Induction of prolonged excitability in myometrium of pregnant guinea-pigs by prostaglandin F2 alpha

Uterine contractions in rodent models and humans

Aberrant uterine contractions can lead to preterm birth and other labour complications and are a significant cause of maternal morbidity and mortality. To investigate the mechanisms underlying dysfunctional uterine contractions, researchers have used experimentally tractable small animal models. However, biological differences between humans and rodents change how researchers select their animal model and interpret their results. Here, we provide a general review of studies of uterine excitation and contractions in mice, rats, guinea pigs, and humans, in an effort to introduce new researchers to the field and help in the design and interpretation of experiments in rodent models.

Bioelectrical activity of porcine oviduct and uterus during spontaneous and induced estrus associated with cyclic hormone changes

It is widely accepted that uterine contraction is initiated by spontaneous generation of electrical activity at a cellular level in the form of action potentials. Such action potential events, when they involve many myometrial cells and occur in immediate succession, are described by their amplitude and duration. In an effort to improve clinical management of uterine contractions, research has focused on determination of the properties of the reproductive tract's electrical activity under hormonal stimulation. The aim of this study was to evaluate the myoelectric activity (amplitude and duration) of the oviduct and the uterus in relation to plasma concentration of LH, estradiol (E2), and progesterone (P4) during spontaneous and induced estrus in gilts. The course of the experiment was divided into eight periods defined by hormone concentrations (LH, P4, and E2) and time intervals before and after the start of the LH surge. Myoelectric signals were recorded, and the hormone levels were measured during proestrus and estrus in natural and hormone-induced estrus cycle. During the natural estrus, the LH surge was longer than after hormonal stimulation (28 vs. 20 hours) and suggested an inverse relationship between the LH concentration and the duration of myoelectric activity (S_R = -0.68). Analyses of the records of the amplitudes and durations of the electromyography activity in uterine horns and oviducts showed significant differences between spontaneous and induced estrus (P < 0.05). During induced estrus, the LH surge began earlier (T1 vs. T2) and increased more (7.46 vs. 6.50 ng/mL) than during spontaneous estrus. This observation suggests a direct relationship between the LH concentration and the amplitude of the myoelectric activity (Spearman rank correlation = 0.71). The significantly higher duration and amplitude of the activity in the isthmus of the oviduct and the uterus during induced estrus shortly after the onset of standing heat (4-8 hours after the LH surge) suggested more favorable conditions for effective artificial insemination.

Progress in understanding electro-mechanical signalling in the myometrium

In this review, we give a state-of-the-art account of uterine contractility, focussing on excitation-contraction (electro-mechanical) coupling (ECC). This will show how electrophysiological data and intracellular calcium measurements can be related to more modern techniques such as confocal microscopy and molecular biology, to advance our understanding of mechanical output and its modulation in the smooth muscle of the uterus, the myometrium. This new knowledge and understanding, for example concerning the role of the sarcoplasmic reticulum (SR), or stretch-activated K channels, when linked to biochemical and molecular pathways, provides a clearer and better informed basis for the development of new drugs and targets. These are urgently needed to combat dysfunctions in excitation-contraction coupling that are clinically challenging, such as preterm labour, slow to progress labours and post-partum haemorrhage. It remains the case that scientific progress still needs to be made in areas such as pacemaking and understanding interactions between the uterine environment and ion channel activity.

Store-operated Ca²⁺ entry and depolarization explain the anomalous behaviour of myometrial SR: effects of SERCA inhibition on electrical activity, Ca²⁺ and force

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SERCA pump inhibition by CPA caused membrane depolarization, activation of action potentials, Ca²⁺ spikes and force.

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Depletion of Ca²⁺ store by agonists leads to membrane depolarization and activation of electrical and mechanical activity.

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Ca²⁺ release/Ca²⁺ entry coupling is playing a key role in control of spontaneous electrical and mechanical activity in rat pregnant myometrium.

In the myometrium SR Ca²⁺ depletion promotes an increase in force but unlike several other smooth muscles, there is no Ca²⁺ sparks-STOCs coupling mechanism to explain this. Given the importance of the control of contractility for successful parturition, we have examined, in pregnant rat myometrium, the effects of SR Ca²⁺-ATPase (SERCA) inhibition on the temporal relationship between action potentials, Ca²⁺ transients and force. Simultaneous recording of electrical activity, calcium and force showed that SERCA inhibition, by cyclopiazonic acid (CPA 20 μM), caused time-dependent changes in excitability, most noticeably depolarization and elevations of baseline [Ca²⁺]_i and force. At the onset of these changes there was a prolongation of the bursts of action potentials and a corresponding series of Ca²⁺ spikes, which increased the amplitude and duration of contractions. As the rise of baseline Ca²⁺ and depolarization continued a point was reached when electrical and Ca²⁺ spikes and phasic contractions ceased, and a maintained, tonic force and Ca²⁺ was produced. Lanthanum, a non-selective blocker of store-operated Ca²⁺ entry, but not the L-type Ca²⁺ channel blocker nifedipine (1–10 μM), could abolish the maintained force and calcium. Application of the agonist, carbachol, produced similar effects to CPA, i.e. depolarization, elevation of force and calcium. A brief, high concentration of carbachol, to cause SR Ca²⁺ depletion without eliciting receptor-operated channel opening, also produced these results. The data obtained suggest that in pregnant rats SR Ca²⁺ release is coupled to marked Ca²⁺ entry, via store operated Ca²⁺ channels, leading to depolarization and enhanced electrical and mechanical activity.

Calcium-activated chloride currents prolongs the duration of contractions in pregnant rat myometrial tissue

We investigated the importance of pharmacologically blocking calcium-activated chloride (I(Cl(Ca))) and L-type calcium currents on isometric contractions of strips of D21 pregnant rat myometrial tissue, while simultaneously measuring the electrical activity of the tissue strips with extracellular contact electrodes. When measured with contact electrodes, the duration of the spiking activity directly reflects the duration of the tissue-level plateau potential. We correlated the number of spikes, durations of spiking activity, and the spiking frequencies with changes of the area under the force curves as a function of exposure to low doses of anthracene-9-carboxylate (9-AC, a non-specific Cl channel blocker), chlorotoxin (a specific I(Cl(Ca)) blocker) and nifedipine (an L-type calcium channel blocker). The area under the force curve was measured only during spiking electrical activity, thereby separating pharmacological effects on tissue relaxation from those that modulate force production. Blocking chloride channels reduced impulse, shortened the duration of spiking activity, and reduced the number of spikes generated in each contraction. This was observed without a change in the frequency of spike production or a reduction of peak force. Nifedipine reduced impulse, shortened the duration of spiking activity, and reduced the number of spikes. In contrast to chloride channel blockade, nifedipine reduced maximum spike frequency and peak force. Taken together, our data suggest that blocking L-type calcium channels reduces impulse directly by reducing peak force, and indirectly by reducing activation of I(Cl(Ca)) , which shortens the duration of the contraction.

Temporal and spatial variations in spontaneous Ca events and mechanical activity in pregnant rat myometrium

OBJECTIVES

The aim of this study was to investigate the temporal and spatial characteristics of spontaneous Ca signals in pregnant rat myometrium.

STUDY DESIGN

Confocal imaging of longitudinal strips of 21-day pregnant rats loaded with the Ca sensitive indicator Fluo-4, was combined with measurements of mechanical activity in uterine smooth muscle cells, in situ and freshly isolated.

RESULTS

Our results show that the Ca transients in pregnant uterine tissue are composed of Ca spikes, which are associated with the spike-like action potentials. There is large variation in the pattern of spontaneous activity in myometrium, ranging from non-propagating Ca spikes confined to individual smooth muscle cells, through to regional and global propagating Ca spikes. Irrespective of the pattern of activity displayed, the Ca signals were always in the form of Ca spikes, singularly or in bursts. These Ca spikes did not show fixed initiations sites, propagated in longitudinal and transverse directions from the initiation regions, and had a variable pattern of propagation in preparations which were not synchronously active. In preparations which showed synchronous activity, Ca spikes singularly or bursts propagated mainly in the transverse direction from the initiation regions. The amplitude of force generated by single spikes was dependent on the number of bundles recruited by the propagating Ca spike within the strip, and was about 30-40% of the maximal force produced by carbachol or high-K stimulation. If Ca spikes appeared in the form of bursts they generated longer lasting fused contractions, the amplitudes of which were dependent on the number and the frequency of Ca spikes in the burst.

CONCLUSIONS

Longitudinal myometrium from pregnant rats generates spontaneous Ca spikes which vary in their initiation sites, spatial spread and frequency and are associated with the spike-like action potentials. They are sensitive to the L-type Ca channel blocker, nifedipine. Contractile activity was dependent on the spatial spread of individual Ca spikes and when fully synchronized, produced single submaximal phasic contraction. The number and frequency of bursts of Ca spikes controlled the amplitude and duration of contraction.

Preterm labour. Myometrial function in prematurity

The primary function of the uterus during gestation is to harbour the growing conceptus in a largely quiescent environment. Upon maturation of the fetus to a point sufficient for extrauterine survival, the uterus must remodel itself sufficiently to generate forceful contractions during labour. During preterm delivery, the process of remodelling of the myometrium occurs early due to a number of different causes, although the underlying basis for myometrial contraction remains the same. This review summarises the anatomical, physiological and molecular basis for contraction. We describe the fibre structure of the human uterus and how this relates to the spread of electrical excitation during a contraction. The process of excitation within a single myometrial cell is described, as well as how this relates to contraction. We then focus on how excitation-contraction coupling is modulated by intercellular communication, pharmacomechanical-coupling and hormonal milieu. Lastly, we consider the actions of the commonly accepted uterine agonists oxytocin, prostaglandin F(2alpha), and prostaglandin E(2), and the tocolytic ritodrine.

Chorion releases a factor that inhibits oxytocin-stimulated myometrial contractility in the pregnant guinea pig

It was postulated that chorion releases a substance necessary for the maintenance of uterine quiescence during pregnancy. A decrease in the release of this substance at the end of the pregnancy would be necessary for normal myometrial activation. This hypothesis was tested by demonstrating the ability of chorion to inhibit oxytocin-stimulated myometrial contractility in vitro. Tissues were obtained from timed pregnant Duncan-Hartley guinea pigs either at pre-term or near-term gestation. Myometrial strips were placed in organ baths for isometric tension measurement and contractions stimulated by oxytocin (10(-8) mol/l). Fetal membranes or conditioned medium from chorion were added directly to the organ bath. Near-term chorion and chorion conditioned-medium decreased oxytocin-stimulated contractile activity to 39% and 49% respectively. Neither pre-term nor near-term amnion reduced oxytocin-stimulated myometrial contractile activity. Relaxation induced by pre-term chorion was greater than near-term chorion (23% and 41% of the oxytocin-induced basal level respectively; P < 0.05). Further, chorion-induced relaxation was independent of the gestational age of the myometrium. Human chorion from a term, not-in-labour woman also inhibited oxytocin-stimulated guinea pig myometrial contractility. It was concluded that the chorion releases a substance or substances that reduce oxytocin-stimulated myometrial contractility and may be involved in the maintenance of uterine quiescence during pregnancy.

The effects of P2Y receptor agonists and adenosine on prostaglandin production by the guinea-pig uterus

1. This study has investigated the effects adenosine 5'-triphosphate (ATP), analogues of ATP, uridine 5'-triphosphate (UTP) and adenosine on prostaglandin output from the guinea-pig uterus superfused in vitro, and from guinea-pig endometrium and myometrium cultured for 24 h. 2. ATP, 2-methylthio ATP and adenosine increased the outputs of prostaglandin F(2 alpha) (PGF(2 alpha)) and 6-keto-PGF(1 alpha) (reflecting PGI(2) production), and UTP increased the output of PGF(2 alpha) from the superfused guinea-pig uterus. These findings support the hypothesis that the contractile effects of ATP, 2-methylthio ATP, UTP and adenosine are mediated by prostaglandins. 3. Suramin (a P2 receptor antagonist) and 8-sulphophenyltheophylline (an A receptor antagonist) blocked the stimulatory actions of ATP and adenosine, respectively, on PGF(2 alpha) output, suggesting that ATP acts on P2 receptors (probably of the P2Y type) and adenosine acts on A receptors in the guinea-pig uterus to increase PGF(2 alpha) production. 4. ATP, 2-methylthio ATP, alpha,beta-methylene ATP, beta,gamma-methylene ATP, UTP and adenosine increased the output of PGF(2 alpha) from guinea-pig endometrium and myometrium after 24 h of culture, with a greater stimulatory effect being exerted on the endometrium than on the myometrium. Little or no stimulatory effect was seen after 2 and 8 h of culture. In addition the effects of ATP, ATP analogues, UTP and adenosine on the outputs of PGE(2) and 6-keto-PGF(1 alpha) from cultured endometrium and myometrium were more variable, with both stimulation and inhibition being observed. 5. The stimulatory effects of ATP and adenosine on PGF(2 alpha) output from the endometrium and myometrium were associated with an increase in the prostaglandin synthesizing capacity of both tissues, due probably to an increase in the amount of prostaglandin H synthase present.

Hyperpolarization and slowing of the rate of contraction in human uterus in pregnancy by prostaglandins E2 and f2alpha: involvement of the Na+ pump

1. The effects of prostaglandins E2 (PGE) and F2alpha (PGF) on membrane potential and isometric tension and cytoplasmic free calcium concentration ([Ca2+]i) and tension were studied in strips of uterine smooth muscle obtained from women undergoing Caesarean delivery at term and during established labour. 2. Prostaglandins (PGs) evoked a biphasic response. The excitatory component consisted of depolarization of the membrane, which initiated spike action potentials, an increase in [Ca2+]i and tension development. The membrane remained depolarized at -19 +/- 1 mV for about 2 min, then repolarized abruptly, [Ca2+]i promptly returned to basal levels, and tension development ceased. 3. This component of the response to PGE or PGF was followed by a slow hyperpolarization which reached -85 +/- 2 mV (n = 22) at term and -70 +/- 2 mV (n = 9) during labour, and during which spontaneous action potentials and tension development did not occur. 4. Nifedipine (10-6 M) abolished spontaneous activity, abolished PG-induced action potentials and reduced the increase in [Ca2+]i (9 +/- 3 %, n = 6), the depolarization (10 +/- 1 mV, n = 14), the tension (2 +/- 1 %, n = 14) and the hyperpolarization (9 +/- 1 mV, n = 14, at term). 5. A variety of K+ channel blockers were without effect on the peak amplitude of the PG-induced hyperpolarization but the latter did not occur in the presence of ouabain (10-6 M) or in K+-free or low-Na+ solutions, suggesting an involvement of the Na+-K+-ATPase pump. 6. In conclusion, a substantial dependence on Ca2+ influx through voltage-operated Ca2+ channels accounts for the importance of membrane potential in regulating contractions in human uterine smooth muscle. The classical excitatory effect of PGE and PGF is followed by hyperpolarization involving the Na+-K+-ATPase pump. The hyperpolarization restricts the response to a single contraction and decreases the frequency of subsequent contractions. The amplitude of the hyperpolarization decreases during labour, allowing contraction frequency to increase. Its persistence at this time ensures complete relaxation between each single robust contraction, preventing spasm of the uterus that would restrict blood flow to the fetus during delivery.

Force, membrane potential and cytoplasmic Ca2+ responses to cyclic nucleotides in rat anococcygeus muscle

Simultaneous recordings of membrane potential and force, and cytoplasmic calcium ([Ca2+]i) and force were made in rat anococcygeus to determine whether membrane hyperpolarisation plays a role in cyclic nucleotide-induced relaxation. In the presence of phenylephrine (0.2 microM), which evoked sustained contraction, an elevation in [Ca2+]i, and depolarisation, nitroprusside (5 microM) caused 96+/-3% relaxation, 77+/-3% decrease in suprabasal [Ca2+]i, and 16+/-2 mV hyperpolarisation. Forskolin (1 microM) caused 98+/-1% relaxation, 92+/-2% decrease in suprabasal [Ca2+]i, and 18+/-1 mV hyperpolarisation. These responses persisted in the presence of a variety of K+ channel blockers or in ouabain. The decrease in [Ca2+]i preceded the commencement of relaxation whereas the onset of hyperpolarisation lagged behind. Thus, cyclic nucleotide-mediated relaxation in rat anococcygeus is not dependent on hyperpolarisation mediated by the opening of K+ channels. Rather, it is suggested that the decrease in [Ca2+]i gives rise to hyperpolarisation, which reflects a decline in the Ca2+ dependent conductance(s) activated by phenylephrine.

Oxytocin enhances action potentials in pregnant human myometrium--a study with microelectrodes

OBJECTIVE

Our purpose was to quantitatively assess the effects of oxytocin on membrane properties in the pregnant human myometrium.

STUDY DESIGN

Specimens were obtained from the lower uterine segment during cesarean section at term. Electrical activity was recorded from individual cells by a conventional microelectrode method and the membrane functions were analyzed.

RESULTS

Two types of spontaneous action potentials were seen: a long plateau potential and a spike-like action potential. With no change in the resting membrane potential, low concentrations of oxytocin either evoked an action potential with a plateau phase, increased the amplitude and duration of the plateau potential, or increased the frequency of generation of action potentials. Oxytocin also lowered the threshold for evoking an action potential. Higher concentrations depolarized the membrane with an associated reduction in membrane resistance.

CONCLUSION

Oxytocin augments the excitability of pregnant human myometrial cells by multiple actions on the membrane, affecting both frequency and amplitude of action potentials.

Distribution of mast cells and the effect of their mediators on contractility in human myometrium

OBJECTIVE

To examine the distribution of mast cells in human uterine tissue and to study the interactions between mast cell mediators (histamine and serotonin) and PGF2 alpha in human myometrium contractile activity.

DESIGN

Distribution of mast cells were analysed in cryostat sections of myometrium samples stained with Toluidine blue. Contractile activity was evaluated in an isolated organ bath preparation on myometrial strips obtained from women whose pregnancies ended in elective caesarean section.

SETTING

Biological Science Laboratories at the University of Concepción.

SUBJECTS

Twenty women undergoing elective caesarean and 10 women undergoing hysterectomy at the G. Grant Hospital.

MAIN OUTCOME MEASUREMENTS

Cumulative concentration-response curves for histamine and serotonin before and after the addition of subumbral concentrations of the PGF2 alpha or serotonin were performed.

RESULTS

Serotonin was more active than histamine to evoke contractions (EC50:0.20 (SE 0.02) mumol/l vs 1.5 (SE 0.2) mumol/l, respectively). Furthermore, threshold concentrations of serotonin (0.05 mumol/l) potentiated the contractile effect of histamine (EC50:0.3 (SE 0.06) mumol/l and 50% increase in E(max)). PGF2 alpha had a poor contractile effect, but threshold concentrations (0.05 and 0.10 mumol/l) enhanced the contractile effect of both serotonin and histamine. A population of mast cells was found in close apposition to smooth muscle fibres.

CONCLUSIONS

It is postulated that the simultaneous release of mast cell mediators (histamine and serotonin) in myometrium could be an important stimulus for evoking strong contractions in the human uterus. PGF2 alpha may have indirect effects in myometrium by amplifying the effects of histamine and serotonin.

Mast cells mediators evoke contractility and potentiate each other in mouse uterine horns

1. The ability of prostaglandin F2 alpha (PGF2 alpha), histamine and serotonin to induce and/or potentiate contractions in mouse uterine horns was studied in an attempt to analyze whether uterine mast cells degranulation could favor contractions during labor. 2. PGF2 alpha was the most potent of the three compounds (EC50 = 0.7 microM), being followed by serotonin (EC50 = 1.2 microM) and histamine. Between 10 and 100 microM histamine only evoked weak contractions, not higher than 50% of maximal amplitude of contractions. 3. Serotonin (0.1 microM) potentiated the contractile effect of both histamine and PGF2 alpha when added simultaneously with the corresponding compound to the isolated organ bath. Also, histamine at threshold contractile concentration (3 microM) was able to potentiate the contractile effect of both serotonin and PGF2 alpha. 4. The potentiating effect of PGF2 alpha on both histamine and serotonin evoked contractions was recorded after treating the preparation with 10 microM of the compound for at least 10 min before the corresponding concentration-effect curves were performed. 5. It is postulated that the simultaneous presence of these mast cells mediators in myometrium could be an important stimulus for both to trigger and/or to maintain contractions during labor.

Response of the rat myometrium to phenylephrine in early pregnancy and the effects of 6-hydroxydopamine

1. The contractile responses of the longitudinal and circular muscle layers of the rat uterus to the alpha 1-adrenoceptor agonist phenylephrine were measured on days 3-6 of gestation. There was a progressive increase in sensitivity to phenylephrine in both muscle layers between days 3 and 6 of gestation. Overall, this amounted to a 13 and 9 fold increase in sensitivity in longitudinal and circular muscles, respectively. In longitudinal muscle the slope of the Hill plot was 2 on day 3 of pregnancy and was decreased to 1 thereafter. 2. The sympathetic nerve terminals innervating the smooth muscle of the uterus were destroyed by administration of 6-hydroxydopamine (2 x 50 mg kg-1) 4-7 days before testing with phenylephrine. Following this treatment there was a significant increase in sensitivity to phenylephrine on day 3 in both muscle layers. After day 4, the longitudinal muscle was less sensitive to phenylephrine. 3. In the longitudinal muscle there was a progressive increase in the contractile response to maximal concentrations of phenylephrine and to high potassium (100 mM) between days 3 and 6 of pregnancy. In the circular muscle the responsiveness to both phenylephrine and potassium remained unchanged between days 3 and 6 of gestation. 6-Hydroxydopamine had no effect on the maximal responses to phenylephrine or high potassium in either muscle layer. 4. In conclusion, denervation supersensitivity of uterine smooth muscle following injection of 6-hydroxydopamine is observed only on day 3 of pregnancy and appears to be replaced by subsensitivity by day 6. The decrease in the slope of the Hill plot in longitudinal muscle after day 3 may be explained by changes in events between activation of alpha 1-adrenoceptors and contraction.

Passively propagated spikes from the soma of cells in pineal gland of guinea pigs

The membrane properties of cells within the pineal gland of guinea pigs were studied using intracellular electrophysiological techniques. The electrotonic responses to intracellular current injection decayed with a single exponential in approximately 60% of cells but was preceded by a quicker component in the remainder. The membrane time constant was 2.8 ms. Depolarization beyond -29 mV activated an outward current that reversed at around the value of the resting potential. Hyperpolarization activated a slow inward current. Spikes occurred in response to activation of alpha-adrenoceptors. They were resistant to tetrodotoxin but were abolished by nifedipine and verapamil, suggesting that calcium carries the current during their upstroke. Spikes could not be evoked by depolarizing current pulses of 1-ms to 2-s duration. The responses to hyperpolarizing current steps or voltage-clamp steps applied during the peak of spikes evoked by nerve stimulation were indistinguishable from the responses to those applied between spikes. During nerve stimulation, fluctuations were observed in the current trace of cells under voltage clamp, indicating that the spikes could not be voltage clamped successfully. It is concluded that the spikes occurring in response to nerve stimulation are generated on the processes of the pinealocytes and are passive in the soma.

Differential modulation by pulmonary stretch afferents of some reflex cardioinhibitory responses in the cat

1. Cats were anaesthetized with a mixture of chloralose and urethane, and were artificially ventilated, the thorax being opened via a medial sternotomy. 2. Various cardiovascular receptors were stimulated during reflex inhibition of the central inspiratory neurones produced by electrical stimulation of both superior laryngeal nerves simultaneously, while the afferent input from the lungs was held constant by temporarily interrupting artificial respiration, the lungs being held static in their expiratory position. 3. Reflex cardioinhibitory responses were elicited by stimulation of (a) the carotid body chemoreceptors by intracarotid injections of cyanide; (b) the arterial baroreflex by controlled elevations of the blood pressure; (c) the carotid sinus baroreceptors by raising the pressure in isolated perfused carotid sinus preparations; (d) cardiac receptors by left atrial injections of veratridine, and (e) pulmonary C fibres (including J receptors) by right atrial injections of phenyl biguanide. 4. The effects of single inflations of the lungs on these reflex cardioinhibitory responses were studied and compared with the effects of the various inputs alone. 5. Stepwise increases in lung volume, while having no consistent effect on arterial blood pressure, progressively diminished the arterial chemoreceptor-induced bradycardia to a value of about 7% of the control response without lung inflation. The pulmonary afferents were less effective on reflex responses from other inputs, the corresponding values being: arterial baroreflex, 66%; carotid baroreceptors, 66%, and cardiac receptors, 70%. These effects of lung inflation were abolished by selective denervation of the lungs. 6. In contrast, the size of the bradycardia evoked by pulmonary C fibre stimulation was, on average, unaffected by inflation of the lungs. In some tests the response was actually increased. 7. The differential modulation by lung inflation of these reflex cardioinhibitory responses were the same after upper thoracic sympathectomy indicating that pulmonary afferents and cardiac efferents involved fibres in the vagus nerves. 8. The possible central mechanisms responsible for the differential modulation by lung inflation of cardioinhibitory reflexes are discussed.

Cardiovascular responses to stimulation of cardiac receptors in the cat and their modification by changes in respiration

1. In cats anaesthetized with a mixture of chloralose and urethane, stimulation of cardiac receptors by left atrial injections of veratridine had little or no effect on pulmonary ventilation but caused bradycardia, systemic hypotension and hindlimb vasodilation with a latency of 3.3 s. 2. The hindlimb vasodilatation was due largely, if not entirely, to a reduction in sympathetic vasoconstrictor activity. 3. Similar cardiovascular responses occurred when the arterial blood pressure was maintained constant and also in artificially ventilated animals. 4. When the cardiac receptors were excited during a period of apnoea which was induced reflexly by electrical stimulation of the central cut end of a superior laryngeal nerve, the cardio-inhibitory response to left atrial injections of veratridine was enhanced but the size of the vasodilator response was unaffected. 5. In contrast, the cardiovascular effects of stimulation of the carotid body chemoreceptors, bradycardia and hindlimb vasoconstriction were enhanced by the laryngeal input. 6. The possible central mechanism responsible for the differential modulation of cardiac receptor and carotid chemoreceptor reflexes by respiration are discussed.

Cardiovascular responses to stimulation of pulmonary C fibres in the cat: their modulation by changes in respiration

1. In cats anaesthetized with a mixture of chloralose and urethane, stimulation of pulmonary C fibres by right atrial injections of phenylbiguanide caused, after a latency of about 3 s, a reduction in pulmonary ventilation or apnoea, bradycardia and systemic hypotension, confirming previous work. 2. A decrease in femoral artery perfusion pressure also occurred under conditions in which the hindlimb was vascularly isolated, the blood flow was maintained constant and the inferior vena caval pressure did not change. This indicates a reduction in vascular resistance due to vasodilatation. The response was unaffected by atropine and propranolol, but was reduced or abolished by guanethidine, hexamethonium and denervation of the limb, indicating that it is due to a reduction in activity in sympathetic vasoconstrictor fibres. 3. Similar cardiovascular responses were observed when the arterial blood pressure was maintained constant, and also in artificially ventilated animals. 4. Evidence is presented that the receptors responsible for the respiratory and cardiovascular responses to right atrial injections of phenylbiguanide lie in the pulmonary vascular bed. 5. When the pulmonary C fibres were excited during a period of apnoea which was induced reflexly by electrical stimulation of the central end of a superior laryngeal nerve, there were no consistent differences in the size of the cardiac or vascular responses compared to the control responses in the absence of the laryngeal input. This result occurred irrespective of the size of the control ventilatory response to phenylbiguanide. 6. By contrast in the same experiments, the cardio-inhibitory and vasoconstrictor responses to excitation of the carotid body chemoreceptors were invariably potentiated by electrical stimulation of a superior laryngeal nerve, as found previously. 7. The possible central mechanisms responsible for the differential modulation of pulmonary C fibre and carotid chemoreceptor reflexes by respiration are discussed.