Role of hypotension induced by PGI2 on the control of depth and frequency of breathing

Prostaglandins and Medicine 5: 415-423, 1980

ROLE OF HYPOTENSION INDUCED FREQUENCY OF BREATHING

BY PGI2

ON

THE

CONTROL

OF DEPTH

AND

M.G.Clement, G.Aguggini.-Istituto di M.O.TriulziO,G.C.MaggiOand Fisiologia Veterinaria e Biochimica, Universita’ di Milano, via Celoria 10,20133,Milano,Italia. (reprint request to MGC). O(Present address: Patologia Medica,Universita’ di Milano,Ospedale Bassini, Milano, Italia) ABSTRACT We studied the respiratory response to infusion of prostacyclin (PGI,)in 7 anaesthetized pigs in an attempt to understand whether the changes in pattern ofbreathing were due to a direct action of the substance or to the concomitant hypotensive effect. The depth and frequency of breathing were analyzed in terms of the threshold-inhibition curve for termination of inspiration (VT/TI relationship ) and of bulbo-pontine rhythm, estimated from inspiratory and expiratory time during occlusion of the airways at the end expiratory level (TE/TI relationship ). This provide the central respiratory rhythm when the phasic lung volume receives no input from pulmonary stretch receptors. The hypotension induced by PGI2 increased pulmonary ventilation mainly through a change in VT and caused a slight rightward displacement of VT/T1 relationship without modifying the slope of the curve.This effect seems to be vagally mediated. PG12 also changed the bulbo-pontine rhythm.Our results show that PG12 modifies the vagal discharge and bulbo-pontine rhythm by two opposite mechanisms: hypotension and an apparent direct action on carotid and aortic baroreceptors.

INTRODUCTION

Prostacyclin (PG12) is a circulating hormone (l-2) with antiaggregating properties and a powerful hypotensive effect due to a dilation of arterial and venous vessels (3-4-5). It has also observed that during hyperventilation PGI synthesis (6-7) in the lungs is increased. Changes in pattern of breathing in the anaes i; etized pig during PG12 infusion were previously studied (51, but it is still not clear whether the changes are due to a direct action of the substance or to the concomitant blood pressure reduction. It is also known that in almost all species hypotension induces an increase in ventilation (8-9-10). This behaviour can be evoked principally by two processes :a decrease in rate of firing of carotid baroreceptor afferents and a chemoreceptor stimulation caused by decreased blood flow (11). Both these mechanisms can modify the bulbo-pontine rhythm and the sensitivity of the

415

respiratory center to phasic vagal input (9-12). In this study we investigated the relationship between the hypotension induced by PGI and the respiratory changes , evaluated from depth and frequency of breathing accordins to the model suggested by Clark and von Euler (13). Von Euler’s relationship tidal volume versus inspiratory time represents the central threshold for inspiratory inhibition on which vagal afferents work. The bulbo-pontine rhythm has been studied by Grunstein (14) in terms of timing of breathing during airways occlusion at the end expiratory level: with this we can evaluate the timing of breathing in the absence of phasic lung volume changes,which are modulated by vagal stretch receptor information. MATERIALS

AND METHODS

Seven pigs of both sexes, weighing 16-25 Kg ,were premeditated with 0.05 ml/Kg of 1% propionylpromazine given intramuscularly and then anaesthetized with 30-40 mg/Kg of Pentothal,injected into the cranial cava vein through the jugular sulcus. Supplementary doses of Pentothal were given through a catheter inserted in the femoral vein. Depth of anaesthesia was monitored from respiration rate and depth and heart rate. These of course changed after vagosympathectomy and care was taken not to cut nerves after a dose of anaesthetic. The pigs were fastened in a supine position. Systemic blood pressure (BP) was recorded from the right femoral artery through a polyethylene catheter using a capacitance manometer (Bell & Howell). A tracheal cannula was inserted just below the cricoid cartilage. Tracheal pressure was measured with a differential capacitance manometer (Bell & Howell 4104 > connected to a side arm of the tracheal cannula. Tidal volume (VT) and air flow (\5 ) were measured by Fleisch No2 pneumotachograph head, internal diameter 28 mm, attached to the tracheal cannula and to a differential conductance transducer ( Statham 10897) with electrical integration. Arterial and venous PO , PC02 and pH were measured in 1.5 ml blood samples taken from the carotid artery ?Jlood pressure cannula and from jugular vein, using a Blood Gas Analyser 413 (Instrumentation Laboratory). The animal’s temperature was measured with a rectal probe and maintained by an electric blanket. Vagosympathectomy was performed bilaterally in the mid-cervical region, where the vagus nerve joins the sympathetic trunk and they are held in close association by a common connective tissue sheath down to the level of the seventh cervical vertebra. In experiments involving vagosympathectomy, 20-30 min were allowed to elapse after cutting the nerves before further procedures were carried out. Blood pressure and respiratory measurements were made in the steady state before and after vagosympathectomy and at 1,2,3,4,5,10,15 and 20 min after the beginning of perfusion with PG12 ,2pg/Kg/min for 4 min. At the moment of use,prostacyclin, as the sodium salt (5 mg> ,was dissolved in 0.5 ml of anhydrous ethanol and diluted l/10 with Tris buffer pH 9. This solution was diluted to the desired concentration with Krebs bicarbonate buffer pH 7.6. We studied the pattern of breathing in terms of tidal volume and inspiratory and expiratory times (TI and TE) and the bulbo-pontine rhythm as the timing of breaths following occlusion of the airways at the end expiratory level. This last provided an index of the timing of breathing in the absence of phasic lung volume-related vagal which are known to modulate the information from pulmonary stretch receptors, inspiratory and expiratory times. We obtained the occluded inspiratory time (TIo) from the tracheal pressure record, measured from the onset of the inspiratory effort to peak negative pressure developed in the airways, and the occluded expiratory time (TEO) from peak value to the onset of the next breath. Five to ten respiratory cycles were analyzed

416

(ml)

xl

BP(mmHg) 101.91 +6.77

13.85 +2.27

15.81 +2.20

A PlnTIo

3.93* +0.50

3.16 +0.53

TEO (see1

86.62 +4.90

2.16* +0.18

1.91 +0.22

TIo(set)

1.91 +0.22

0.97 +0.06 -

5.39 +0.56

1.69 +0.29

0.96 +0.09

5.70 +0.71

21.77 +1.81 -

247" +12

1'

TE ( set 1

P TI(sec 1

VM(l/min)

24.98 -+2.82

229 +14

Control

L

76.99 +9.45

14.41 -+2.19

4.06" +0.54

2.18* +0.14

1.80 +O.lO

1.03 +0.06

6.03 +0.37 -

21.61 -+0.93

279" +12

2’

69.75" +7.48

4.47* -+0.74 14.89 +2.36

2.12 +O.lO

1.76 +O.lO

1.03 +0.06

6.17 +0.42

21.86 +1.08

282* +13

3'

64.55" +6.84

14.89 +2.36 -

4.49" +0.76

2.10 +0.08

1.79 +0.14

1.03 +0.07

6.20 -+0.51

21.77 +1.14

284* +15

4'

65.48" +7.12

14.51 -+2.28

4.27" +0.63

2.16 +O.lO

1.73 +0.14

1.04 +0.07

6.38" 0.53

22.26 -+1.49

287" +13

5’

96.92 +4.99

14.61 +1.96

3.37 +0.56

1.93 +0.19

1.81 +0.28 -

1.03 to.10

5.20 20.55

22.79 +2.21

228 +ll

10'

Variables inpigduringand afterPGI, infusion.Values are mean + S.E..Significance at PdO.05".

FR ( per min 1

VT

Table I

99.36 +7.62

15.78 +0.80

3.27 +0.48

1.93 +0.19

1.60 +0.16

0.98 +0.08

5.69 +0.52

24.20 +1.77

238 -+18

15'

102.29 +6.33

14.27 +1.69

3.39 +0.39

1.96 +0.17

1.59 +0.13

1.00 +0.07

5.86 +0.56

24.13 +1.92

244 +18

20'

p ;

BP ( mm Hg >

AP/ATIo

TEo ( set )

64.10 24.43

52.68" 26.45

47.27" +5.17

16.59 +2.11

15.99 +2.27

16.30 +2.30

16.59 +2.52 96.85 i3.27

2.69" 20.23

2.60* +0.22

1.76 +0.12

2.29” +0.13

2.02

1.79 +O.lO

LO.11

1.79 +0.08

1.81 +0.09

TIo ( set>

2.54" +U.27

2.38* +0.19

2.02 +0.09

1.98 +O.lO

1.89 +0.14

1.93 +014

TE ( set >

4.43 +0.28

4.51 +0.34

4.84 +0.42 1.91 +0.09

5.08 +0.62

45.64" +5.19

17.41 +2.05

2.70" +0.28

1.79 +0.12

2.66 iO.32

1.83 +O.ll

4.61 +0.37

13.79* +0.89 -

336.78 ~20.49

322.14 218.53 13.86" +0.86

4'

3'

L

PGI,infusion.Values

14.22" +0.78

318.27 219.81

2'

and after

15.47* +0.70

312.07 222.98

1’

pig during

1.85 +0.09

1

16.63 +0.82

302.38 231.43

control

in vagosympathectomized

TI ( set >

VM Wmin

FR ( per min)

VT (ml)

Table II Variables

58.90* +5.60

16.70 +2.31

2.47* +0.13

1.83 +O.ll

2.34" +0.12

1.93 +O.ll

4.73 +0.29

14.25" +0.71

335.24 +22.04 -

5'

96.55 +2.87 -

15.38 i2.26

2.14 +O.ll

1.70 +0.09

1.95 to.12

1.82 +O.lO

4.99 +0.54

16.41 +0.92

303.05 +26.37

10'

96.95 +3.76

16.27 +2.56

2.05 +0.13

1.76 to.11

1.86 +0.13 -

1.93 +0.12

5.18 ~0.69

16.21 21.07

97.84 +3.18

2.03 -+0.16 17.35 +2.83

1.72 LO.14

1.81 +0.16

1.87 +0.13

5.41 20.82

16.92 +1.37

311.62 +29.52

20'

at PdO.05”.

313.93 230.85

15'

are mean + S.E..Significance

for both unoccluded

and occluded breaths.

RESULTS

Our results show a remarkable deorease in systemic arterial pressure during PGI2 infusion (from 101,91 + 6.77 to 64.55 + 6.84 mm Hg) ( Table I). The decrease was pa%cularly striking in the vagosympathectomized animal, in which mean arterial pressure fell to 45.64 + 5.19 mm Hg (Table 2). Perfusion with prostacyclin also induced changes in the minute ventilation, mainly due to an increase in tidal volume, since the respiratory frequency did not significantly change ( Table 1). After vagosympathectomy , the respiratory rate significantly decreased, with a slight increase in tidal volume. However, there was no statistically significant change in minute volume (Table 2). The output ( A P/A T ) to the respiratory muscles, evaluated as the rate of change of subatmospheric pressure developed in the trachea of the animal during the inspiratory effort against closed airways, did not change during infusion in either experimental condition (Table I and 2). We evaluated the nervous control of the pattern of breathing from the relationship V versus T described by von Euler, which shows the Hering-Breuer threshold curve for .x... f. in ibition o inspiratory activity based on the firing of pulmonary stretch receptors. To obtain this relationship we pooled the coordinates of unoccluded and occluded breaths (expression of bulbo-pontine rhythm). Fig.1 shows the results under control conditions and after 4 min of perfusion before

‘I set Fig.lV vs TI relationship symbols Jand occluded breaths vagosympathectomy ( O---J infusion ( triangles). Points refer to mean values 2

obtained by pooling the coordinates of unoccluded (open (half-filled symbols) before (&---@I A ----A> and after A-A 1. Control conditions ( circles) and after 4’ PG12 S.E. for all the animals studied.

419

and after vagosympathectomy . PGI2 caused a slight rightward displacement of the V /T V and TTo I. relationship without modifying the slope for similar increases of Vagosympathectomy abolished the negative slope of the curve, due to supp&ssion of’the pulmonary receptors. PGI2 input from stretch infusion phasic vagal postvagosympathectomy did not change the data. Fig.2 reports the expiratory versus inspiratory time relationship obtained by pooling the coordinates of the unoccluded (open symbols) and occluded (half-filled symbols) breaths. It shows how the phasic vagal afferents from pulmonary stretch receptors affect the firing frequency of the bulbo-pontine respiratory pacemaker. After 4 min of perfusion (A), PG12 induced a leftward displacement of the curve from the control position ( 0 ) , also modifying the slope . This is mainly due to an increase in T o O increases to a lesser extent. Prostacyclin infusion did not change TI or ?kzeTi and fig.2). Postvagosympathectomy T was essentially the same as the prevagosympathectomy inspiratory time during occ I uded breath. Conversely, the postvagosympathectomy T was significantly shorter than the TE O during occluded breath under control conditions L!?able 2, fig.2). After vagosympathectomy PG12 perfusion induced a lengthening of TEo of about 0.6 seconds. The hypotension induced by prostacyclin did not cause any change in pH, PO2 and PC02. All parameters returned to control values by 15 min after the beginning of the perfusion.

1

1.5

2

2.5 7.

Fig.2- T vs TI relationship obtained by pooling the coordinates of unoccluded (open symbols f and occluded (half-filled symbols )breaths before and after vagosympathectomy ( closed symbols) ,control condition ( circles) and after 4 min PG12 infusion ( triangles). Points refer to mean values + S.E. for all the animals studied.

420

DISCUSSION

Our results show that the respiratory response to hypotension evoked by PGI2 in the anaesthetized pig is characterized by an adjustment of central threshold and by a change in timing. It is well known that hypotension induces an increase in pulmonary ventilation, principally caused by the hypoxic stimulation of carotid chemoreceptors because of reduced blood flow .Some investigators (15, 16) have noted an increase in impulse activity of carotid and aortic chemoreceptor fibres during hypotension. Since the blood gases do not change during PGI2 infusion, the increase in ventilation and specially the variations in tidal volume threshold can also be correlated with the hypoxic stimulus on carotid and aortic chemoreceptors, due to decreased blood flow. Miserocchi (12) maintains that for the tidal volume the central threshold depends on centres , with their balance of three mechanisms : 1) the output of the respiratory peripheral and central components; 2) the central threshold for inhibition of inspiration and 3) the effect of vagal feed back on the threshold-sensitivity curve. Our results show that PG12 infusion does not modify the respiratory centres output, while it shifts the VT/TI relationship to the right, and also modifies the central threshold for inspiratory inhibition without changing the firing of pulmonary afferents since the tidal volume increases while the T does not change. The respiratory rate probably decreases because of an inhibitory effect on bulbopontine respiratory timing, as evaluated from the breathing timing obtained after airways occlusion at the end expiratory volume. Our results show that T O and TEo progressively increase, with greatest change after 4 min of perfusion, when t F,e blood pressure is about 64 mm Hg. The threshold and frequency changes after PG12 infusion appear to be principally mediated by vagus nerve, because after bilateral vagosympathectomy, PGI does not modify theVT/TI relationship and causes only a slight increase in TEo., &ilva (8) observed in the cat , after hypotension induced by hemorrhage, an increase in pulmonary ventilation due to an increase in tidal volume, without changes in respiratory rate because of chemoreceptor stimulation. The same author suggests that the decrease in firing of baroceptor afferents does not significantly contribute to the respiratory response . On the contrary, Miserocchi (9) feels that after bleeding, when the blood pressure reaches the aortic baroreceptor activity threshold, the respiratory rate increases because of withdrawal of inhibitory vagal afferents. When blood pressure is below 100 mmHg, the ventilatory response is mainly due to the chemoreceptor stimulation by reduced blood flow, while the decrease in inhibitory afferents from carotid baroreceptors plays a lesser role. In the pig, unlike in other species, when blood pressure drops to aortic baroreceptor activation threshold , the TEo becomes shorter without changes in the VT/T1 relationship ( 17). Therefore, T E” shortening after vagosympathectomy can be due to the withdrawal of inhibitory vagal input from aortic baroreceptor afferents (fig.2). The hypotensive effect of PG12 should thus have an excitatory action on expiratory timing, but we found similar inhibitory effects equal postvagosympathectomy during perfusion. Since recently Aguggini et al. (18) found that hypoxic stimulation of pigs did not affect the TE versus TI relationship, the change in T O could be related to some direct This stimulus should stimulation by prostacyclin of aortic and caroti 5 baroreceptors. lead to a strengthening of the inhibitory effect on the respiratory center.

421

The lesser lengthening of the TE” after vagosympathectomy can be ascribed to the absence of vagal inhibitory action. PGI2 therefore seems to be able to modify the vagal discharge and bulbo-pontine pacemaker through two opposite effects:the first a blood pressure reduction, the second a possible direct action of prostacyclin on carotid and aortic baroreceptors.

ACKNOWLEDGEMENT We are grateful Milano, Italy,for

to Professor C.Gandolfi supplying the PGI2.

of Research

Institute

of Farmitalia-Carlo

Erba

REFERENCES

1.

Gryglewski RJ,Bunting S,Moncada S,Flower RJ.Arterial walls are protected against deposition of platelet thrombi by a substance (prostaglandin X > which they make from prostaglandin endoperoxides.Prostaglandin 12:685,1976.

2.

Moncada S,Korbut R,Bunting Nature (Land) 273:767,1978.

3.

Armstrong JM,Lattimer N,Moncada S,Vane JR.Comparison of the vasodepressor effects of prostacyclin and 6-oxo-prostaglandin F with those of prostaglandin E2 in rats and rabbits. Brit.J.Pharmacol.62:125,19&.

4.

Weeks JR.The general pharmacology of especially activity prostaglandin system.Pol.J.Pharmacol.Pharm. 30:215,1978.

5.

Clement MG,Triulzi MO,Maggi GC,Aguggini G.Analysis of hemodinamic respiratory effects of PG12 in pigs.Prostaglandins and Medicine: in press.

6.

Griglewski RJ,Korbut R,Ocetkiewicz A.Generation vivo and its release into the arterial circulation.Nature

7.

Gryglewski RJ,Korbut R,Ocetkiewicz A,Splawinski J,Wojtaszek B,Swies J. Lungs on physiological significance. as a generator of prostacyclin - hypothesis Naunyn-Schmiedeberg’s.Arch.Pharmac.304:45,1978.

8.

D’Silva JL,Gill D,Mendel D.The effects cat.J.Physiol. (Land) 187:369,1966.

9.

Miserocchi G,Quinn B.Control of breathing anaesthetized cats.Resp.Physiol.:in press 1980.

10.

Chien S.Role 47:214,1967.

of

the

S,Vane JR.Prostacyclin

sympathetic

hormone.

prostacyclin (PGI PGX):a new 2c)ardiovascular the on

during

system

and

of prostacyclin by lungs in (Land) 273:765,1978.

of acute hemorrhage

nervous

422

is a circulating

on respiration

acute

in the

hemorrhage

in hemorrhage.Physiol.Erg.

in

11.

Comroe JHJ.The location J.Physio1.27:176,1939.

and function

of the chemoreceptors

12.

Miserocchi G.Role of peripheral and central chemosensitive control of depth and frequency of breathing.Resp.Physiol.26:101,1976.

afferents

13.

Clark FG,von Euler C. On the regulation J. Physiol. (Land) 222:267,1972.

of breathing.

14.

Grunstein frequency

15.

Daly M de B,Lambertsen CJ,Schweitzer A.Observations on the volume of blood flow and oxigen utilisation of the carotid body in the cat.J.Physiol.(Lond) 125:67,1954.

16.

Landgren S,Neil E.Chemoreceptors Physiol.Scand.23:58,1951.

17.

Clement MG,Aguggini G,Miserocchi G.Respiratory in pigs.Quart.J.Exp.Physiol.and Cogn.Medicine:in

18.

Aguggini G,Clement MG,Davies asphyxia.Res.Vet.Sc.26:267,1979.

of detph and frequency

MM,Younes M,Milic-Emili J.Control of tidal in anaesthetized cats.Appl.Physiol.35:463,1973.

impulse

A.Unusual

423

of the aorta.Am.

activity

volume

following

modulation press.

response

in the

and respiratory

hemorrhage.Acta

by vagal afferents

of anaesthetized

pigs

to