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High Pressure and Anesthesia: Compressibility, Molal Volume, and Partial Molal Volume of Volatile Anesthetics
High Pressure and Anesthesia: Compressibility, Molal Volume, and Partial Molal Volume of Volatile Anesthetics KOHSUKE FUKUSHIMA**, HIROSHIKAMAYA',
AND ISSAKU UEDA*'
Received June 16, 1989, from the 'Department of Anesthesia, University of Utah School of Medicine, and Anesthesia Service, Veterans Administration Medical Ceflter, Salt Lake Cit , Utah 84148. Accepted for publication December 14, 1989. *On leave from the Department of Chemistry, Faculty of Science, Fukuoka diversity, Nanakuma, Jonan-ku, Fukuoka 814-01, Japan. Abstract 0 This study was undertaken to provide volume data of volatile anesthetics under high pressure. Molal volumes of liquid halothane, enflurane, and isoflurane and their partial molal volumes in water and in 1-0ctanol were determined by high-precision solution densimetry at 25.000 f 0.0005 "C over the pressure range from ambient to 34.56 MPa (341 atm). The isothermal compressibilities of the pure anesthetics and their isothermal partial molal compressibilities at their infinite dilution in water and in 1-octanol have also been calculated at 0.1013 MPa (1 atm).
The pressure-reversal of anesthesia was first reported in 1942 by Johnson, Eyring, and co-workers182 in the light intensity of the luminous bacteria Photobacterium fischeri. They found that general anesthetics decreased the bacterial light intensity, and hydrostatic pressure in the range of 100 to 150 atm completely restored the luminosity. Johnson and Flagler3 further demonstrated that anesthetized tadpoles started swimming again by application of pressure in the similar range. These results were confirmed by Lever et al.4 in newts and mice in 1971. Numerous reports on the pressure reversal of anesthesia followed.5-8 Despite the abundance of reports on the pressure antagonism of anesthesia, studies on the physical properties of anesthetics under high pressure are few. Because pressure is paired with volume in thermodynamics, the antagonism of anesthesia by this simple physical force suggests involvement of volume changes in anesthesia mechanisms. The densities of volatile anesthetics in the pure liquid state at ambient pressure were reported by Bottomley and Seiflow,9 Stern and Shiah,"J Korman and Ritchie,ll and Mori et a1.12 The partial molal volumes of anesthetics in water and in organic phases were reported by Kita et al.13 and Mori et al.,lz also at ambient pressure. The present study measured densities of halothane, enflurane, and isoflurane at high pressures, up to 34.56 MPa (341 atm), to provide a data base for analyses of the pressure reversal of anesthesia. The data are presented as (I)the molal volume for the pure liquid state, (2) the partial molal volume in water, (3) the partial molal volume in an organic solvent, 1-octanol, and ( 4 ) the isothermal compressibilities. 1-Octanol was used to represent the organic phase because solubility in this solvent has been shown to correlate well with drug potencies.14
confirmed that the preparations are free from water contamination. 1-Octanol was obtained from J. T. Baker (Phillipsburg, NJ) and was also treated with activated aluminum oxide columns. The purity of 1-octanol was confirmed by a Shimadzu gas chromatograph (Columbia, MD) with a flame ionization detector. The column was 1/8 in x 6 ft, stainless steel, containing Porapak Q 80/100mesh. The operating conditions were isothermal at 160 "C and at a flow rate of 20 mL/min of helium as the carrier gas. Water was purified by distillation, then treated by two mixed-bed ion-exchanger columns, an activated charcoal column, and an ultrafilter (Millipore, Bedford, MA). Water:anesthetic mixtures were prepared in 30-mL Teflon tubes equipped with air-tight screw caps, O-rings, and plugs. The amount of water and anesthetic contained in the bottle was measured by weighing it with a Mettler semimicrobalance (Hightstown, NJ) to 5* g. The mixture was kept in a constant temperature water bath at 25 "C for 3 days with shaking. With the 1-octano1:anesthetic mixture, the equilibration time was 24 h. After the incubation, the bottles were weighed again to check possible leaks. The densities of sample solutions were measured by an Anton-Paar oscillation densimeter DMA60/512 (Mettler-Paar, Hightstown, NJ). The temperature of the densimeter cuvette was maintained at 298.150 f 0.0005 K by circulating water from a Hart 5003 Iso-Them water bath (Hart Scientific, Provo, UT), and monitored by a microprocessor-controlled thermistor thermometer (Micro-Therm 1006, Hart Scientific, Provo, UT) with 0.0001 K resolution. The millidegree stability was necessary to obtain reproducible results. The temperature of the water bath was monitored by a Hewlett-Packard quartz thermometer 2804 A (Palo Alto, CA), also with 0.0001 K resolution. Hydrostatic pressure was generated by a hand-operated hydraulic pump, and was transmitted to the two ends of the stainless steel vibration tube of the densimeter by stainless steel pipes. The pressure was monitored by an Autoclave Digital Pressure Transducer (model DPS-0201; Erie, PA). The cell constant of the densimeter was determined by using the densities of water and NaCl s0lutions.15~16 Before introducing the test solution into the densimeter, the solution was sonicated by immersing the container tube in a sonicator water bath (Sonicor Instrument, Copiague, NY), and heated 2°C above the measuring temperature for 10 min to prevent microbubble formation in the test solution during the density measurement. The density was measured in triplicate to confirm the accuracy of the procedure. The deviations were almost null.
Results The molal volumes of pure anesthetics were calculated from the solution densities as follows:
M
Experimental Section
U0,P = -
Isoflurane (1-chloro-2,2,2,-trifluoroethyl difluoromethyl ether) and enflurane (2-chloro-1,1,2,-trifluoroethyl difluoromethyl ether) were obtained from Anaquest (Madison, WI) and halothane (2-bromo-2chloro-l,l,l,-trifluoroethane) from Ayerst (New York, NY). They were treated with activated aluminum oxide columns (Woelen Pharma, West Germany) to remove traces of water. The stabilizer contained in halothane (0.01% thymol) was removed by this procedure. The water contents were checked by Fourier-transform IR spectroscopy (FTIR model 1750; Perkin-Elmer, Norwalk, CT), which
where v , , ~ M , , and do,p are the molal volume, molecular weight, and the density of anesthetics, respectively. The subscript o signifies the pure state. The molal volumes of anesthetics, calculated from the density data a t 298.150 K (25.000 "C) and at various pressures up to 34.56 MPa, are listed in Table I. At the location where the experiments were performed, the ambient pressure is 0.085 MPa (0.84 atm)
0022-3549/90/1000-0893$01.001'0 0 1990,American Pharmaceutical Association
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d0,P
Journal of Pharmaceutical Sciences / 893 Vol. 79, No. 10, October 1990
Table C l o l a l Volume of Pure Anesthetlcs at 298.150 K under Varlous Pressures and the isothermal Compresslblllty, k, at 0.1013 YPa
Halothane
Molal Volume, cm3.mol-’ Enflurane
lsoflurane
0.085 3.53 6.98 13.88 20.77 27.66 34.56
106.3 105.8 105.3 104.3 103.4 102.6 101.9
122.0 121.3 120.8 1 1 9.5 118.5 117.5 116.6
123.7 122.9 122.2 120.9 119.8 118.8 117.8
k f SE, MPa-’
1.47-10-3 -c 3.37.10-5
1.56.10-3 3.51 .10-5
1.75.10-324.70.10-5
Pressure, MPa
rather than 0.1013 MPa. The density data of the anesthetics as a function of pressure were fitted by the least squares method to the following equation: n
In uo,p =
1.0127
C aiPi
v)
i=O
1.0124:
where aiis a constant. The isothermal compressibility, k, of the anesthetics is given by eq 3:
z
1.0121 0.982 0.980
t
m
where V is the system volume. The compressibilities of pure anesthetics a t 0.1013 MPa and 298.150 K were calculated according to eqs 2 and 3 by second-degreepolynomial fitting. The densities of water:anesthetic solutions at 298.150 K under ambient and 34.56 MPa pressures are shown in Figure 1as a function of anesthetic molalities (moles of solute in 1000 g of solvent). The molality instead of molarity (moles of solute in 1000 mL of solution) was used because molarity values vary according to temperature and pressure. The values extrapolated to zero anesthetic concentration were identical with those of pure water at each pressure. The densities of 1-octano1:anesthetic mixtures under ambient and 34.56 MPa pressures are shown in Figure 2 as a function of anesthetic molalities. The apparent molal volume of solute (&, where subscript 2 signifies solute) in a two-component system is expressed as follows:
\
0.978
D Y
> 0.976
k -
z w
0.974
0 .9 7 2 0.970
0
MOLALITY
where m, and M, are the molality and molecular weight of anesthetics, respectively, and d and doare the densities of the solution and the solvent, respectively. The partial molal volume of the solute is defined as follows: 894 I Journal of Pharmaceutical Sciences Vol. 79, No. 10,October 1990
( x 1O - * )
Figure 1-Densities of anesthetics in water at 298.150 K (25.000“C).(A) Ambient pressure (0.085MPa). (8) Pressure of 34.56 MPa. Key: (A) halothane; (0)enflurane; and (El) isoflurane.
02 =
where n, and n2 are the number of moles of the solvent and solute, respectively, and us is the molal volume of the pure solvent. The apparent molal volume, &, is related to the density of the solution as follows:
2
1
(dVIWTp,n,
(6)
where V and n2 are the volume of the system and the number of moles of the solute, respectively. From eqs 4 and 6, eq 7 can be written as follows:
Equation 7 indicates that V , a t infinite dilution is obtaincd from the intersect of the plot between 4 and molality, and u2 at nonzero concentration is obtained by ‘taking the first derivative. The partial molal volumes of anesthetics at infinite dilution in water and in 1-octanol as a function of pressure are listed in Tables I1 and 111, respectively.
0.845
-5 m
0.844
\
0, U
0.843
>-
k 0.842 W
n
0.84 1 0.825
1
- A
m^ 0.824
E0
\
3 0.823 >. k 0.822 v)
Z
0.821 0.820 1 0
I
I
I
I
I
1
2
3
4
5
( x 1O-*)
MOLALITY
Figure 2-Densities of anesthetics in 1-octanol at 298.150 K (25.000"C).(A) Ambient pressure (0.085MPa). (B) Pressure of 34.56 MPa. Key: (A)halothane; (0)enflurane; and (0)isoflurane.
The isothermal partial molal compressibility, K, is defined as follows: I
-\
-
k
= T
The values for K a t infinite dilution in water and in 1-octanol are listed in Tables I1 and 111, respectively. Notice that the definition of the isothermal partial molal compressibility in a solvent differs from the isothermal compressibility of pure liquid anesthetics. Accordingly, the dimension of the compressibility is also different between the two.
Discussion The present results on liquid anesthetics at ambient pressure and at 298.15 K are in good agreement with reported
values.11J2 The possible loss of anesthetics from water and 1-octanol during transfer from the incubation vessel to the densimeter was not determined. The escaped amount, however, appears to be negligible because extrapolation of the density data to zero anesthetic concentration transected the density coordinate at the identical value of the solvent. Escape of the anesthetic from the densimetry cuvette during the measurement was unfeasible because this is a closed system under high pressure. The partial molal volume of halothane in water decreased with increasing pressure, whereas those of enflurane and isoflurane increased. The mechanism of the difference in the sign of the isothermal partial molal compressibility in water between the ether-type anesthetics and the alkane-type anesthetic is unknown. It may be caused by the difference in water structure clustered around the anesthetic molecules. Partial molal volume in water is strongly influenced by the water structure. The hydrogen-bond breaking activity of volatile anesthetics is amply documented by Sandorfy and CO-workers.17-20 They have shown that anesthetics form a proton donor:acceptor complex in competition with water molecules so that the H20:H20 dimer is broken to form an anesthetic:H,O dimer. Another feature of anesthetics is to self-aggregate in water. Yoshida et a1.21 reported that enflurane started to self-aggregate a t -4 mM in water and formed an octamer before visually saturating the aqueous phase. For these reasons, the density of the anesthetic:water mixture is strongly nonlinear with respect to anesthetic concentrations. Accordingly, the partial molal volume of anesthetics extrapolated to infinite dilution in water is subject to substantial error. Because the partial molal compressibilities of anesthetics in water are computed on the extrapolated values, the parameter is associated with relatively large standard error. Cabani et a1.22 reported that the partial molal compressibilities of n-alcohols (ethanol to n-hexanol) are negative at 10 "C but positive a t 25 "C. Their data do not show the error range, but may not be free from deviations. The exact cause of these discrepancies in the sign of the partial molal compressibility in water requires further studies. The larger partial molal volume of anesthetics in octanol than in water indicates that high pressure will squeeze out these anesthetic molecules from the organic phase to the water phase. If one accepts that octanol represents anesthetic binding sites reasonably well, pressure-reversal of anesthesia may involve elimination of anesthetic molecules from the action sites. Kaneshina et al.,23 however, reported that the portions of anesthetic molecules squeezed out from phospholipid bilayer membranes were 10.4%for halothane, 6.4% for chloroform, 9.6% for enflurane, and 8.5% for methoxyflurane at 100 atm (10.13 MPa). Similar "squeeze-out" results were obtained with a desorption study of anesthetics by pressure in ionic surfactant micelles.24 Although the squeeze-out of an-
Table ICPartlal Molal Volume of Anesthetlcs at Infinite Dilution In Water at 298.150 K under Varlous Pressures and the Isothermal Partlal Molal Compresslbillty (3at lnfinlte Dilution at 0.1013 MPa Pressure, MPa
0.085 3.53 6.98 13.88 20.77 27.66 34.56
iG f SE, cm3 mot-' MPa-' a
Partial Molal Volume, cm3.mol-' Halothane
Enflurane
isoflurane
102.6 101.3 101.1 100.8 100.3 100.5 99.3
108.0 106.5 106.3 107.5 108.6 109.6 108.7
113.0 112.8 113.0 113.0 115.8 116.3 115.7
7.42.lo-' t 1.51
lo-'
-6.89 . 1 0 - 2 t 2.74.1O-'
-1.06.lo-'
t
2.53.
Journal of Pharmaceutical Sciences I 895 Vol. 79,No. 70, October 7990
Table IlCPartlai Molal Volume of Anesthetics at lnflnlte Dilution In 1-Octanol at 298.150 K Under Various Pressures and the Isothermal Partlal Molal Compresslblllty, k,at lnflnlte.Dllut1onat 0.1013 MPa Partial Molal Volume, CKI~"IOI-~ Pressure, MPa Halothane Enflurane lsoflurane 0.085 3.53 6.98 13.88 20.77 27.66 34.56
104.6 104.1 103.6 103.0 102.1 101.1 100.2
b r SE, cm3. rnol-' * MPa-'
1.25 * lo-' f 3.41 *
esthetic molecules from the tissue binding site undoubtedly contributes to the pressure reversal of anesthesia to a certain extent, the obtained numerical squeeze-out values are too modest to account for the full recovery from anesthesia. Additional mechanisms must be operative for the pressure reversal phenomenon, and their elucidation may lead to the understanding of anesthesia mechanisms.
References and Notes 1. Johnson, F. H.; Eyring, H.; Williams, R. B. J.Cell. Comp.Physiol. 1942,20,247-268. 2. E in H.; Magee, J. L. J. Cell. Comp. Physiol. 1942, 20, lglV7. 3. Johnson, F. H.; Flagler, E. A. Science 1951,112,91-92. 4. Lever, M. J.; Miller, K. W.; Paton, W.D. M.; Smith, E. B. Nature 1971,231,368-371. 5. Halsey, M. J. Physiol. Reu.1982, 62, 1341-1377. 6. MacDonald, A.G. Phil. Trans. Roy. Soc. Lond. B 1984, 304, 47-68. 7. Kendig, J. J. Am. J. Physiol. 1984,246, C91495. 8. Smith, E. B.; Bower-Rile F.; Daniels, S. Molecular and Cellular Mechanisms ofAnest&cs; Roth, S. H.; Miller, K. W., Eds.; Plenum: New York, 1986; pp 341-353. 9. Bottomley, G.A.; Seiflow, G. J. Appl. Chem. 1963,13,339-343. 10. Stem,S. A.; Shiah, S. P. Anesthesiology 1979, 50, 538. 11. Korman, B.; Ritchie, I. M. Anesthesiology 1982,57,4243.
896 1 Journal of Pharmaceutical Sciences Vol. 79, No. 10, October 1990
122.7 122.1 121.5 120.3 119.8 119.1 117.7 1.36 * lo-'
* 7.71
125.0 124.2 123.7 122.6 121.9 120.9 120.0 *
10-3
1.40 * lo-' f 3.41 * low3
12. Mori, T.; Matubayasi, N.; Ueda, I. Mol. Pharmacol. 1984, 25, 123-130. 13. Kita, Y.; Bennett, L. J.; Miller, K. W. Biochim. Biophys. Acta 1981,647,130-139. 14. Hansch, C.; Clayton, J. M. J.Pharm. Sci.1973, 62, 1-21. 15. Fine, A. R.; Millero, F. J. J. Chem. Phys. 1973,59, 5529-5536. 16. Gates, J. A.; Wood, R. H. J. Chem. Eng. Data 1985,30,44-49. 17. DiPaolo, T.; Sandorfy, C. J. Med. Chem. 1974,17,809-814. 18. Hobza, P.; Mulder, F.; Sandorfy, C. J.Am. Chem. SOC.1981,103, 1360-1366. 19. Hobza, P.; Mulder, F.; Sandorfy, C. J.Am. Chem. SOC.1982,104, 925-928. 20. Trudeau, G.; Dumas, J M . ; Dupuis, P.; Guerin, M.; Sandorfy, C. Topics Current Chemistry 1980,93, 91-125. 21. Yoshida, T.; Okabayashi, H.; Kamaya, H.; Ueda, I. Biochim. Biophys. Acta 1989,979,287-293. 22. Cab!a_ni, S.; Conti, G.; Matteoli, E. J. Solution Chem. 1979, 8, 11-m.
23. Kaneshina, S.; Kamaya, H.; Ueda, I. Biochim. Biophys. Acta 1982,685,3074314. 24. Kaneshina, S., Kamaya, H., Ueda, I. J. Colloid InteTface Sci. 1983,93,215-224.
Acknowledgments This stud was supported b the Medical Research Service of the Veterans A&ninistration, a n J N I H grants GM25716 and GM27670.
AND ISSAKU UEDA*'
Received June 16, 1989, from the 'Department of Anesthesia, University of Utah School of Medicine, and Anesthesia Service, Veterans Administration Medical Ceflter, Salt Lake Cit , Utah 84148. Accepted for publication December 14, 1989. *On leave from the Department of Chemistry, Faculty of Science, Fukuoka diversity, Nanakuma, Jonan-ku, Fukuoka 814-01, Japan. Abstract 0 This study was undertaken to provide volume data of volatile anesthetics under high pressure. Molal volumes of liquid halothane, enflurane, and isoflurane and their partial molal volumes in water and in 1-0ctanol were determined by high-precision solution densimetry at 25.000 f 0.0005 "C over the pressure range from ambient to 34.56 MPa (341 atm). The isothermal compressibilities of the pure anesthetics and their isothermal partial molal compressibilities at their infinite dilution in water and in 1-octanol have also been calculated at 0.1013 MPa (1 atm).
The pressure-reversal of anesthesia was first reported in 1942 by Johnson, Eyring, and co-workers182 in the light intensity of the luminous bacteria Photobacterium fischeri. They found that general anesthetics decreased the bacterial light intensity, and hydrostatic pressure in the range of 100 to 150 atm completely restored the luminosity. Johnson and Flagler3 further demonstrated that anesthetized tadpoles started swimming again by application of pressure in the similar range. These results were confirmed by Lever et al.4 in newts and mice in 1971. Numerous reports on the pressure reversal of anesthesia followed.5-8 Despite the abundance of reports on the pressure antagonism of anesthesia, studies on the physical properties of anesthetics under high pressure are few. Because pressure is paired with volume in thermodynamics, the antagonism of anesthesia by this simple physical force suggests involvement of volume changes in anesthesia mechanisms. The densities of volatile anesthetics in the pure liquid state at ambient pressure were reported by Bottomley and Seiflow,9 Stern and Shiah,"J Korman and Ritchie,ll and Mori et a1.12 The partial molal volumes of anesthetics in water and in organic phases were reported by Kita et al.13 and Mori et al.,lz also at ambient pressure. The present study measured densities of halothane, enflurane, and isoflurane at high pressures, up to 34.56 MPa (341 atm), to provide a data base for analyses of the pressure reversal of anesthesia. The data are presented as (I)the molal volume for the pure liquid state, (2) the partial molal volume in water, (3) the partial molal volume in an organic solvent, 1-octanol, and ( 4 ) the isothermal compressibilities. 1-Octanol was used to represent the organic phase because solubility in this solvent has been shown to correlate well with drug potencies.14
confirmed that the preparations are free from water contamination. 1-Octanol was obtained from J. T. Baker (Phillipsburg, NJ) and was also treated with activated aluminum oxide columns. The purity of 1-octanol was confirmed by a Shimadzu gas chromatograph (Columbia, MD) with a flame ionization detector. The column was 1/8 in x 6 ft, stainless steel, containing Porapak Q 80/100mesh. The operating conditions were isothermal at 160 "C and at a flow rate of 20 mL/min of helium as the carrier gas. Water was purified by distillation, then treated by two mixed-bed ion-exchanger columns, an activated charcoal column, and an ultrafilter (Millipore, Bedford, MA). Water:anesthetic mixtures were prepared in 30-mL Teflon tubes equipped with air-tight screw caps, O-rings, and plugs. The amount of water and anesthetic contained in the bottle was measured by weighing it with a Mettler semimicrobalance (Hightstown, NJ) to 5* g. The mixture was kept in a constant temperature water bath at 25 "C for 3 days with shaking. With the 1-octano1:anesthetic mixture, the equilibration time was 24 h. After the incubation, the bottles were weighed again to check possible leaks. The densities of sample solutions were measured by an Anton-Paar oscillation densimeter DMA60/512 (Mettler-Paar, Hightstown, NJ). The temperature of the densimeter cuvette was maintained at 298.150 f 0.0005 K by circulating water from a Hart 5003 Iso-Them water bath (Hart Scientific, Provo, UT), and monitored by a microprocessor-controlled thermistor thermometer (Micro-Therm 1006, Hart Scientific, Provo, UT) with 0.0001 K resolution. The millidegree stability was necessary to obtain reproducible results. The temperature of the water bath was monitored by a Hewlett-Packard quartz thermometer 2804 A (Palo Alto, CA), also with 0.0001 K resolution. Hydrostatic pressure was generated by a hand-operated hydraulic pump, and was transmitted to the two ends of the stainless steel vibration tube of the densimeter by stainless steel pipes. The pressure was monitored by an Autoclave Digital Pressure Transducer (model DPS-0201; Erie, PA). The cell constant of the densimeter was determined by using the densities of water and NaCl s0lutions.15~16 Before introducing the test solution into the densimeter, the solution was sonicated by immersing the container tube in a sonicator water bath (Sonicor Instrument, Copiague, NY), and heated 2°C above the measuring temperature for 10 min to prevent microbubble formation in the test solution during the density measurement. The density was measured in triplicate to confirm the accuracy of the procedure. The deviations were almost null.
Results The molal volumes of pure anesthetics were calculated from the solution densities as follows:
M
Experimental Section
U0,P = -
Isoflurane (1-chloro-2,2,2,-trifluoroethyl difluoromethyl ether) and enflurane (2-chloro-1,1,2,-trifluoroethyl difluoromethyl ether) were obtained from Anaquest (Madison, WI) and halothane (2-bromo-2chloro-l,l,l,-trifluoroethane) from Ayerst (New York, NY). They were treated with activated aluminum oxide columns (Woelen Pharma, West Germany) to remove traces of water. The stabilizer contained in halothane (0.01% thymol) was removed by this procedure. The water contents were checked by Fourier-transform IR spectroscopy (FTIR model 1750; Perkin-Elmer, Norwalk, CT), which
where v , , ~ M , , and do,p are the molal volume, molecular weight, and the density of anesthetics, respectively. The subscript o signifies the pure state. The molal volumes of anesthetics, calculated from the density data a t 298.150 K (25.000 "C) and at various pressures up to 34.56 MPa, are listed in Table I. At the location where the experiments were performed, the ambient pressure is 0.085 MPa (0.84 atm)
0022-3549/90/1000-0893$01.001'0 0 1990,American Pharmaceutical Association
(1)
d0,P
Journal of Pharmaceutical Sciences / 893 Vol. 79, No. 10, October 1990
Table C l o l a l Volume of Pure Anesthetlcs at 298.150 K under Varlous Pressures and the isothermal Compresslblllty, k, at 0.1013 YPa
Halothane
Molal Volume, cm3.mol-’ Enflurane
lsoflurane
0.085 3.53 6.98 13.88 20.77 27.66 34.56
106.3 105.8 105.3 104.3 103.4 102.6 101.9
122.0 121.3 120.8 1 1 9.5 118.5 117.5 116.6
123.7 122.9 122.2 120.9 119.8 118.8 117.8
k f SE, MPa-’
1.47-10-3 -c 3.37.10-5
1.56.10-3 3.51 .10-5
1.75.10-324.70.10-5
Pressure, MPa
rather than 0.1013 MPa. The density data of the anesthetics as a function of pressure were fitted by the least squares method to the following equation: n
In uo,p =
1.0127
C aiPi
v)
i=O
1.0124:
where aiis a constant. The isothermal compressibility, k, of the anesthetics is given by eq 3:
z
1.0121 0.982 0.980
t
m
where V is the system volume. The compressibilities of pure anesthetics a t 0.1013 MPa and 298.150 K were calculated according to eqs 2 and 3 by second-degreepolynomial fitting. The densities of water:anesthetic solutions at 298.150 K under ambient and 34.56 MPa pressures are shown in Figure 1as a function of anesthetic molalities (moles of solute in 1000 g of solvent). The molality instead of molarity (moles of solute in 1000 mL of solution) was used because molarity values vary according to temperature and pressure. The values extrapolated to zero anesthetic concentration were identical with those of pure water at each pressure. The densities of 1-octano1:anesthetic mixtures under ambient and 34.56 MPa pressures are shown in Figure 2 as a function of anesthetic molalities. The apparent molal volume of solute (&, where subscript 2 signifies solute) in a two-component system is expressed as follows:
\
0.978
D Y
> 0.976
k -
z w
0.974
0 .9 7 2 0.970
0
MOLALITY
where m, and M, are the molality and molecular weight of anesthetics, respectively, and d and doare the densities of the solution and the solvent, respectively. The partial molal volume of the solute is defined as follows: 894 I Journal of Pharmaceutical Sciences Vol. 79, No. 10,October 1990
( x 1O - * )
Figure 1-Densities of anesthetics in water at 298.150 K (25.000“C).(A) Ambient pressure (0.085MPa). (8) Pressure of 34.56 MPa. Key: (A) halothane; (0)enflurane; and (El) isoflurane.
02 =
where n, and n2 are the number of moles of the solvent and solute, respectively, and us is the molal volume of the pure solvent. The apparent molal volume, &, is related to the density of the solution as follows:
2
1
(dVIWTp,n,
(6)
where V and n2 are the volume of the system and the number of moles of the solute, respectively. From eqs 4 and 6, eq 7 can be written as follows:
Equation 7 indicates that V , a t infinite dilution is obtaincd from the intersect of the plot between 4 and molality, and u2 at nonzero concentration is obtained by ‘taking the first derivative. The partial molal volumes of anesthetics at infinite dilution in water and in 1-octanol as a function of pressure are listed in Tables I1 and 111, respectively.
0.845
-5 m
0.844
\
0, U
0.843
>-
k 0.842 W
n
0.84 1 0.825
1
- A
m^ 0.824
E0
\
3 0.823 >. k 0.822 v)
Z
0.821 0.820 1 0
I
I
I
I
I
1
2
3
4
5
( x 1O-*)
MOLALITY
Figure 2-Densities of anesthetics in 1-octanol at 298.150 K (25.000"C).(A) Ambient pressure (0.085MPa). (B) Pressure of 34.56 MPa. Key: (A)halothane; (0)enflurane; and (0)isoflurane.
The isothermal partial molal compressibility, K, is defined as follows: I
-\
-
k
= T
The values for K a t infinite dilution in water and in 1-octanol are listed in Tables I1 and 111, respectively. Notice that the definition of the isothermal partial molal compressibility in a solvent differs from the isothermal compressibility of pure liquid anesthetics. Accordingly, the dimension of the compressibility is also different between the two.
Discussion The present results on liquid anesthetics at ambient pressure and at 298.15 K are in good agreement with reported
values.11J2 The possible loss of anesthetics from water and 1-octanol during transfer from the incubation vessel to the densimeter was not determined. The escaped amount, however, appears to be negligible because extrapolation of the density data to zero anesthetic concentration transected the density coordinate at the identical value of the solvent. Escape of the anesthetic from the densimetry cuvette during the measurement was unfeasible because this is a closed system under high pressure. The partial molal volume of halothane in water decreased with increasing pressure, whereas those of enflurane and isoflurane increased. The mechanism of the difference in the sign of the isothermal partial molal compressibility in water between the ether-type anesthetics and the alkane-type anesthetic is unknown. It may be caused by the difference in water structure clustered around the anesthetic molecules. Partial molal volume in water is strongly influenced by the water structure. The hydrogen-bond breaking activity of volatile anesthetics is amply documented by Sandorfy and CO-workers.17-20 They have shown that anesthetics form a proton donor:acceptor complex in competition with water molecules so that the H20:H20 dimer is broken to form an anesthetic:H,O dimer. Another feature of anesthetics is to self-aggregate in water. Yoshida et a1.21 reported that enflurane started to self-aggregate a t -4 mM in water and formed an octamer before visually saturating the aqueous phase. For these reasons, the density of the anesthetic:water mixture is strongly nonlinear with respect to anesthetic concentrations. Accordingly, the partial molal volume of anesthetics extrapolated to infinite dilution in water is subject to substantial error. Because the partial molal compressibilities of anesthetics in water are computed on the extrapolated values, the parameter is associated with relatively large standard error. Cabani et a1.22 reported that the partial molal compressibilities of n-alcohols (ethanol to n-hexanol) are negative at 10 "C but positive a t 25 "C. Their data do not show the error range, but may not be free from deviations. The exact cause of these discrepancies in the sign of the partial molal compressibility in water requires further studies. The larger partial molal volume of anesthetics in octanol than in water indicates that high pressure will squeeze out these anesthetic molecules from the organic phase to the water phase. If one accepts that octanol represents anesthetic binding sites reasonably well, pressure-reversal of anesthesia may involve elimination of anesthetic molecules from the action sites. Kaneshina et al.,23 however, reported that the portions of anesthetic molecules squeezed out from phospholipid bilayer membranes were 10.4%for halothane, 6.4% for chloroform, 9.6% for enflurane, and 8.5% for methoxyflurane at 100 atm (10.13 MPa). Similar "squeeze-out" results were obtained with a desorption study of anesthetics by pressure in ionic surfactant micelles.24 Although the squeeze-out of an-
Table ICPartlal Molal Volume of Anesthetlcs at Infinite Dilution In Water at 298.150 K under Varlous Pressures and the Isothermal Partlal Molal Compresslbillty (3at lnfinlte Dilution at 0.1013 MPa Pressure, MPa
0.085 3.53 6.98 13.88 20.77 27.66 34.56
iG f SE, cm3 mot-' MPa-' a
Partial Molal Volume, cm3.mol-' Halothane
Enflurane
isoflurane
102.6 101.3 101.1 100.8 100.3 100.5 99.3
108.0 106.5 106.3 107.5 108.6 109.6 108.7
113.0 112.8 113.0 113.0 115.8 116.3 115.7
7.42.lo-' t 1.51
lo-'
-6.89 . 1 0 - 2 t 2.74.1O-'
-1.06.lo-'
t
2.53.
Journal of Pharmaceutical Sciences I 895 Vol. 79,No. 70, October 7990
Table IlCPartlai Molal Volume of Anesthetics at lnflnlte Dilution In 1-Octanol at 298.150 K Under Various Pressures and the Isothermal Partlal Molal Compresslblllty, k,at lnflnlte.Dllut1onat 0.1013 MPa Partial Molal Volume, CKI~"IOI-~ Pressure, MPa Halothane Enflurane lsoflurane 0.085 3.53 6.98 13.88 20.77 27.66 34.56
104.6 104.1 103.6 103.0 102.1 101.1 100.2
b r SE, cm3. rnol-' * MPa-'
1.25 * lo-' f 3.41 *
esthetic molecules from the tissue binding site undoubtedly contributes to the pressure reversal of anesthesia to a certain extent, the obtained numerical squeeze-out values are too modest to account for the full recovery from anesthesia. Additional mechanisms must be operative for the pressure reversal phenomenon, and their elucidation may lead to the understanding of anesthesia mechanisms.
References and Notes 1. Johnson, F. H.; Eyring, H.; Williams, R. B. J.Cell. Comp.Physiol. 1942,20,247-268. 2. E in H.; Magee, J. L. J. Cell. Comp. Physiol. 1942, 20, lglV7. 3. Johnson, F. H.; Flagler, E. A. Science 1951,112,91-92. 4. Lever, M. J.; Miller, K. W.; Paton, W.D. M.; Smith, E. B. Nature 1971,231,368-371. 5. Halsey, M. J. Physiol. Reu.1982, 62, 1341-1377. 6. MacDonald, A.G. Phil. Trans. Roy. Soc. Lond. B 1984, 304, 47-68. 7. Kendig, J. J. Am. J. Physiol. 1984,246, C91495. 8. Smith, E. B.; Bower-Rile F.; Daniels, S. Molecular and Cellular Mechanisms ofAnest&cs; Roth, S. H.; Miller, K. W., Eds.; Plenum: New York, 1986; pp 341-353. 9. Bottomley, G.A.; Seiflow, G. J. Appl. Chem. 1963,13,339-343. 10. Stem,S. A.; Shiah, S. P. Anesthesiology 1979, 50, 538. 11. Korman, B.; Ritchie, I. M. Anesthesiology 1982,57,4243.
896 1 Journal of Pharmaceutical Sciences Vol. 79, No. 10, October 1990
122.7 122.1 121.5 120.3 119.8 119.1 117.7 1.36 * lo-'
* 7.71
125.0 124.2 123.7 122.6 121.9 120.9 120.0 *
10-3
1.40 * lo-' f 3.41 * low3
12. Mori, T.; Matubayasi, N.; Ueda, I. Mol. Pharmacol. 1984, 25, 123-130. 13. Kita, Y.; Bennett, L. J.; Miller, K. W. Biochim. Biophys. Acta 1981,647,130-139. 14. Hansch, C.; Clayton, J. M. J.Pharm. Sci.1973, 62, 1-21. 15. Fine, A. R.; Millero, F. J. J. Chem. Phys. 1973,59, 5529-5536. 16. Gates, J. A.; Wood, R. H. J. Chem. Eng. Data 1985,30,44-49. 17. DiPaolo, T.; Sandorfy, C. J. Med. Chem. 1974,17,809-814. 18. Hobza, P.; Mulder, F.; Sandorfy, C. J.Am. Chem. SOC.1981,103, 1360-1366. 19. Hobza, P.; Mulder, F.; Sandorfy, C. J.Am. Chem. SOC.1982,104, 925-928. 20. Trudeau, G.; Dumas, J M . ; Dupuis, P.; Guerin, M.; Sandorfy, C. Topics Current Chemistry 1980,93, 91-125. 21. Yoshida, T.; Okabayashi, H.; Kamaya, H.; Ueda, I. Biochim. Biophys. Acta 1989,979,287-293. 22. Cab!a_ni, S.; Conti, G.; Matteoli, E. J. Solution Chem. 1979, 8, 11-m.
23. Kaneshina, S.; Kamaya, H.; Ueda, I. Biochim. Biophys. Acta 1982,685,3074314. 24. Kaneshina, S., Kamaya, H., Ueda, I. J. Colloid InteTface Sci. 1983,93,215-224.
Acknowledgments This stud was supported b the Medical Research Service of the Veterans A&ninistration, a n J N I H grants GM25716 and GM27670.