Acta Physiol Scand 1990, 139, 241-242
Olsalazine sodium stimulates chloride transport across the bullfrog cornea B. O D L I N D a n d 0. E R I K S S O N " Department of Medical Pharmacology, Biomedicum, and Chemistry, University of Uppsala, Sweden Olsalazine sodium consists of two molecules of 5aminosalicylic acid (5-ASA) coupled with an azobond. After bacterial splitting of the azo-bond in the colon, 5-ASA can exert its anti-inflammatory action, e.g. in ulcerative colitis (Sandberg-GertzCn et al. 1986).However, the unsplit olsalazine molecule has in high concentrations been shown to act as a secretagogue in the rat intestine (Goerg et al. 1987);in clinical trials the drug sometimes results in loose stools or even diarrhoea. The mechanism behind this effect is unclear; a stimulation of chloride secretion has been suggested (Goerg et al. 1987).The purpose of the present study was to examine this possibility by exposing olsalazine to the 'pure ' chloride-transporting epithelium of the bullfrog cornea. Frogs (Rana catesbeiana) of both sexes were purchased from Graska Biological Supply Co., Oshkosh, WI, USA, and kept in tap water for at least 2 weeks after delivery. The water temperature was 8-10 "C. T h e toads were fed manually with pieces of ox liver once a week. Corneas from doubly pithed frogs were mounted as described earlier (Eriksson & Wistrand 1986). The transport of sodium and chloride was determined electrophysiologically by measuring the transepithelial potential difference (PD ; mV), the short-circuit current (SCC; ,uA cm-') and the DC resistance (R; ohm cm'). These methods have been described in detail earlier (Eriksson & Wistrand 1986). The epithelia displayed a transepithelial P D of 16.9f2.4mV (tear side negative), an SCC of I I .7& I .o pA cm-' and a transepithelial D C resistance of 1504$zoz ohm cm' (mean+SE; n = 8). Results are given as arithmetic means+SE. T h e differences between predrug and post-drug values for frog cornea were evaluated by Student's t-test (twosided test) for paired variables and considered significant at P < 0.05 (").
* Department
of Clinical
Olsalazine sodium ( I .2-5.0 mM) added to the aqueous solution stimulated the short-circuit current (SCC) and the transepithelial P D in the isolated cornea of the frog, Rana catesbeiana, by about 20% (o.001 < P < 0.025), while the D C resistance was unchanged (-0.8%; P > 0.70; Table I). After the initial significant increase of SCC and PD, the values levelled out (Fig. I ) . Lower concentrations of olsalazine added to the aqueous side had no effect (Table I). When added to the contralateral solution in low concentration (0.3 mM), the drug had no effect, while higher concentrations of the drug stimulated the short-circuit current without affecting the PD (Table 1).
About 95 yo of the transepithelial potential difference and short-circuit current of the amphibian cornea is carried by chloride ions, which are transported from the aqueous (endothelial) to the tear (epithelial) side (Zadunaisky 1966,Candia & Askew I 968). Thus, there is no doubt that olsalazine sodium has a stimulating effect on the chloride secretion of frog
30t
T
Il* \ I
20
Received 30 January 1990,accepted 14 February '990. Key words : bullfrog cornea, chloride transport, olsalazine. Correspondence : Bo Odlind MD, PhD, Department of Medical Pharmacology, Biomedicum, Box 593, S-75124 Uppsala, Sweden.
I i M t \minutesiAb I t
T
.
40 60 n AUUI I IUN
OF OLSALAZINE SODIUM
Fig. I . Effects of olsalazine sodium (1.2-5mM) on potential difference (PD) and short-circuit current (SCC) in frog corneal epithelium. The drug was added to the aqueous solution. Values are means f SE. * denotes significant ( P < 0.05) changes from baseline.
24 1
242
B . Odlind and
0’. Eriksson
Table I . Effects of olsalazine sodium on electrophysiological variables in the cornea of Rana catesbeiana. The drug was added to the aqueous (A) or tear (T) side and the effects were recorded 8 m i n thereafter. The values are mean$SE. The aqueous side was positive with respect to the tear side. Numbers within parentheses are the numbers of corneas tested Per cent change after drug Drug concentration (mM) 0.3 (A) I .2-5 (A) 0.3 (T) 5 (TI
PD
+ 3.4 (2)
+ 16.5 k4.2(6)* -4.4 ( 2 ) -8.6 (I)
SCC
Resistance
- 3.4 +21.5f7.5* -2.4 + ‘9.3
+ 7.5 -0.8+ 5.1 - 2.0 -23.4
cornea. This is in line with the effect of this drug observed by Goerg et al. (1987). They observed that olsalazine sodium stimulated the chloride secretion and, to a lesser degree, the sodium secretion in the rat ileum and colon. In the cornea of amphibians, SCC is increased by 3-isobutyl-~-methylxanthine (IBMX), prostaglandins (PGE,) and dibutyryl cyclic AMP, added to the aqueous side (Chalfie et a/. 1972, Beitch et al. 1975, Eriksson & Wistrand 1986). However, we do not know at present if the stimulatory effect of olsalazine sodium on chloride secretion is mediated by cyclic AMP, prostaglandins or some other mediator system. This study, however, shows that the drug has a stimulatory effect on the chloride transport of a ‘model tissue’, the frog corneal epithelium.
REFERENCES BEITCH,B.R., BEITCH,I. & ZADUNAISKY, J.A. 1975. The stimulation of chloride transport by prostaglandins and their interaction with epinephrine, theophylline and cyclic AMP in corneal epithelium. 3 Membr Bid 19, 381-396. CANDIA, O.A. & ASKEW,W.A. 1968. Active sodium
transport in the isolated bullfrog cornea. Biochim Bioph.ys Acta 163, 262-265. CHAI.FIE,M., NEUFELD,A.11. 8i ZADUNAISKY, J.A. 1972. Action of epinephrine and other cyclic AMPmediated agents on chloride transport of frog cornea. Invest Ophthalmol I I , 644-650. ERIKSSON, 0.& WISTRAND, P. 1986. Inhibitory effects of chemically different ‘loop’ diuretics on chloride transport across the bullfrog cornea. Acta Physiol Scand 127, 137-144. GOERG,K.J., WANITSCHKE, R., GABBERT, H., BREILING, J., FRANKE, M. & MEYERZUM B~SCHENFELDE, K.-H. 1987. T h e effect of disodium (DSA) on water and electrolyte transfer of the rat ileum and colon in vivo compared with sulfasalazine (SASP), 5 aminosalicylic acid (5ASA) and sulfapyridine (SP). Digestion 37, 79-87. SANDBERG-GERTZEN, H., JARNEROTH, G. 8i GRAAZ, W. 1986. Azodisalsodium in the treatment of ulcerative colitis. A study of tolerance and relapse prevention properties. Gastroenterology 90, 1024. ZADUNAISKY, J.A. 1966. Active transport of chloride in frog cornea. A m 3 Physiol 211, 505-512.
Acta Physiol Scand 1990, 139, 243-244
Decreased Ca2+ buffering contributes t o slowing of relaxation in fatigued Xenopus muscle fibres H. W E S T E R B L A D and J. L A N N E R G R E N Department of Physiology 11, Karolinska Institute, Stockholm, Sweden Besides a decline in force, slowing of relaxation is a well-known attribute of fatigue in skeletal muscle. The slowing may be due to a decreased rate of crossbridge cycling and/or less efficient removal of calcium from the troponin-tropomyosin complex. A major determinant of Ca2+removal is the activity of the Ca2+ pump in the sarcoplasmic reticulum (SR), but interaction ofCa2+with parvalbumin (PA), a cytosolic Ca2+-binding protein, may also be important. In the present study we have tried to evaluate the importance of such Ca2+-PA interaction versus acidification (which affects both cross-bridges and SR Ca2+pumps) for the slowing of relaxation developing with fatigue. Single large, transparent fibres (type I ;Westerblad & Lannergren 1986) were dissected from lumbrical muscles of Xennpus laevis, mounted in a perfusion chamber for stimulation and isometric tension recording and superfused with standard Ringer solution ( 2 2 . 5 "C). Intracellular acidosis was produced by changing to Ringer solution bubbled with 5% CO, +95 y) 0,. Fatiguing stimulation consisted of 350 ms, 70 Hz tetani given every 3.8 s and was continued until tetanic tension was down to about 70% of the original. The Ca2+-PA interaction was evaluated by interrupting stimulation for a short period at various t i m k during a long tetanus and measuring the ensuing brief relaxation (see Peckham & Woledge 1986). These interrupted tetani had a total duration of about 2 s and consisted of recurring IOOms on-periods (70 Hz stimulation) and 50-ms offperiods (no stimulation). Measurements were made from digitized tension records. The relaxation rate was evaluated by measuring the slope during a Is-ms interval of the initial, nearly linear, part of force decline. In each experiment the relaxation rate after a rested 350-m~tetanus was set as I O O ~ ( ; this rate ranged between 984 and 1200kPa s-' in the four fibres studied. For statistical analysis paired t-tests were used and the significance level was set at 0.01 throughout. Received z February 1990, accepted 14 February 1990. Key mords: Ca2+,fatigue, parvalbumin, pH, relaxation, skeletal muscle, Xennpus luevzs. Correspondence : Jan Lannergren, Department of Physiology, Karolinska Institute, S-104 0 1 Stockholm, Sweden.
Figure I (a) shows interrupted contractions of one fibre produced in the three different states. Relaxation became progressively slower during the contraction in the rested state as well as when the fibre was acidified by 5 TICO,. In the fatigued state, however, relaxation was already very slow at the beginning and remained unchanged throughout the contraction. The same (a)
J
L
300 rns
0.0
0.5
1 .o
1.5
2.0
Duration (s)
Fig. I . (a) Force production during an interrupted tetanus in the rested (top record), acidified (middle) and fatigued (bottom) state. In this fibre fatigue was produced by 46 short tetani and the interrupted contraction was started 1 . 8 s after the last fatiguing tetanus. (b) Mean values of relaxation rate at various times during interrupted tetani in the rested ( O ) , acidified ( 0 )and fatigued (A) state; SEM indicated by bars when larger than symbol size. The relaxation rate at the end of a rested, 350-m~tetanus was set as 10o0/, and is indicated by the dotted lines. Curves were constructed from the mean values in Table I .
243
244
W.Westerblad and J, Lannergren
Table I. Mean values (fSEM, n = 4) of the initial, extra rate of relaxation (yo), the final, steady relaxation rate (v,) and the time constant of the slowing (7).In each experiment values of yo, y , and 7 were obtained by fitting data points to the equation given in the text
y,(o/6) Y , (%) 7 (S)
Rested
Acidified
Fatigued
99.8k6.4 59.8 f3.6 0.43f0.03
80.8f9.0 34.3 f3.5 0.36f0.02
o 26.3 k 1.7 -
observations were made in the other three fibres, and I (b) and Table I. In Fig. ~ ( b the ) symbols represent the mean rate of relaxation, expressed as per cent of the rate after a rested 350-m~tetanus, plotted against the time from the onset of contraction (t). The solid lines in Fig. I (b) and the values in Table I were obtained by fitting experimental data with the equation : the results are summarized in Fig.
+
the period of fatiguing stimulation the Ca2+ release from the SR must have been large enough to fully saturate PA. The present results show that it is necessary to take Ca*+ buffering into account when investigating the cause of slowed relaxation in fatigue. If, for example, the effects of fatigue and acidosis in our case had been compared by measuring the rate of relaxation after a standard 350-m~tetanus, PA would have been fully saturated with Ca2+ in the fatigued state but only about half-saturated in the acidified state. This would then give a considerably faster relaxation rate in the latter case. However, with longer tetani PA also gets saturated with Ca2+ in the acidified state and the relaxation rates become similar. Thus, at least in amphibian muscle the combined effect of CaZ+ saturation of PA and reduced pH, can explain the slowing of relaxation during fatigue. This work was supported by the Swedish MRC (project no. 3642) and by the Bergvall Foundation.
.y = y oe&” y ,
REFERENCES
where y , is the final, steady rate of relaxation and y o is the ‘extra’ relaxation rate at t = o and 7 is the time constant for the slowing. ymwas significantly lower in the acidified than in the rested state. yo and 7,on the other hand, did not differ significantly between these two states, which is in agreement with the finding that Ca2+ buffering of PA is not affected by p H (Pechere et al. 1977). In the fatigued condition the relaxation rate was uniformly low throughout the interrupted tetani (yo = 0) and the rate was not significantly different from y , in the acidified state. Thus, during
PECHERE,J.F., DERANCOURT, J. & HAIECH, J. 1977. The participation of parvalbumins in the activation-relaxation cycle of vertebrate fast skeletal muscle. FEBS Lett 75, 111-114. PECKHAM, M . & WOLEDGE, R.C. 1986. Labile heat and changes in rate of relaxation of frog muscles. 3’ Physiol 374, 123-135. WESTERBLAD, H. & LANNERGREN, J. 1986. Force and membrane potential during and after fatiguing, intermittent tetanic stimulation of single Xenopus muscle fibres. Acta Physiol Scand 128,369-378.
Olsalazine sodium stimulates chloride transport across the bullfrog cornea B. O D L I N D a n d 0. E R I K S S O N " Department of Medical Pharmacology, Biomedicum, and Chemistry, University of Uppsala, Sweden Olsalazine sodium consists of two molecules of 5aminosalicylic acid (5-ASA) coupled with an azobond. After bacterial splitting of the azo-bond in the colon, 5-ASA can exert its anti-inflammatory action, e.g. in ulcerative colitis (Sandberg-GertzCn et al. 1986).However, the unsplit olsalazine molecule has in high concentrations been shown to act as a secretagogue in the rat intestine (Goerg et al. 1987);in clinical trials the drug sometimes results in loose stools or even diarrhoea. The mechanism behind this effect is unclear; a stimulation of chloride secretion has been suggested (Goerg et al. 1987).The purpose of the present study was to examine this possibility by exposing olsalazine to the 'pure ' chloride-transporting epithelium of the bullfrog cornea. Frogs (Rana catesbeiana) of both sexes were purchased from Graska Biological Supply Co., Oshkosh, WI, USA, and kept in tap water for at least 2 weeks after delivery. The water temperature was 8-10 "C. T h e toads were fed manually with pieces of ox liver once a week. Corneas from doubly pithed frogs were mounted as described earlier (Eriksson & Wistrand 1986). The transport of sodium and chloride was determined electrophysiologically by measuring the transepithelial potential difference (PD ; mV), the short-circuit current (SCC; ,uA cm-') and the DC resistance (R; ohm cm'). These methods have been described in detail earlier (Eriksson & Wistrand 1986). The epithelia displayed a transepithelial P D of 16.9f2.4mV (tear side negative), an SCC of I I .7& I .o pA cm-' and a transepithelial D C resistance of 1504$zoz ohm cm' (mean+SE; n = 8). Results are given as arithmetic means+SE. T h e differences between predrug and post-drug values for frog cornea were evaluated by Student's t-test (twosided test) for paired variables and considered significant at P < 0.05 (").
* Department
of Clinical
Olsalazine sodium ( I .2-5.0 mM) added to the aqueous solution stimulated the short-circuit current (SCC) and the transepithelial P D in the isolated cornea of the frog, Rana catesbeiana, by about 20% (o.001 < P < 0.025), while the D C resistance was unchanged (-0.8%; P > 0.70; Table I). After the initial significant increase of SCC and PD, the values levelled out (Fig. I ) . Lower concentrations of olsalazine added to the aqueous side had no effect (Table I). When added to the contralateral solution in low concentration (0.3 mM), the drug had no effect, while higher concentrations of the drug stimulated the short-circuit current without affecting the PD (Table 1).
About 95 yo of the transepithelial potential difference and short-circuit current of the amphibian cornea is carried by chloride ions, which are transported from the aqueous (endothelial) to the tear (epithelial) side (Zadunaisky 1966,Candia & Askew I 968). Thus, there is no doubt that olsalazine sodium has a stimulating effect on the chloride secretion of frog
30t
T
Il* \ I
20
Received 30 January 1990,accepted 14 February '990. Key words : bullfrog cornea, chloride transport, olsalazine. Correspondence : Bo Odlind MD, PhD, Department of Medical Pharmacology, Biomedicum, Box 593, S-75124 Uppsala, Sweden.
I i M t \minutesiAb I t
T
.
40 60 n AUUI I IUN
OF OLSALAZINE SODIUM
Fig. I . Effects of olsalazine sodium (1.2-5mM) on potential difference (PD) and short-circuit current (SCC) in frog corneal epithelium. The drug was added to the aqueous solution. Values are means f SE. * denotes significant ( P < 0.05) changes from baseline.
24 1
242
B . Odlind and
0’. Eriksson
Table I . Effects of olsalazine sodium on electrophysiological variables in the cornea of Rana catesbeiana. The drug was added to the aqueous (A) or tear (T) side and the effects were recorded 8 m i n thereafter. The values are mean$SE. The aqueous side was positive with respect to the tear side. Numbers within parentheses are the numbers of corneas tested Per cent change after drug Drug concentration (mM) 0.3 (A) I .2-5 (A) 0.3 (T) 5 (TI
PD
+ 3.4 (2)
+ 16.5 k4.2(6)* -4.4 ( 2 ) -8.6 (I)
SCC
Resistance
- 3.4 +21.5f7.5* -2.4 + ‘9.3
+ 7.5 -0.8+ 5.1 - 2.0 -23.4
cornea. This is in line with the effect of this drug observed by Goerg et al. (1987). They observed that olsalazine sodium stimulated the chloride secretion and, to a lesser degree, the sodium secretion in the rat ileum and colon. In the cornea of amphibians, SCC is increased by 3-isobutyl-~-methylxanthine (IBMX), prostaglandins (PGE,) and dibutyryl cyclic AMP, added to the aqueous side (Chalfie et a/. 1972, Beitch et al. 1975, Eriksson & Wistrand 1986). However, we do not know at present if the stimulatory effect of olsalazine sodium on chloride secretion is mediated by cyclic AMP, prostaglandins or some other mediator system. This study, however, shows that the drug has a stimulatory effect on the chloride transport of a ‘model tissue’, the frog corneal epithelium.
REFERENCES BEITCH,B.R., BEITCH,I. & ZADUNAISKY, J.A. 1975. The stimulation of chloride transport by prostaglandins and their interaction with epinephrine, theophylline and cyclic AMP in corneal epithelium. 3 Membr Bid 19, 381-396. CANDIA, O.A. & ASKEW,W.A. 1968. Active sodium
transport in the isolated bullfrog cornea. Biochim Bioph.ys Acta 163, 262-265. CHAI.FIE,M., NEUFELD,A.11. 8i ZADUNAISKY, J.A. 1972. Action of epinephrine and other cyclic AMPmediated agents on chloride transport of frog cornea. Invest Ophthalmol I I , 644-650. ERIKSSON, 0.& WISTRAND, P. 1986. Inhibitory effects of chemically different ‘loop’ diuretics on chloride transport across the bullfrog cornea. Acta Physiol Scand 127, 137-144. GOERG,K.J., WANITSCHKE, R., GABBERT, H., BREILING, J., FRANKE, M. & MEYERZUM B~SCHENFELDE, K.-H. 1987. T h e effect of disodium (DSA) on water and electrolyte transfer of the rat ileum and colon in vivo compared with sulfasalazine (SASP), 5 aminosalicylic acid (5ASA) and sulfapyridine (SP). Digestion 37, 79-87. SANDBERG-GERTZEN, H., JARNEROTH, G. 8i GRAAZ, W. 1986. Azodisalsodium in the treatment of ulcerative colitis. A study of tolerance and relapse prevention properties. Gastroenterology 90, 1024. ZADUNAISKY, J.A. 1966. Active transport of chloride in frog cornea. A m 3 Physiol 211, 505-512.
Acta Physiol Scand 1990, 139, 243-244
Decreased Ca2+ buffering contributes t o slowing of relaxation in fatigued Xenopus muscle fibres H. W E S T E R B L A D and J. L A N N E R G R E N Department of Physiology 11, Karolinska Institute, Stockholm, Sweden Besides a decline in force, slowing of relaxation is a well-known attribute of fatigue in skeletal muscle. The slowing may be due to a decreased rate of crossbridge cycling and/or less efficient removal of calcium from the troponin-tropomyosin complex. A major determinant of Ca2+removal is the activity of the Ca2+ pump in the sarcoplasmic reticulum (SR), but interaction ofCa2+with parvalbumin (PA), a cytosolic Ca2+-binding protein, may also be important. In the present study we have tried to evaluate the importance of such Ca2+-PA interaction versus acidification (which affects both cross-bridges and SR Ca2+pumps) for the slowing of relaxation developing with fatigue. Single large, transparent fibres (type I ;Westerblad & Lannergren 1986) were dissected from lumbrical muscles of Xennpus laevis, mounted in a perfusion chamber for stimulation and isometric tension recording and superfused with standard Ringer solution ( 2 2 . 5 "C). Intracellular acidosis was produced by changing to Ringer solution bubbled with 5% CO, +95 y) 0,. Fatiguing stimulation consisted of 350 ms, 70 Hz tetani given every 3.8 s and was continued until tetanic tension was down to about 70% of the original. The Ca2+-PA interaction was evaluated by interrupting stimulation for a short period at various t i m k during a long tetanus and measuring the ensuing brief relaxation (see Peckham & Woledge 1986). These interrupted tetani had a total duration of about 2 s and consisted of recurring IOOms on-periods (70 Hz stimulation) and 50-ms offperiods (no stimulation). Measurements were made from digitized tension records. The relaxation rate was evaluated by measuring the slope during a Is-ms interval of the initial, nearly linear, part of force decline. In each experiment the relaxation rate after a rested 350-m~tetanus was set as I O O ~ ( ; this rate ranged between 984 and 1200kPa s-' in the four fibres studied. For statistical analysis paired t-tests were used and the significance level was set at 0.01 throughout. Received z February 1990, accepted 14 February 1990. Key mords: Ca2+,fatigue, parvalbumin, pH, relaxation, skeletal muscle, Xennpus luevzs. Correspondence : Jan Lannergren, Department of Physiology, Karolinska Institute, S-104 0 1 Stockholm, Sweden.
Figure I (a) shows interrupted contractions of one fibre produced in the three different states. Relaxation became progressively slower during the contraction in the rested state as well as when the fibre was acidified by 5 TICO,. In the fatigued state, however, relaxation was already very slow at the beginning and remained unchanged throughout the contraction. The same (a)
J
L
300 rns
0.0
0.5
1 .o
1.5
2.0
Duration (s)
Fig. I . (a) Force production during an interrupted tetanus in the rested (top record), acidified (middle) and fatigued (bottom) state. In this fibre fatigue was produced by 46 short tetani and the interrupted contraction was started 1 . 8 s after the last fatiguing tetanus. (b) Mean values of relaxation rate at various times during interrupted tetani in the rested ( O ) , acidified ( 0 )and fatigued (A) state; SEM indicated by bars when larger than symbol size. The relaxation rate at the end of a rested, 350-m~tetanus was set as 10o0/, and is indicated by the dotted lines. Curves were constructed from the mean values in Table I .
243
244
W.Westerblad and J, Lannergren
Table I. Mean values (fSEM, n = 4) of the initial, extra rate of relaxation (yo), the final, steady relaxation rate (v,) and the time constant of the slowing (7).In each experiment values of yo, y , and 7 were obtained by fitting data points to the equation given in the text
y,(o/6) Y , (%) 7 (S)
Rested
Acidified
Fatigued
99.8k6.4 59.8 f3.6 0.43f0.03
80.8f9.0 34.3 f3.5 0.36f0.02
o 26.3 k 1.7 -
observations were made in the other three fibres, and I (b) and Table I. In Fig. ~ ( b the ) symbols represent the mean rate of relaxation, expressed as per cent of the rate after a rested 350-m~tetanus, plotted against the time from the onset of contraction (t). The solid lines in Fig. I (b) and the values in Table I were obtained by fitting experimental data with the equation : the results are summarized in Fig.
+
the period of fatiguing stimulation the Ca2+ release from the SR must have been large enough to fully saturate PA. The present results show that it is necessary to take Ca*+ buffering into account when investigating the cause of slowed relaxation in fatigue. If, for example, the effects of fatigue and acidosis in our case had been compared by measuring the rate of relaxation after a standard 350-m~tetanus, PA would have been fully saturated with Ca2+ in the fatigued state but only about half-saturated in the acidified state. This would then give a considerably faster relaxation rate in the latter case. However, with longer tetani PA also gets saturated with Ca2+ in the acidified state and the relaxation rates become similar. Thus, at least in amphibian muscle the combined effect of CaZ+ saturation of PA and reduced pH, can explain the slowing of relaxation during fatigue. This work was supported by the Swedish MRC (project no. 3642) and by the Bergvall Foundation.
.y = y oe&” y ,
REFERENCES
where y , is the final, steady rate of relaxation and y o is the ‘extra’ relaxation rate at t = o and 7 is the time constant for the slowing. ymwas significantly lower in the acidified than in the rested state. yo and 7,on the other hand, did not differ significantly between these two states, which is in agreement with the finding that Ca2+ buffering of PA is not affected by p H (Pechere et al. 1977). In the fatigued condition the relaxation rate was uniformly low throughout the interrupted tetani (yo = 0) and the rate was not significantly different from y , in the acidified state. Thus, during
PECHERE,J.F., DERANCOURT, J. & HAIECH, J. 1977. The participation of parvalbumins in the activation-relaxation cycle of vertebrate fast skeletal muscle. FEBS Lett 75, 111-114. PECKHAM, M . & WOLEDGE, R.C. 1986. Labile heat and changes in rate of relaxation of frog muscles. 3’ Physiol 374, 123-135. WESTERBLAD, H. & LANNERGREN, J. 1986. Force and membrane potential during and after fatiguing, intermittent tetanic stimulation of single Xenopus muscle fibres. Acta Physiol Scand 128,369-378.
