 20 micrograms/kg VVA elicited decreases in AoP and heart rate that were similar in magnitude to responses obtained with only 5 micrograms/kg VVA in dogs. Cardiovascular responses to VVA were abolished after bilateral vagotomy but were not altered after denervation of the carotid sinuses. The results indicate postnatal maturation of the Bezold-Jarisch reflex in swine.

Circulatory effects of splanchnic nerve stimulation in developing swine

The relative maturity of immediate intestinal circulatory responses to efferent splanchnic nerve stimulation for 20 s was tested in 34 piglets (1 day-1 mo old) under pentobarbital anesthesia (15-30 mg/kg). Aortic pressure, heart rate, and superior mesenteric arterial flow (measured by electromagnetic flow transducers) were recorded simultaneously. Intestinal vascular resistance was calculated as mean aortic pressure/mean arterial flow. Resistance increased in most animals during experimental inhibition of the carotid sinus baroreceptor reflex. Transection of the splanchnic nerve decreased mesenteric resistance in all animals. Nerve stimulation at 2 Hz was not effective in all of the youngest animals; at 3 Hz, mesenteric resistance increased in all animals. The latency of this response was shorter at each increase in stimulation frequency. The aortic pressure rise associated with splanchnic nerve stimulation was greater with each increase in frequency. This rise was attenuated or absent after adrenalectomy and during postganglionic fiber stimulation. The vasoconstrictor effect of splanchnic nerve stimulation was attenuated after alpha-adrenergic receptor blockade with phentolamine in a dose (0.25-0.75 mg/kg) that blocked vasoconstrictor effects of norepinephrine (0.5 micrograms/kg). We conclude that alpha-adrenergic mechanisms are functional, although not mature, and that sympathetic vasoconstrictor tone is present in the intestinal circulation of swine at birth.

Cardiovascular responses to arterial and venous hemorrhage in neonatal swine

The cardiovascular effects of graded arterial or venous hemorrhage were evaluated in developing swine (less than or equal to 1 day, 2-5 days, 1 wk, and 2 wk of age) anesthetized with halothane in 50% N2O-50% O2. Serial 5-ml/kg aliquots of arterial or venous blood were removed at 3- to 4-min intervals to a cumulative total of 20 ml/kg. Tachycardia occurred in most animals. Decreases in aortic pressure to arterial, but not to venous, hemorrhage were age dependent. Renal, femoral, and carotid arterial flows decreased with hemorrhage in all animals; the decreases in blood flow did not differ among the three circulations and were not age dependent. Increases in femoral resistance were obtained to both arterial and venous hemorrhage only in the 2-wk-old group. As the degree of hemorrhage was increased, aortic pressure, regional blood flow, and femoral resistance (2 wk olds) responses were larger in magnitude. Aortic pressure in piglets 1-5 days of age could not be restored to within 20% of the prehemorrhage level at a smaller bleeding volume with arterial than with venous hemorrhage. Pressor responses to norepinephrine (0.5 micrograms/kg) and to bilateral common carotid occlusion were still present after 20-ml/kg hemorrhage. These responses indicate the presence of a progressive maturation-related compensation to the stress of arterial but not venous hemorrhage.

Brainstem cells of origin of the cervical vagus and cardiopulmonary nerves in the neonatal pig (Sus scrofa)

The distribution of the cells of origin of the cervical vagus and cardiopulmonary nerves has been studied in neonatal piglets (Sus scrofa) ranging in age from 1 to 60 days. Cardiopulmonary nerves were identified physiologically and anatomically prior to injection of horseradish peroxidase (HRP) into the nerves. Following injection of HRP into the cervical vagus nerve retrogradely labeled neurons were present in the dorsal motor nucleus of the vagus nerve (DMV), the nucleus of the solitary tract, the nucleus ambiguus (NA), ventrolateral to the NA and in an intermediate zone between the DMV and the NA. Two unique clusters of neurons were also retrogradely labeled after injections into the vagus nerve. One group was located lateral to the most caudal levels of the DMV and extended as far caudally as the C1 spinal segment. The second distinctive group was located ventrolateral to the nucleus ambiguus in a cell column identified as the ventrolateral nucleus ambiguus (VLNA). After injections of HRP into cardiopulmonary nerves, the majority of neurons were found in the VLNA and the distinct clusters of neurons in this cell column were particularly heavily labeled. Small numbers of cells were labeled in the DMV and NA and none were labeled in the solitary nucleus after cardiopulmonary nerve injections. There were no apparent age-related differences in the degree or distribution of retrograde labeling. The distribution of neurons in the medulla oblongata projecting into cardiopulmonary nerves in the piglet is similar to that described in other species, i.e., the nucleus ambiguus, particularly its ventrolateral cell column, is the primary site of cardiomotor neurons. In addition, in the piglet there is a morphologically distinct cluster of cells related to the heart, and possibly the lungs, which does not appear to be present in other species.

Cardiac effects of esophageal stimulation: possible relationship between gastroesophageal reflux (GER) and sudden infant death syndrome (SIDS)

Twenty-six artificially ventilated newborn pigs were subjected to simulated gastroesophageal reflux; saline (10 cc) of varying pH was flushed through the esophagus from below. At a given pH threshold, reflex bradycardia, which could be blocked by atropine, was elicited. Transecting of the superior laryngeal nerves, the recurrent laryngeal nerves, and the pharyngeal plexus nerves did not block the reflex bradycardia. However, bypassing the regions superior to the esophagus with a shunt prevented the bradycardia. These results indicate that bradycardia caused by gastroesophageal reflux is independent of changes in ventilation and may be an important cause of sudden infant death.

Inhibitory effects on fast sympathetic rhythms

In decerebrate cats, the effects of barbiturate anesthesia and of pulmonary afferent activity (produced by lung inflation) on fast (ca. 10/s) rhythms in sympathetic efferent discharges (cervical sympathetic and splanchnic) were analyzed. These inhibitory influences, in addition to reducing overall activity, eliminated or drastically reduced the highest frequency oscillations. Barbiturate anesthesia also markedly reduced high frequency oscillations (ca. 100/s) in phrenic discharge. Apparently, then, the occurrence of fast neural rhythms is dependent on the level of tonic excitatory activity.

Maturation of central autonomic control of the circulation

Postnatal maturation of central neural regulation of cardiovascular function is being examined in developing swine. Three major types of investigations have been undertaken: 1) alterations of inputs from visceral and somatic afferents, e.g., baroreceptor manipulation, stimulation of sciatic nerves; 2) stimulation of central vasoactive sites; 3) subjecting the animals to the stresses of hemorrhage, hypoxia, or hypercapnia. Our findings indicate that cardiovascular reflexes mature at different postnatal ages. For example, the J-receptor reflex had almost the adult pattern of response at birth, whereas the Bezold-Jarisch reflex had a markedly delayed postnatal maturation. Recordings of spontaneous discharge in a major efferent sympathetic supply, i.e., the greater splanchnic nerve, have indicated that neural innervation to the adrenal medulla and splanchnic vasculature is present at birth in piglets.

Maturation of circulatory responses to adrenergic stimuli

The postnatal maturation of renal and femoral circulatory responses to catecholamines and to stimulation of their efferent sympathetic nerve supply has been examined in developing swine. Catecholamine dose-response experiments were carried out in intact animals under pentobarbital anesthesia. Effects of denervation and nerve stimulation were studied in intact animals and effects of neurotransmitter infusions were studied in preparations for in situ perfusion of kidney or hind limb. The renal circulation was found to be relatively sensitive to norepinephrine and under tonic neural vasoconstrictor influence at birth, and the femoral circulation was not. The femoral circulation was found to be relatively sensitive to isoproterenol at birth, whereas the renal circulation exhibited beta-adrenergic vasodilation by the end of the first postnatal week. The basic pressure-flow relationship in either circulation at any age was not altered by infusions of neurotransmitters into the arterial circuit after denervation of the perfused kidney or hind limb. Adrenergic innervation continued to mature rapidly in both circulations during the first postnatal week, as evidenced by the decreasing threshold and increasing magnitude of vasoconstrictor responses to electrical stimulation of the renal or lumbar nerves. A cholinergic component of the response to lumbar nerve stimulation became functional at 1 month after birth.

Maturation-related differences in regional circulatory effects of dopamine infusion in swine

The cardiovascular effects of dopamine (DPA) infusions were evaluated in developing swine, less than or equal to 1 day, 2 weeks, and 2 months of age, anesthetized with halothane in 50% N2O and O2. DPA was given by intravenous infusion in randomized doses of 2, 5, 10 and 20 micrograms/kg/min for 10 min. Depressor responses and bradycardia were observed during 2 micrograms/kg/min in 2-week-olds and during 2, 5 and 10 micrograms/kg/min in 2-month-olds. During infusion of 20 micrograms/kg/min, DPA pressor responses were observed in animals less than or equal to 2 weeks of age; in addition, tachycardia occurred in 1-day-old swine. Renal resistance decreased during infusion of 2 micrograms/kg/min in all swine. It increased during 10 and 20 micrograms/kg/min in the 1-day and 2-week groups, but not in the 2-month group. Mesenteric resistance decreased during infusion of 2, 5 and 10 micrograms/kg/min in less than or equal to 2-week-olds and at all doses in 2-month-olds. Following combined alpha- and beta-adrenergic receptor blockade, infusion of 20 micrograms/kg/min DPA inhibited renal vasoconstriction in most animals and elicited mesenteric vasodilation in all. The results indicate that dopaminergic responses undergo a postnatal maturation which has a different time course for different regional circulations.

Age-related effects of single injections of dopamine on cardiovascular function in developing swine

The cardiovascular effects of single intravenous injections of dopamine (DPA; 2, 5, 10 and 25 micrograms/kg) were evaluated in swine, less than or equal to 1 day to 2 months of age, anesthetized with halothane in 50% N2O-O2. Mean aortic pressure increased following all doses of DPA in swine less than or equal to 1 month of age. Renal vasoconstriction was obtained with 5-25 micrograms/kg in the younger animals and with 25 micrograms/kg in the oldest. After 2 micrograms/kg, renal vasodilation occurred in 2-month-old swine. Femoral and carotid vasoconstriction was elicited in day-old swine after 5-25 micrograms/kg DPA, but carotid vasodilation occurred in the oldest swine. Responses to 25 micrograms/kg DPA after combined alpha and beta adrenergic receptor blockade were generally vasodilator. Thus, dopaminergic receptors were unmasked and a postnatal maturation of these receptors was revealed.

Sympathetic rhythms in spinal cats

Simultaneous recordings from preganglionic sympathetic nerves at different spinal levels, cervical sympathetic and greater splanchnic, reveal the presence of common periodicities as shown by cross-correlation and power spectral analysis; the major types of periodicities are cardiac, respiratory and 10/sec rhythm. These common periodicities could be explained in two ways: (1) there are common periodic inputs to the two types of preganglionic neurons; and (2) there are feedback connections in the spinal cord between the two groups of neurons. To distinguish between these two possibilities, spinal cord transections at C2-C3 were performed on decerebrate unanesthetized cats; recordings were then taken at hourly intervals for more than 12 h, during which time activity gradually increased but still remained small compared to pre-section levels. This low level activity showed no sign of periodicity. Asphyxia of sufficient duration produced increased activity in sympathetic nerves. Splanchnic activity during asphyxia had 2-3/sec oscillations; but the cross-correlation histograms (CCHs) of cervical sympathetic and splanchnic activity were almost flat. Strychnine excited spinal cord neurons more effectively than asphyxia; the CCHs showed locking of activity in phrenic, cervical sympathetic and splanchnic nerves on a slow time-scale (1-5 sec), but no appreciable locking of cervical sympathetic and splanchnic activity on a faster time-scale (100-500 msec) such as occurs in the intact animal. Thus, while there can be oscillation of sympathetic activity at the spinal cord level, the normally occurring synchrony of oscillations between different segmental levels is dependent on inputs from the brain stem.

Femoral circulatory responses to lumbar nerve stimulation in developing swine

The maturation of femoral circulatory responses to efferent lumbar nerve stimulation was tested in 51 developing swine (1 day-3 mo old) under pentobarbital sodium anesthesia (10-30 mg/kg). Aortic pressure, heart rate, and femoral and carotid arterial flows (measured by electromagnetic flow transducers) were recorded simultaneously. Femoral vascular resistance was calculated as mean aortic pressure/mean flow. Transection of the lumbar nerve fibers below the last ganglion in the sympathetic chain did not after femoral resistance in day-old animals but decreased femoral resistance in swine 1 wk of age and older. Efferent lumbar nerve stimulation at various combinations of frequencies and intensities revealed an atropine-blockable vasodilator component in the femoral circulatory response in swine 1 mo of age and older. After alpha-adrenergic receptor blockade with phentolamine (0.25 or 0.5 mg/kg), femoral vasodilation occurred during low-frequency and -intensity stimulation of the lumbar nerve only in animals 1 mo of age and older. Acetylcholine (2 micrograms ia) caused a decrease in femoral resistance at all ages. Vasoconstrictor effects of high-frequency stimulation (5-10 Hz) were present at all ages and were age dependent. The results of these experiments suggest that the femoral circulation in swine at birth in innervated by functionally active vasoconstrictor fibers, which do not provide a tonic influence on femoral resistance until late in the first postnatal week. Furthermore, although femoral vascular cholinergic receptors are demonstrable at birth, there appears to be a delay in the maturation of functionally active vasodilator fibers.

Renal circulatory effects of adrenergic stimuli in anesthetized piglets and mature swine

The relative maturity of renal circulatory responses to efferent renal nerve stimulation, and to exogenous norepinephrine and isoproterenol, was tested in 62 piglets (1--16 days old) under pentobarbital anesthesia (10--25 mg/kg). Aortic pressure, heart rate, and renal and femoral arterial flows (measured by electromagnetic flow transducers) were recorded simultaneously. Renal vascular resistance was calculated as mean aortic pressure/mean flow. Transection of the renal nerve resulted in decreased renal resistance in all animals. Efferent renal nerve stimulation at increasing frequencies (2--12.5 Hz, at 1.2 ms pulse duration and 1.0 mA current) showed age-dependent differences in the threshold and also in the magnitude of increase in renal resistance. Norepinephrine (0.05--1.0 microgram/kg) caused age-dependent increases in renal resistance. Restoration of renal flow toward control level occurred during the peak pressor effect of norepinephrine only in older piglets. Isoproterenol (0.05--1.0 microgram/kg) did not alter renal resistance consistently in piglets younger than 1 wk. Phentolamine (0.25 mg/kg) attenuated or blocked resistance increases to 0.5 microgram norepinephrine/kg or to renal nerve stimulation at 12.5 Hz in all animals. Propranolol (0.1 mg/kg) attenuated or blocked resistance decreases to 0.1 microgram isoproterenol/kg, which occurred only in older piglets. These results indicate the presence of an active alpha-adrenergic vasoconstrictor mechanism and absence of the beta-adrenergic vasodilator mechanism in the renal circulation of swine at birth.

Cardiovascular and respiratory responses to right atrial injections of phenyl diguanide in pentobarbital-anesthetized newborn piglets

The present series of experiments was designed to determine whether the piglet has mature cardiorespiratory responses to the administration of phenyl diguanide (PDG) similar to those reported in adult mammals. A total of 26 acutely instrumented piglets aged 2--23 days were lightly anesthetized with sodium pentobarbital. After control conditions were established, PDG was injected into the right atrium. Each animal was its own control for determination of presence and magnitude of aortic pressure, heart rate, respiratory rate and volume, and blood flow responses to test doses of PDG. A fall in blood pressure was observed in all animals within 2--4 sec after right atrial injection of PDG. Bradycardia occurred and a transient cardiac arrhythmia, consisting of 2nd and 3rd degree heart block, was observed in most animals; the latter has not been previously reported. Apnea followed by rapid shallow breathing was observed in most animals. This respiratory effect was more pronounced in the younger animals. After atropine, PDG elicited a monophasic rise in aortic pressure; the cardiac rhythm and rate changes were abolished. However, the apneic response was retained. Subsequent bilateral vagotomy abolished the hypotensive effect on PDG. Such results suggest the possibility that the piglet,s cardiorespiratory response to the administration of PDG may be evoked by stimulation of type J pulmonary receptors. These have been postulated to be responsible for the triad of responses of hypotension, bradycardia, and apnea seen in other species after PDG administration.

Age-related cardiovascular effects of catecholamines in anesthetized piglets

We studied cardiac and peripheral circulatory effects of graded doses of catecholamines (0.05-1.0 microgram/kg) in piglets aged less than or equal to 1 day, 2--4 days, 1 wweek, 2 weeks, and 2.5-3 months, under anesthesia with pentobarbital. We evaluated cardiovascular function from simultaneous recordings of aortic pressure, ventricular pressure and its first derivative, heart rate, and phasic carotid and femoral blood flows. We calculated vascular resistance as the ratio of mean aortic pressure to mean flow. The age of onset of a given cardiovascular response was determined, and magnitudes of each type of response were compared among the age groups. Norepinephrine elevated the blood pressure at all doses in piglets of all ages, elicited reflex bradycardia only in older piglets, and increased carotid resistance. Epinephrine elevated the blood pressure at all doses in piglets less than 1 week old, but low doses lowered the blood pressure in piglets older than 1 week of age; resistance changes in the femoral and carotid circulations were variable except in the 2.5-3 month age group. Isoproterenol increased cardiac contractility at all doses in piglets of all ages and increased heart rate at low doses in piglets older than 2 days of age; however, blood pressure and femoral resistance decreases were age and dose dependent. There were age-related differences in the catecholamine dose required to elicit a given cardiac or peripheral circulatory effect and age-related differences in the direction and magnitude of such effects. These results provide evidence for differing rates of postnatal maturation of cardiovascular alpha- and beta- adrenergic mechanisms in swine.

Integrated cardiovascular responses to combined somatic and visceral afferent stimulation in newborn piglets

Mean aortic pressure, arterial flow and heart rate response to interactions of afferent stimulation of the sciatic nerve (SNS) and carotid sinus stimulation by infusion (CSI) or inhibition by bilateral common carotid occlusion (BCCO) were studied in 25 newborn piglets under halothane-N2O anesthesia. Combined stimulation of the somatic and visceral afferents produced no heart rate changes. Combinations of pressor stimuli (BCCO and high frequency SNS) showed facilitation; femoral and renal resistances increased. However, combinations of depressor stimuli (CSI and low frequency SNS) showed occlusion. In combinations of pressor with depressor stimuli, the pressor pattern of responses dominated. Thus, dissimilar types of excitatory afferent inputs are capable of evoking facilitation in the neonatal cardiovascular regulatory system.

Microvascular responses to norepinephrine and vasopressin during halothane anesthesia in the rat

This experiment was designed to determine the microvascular responses to the two known naturally occurring vasoconstrictors, norepinephrine (NE) and vasopressin, at known levels of central vasomotor activity before, during and after halothane anesthesia. The responses to topical application of NE and vasopressin were studied in the microvasculature of the mesentery and cremaster muscle, using microscopic methods. Neural (CNS) stimulation was accomplished through electrodes chronically implanted in vasoactive sites of the forebrain and midbrain. The increase in blood pressure in response to CNS stimulation was decreased during halothane anesthesia (32.4 +/- 5.4 per cent before and 24.7 +/- 6.1 per cent during; P less than 0.001). There was no significant change in the steady-state diameter of the microvasculature under study during or after halothane anesthesia. Marked abatement of arteriolar vasoconstriction in response to CNS stimulation was seen prior to halothane. However, the same target vessel showed increased constriction in response to topically applied NE (from 32.3 +/- 4.7 to 53.2 +/- 7.8 per cent; P less than 0.01) during halothane anesthesia. By contrast, the response to vasopressin decreased (from 42.4 +/- 5.7 to 1.0 +/- 6 per cent; P less than 0.001) with halothane. The precise mechanism(s) underlying the described hypersensitivity to NE and hyposensitivity to vasopressin in the same vascular structure during halothane anesthesia remains undetermined.

Integrated cardiovascular responses to combined somatic afferent stimulation in newborn piglets

Heart rate, mean aortic pressure and arterial flow responses to interactions of afferent stimulation of the sciatic (SNS) and median nerve of the brachial plexus (BNS) were studied in 30 newborn piglets under halothane-N2O anesthesia. High frequency or intensity SNS or BNS stimulation resulted in pressor responses accompanied by significant increases in the femoral flow and carotid and renal resistance. Combined stimulation produced responses smaller than the estimated sum of the responses. Low frequency or intensity SNS or BNS resulted in depressor responses accompanied by significant decreases in mean femoral flow without change in heart rate; combined stimulation produced responses smaller than the estimated sum. When pressor and depressor patterns of stimulation were combined, pressor responses always dominated. The absence of facilitation to any combination of interactions implies that the cardiovascular regulatory system is not fully developed at birth and undergoes, therefore, postnatal maturation.