Effects of isoflurane, halothane, and chloroform on the interactions and lateral organization of lipids in the liquid-ordered phase

Abstract

The first quantitative insight has been obtained into the effects that volatile anesthetics have on the interactions and lateral organization of lipids in model membranes that mimic "lipid rafts". Specifically, nearest-neighbor recogntion measurements, in combination with Monte Carlo simulations, have been used to investigate the action of isoflurane, halothane, and chloroform on the compactness and lateral organization of cholesterol-rich bilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in the liquid-ordered (l(o)) phase. All three anesthetics induce a similar weakening of sterol-phospholipid association, corresponding to ca. 30 cal/mol of lipid at clinically relevant concentrations. Monte Carlo lattice simulations show that the lateral organization of the l(o) phase, under such conditions, remains virtually unchanged. In sharp contrast to their action on the l(o) phase, these anesthetics have been found to have a similar strengthening effect on sterol-phospholipid association in the liquid-disordered (l(d)) phase. The possibility of discrete complexes being formed between DPPC and these anesthetics and the biological relevance of these findings are discussed.

Figure 1

A stylized illustration showing the exchangeable homodimers, AA and BB and the corresponding heterodimer, AB, plus the equations that describe the dimer equilibrium and the relationship between the equilibrium constant, K, and the corresponding nearest-neighbor interaction free energy, ω_AB, between A and B.

Figure 2

Reaction vessel used for carrying out binding measurements.

Figure 3

(A) Plot of X_A/P versus isoflurane concentration in buffer at 45°C for cholesterol-rich and cholesterol-poor bilayers. (B) Plot of free energy of interaction between 1 and 2 (i.e., ω_AB) as a function of X_A/P. Error bars that are not visible lie within the symbols themselves. For each of these measurements, the total concentration of lipid was 4.1 mM.

Figure 4

(A) Plot of X_A/P versus halothane concentration in buffer at 45°C for cholesterol-rich and cholesterol-poor bilayers. (B) Plot of free energy of interaction between 1 and 2 (i.e., ω_AB) as a function of X_A/P. Error bars that are not visible lie within the symbols themselves. For each of these measurements, the total concentration of lipid was 4.1 mM.

Figure 5

(A) Plot of X_A/P versus chloroform concentration in buffer at 45°C for cholesterol-rich and cholesterol-poor bilayers. (B) Plot of free energy of interaction between 1 and 2 (i.e., ω_AB) as a function of X_A/P. Error bars that are not visible lie within the symbols themselves. For each of these measurements, the total concentration of lipid was 4.1 mM.

Figure 6

Plot of Kp versus the concentration of chloroform in buffer, where K_p = [X_A/P]_{liquid-disordered}/[X_A/P]_{liquid-ordered}.

Figure 7

Composite plot of ω_AB as a function of X_A/P for isoflurane, halothane and chloroform (data taken from Figures 3,4 and 5).

Chart 1

Chart 2