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Generation and Propagation of Uterine Activity in Situ
Generation and Propagation of Uterine Activity in Situ HIROSHI TAKEDA, M.D.
have studied the generation and propagation of the electrical activity of the uterus. 4 • 5. 8. 12 In these pioneering studies, excised strips were examined in vitro so as to characterize the nature of the excitation process in this special tissue. Recently, a new method6 has been developed which allowed us to examine the excitation process during chronic experiments in the living, unanesthetized animal. The present report describes a continuation of this work. The parturient and postpartum rabbit uterus is electrically and mechanicallyactive. 6 Activity readily propagates along the entire uterine horn. Direct visual observation during spontaneous labor suggests that parturient activity in rabbits is initiated at the cervical portion. When the entire uterine contents are expelled, activity subsides. However, when the uterine cavity is refilled by a condom rubber balloon containing fluid, activity returns and becomes a function of uterine volume. 6 Under these conditions, electrical activity is usually generated at the ovarian end of the uterus, although occasionally the cervical end remains the pacemaker; at least temporarily. A large number of records collected during a 2-year period show that all portions of the postpartum rabbit uterus are capable of becoming pacemakers. Indeed, we frequently observed that pacemaker activity shifted from one uterine portion to the other during a few hours of observation. The propagation velocity shows considerable fluctuation from one animal to the other. In all animals, it increases with time elapsed after delivery. Propagation velocity in the parturient animal is greatly accelerated by an increase in uterine volume. Conversely, velocity can be greatly reduced, and the activity wave blocked, by drastic volume reduction. The present report demonstrates the potentiality of the uterus for multiple pacemaker activity and the shift in the pacemaker activity of the parturient
SEVERAL INVESTIGATORS
From The Rockefeller Institute, New York, N. Y. The study described in this article was supported by Grant RG-7942 from the National Institutes of Health and by the Population Council.
113
114
TAKEDA
FERTILITY
& STERILITY
uterus. The triggering effect of sudden volume increase and its facilitating action on synchronic activity are also illustrated. METHOD
The simultaneous recording of the electrical and mechanical activity of the rabbit uterus in situ has been described previously.6 The same method was employed in the present experiments. One balloon filled the uterine cavity, and three electrodes were inserted into the uterine wall. In a second type of experiment, a double balloon with a connecting tube was inserted into the uterine cavity, rather than one balloon, and only two electrodes were used. Figure I illustrates the second experimental arrangement. The two balloons, a and b, communicate through a polyethylene tube, c, of I-cm. diame-
Fig. 1. Schematic illustration of experimental arrangement.
ter. Each balloon is connected to a pressure transducer by polyethylene tubes, d and e. An electrode is inserted into the myometrial wall adjacent to the middle portion of each balloon at f and g. These are chronic, external electrodes. RESULTS
Before describing the experiments with the double7balloon assembly, we will show the effect of mechanical block in propagation and pacemaker activity. Figure 2 illustrates recordings made during a typical experiment. As in earlier experiments, a single balloon filled the uterine cavity, and three electrodes were inserted into the wall of the parturient uterus. Record A shows that the ovarian, middle, and cervical uterine portions work in synchrony and that activity is generated at the ovarian end of the uterus. The uterus and the balloon are then ligated with a thick thread between the ovarian and middle portion (B) . Propagation is blocked at the site of the ligature, and the two uterine segments now function independently. It is evident that the activity at the ovarian end is not disturbed and that it keeps its original rhythm. The lower segments are only slightly disturbed, for the middle portion promptly assumes pacemaker activity and generates regular train discharges. The only change is a reduced rhythm.
VOL.
115
UTERINE ACTIVITY
16, No.1, 1965
When the ligature is released and is placed between the middle and cervical portions (C), the ovarian and middle portions again work in synchrony, propagation between them being restored, and the cervical portion assumes a new rhythm dictated by its own pacemaker activity. These records illustrate that propagation is blocked by a mechanical procedure such as ligation of the uterine horn; the effect is reversible. They
A
..
B
~ 1(1 • *' ~ .. '" ~. .. I~ M • ~ •
• , •
I~*
i11, ~ ~ ~11\t ~ , ~ + t ; ~ ; t ~ ~
10 see.
c
Fig. 2. Effect of ligation on synchronic electric activity of parturient rabbit uterus. See text.
also show that a uterine portion which is normally under the influence of the pacemaker activity of another uterine portion becomes a pacemaker when the superimposed influence is eliminated. During the experiment described, two types of changes were imposed upon the uterus when the uterine horn and the balloon in its cavity were ligated. Before ligation, the upper and lower uterine portions had an electrical contact through propagation in the muscle fibers and also a fluid contact in the balloon. It is of interest to study the relative contribution to synchronic activity of these two factors. This can be done by the double or "dumbbell" balloon technic shown in Fig. 1. Figure 3, A, shows shifting pacemaker activity. The first, third, fourth, fifth, and seventh excitation waves originate at the ovarian end, whereas the second and sixth do so at the cervical end of the uterus. In spite of this shifting pacemaker activity, propagation is fair, and the average time delay between the activity of the two uterine portions is 10.1 sec./IO cm. (Table 1).
116
TAKEDA
FERTILITY
& STERILITY
When fluid contact between the two uterine portions is interrupted by closing the connecting tube (Fig. 3 B), only the electric communication through the muscle fibers remains between the uterine portions. The effect of the suspension of fluid contact is manifested by a delay in propagating activity. The average delay increases from 10.1 sec.flO cm. to 15.7 sec.flO cm. (Table 1). The significance of a fluid contact between different uterine portions is best disclosed by the experiment illustrated in Fig. 4. The procedure is the same as before. At A both electrical and fluid contacts operate, and there is good synchrony. At B both contacts have been interrupted by cutting the hom into two segments and by closing the tube connecting the two balloons. There is complete asynchrony, and the two segments function inde-
TABLE 1. Time Delays in Experimental Conditions Illustrated in Fig. 3 and 4 Mode of connection Figure No.
3A 3B 4A 4B 4C
Time delay (sec.) Myometrial
+ + +
Hydrodynamic
I
II
III
IV
V
VI
VII
Mean
8 20 4
12 22 2
18 16 6
10 12
6 16
6 10
+
8 14 2
10.1 15.7 3.5
+
6
8
2
+
Independent
8
6.0
VOL.
16, No.1, 1965
UTERINE ACTIVITY
117
pendently. At C the fluid contact has been restored by opening the tube. Synchronic activity promptly returns (Table 1). A reasonable explanation for this effect of fluid movement is the following. When one uterine segment becomes active, fluid escapes rapidly from the
A Fig. 4. Effect on synchronic activity of cutting uterine horn into two segments, and suspending and restoring fluid contact between separated segments. See text.
B
c
underlying balloon to the balloon of the inactive segment. While the pacemaker segment contracts, the inactive segment is stretched. Rapid stretch activates the inactive segment during the contraction time of the pacemaker segment, inducing activity in the second segment shortly after the first segment. DISCUSSION ._'\
All uterine portions of the parturient uterus are potential pacemakers. During spontaneous labor, the cervical portion sets the rhythm, as a rule, while after delivery, it is taken over by the ovarian portion. In in-vitro studies ovarian-end dominance was observedY·13 Theoretically, pacemaker activity would arise from portions of the lowest threshold. As there is no
llB
TAKEDA
FERTILITY
& STERILITY
apparent anatomic difference between cells of different uterine portions, local stretch, local hormone effects, differences in innervation, and related tactors may affect the threshold. The present experiments show that when a new portion becomes the pacemaker, activity is then generated less frequently at least for a time. This suggests that normally, the pacemaker activity originates from the uterine portion that has the higher rhythm. The automaticity of the remaining uterine portions then becomes masked. The mechanism of propagation in the uterus is uncertain. Electrical, mechanical, chemical, and nerve-controlled mechanisms are discussed in the literature. Recent studies 1 endorse electrical continuity between cells as the most likely mechanism. In the case of the whole, pregnant uterus, the mechanical trigger surely operates, as demonstrated here. It is well known that smooth muscles are readily activated by stretch. 2 • 3 The effect is ascribed to sudden depolarization and increased Na permeability. The membrane potential most readily reaches firing level when potential fluctuation is exaggerated by moderate depolarization. Depolarization without fluctuation is thought to be insufficient to cause spike discharge. 9 However, depolarization triggers train discharge, no matter how achieved, in a uterus capable of train discharge. 5 The fact that without direct structural contact (i.e., when the uterus is cut into two segments) the myometrium can work in synchrony through a stretch effect is interesting, for it is evident that a mechanical and hydrodynamic contact does exist during pregnancy between different uterine portions. The placental uterine portion is known to have a high threshold and relative inactivity/o and because of this, its sensitivity to stretch is low. 7 The contraction wave at this portion may be blocked only when, in addition to electrical contact, the effects of mechanical and hydrodynamic contacts are minimal. SUMMARY Propagation in the parturient uterus in situ is facilitated by alternate mechanisms. Synchronic activity at distant uterine portions is achieved by the spread of electrical activity from one cell to the next, by the stretching effect of the active portion on the passive region, and by the stretching effect of fluid, moving from the active toward the passive region. When one of these factors is eliminated, the propagation velocity decreases. When both the cell-to-cell contact and the stretching effects are suspended, the isolated uterine segments function independently. Shionogi Research Laboratories Osaka, Japan
VOL.
16, No.1, 1965
UTERINE ACTIVITY
119
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
BARR, L. /. Theoret. Biol. 4:73, 1963. BOZLER, E. Am. J. Physiol. 149:299, 1947. BULBRING, E. J. Physiol. 128:200, 1955. BURNSTOCK, G., et al. Physiol. Rev. 43:482, 1963. CSAPO, A. I. Physiol. Rev. 42 (Suppl. 5) :7, 1962. CSAPO, A., TAKEDA, H., and WOOD, C. Am. J. Obst. & Gynec. 85:813, 1963. DANIEL, E. G. Am. J. Obst. & Gynec. 80:229, 1960. GOTO, M., and TAMAI, T. Kinpodo Publishing Co., Tokyo, 1960. GOTO, M., and WOODBURY, J. W. Fed. Proc. 17:48, 1958. KURIYAMA, H., and CSAPO, A. Am. /. Obst. & Gynec. 82:592, 1961. MARSHALL, J. M. Am. J. Physiol. 197:935, 1959. MARSHALL, J. M. Physiol. Rev. 42 (Suppl. 5) :213, 1962. REYNOLDS, A. M. R. Physiology of the Uterus, ed. 2. Hoeber, New York, 1949.
have studied the generation and propagation of the electrical activity of the uterus. 4 • 5. 8. 12 In these pioneering studies, excised strips were examined in vitro so as to characterize the nature of the excitation process in this special tissue. Recently, a new method6 has been developed which allowed us to examine the excitation process during chronic experiments in the living, unanesthetized animal. The present report describes a continuation of this work. The parturient and postpartum rabbit uterus is electrically and mechanicallyactive. 6 Activity readily propagates along the entire uterine horn. Direct visual observation during spontaneous labor suggests that parturient activity in rabbits is initiated at the cervical portion. When the entire uterine contents are expelled, activity subsides. However, when the uterine cavity is refilled by a condom rubber balloon containing fluid, activity returns and becomes a function of uterine volume. 6 Under these conditions, electrical activity is usually generated at the ovarian end of the uterus, although occasionally the cervical end remains the pacemaker; at least temporarily. A large number of records collected during a 2-year period show that all portions of the postpartum rabbit uterus are capable of becoming pacemakers. Indeed, we frequently observed that pacemaker activity shifted from one uterine portion to the other during a few hours of observation. The propagation velocity shows considerable fluctuation from one animal to the other. In all animals, it increases with time elapsed after delivery. Propagation velocity in the parturient animal is greatly accelerated by an increase in uterine volume. Conversely, velocity can be greatly reduced, and the activity wave blocked, by drastic volume reduction. The present report demonstrates the potentiality of the uterus for multiple pacemaker activity and the shift in the pacemaker activity of the parturient
SEVERAL INVESTIGATORS
From The Rockefeller Institute, New York, N. Y. The study described in this article was supported by Grant RG-7942 from the National Institutes of Health and by the Population Council.
113
114
TAKEDA
FERTILITY
& STERILITY
uterus. The triggering effect of sudden volume increase and its facilitating action on synchronic activity are also illustrated. METHOD
The simultaneous recording of the electrical and mechanical activity of the rabbit uterus in situ has been described previously.6 The same method was employed in the present experiments. One balloon filled the uterine cavity, and three electrodes were inserted into the uterine wall. In a second type of experiment, a double balloon with a connecting tube was inserted into the uterine cavity, rather than one balloon, and only two electrodes were used. Figure I illustrates the second experimental arrangement. The two balloons, a and b, communicate through a polyethylene tube, c, of I-cm. diame-
Fig. 1. Schematic illustration of experimental arrangement.
ter. Each balloon is connected to a pressure transducer by polyethylene tubes, d and e. An electrode is inserted into the myometrial wall adjacent to the middle portion of each balloon at f and g. These are chronic, external electrodes. RESULTS
Before describing the experiments with the double7balloon assembly, we will show the effect of mechanical block in propagation and pacemaker activity. Figure 2 illustrates recordings made during a typical experiment. As in earlier experiments, a single balloon filled the uterine cavity, and three electrodes were inserted into the wall of the parturient uterus. Record A shows that the ovarian, middle, and cervical uterine portions work in synchrony and that activity is generated at the ovarian end of the uterus. The uterus and the balloon are then ligated with a thick thread between the ovarian and middle portion (B) . Propagation is blocked at the site of the ligature, and the two uterine segments now function independently. It is evident that the activity at the ovarian end is not disturbed and that it keeps its original rhythm. The lower segments are only slightly disturbed, for the middle portion promptly assumes pacemaker activity and generates regular train discharges. The only change is a reduced rhythm.
VOL.
115
UTERINE ACTIVITY
16, No.1, 1965
When the ligature is released and is placed between the middle and cervical portions (C), the ovarian and middle portions again work in synchrony, propagation between them being restored, and the cervical portion assumes a new rhythm dictated by its own pacemaker activity. These records illustrate that propagation is blocked by a mechanical procedure such as ligation of the uterine horn; the effect is reversible. They
A
..
B
~ 1(1 • *' ~ .. '" ~. .. I~ M • ~ •
• , •
I~*
i11, ~ ~ ~11\t ~ , ~ + t ; ~ ; t ~ ~
10 see.
c
Fig. 2. Effect of ligation on synchronic electric activity of parturient rabbit uterus. See text.
also show that a uterine portion which is normally under the influence of the pacemaker activity of another uterine portion becomes a pacemaker when the superimposed influence is eliminated. During the experiment described, two types of changes were imposed upon the uterus when the uterine horn and the balloon in its cavity were ligated. Before ligation, the upper and lower uterine portions had an electrical contact through propagation in the muscle fibers and also a fluid contact in the balloon. It is of interest to study the relative contribution to synchronic activity of these two factors. This can be done by the double or "dumbbell" balloon technic shown in Fig. 1. Figure 3, A, shows shifting pacemaker activity. The first, third, fourth, fifth, and seventh excitation waves originate at the ovarian end, whereas the second and sixth do so at the cervical end of the uterus. In spite of this shifting pacemaker activity, propagation is fair, and the average time delay between the activity of the two uterine portions is 10.1 sec./IO cm. (Table 1).
116
TAKEDA
FERTILITY
& STERILITY
When fluid contact between the two uterine portions is interrupted by closing the connecting tube (Fig. 3 B), only the electric communication through the muscle fibers remains between the uterine portions. The effect of the suspension of fluid contact is manifested by a delay in propagating activity. The average delay increases from 10.1 sec.flO cm. to 15.7 sec.flO cm. (Table 1). The significance of a fluid contact between different uterine portions is best disclosed by the experiment illustrated in Fig. 4. The procedure is the same as before. At A both electrical and fluid contacts operate, and there is good synchrony. At B both contacts have been interrupted by cutting the hom into two segments and by closing the tube connecting the two balloons. There is complete asynchrony, and the two segments function inde-
TABLE 1. Time Delays in Experimental Conditions Illustrated in Fig. 3 and 4 Mode of connection Figure No.
3A 3B 4A 4B 4C
Time delay (sec.) Myometrial
+ + +
Hydrodynamic
I
II
III
IV
V
VI
VII
Mean
8 20 4
12 22 2
18 16 6
10 12
6 16
6 10
+
8 14 2
10.1 15.7 3.5
+
6
8
2
+
Independent
8
6.0
VOL.
16, No.1, 1965
UTERINE ACTIVITY
117
pendently. At C the fluid contact has been restored by opening the tube. Synchronic activity promptly returns (Table 1). A reasonable explanation for this effect of fluid movement is the following. When one uterine segment becomes active, fluid escapes rapidly from the
A Fig. 4. Effect on synchronic activity of cutting uterine horn into two segments, and suspending and restoring fluid contact between separated segments. See text.
B
c
underlying balloon to the balloon of the inactive segment. While the pacemaker segment contracts, the inactive segment is stretched. Rapid stretch activates the inactive segment during the contraction time of the pacemaker segment, inducing activity in the second segment shortly after the first segment. DISCUSSION ._'\
All uterine portions of the parturient uterus are potential pacemakers. During spontaneous labor, the cervical portion sets the rhythm, as a rule, while after delivery, it is taken over by the ovarian portion. In in-vitro studies ovarian-end dominance was observedY·13 Theoretically, pacemaker activity would arise from portions of the lowest threshold. As there is no
llB
TAKEDA
FERTILITY
& STERILITY
apparent anatomic difference between cells of different uterine portions, local stretch, local hormone effects, differences in innervation, and related tactors may affect the threshold. The present experiments show that when a new portion becomes the pacemaker, activity is then generated less frequently at least for a time. This suggests that normally, the pacemaker activity originates from the uterine portion that has the higher rhythm. The automaticity of the remaining uterine portions then becomes masked. The mechanism of propagation in the uterus is uncertain. Electrical, mechanical, chemical, and nerve-controlled mechanisms are discussed in the literature. Recent studies 1 endorse electrical continuity between cells as the most likely mechanism. In the case of the whole, pregnant uterus, the mechanical trigger surely operates, as demonstrated here. It is well known that smooth muscles are readily activated by stretch. 2 • 3 The effect is ascribed to sudden depolarization and increased Na permeability. The membrane potential most readily reaches firing level when potential fluctuation is exaggerated by moderate depolarization. Depolarization without fluctuation is thought to be insufficient to cause spike discharge. 9 However, depolarization triggers train discharge, no matter how achieved, in a uterus capable of train discharge. 5 The fact that without direct structural contact (i.e., when the uterus is cut into two segments) the myometrium can work in synchrony through a stretch effect is interesting, for it is evident that a mechanical and hydrodynamic contact does exist during pregnancy between different uterine portions. The placental uterine portion is known to have a high threshold and relative inactivity/o and because of this, its sensitivity to stretch is low. 7 The contraction wave at this portion may be blocked only when, in addition to electrical contact, the effects of mechanical and hydrodynamic contacts are minimal. SUMMARY Propagation in the parturient uterus in situ is facilitated by alternate mechanisms. Synchronic activity at distant uterine portions is achieved by the spread of electrical activity from one cell to the next, by the stretching effect of the active portion on the passive region, and by the stretching effect of fluid, moving from the active toward the passive region. When one of these factors is eliminated, the propagation velocity decreases. When both the cell-to-cell contact and the stretching effects are suspended, the isolated uterine segments function independently. Shionogi Research Laboratories Osaka, Japan
VOL.
16, No.1, 1965
UTERINE ACTIVITY
119
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
BARR, L. /. Theoret. Biol. 4:73, 1963. BOZLER, E. Am. J. Physiol. 149:299, 1947. BULBRING, E. J. Physiol. 128:200, 1955. BURNSTOCK, G., et al. Physiol. Rev. 43:482, 1963. CSAPO, A. I. Physiol. Rev. 42 (Suppl. 5) :7, 1962. CSAPO, A., TAKEDA, H., and WOOD, C. Am. J. Obst. & Gynec. 85:813, 1963. DANIEL, E. G. Am. J. Obst. & Gynec. 80:229, 1960. GOTO, M., and TAMAI, T. Kinpodo Publishing Co., Tokyo, 1960. GOTO, M., and WOODBURY, J. W. Fed. Proc. 17:48, 1958. KURIYAMA, H., and CSAPO, A. Am. /. Obst. & Gynec. 82:592, 1961. MARSHALL, J. M. Am. J. Physiol. 197:935, 1959. MARSHALL, J. M. Physiol. Rev. 42 (Suppl. 5) :213, 1962. REYNOLDS, A. M. R. Physiology of the Uterus, ed. 2. Hoeber, New York, 1949.