- Email: [email protected]
Propylene glycol-induced skeletal muscle excitation
Fd Chem. Toxic. Vol. 31, No. 9, pp. 647-650, 1993 Printed in Great Britain.All rights reserved
0278-6915/93$6.00+ 0.00 Copyright © 1993PergamonPress Ltd
PROPYLENE GLYCOL-INDUCED SKELETAL MUSCLE EXCITATION T. HATTORI and H. MAEHASHI Department of Dental Pharmacology, Matsumoto Dental College, 1780 Hirooka-Gohbara, Shiojiri 399-07, Japan (Accepted 19 April 1993)
Abstract--The effects of propylene glycol (PG) on frog nerve-muscle preparations were examined to determine whether it has an effect on neuromuscular transmission and, if so, to elucidate the mode of the action. PG (5%, v/v) increased the twitch tension to over twice the control value. PG at concentrations above 0.2% significantlyincreased the amplitude of the endplate potential. PC; (1%) raised the frequency of the miniature endplate potentials and increased their amplitude. These results show that PC; both facilitates transmitter release from the nerve terminals and raises the acetylcholinesensitivityof the muscle endplate.
INTRODUCTION
MATERIALS AND METHODS
Propylene glycol (PG) is a widely used compound with diverse applications. It is used as a solvent for flavouring materials in baking and candy production as well as for inks for printing on food wrappers. It has been used in the pharmaceutical industry as a solvent for drugs, as a stabilizer for vitamins and in pastes for medicinal purposes (Ruddick, 1972). More importantly, it has been used as a solvent for watersoluble drugs in oral and iv dose forms (Yu and Sawchuk, 1987). For example, PG is used in the formulation of commonly used drugs such as phenobarbital, pentobarbital, diazepam and phenytoin, at concentrations up to 40% (v/v) (Speth et al., 1987). However, there are a few reports describing a severe toxic effect of PG on human nervous tissue. Arulanantham and Genel (1978) have reported the clinical course of a patient who developed seizures with the long-term ingestion of a medication with PG as its co-solvent. Martin et al. (1970) and Zaroslinski et al. (1971) have suggested that the main toxic action of PG is depression of the central nervous system. In addition, in reports of experiments on animals, PG as a component of the solvent for benzodiazepines has been proposed to be involved in augmentation of the muscle twitch induced by the drugs (Dretchen et aL, 1973; Driessen et al., 1985; Webb et al., 1973). The mechanism of action of PG was not investigated in these reports. In the study described here, the effects of PG on frog nerve-muscle preparations have been examined to determine whether it has an excitatory effect on muscle and, if so, to elucidate the mode of the action.
Experiments were performed on sciatic nervesartorius muscle preparations from bullfrogs (Rana catesbeiana) weighing 100-150g. The preparation was perfused with Ringer's solution of the following composition (mM): NaCl, l l0; KCI, 1.9; CaCI2, l.l; NaH2PO 4, 0.5; NaHCO3, 2.4 and glucose, 5.6 ( p n = 7.3). To examine the effect of PG on the twitch, the preparation was vertically fixed to the bottom of a Magnus tube containing 10 ml Ringer's solution bubbled with air. The twitch was evoked by electrical stimulation of the nerve with suction electrodes using suprathreshold voltage of 0.1 msec duration, and at a frequency of 2 Hz. The twitch tension was isometrically measured by means of a force-displacement transducer (Hattori and Maehashi, 1986). Conventional intracellular recording techniques were adopted to record the electrical changes at the endplate, by use of glass microelectrodes filled with 3 M KCI (electrical resistance 10-30MD) (Hattori and Maehashi, 1988). When the endplate potential (e.p.p.) was recorded, d-tubocurarine (2#M) was added to the perfusate to prevent the muscle fibres from firing and twitching. The effects of PG on the membrane resistance (m.r.) of the muscle fibre were also examined. Electrotonic hyperpolarizing potentials were evoked by passing the transmembranous current through an electrode separate from that used for recording, and the m.r. was calculated by dividing the electrotonic potential by the current. Experiments were carried out at room temperature (20-25°C). The d-tubocurarine chloride and PG used in this study were obtained from Nacalai Tesque (Japan). Data were expressed as mean value + standard error of the mean and number of experiments (n). Statistical analyses of the data were performed by the Student's two-sided simple t-test in the case of the
Abbreviations: e.p.p.= endplate potential; m.e.p.p.= miniature endplate potential; m.r. = membrane resistance; PG = propylene glycol; r.p. = resting potential.
647
648
T. HATTORIand H. MAEHASHI
resting potential (r.p.) and by the paired t-test in the measurements of e.p.p, amplitude, frequency of miniature endplate potential (m.e.p.p.), and m.e.p.p. amplitude. Differences between mean values were considered significant if the probability of error (P) was less than 0.05.
frequency from 1.47___0.24 to 2.23 +0.37/see ( n = 10, P <0.005) and its amplitude from 0 . 2 6 + 0 . 0 2 to 0 . 2 9 + 0 . 0 2 m V ( n = 1 0 , P < 0 . 0 0 1 ) under conditions in which the KCI concentration in the perfusate was five times the normal value. Significant increases in m.e.p.p, frequency were observed also at normal concentrations of KCI, values obtained before and after PG application being 0.17 + 0.07 and 0.27 + 0.09/see (n = 5, P < 0.02).
RESULTS
PG (5%) began to potentiate the twitch soon after its application and about 3 m i n later the twitch tension reached a plateau at a level over twice the control value. Washing with normal Ringer's solution soon restored the potentiated twitch. The effects of 1% PG on the r.p. and m.r. of the muscle fibre were examined because these influence the electrical phenomena at the neuromuscular junction, but in fact neither was significantly changed by this concentration of PG. The values obtained before and after the application were as follows: r.p. (mV), 92.15 +0.61 and 91.80+0.62 (n=60); m.r. ( x 105 f~), 7.56 _ 0.45 and 7.54 + 0.46 (n = 10). P G increased the e.p.p, in a concentration-dependent manner in the concentration range from 0.1 to 2% (Fig. 1). The differences between the values obtained before and after the application of P G at concentrations above 0.2% were statistically significant. The effect of P G on the m.e.p.p, frequency and its amplitude were investigated to test the actions o f P G on the pre- and postsynaptic membrane at the neuromuscular junction, respectively. As illustrated in Fig. 2, P G (1%) significantly raised the m.e.p.p.
DISCUSSION
PG increased e.p.p, and m.e.p.p, amplitudes and m.e.p.p, frequency. An increase in the r.p. (Takeuchi and Takeuchi, 1959) or the m.r. (Katz and Thesleff, 1957) of a muscle fibre can lead to increased e.p.p. and m.e.p.p, amplitudes; however, since neither of these factors was changed, they do not appear to be involved in the increment in e.p.p. As, therefore, the m.e.p.p, frequency was raised and the m.e.p.p, amplitude was increased, it is suggested that the increased e.p.p, may be due both to enhanced transmitter release and to a rise in acetylcholine sensitivity of the endplate. This conclusion is derived from the discovery by Katz (1962) that m.e.p.p, frequency and m.e.p.p, amplitude are entirely controlled by the condition o f the presynaptic (nerve terminal) and postsynaptic membrane (endplate) at neuromuscular junctions, respectively. Consequently, the twitch may be augmented as a result of the e.p.p, increment inducing repetitive firing of muscle fibres. Driessen et aL (1985) investigated the effect of benzodiazepines on the twitch of rat anterior tibialis
(A) 5mV I Control
50 mscc
PG
(B) ~, 6 E 5 ,-o "~ 4
D
•e~ " 3 E 2 0 _1 0.1
I
I
I
I
0.2
0.5
1.0
2.0
PG concentration
(%)
Fig. 1. Increase in endplate potential (e.p.p.) amplitude produced by propylene glycol (PG). (A): wave form observed before and after application of 1% PG. (B): concentration-dependent increase in e.p.p. amplitude. Open and closed circles represent the e.p.p, amplitudes recorded before and after the application, respectively. Each point shows the mean value + the standard error of the mean of 14--16 observations. *, *** and ****: significantly different from the control value at P < 0.05, 0.005 and 0.001, respectively.
Propylene glycol and neuromuscular junction
649
(A) Control
I
1 lmV 10
0.3 m
:c¢
(B)
0•2 m
0.1
0.0
m
Control
PG
Fig. 2. Increases in both the frequency and the amplitude of m.e.p.p, induced by propylene glycol (PG). m.e.p.p, observed before and after I% PG application, when KC1 content of the perfusate was five times the normal concentration. (B): m.e.p.p, amplitude in the absence and presence ofPG. N = 10, ****" P < 0.001• (A):
muscle, and concluded that the apparent diazepineinduced antagonism of a neuromuscular blocking drug is mainly due to PG, the main organic solvent for the poorly water-soluble benzodiazepines. Dretchen et al. (1973) found that low doses of the commercially available drug given intra-arterially reversed the myoneural blockades produced by both depolarizing and non-depolarizing blockers, and that this property was due to the solvent system of the drug (40% PG, 10% ethyl alcohol, 5% sodium benzoate and benzoic acid, and 1.5% benzyl alcohol in sterile water)• Moreover, Webb et al. (1973) reported that PG, a solvent for diazepam, in a volume that contained doses of 1-5mg diazepam/kg, itself slightly enhanced the rate of recovery from the blockade produced by tubocurarine and gallamine.
FCT31/~-D
These reports are generally in accord with those reported here, in that the responses described by others may be based on the fact that PG facilitates neuromuscular transmission both by increasing the transmitter release from the nerve terminals and by raising the acetylcholine sensitivity of the endplate. From the observation that seizures in a patient occurred only during the period when PG was being ingested, Arulanantham and Genel (1978) suggested that the association seen in this patient could be related to the long-term ingestion of PG. This effect may be explained by analogy with the result found in this study, that PG increased the m.e.p.p, frequency in normal KC1 saline; hence, seizures may be the result of PG, in the central nervous system, enhancing spontaneous transmitter release from the nerve terminals.
650
T. HATTORI and H. MAEHASHI
A l t h o u g h P G has been t h o u g h t to be a relatively non-toxic solvent, this study has d e m o n s t r a t e d t h a t it m a y be toxic at large doses. It is necessary to give serious consideration to the future use of P G as a solvent, since it is used at comparatively high concentrations. T h e results o f o u r investigations suggest t h a t the c o n c e n t r a t i o n s of P G used in vitro should be lower t h a n 0.2%. In addition, o n the basis o f experiments in vivo, Singh et al. (1982) have concluded t h a t P G should be used as a solvent in pharmacological a n d toxicological investigations only at low concentrations o f n o t m o r e t h a n 10%. REFERENCES
Arulanantham K. and Genel M. (1978) Central nervous system toxicity associated with ingestion of propylene glycol. Journal of Pediatrics 93, 515-516. Dretchen K., Ghoneim M. M. and Long J. P. (1973) The interaction of diazepam with myoneural blocking agents. Anesthesiology 34, 463-468. Driessen J. J., Vree T. B., van Egrnond J., Booij L. H. D. J. and Crul J. F. (1985) Interaction of some benzodiazepines and their solvents with vecuronium in the in vivo rat sciatic nerve tibialis anterior muscle preparation. Archives Internationales de Pharmacodynami~ et de Th~rapie 273, 277 288. Hattori T. and Maehashi H. (1986) Enhancement of the twitch of bullfrog sartorius muscle by fluorides. Japanese Journal of Pharmacology 40, 191-193. Hattori T. and Maehashi H. (1988) Anti-curare action of stannous ion in the frog neuromuscular junction. Brain Research 473, 157 160.
Katz B. (1962) The transmission of impulses from nerve to muscle, and subcellular unit of synaptic action. Proceedings of the Royal Society of London, Series B: Biological Sciences 155, 455-477. Katz B. and Thesleff S. (1957) On the factors which determine the amplitude of the 'miniature end-plate potential'. Journal of Physiology 137, 267-278. Martin G. and Finberg L. (1970) Propylene glycol: a potentially toxic vehicle in liquid dosage form. Journal of Pediatrics 77, 877-878. Ruddick J. A. (1972) Toxicology, metabolism, and biochemistry of 1,2-propanediol. Toxicology and Applied Pharmacology 21, 102-11 I. Singh P. P., Junnarkar A. Y., Seshagirirao C., Kaushal R., Naidu M. U. R., Varma R. K., Tripathi R. M. and Shridhar D. R. (1982) A pharmacological study of propane-l,2-diol. ArzneimitteI-Forschung 32, 1443 1446. Speth P. A. J., Vree T. B., Neilen N. F. M., de Mulder P. H. M., Newell D. R., Gore M. E. and de Pauw B. E. (1987) Propylene glycol pharmacokinetics and effects after intravenous infusion in humans. Therapeutic Drug Monitoring 9, 255 258. Takeuchi A. and Takeuchi N. (1959) Active phase of frog's end-plate potential. Journal of Neurophysiology 22, 395-411. Webb S. N. and Bradshaw E. G. (1973) An investigation, in cats, into the activity of diazepam at the neuromuscular junction. British Journal of Anaesthesia 45, 313 318. Yu D. K. and Sawchuk R. J. (1987) Pharmacokinetics of propylene glycol in the rabbit. Journal of Pharmacokinetics and Biopharmaceutics 15, 453-471. Zaroslinski J. F., Browne R. K. and Possley L. H. (1971) Propylene glycol as a drug solvent in pharmacologic studies. Toxicology and Applied Pharmacology 19, 573-578.
0278-6915/93$6.00+ 0.00 Copyright © 1993PergamonPress Ltd
PROPYLENE GLYCOL-INDUCED SKELETAL MUSCLE EXCITATION T. HATTORI and H. MAEHASHI Department of Dental Pharmacology, Matsumoto Dental College, 1780 Hirooka-Gohbara, Shiojiri 399-07, Japan (Accepted 19 April 1993)
Abstract--The effects of propylene glycol (PG) on frog nerve-muscle preparations were examined to determine whether it has an effect on neuromuscular transmission and, if so, to elucidate the mode of the action. PG (5%, v/v) increased the twitch tension to over twice the control value. PG at concentrations above 0.2% significantlyincreased the amplitude of the endplate potential. PC; (1%) raised the frequency of the miniature endplate potentials and increased their amplitude. These results show that PC; both facilitates transmitter release from the nerve terminals and raises the acetylcholinesensitivityof the muscle endplate.
INTRODUCTION
MATERIALS AND METHODS
Propylene glycol (PG) is a widely used compound with diverse applications. It is used as a solvent for flavouring materials in baking and candy production as well as for inks for printing on food wrappers. It has been used in the pharmaceutical industry as a solvent for drugs, as a stabilizer for vitamins and in pastes for medicinal purposes (Ruddick, 1972). More importantly, it has been used as a solvent for watersoluble drugs in oral and iv dose forms (Yu and Sawchuk, 1987). For example, PG is used in the formulation of commonly used drugs such as phenobarbital, pentobarbital, diazepam and phenytoin, at concentrations up to 40% (v/v) (Speth et al., 1987). However, there are a few reports describing a severe toxic effect of PG on human nervous tissue. Arulanantham and Genel (1978) have reported the clinical course of a patient who developed seizures with the long-term ingestion of a medication with PG as its co-solvent. Martin et al. (1970) and Zaroslinski et al. (1971) have suggested that the main toxic action of PG is depression of the central nervous system. In addition, in reports of experiments on animals, PG as a component of the solvent for benzodiazepines has been proposed to be involved in augmentation of the muscle twitch induced by the drugs (Dretchen et aL, 1973; Driessen et al., 1985; Webb et al., 1973). The mechanism of action of PG was not investigated in these reports. In the study described here, the effects of PG on frog nerve-muscle preparations have been examined to determine whether it has an excitatory effect on muscle and, if so, to elucidate the mode of the action.
Experiments were performed on sciatic nervesartorius muscle preparations from bullfrogs (Rana catesbeiana) weighing 100-150g. The preparation was perfused with Ringer's solution of the following composition (mM): NaCl, l l0; KCI, 1.9; CaCI2, l.l; NaH2PO 4, 0.5; NaHCO3, 2.4 and glucose, 5.6 ( p n = 7.3). To examine the effect of PG on the twitch, the preparation was vertically fixed to the bottom of a Magnus tube containing 10 ml Ringer's solution bubbled with air. The twitch was evoked by electrical stimulation of the nerve with suction electrodes using suprathreshold voltage of 0.1 msec duration, and at a frequency of 2 Hz. The twitch tension was isometrically measured by means of a force-displacement transducer (Hattori and Maehashi, 1986). Conventional intracellular recording techniques were adopted to record the electrical changes at the endplate, by use of glass microelectrodes filled with 3 M KCI (electrical resistance 10-30MD) (Hattori and Maehashi, 1988). When the endplate potential (e.p.p.) was recorded, d-tubocurarine (2#M) was added to the perfusate to prevent the muscle fibres from firing and twitching. The effects of PG on the membrane resistance (m.r.) of the muscle fibre were also examined. Electrotonic hyperpolarizing potentials were evoked by passing the transmembranous current through an electrode separate from that used for recording, and the m.r. was calculated by dividing the electrotonic potential by the current. Experiments were carried out at room temperature (20-25°C). The d-tubocurarine chloride and PG used in this study were obtained from Nacalai Tesque (Japan). Data were expressed as mean value + standard error of the mean and number of experiments (n). Statistical analyses of the data were performed by the Student's two-sided simple t-test in the case of the
Abbreviations: e.p.p.= endplate potential; m.e.p.p.= miniature endplate potential; m.r. = membrane resistance; PG = propylene glycol; r.p. = resting potential.
647
648
T. HATTORIand H. MAEHASHI
resting potential (r.p.) and by the paired t-test in the measurements of e.p.p, amplitude, frequency of miniature endplate potential (m.e.p.p.), and m.e.p.p. amplitude. Differences between mean values were considered significant if the probability of error (P) was less than 0.05.
frequency from 1.47___0.24 to 2.23 +0.37/see ( n = 10, P <0.005) and its amplitude from 0 . 2 6 + 0 . 0 2 to 0 . 2 9 + 0 . 0 2 m V ( n = 1 0 , P < 0 . 0 0 1 ) under conditions in which the KCI concentration in the perfusate was five times the normal value. Significant increases in m.e.p.p, frequency were observed also at normal concentrations of KCI, values obtained before and after PG application being 0.17 + 0.07 and 0.27 + 0.09/see (n = 5, P < 0.02).
RESULTS
PG (5%) began to potentiate the twitch soon after its application and about 3 m i n later the twitch tension reached a plateau at a level over twice the control value. Washing with normal Ringer's solution soon restored the potentiated twitch. The effects of 1% PG on the r.p. and m.r. of the muscle fibre were examined because these influence the electrical phenomena at the neuromuscular junction, but in fact neither was significantly changed by this concentration of PG. The values obtained before and after the application were as follows: r.p. (mV), 92.15 +0.61 and 91.80+0.62 (n=60); m.r. ( x 105 f~), 7.56 _ 0.45 and 7.54 + 0.46 (n = 10). P G increased the e.p.p, in a concentration-dependent manner in the concentration range from 0.1 to 2% (Fig. 1). The differences between the values obtained before and after the application of P G at concentrations above 0.2% were statistically significant. The effect of P G on the m.e.p.p, frequency and its amplitude were investigated to test the actions o f P G on the pre- and postsynaptic membrane at the neuromuscular junction, respectively. As illustrated in Fig. 2, P G (1%) significantly raised the m.e.p.p.
DISCUSSION
PG increased e.p.p, and m.e.p.p, amplitudes and m.e.p.p, frequency. An increase in the r.p. (Takeuchi and Takeuchi, 1959) or the m.r. (Katz and Thesleff, 1957) of a muscle fibre can lead to increased e.p.p. and m.e.p.p, amplitudes; however, since neither of these factors was changed, they do not appear to be involved in the increment in e.p.p. As, therefore, the m.e.p.p, frequency was raised and the m.e.p.p, amplitude was increased, it is suggested that the increased e.p.p, may be due both to enhanced transmitter release and to a rise in acetylcholine sensitivity of the endplate. This conclusion is derived from the discovery by Katz (1962) that m.e.p.p, frequency and m.e.p.p, amplitude are entirely controlled by the condition o f the presynaptic (nerve terminal) and postsynaptic membrane (endplate) at neuromuscular junctions, respectively. Consequently, the twitch may be augmented as a result of the e.p.p, increment inducing repetitive firing of muscle fibres. Driessen et aL (1985) investigated the effect of benzodiazepines on the twitch of rat anterior tibialis
(A) 5mV I Control
50 mscc
PG
(B) ~, 6 E 5 ,-o "~ 4
D
•e~ " 3 E 2 0 _1 0.1
I
I
I
I
0.2
0.5
1.0
2.0
PG concentration
(%)
Fig. 1. Increase in endplate potential (e.p.p.) amplitude produced by propylene glycol (PG). (A): wave form observed before and after application of 1% PG. (B): concentration-dependent increase in e.p.p. amplitude. Open and closed circles represent the e.p.p, amplitudes recorded before and after the application, respectively. Each point shows the mean value + the standard error of the mean of 14--16 observations. *, *** and ****: significantly different from the control value at P < 0.05, 0.005 and 0.001, respectively.
Propylene glycol and neuromuscular junction
649
(A) Control
I
1 lmV 10
0.3 m
:c¢
(B)
0•2 m
0.1
0.0
m
Control
PG
Fig. 2. Increases in both the frequency and the amplitude of m.e.p.p, induced by propylene glycol (PG). m.e.p.p, observed before and after I% PG application, when KC1 content of the perfusate was five times the normal concentration. (B): m.e.p.p, amplitude in the absence and presence ofPG. N = 10, ****" P < 0.001• (A):
muscle, and concluded that the apparent diazepineinduced antagonism of a neuromuscular blocking drug is mainly due to PG, the main organic solvent for the poorly water-soluble benzodiazepines. Dretchen et al. (1973) found that low doses of the commercially available drug given intra-arterially reversed the myoneural blockades produced by both depolarizing and non-depolarizing blockers, and that this property was due to the solvent system of the drug (40% PG, 10% ethyl alcohol, 5% sodium benzoate and benzoic acid, and 1.5% benzyl alcohol in sterile water)• Moreover, Webb et al. (1973) reported that PG, a solvent for diazepam, in a volume that contained doses of 1-5mg diazepam/kg, itself slightly enhanced the rate of recovery from the blockade produced by tubocurarine and gallamine.
FCT31/~-D
These reports are generally in accord with those reported here, in that the responses described by others may be based on the fact that PG facilitates neuromuscular transmission both by increasing the transmitter release from the nerve terminals and by raising the acetylcholine sensitivity of the endplate. From the observation that seizures in a patient occurred only during the period when PG was being ingested, Arulanantham and Genel (1978) suggested that the association seen in this patient could be related to the long-term ingestion of PG. This effect may be explained by analogy with the result found in this study, that PG increased the m.e.p.p, frequency in normal KC1 saline; hence, seizures may be the result of PG, in the central nervous system, enhancing spontaneous transmitter release from the nerve terminals.
650
T. HATTORI and H. MAEHASHI
A l t h o u g h P G has been t h o u g h t to be a relatively non-toxic solvent, this study has d e m o n s t r a t e d t h a t it m a y be toxic at large doses. It is necessary to give serious consideration to the future use of P G as a solvent, since it is used at comparatively high concentrations. T h e results o f o u r investigations suggest t h a t the c o n c e n t r a t i o n s of P G used in vitro should be lower t h a n 0.2%. In addition, o n the basis o f experiments in vivo, Singh et al. (1982) have concluded t h a t P G should be used as a solvent in pharmacological a n d toxicological investigations only at low concentrations o f n o t m o r e t h a n 10%. REFERENCES
Arulanantham K. and Genel M. (1978) Central nervous system toxicity associated with ingestion of propylene glycol. Journal of Pediatrics 93, 515-516. Dretchen K., Ghoneim M. M. and Long J. P. (1973) The interaction of diazepam with myoneural blocking agents. Anesthesiology 34, 463-468. Driessen J. J., Vree T. B., van Egrnond J., Booij L. H. D. J. and Crul J. F. (1985) Interaction of some benzodiazepines and their solvents with vecuronium in the in vivo rat sciatic nerve tibialis anterior muscle preparation. Archives Internationales de Pharmacodynami~ et de Th~rapie 273, 277 288. Hattori T. and Maehashi H. (1986) Enhancement of the twitch of bullfrog sartorius muscle by fluorides. Japanese Journal of Pharmacology 40, 191-193. Hattori T. and Maehashi H. (1988) Anti-curare action of stannous ion in the frog neuromuscular junction. Brain Research 473, 157 160.
Katz B. (1962) The transmission of impulses from nerve to muscle, and subcellular unit of synaptic action. Proceedings of the Royal Society of London, Series B: Biological Sciences 155, 455-477. Katz B. and Thesleff S. (1957) On the factors which determine the amplitude of the 'miniature end-plate potential'. Journal of Physiology 137, 267-278. Martin G. and Finberg L. (1970) Propylene glycol: a potentially toxic vehicle in liquid dosage form. Journal of Pediatrics 77, 877-878. Ruddick J. A. (1972) Toxicology, metabolism, and biochemistry of 1,2-propanediol. Toxicology and Applied Pharmacology 21, 102-11 I. Singh P. P., Junnarkar A. Y., Seshagirirao C., Kaushal R., Naidu M. U. R., Varma R. K., Tripathi R. M. and Shridhar D. R. (1982) A pharmacological study of propane-l,2-diol. ArzneimitteI-Forschung 32, 1443 1446. Speth P. A. J., Vree T. B., Neilen N. F. M., de Mulder P. H. M., Newell D. R., Gore M. E. and de Pauw B. E. (1987) Propylene glycol pharmacokinetics and effects after intravenous infusion in humans. Therapeutic Drug Monitoring 9, 255 258. Takeuchi A. and Takeuchi N. (1959) Active phase of frog's end-plate potential. Journal of Neurophysiology 22, 395-411. Webb S. N. and Bradshaw E. G. (1973) An investigation, in cats, into the activity of diazepam at the neuromuscular junction. British Journal of Anaesthesia 45, 313 318. Yu D. K. and Sawchuk R. J. (1987) Pharmacokinetics of propylene glycol in the rabbit. Journal of Pharmacokinetics and Biopharmaceutics 15, 453-471. Zaroslinski J. F., Browne R. K. and Possley L. H. (1971) Propylene glycol as a drug solvent in pharmacologic studies. Toxicology and Applied Pharmacology 19, 573-578.