| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Great Shelford, Cambridge, UK
1 Correspondence: 17 High Green, Great Shelford, Cambridge CB2 5EG, UK. E-mail: alecbangham{at}wwr.co.uk
Bill Bryson, who has the temerity to have written A Short History of Nearly Everything (1)
, claims that little is known about the causes of deep diving narcosis. He acknowledges that it is related to the abundant atmospheric gas, but fails to note that any gas soluble in a nonaqueous phase will partition into the typically phospholipid rich biological cell membrane.
As it happens, a paper on the subject was published from my laboratory at Babraham, many years ago (2)
. Our paper established that nitrogen (and argon) penetrate membrane phospholipids under pressure and the extent of the adsorption was in rough agreement with that for typical volatile anesthetic agents such as ether, chloroform, and halothane, usually administered at atmospheric pressure. Significantly, helium showed no evidence of penetration up to a pressure of 800 pounds per square inch, hence the success of "Heliox". We received little feedback and the significance of our result was not appreciated by us at the time. Now, some 38 years later, I’ve taken the opportunity to remind readers of this Bryson’s modestly titled compendium—and of The FASEB Journal—of the results and conclusions of some 27 peer-reviewed papers on the subject of anesthetic action.
Keith Miller, of the Department of Anesthesia, Massachusetts General Hospital, wrote of liposomes as follows: "To a field whose most powerful model nearly seven decades ago had been a jar of olive oil, the liposome’s arrival was a liberating force." (3)
Sheena Johnson in my laboratory and Miller at Oxford confirmed that just as anaesthetized animals could be reawakened under pressure, so could leaky liposomes (4)
. Sheena Johnson subsequently published substantial experimental data relating to the thermodynamic parameters of anesthetic concentration, temperature, and pressure on the leakiness of liposomes to cations. Martyn Hill summarized her data by the generalization that the cation permeability of liposomes can be analyzed in terms of rate theory, the activation enthalpy DH* being characteristic of the system but being independent of the presence or absence of anesthetics. The fact that the anesthetics tended to increase the permeability and hence the free energy of activation DG*, but not DV* or DH*, means that DS*, the entropy of activation, must change since DG* = DH* + PDV* – TDS*. This indicates that anesthetics and/or a raised temperature operate by changing the entropy of the membrane and that hydrostatic pressure counteracts the ensuing (membrane) disorder (5)
Liposomes were responding, it would appear, as perfect thermodynamic models, a conclusion that immediately bears on the confusion surrounding the mechanism whereby nitrogen mediates the narcosis of deep diving. The conflict is evident: to compromise liposomes a high concentration of nitrogen is required, a concentration achieved only by high pressure. But, pressure reduces solubility into the phospholipid model! Likewise, the whole body of an animal would also be affected by high pressure and is well known to compromise the effect of anesthetics. Little wonder Bill Bryson is bemused. The same conflict arises if and when whole animals are anesthetized at raised temperature. The thermodynamic model would predict lower concentrations of anesthetic to be necessary, contrary to clinical experience, but raised temperature of the whole body system could also be expected to enhance the activity of many enzymes.
Liposomes were appealing to us as models in this regard, because they could be made to resemble synaptic neurotransmitter vesicles both in composition and size. Ironically "synaptosomes" were first isolated by Victor Whittaker and Sheridan at Babraham (Cambridge) in a laboratory adjacent to mine (6)
. We were also aware that neurotransmitters were characteristically weak amines (e.g., adrenaline, dopamine, and 5 HT) and had shown they could be concentrated inside liposomes (7)
. We therefore proposed a proton pump/leak hypothesis to account for a compromised CNS or a state of unconsciousness (8)
.
Could the liposome data support the classic Meyer hypothesis which declared that "the narcotising substance enters into a loose physico-chemical combination with the vitally important lipids of the cell, perhaps with lecithin" and discussed at length by Miller (3)
?
We decided to test it on whole animals (actually, fish). If disordered and leaky synaptosomes compromised normal function of the CNS, warming of the whole body might lower the required anesthetic dose for a given level of unconsciousness, and on the other hand at low body temperature or under high pressure, higher concentrations would be required.
Almost the opposite was observed (9)
! Nevertheless, Deamer and I found ourselves confidently able to reconcile all results and did so in a letter to the British Journal of Anesthesia (10)
We wrote:
Living cells, using metabolic energy and a variety of transport processes, maintain well defined internal compositions that are different from the external environment. A cell uses these gradients in many ways: some are fundamental to survival, whereas others are used for more specialized functions. For instance, specialized intracellular compartments such as mitochondria, lysosomes, and synaptic vesicles in neurones also have defined internal composition. In synaptic vesicles, the homeostatic state is maintained by metabolic activity which supplies ATP to ion transport enzymes. These in turn generate ion gradients across the vesicle membrane. One such ubiquitous process is inward proton transport by ATP dependent enzymes in synaptic vesicle membranes.
The resulting pH gradients, acid inside, are used to drive the accumulation of neurotransmitters into the vesicles. If the metabolic balance between production and loss of energy is modified to the extent that the gradients are not sustained, it follows that the synaptic vesicles will lose some fraction of accumulated neurotransmitters (10)
.
The hypothesis put forward here is that hypoxia (from whatever cause), temperature excursions, pressure, and the colligative effect of anesthetics acting individually or in concert on specific cellular functions of the whole animal, will alter the ionic gradients of synaptic vesicles such that normal function is compromised. A dead animal, after all, has no transmembrane gradients! To be more specific, hypoxia in the nervous system will clearly reduce the availability of ATP generally required by ion pumps. There will be little immediate effect on membrane receptors or the ionic concentration gradients across nerve cell plasma membranes, but it is likely that the much smaller compartments of synaptic vesicles will quickly lose their ability to maintain ion gradients necessary for full function in synaptic transmission, leading to anesthesia.
Similarly, cold temperatures will reduce the rate at which enzymes in the synaptic vesicle membrane can transport ions against a continuing leak, again resulting in compromised function due to loss of neurotransmitters. Higher temperatures in turn will increase lipid bilayer permeability to ions to an extent that cannot be compensated for by increased transport rates, with the same result. Significantly, it is known that volatile anesthetics also increase bilayer permeability to ions (5)
and measurably increase ionic permeability of synaptic vesicles even at concentrations referred to as "clinical" (8)
.
Perhaps most significant for the pump/leak concept is that anesthetic effects in whole animal systems contradicted the thermodynamic prediction of the lipid perturbance model. That is, goldfish, when cooled, actually required increasingly lower concentrations of anesthetics (and at the end none) to produce anesthesia (9)
. It was this observation that prompted our pump/leak hypothesis, and led us to ask whether a lipid site or a protein site by itself could provide a broadly applicable mechanism for anesthesia. We think not.
Although it has been amply demonstrated that a specific anesthetic acting on a specific receptor on a given cell can be understood as leading to cellular anesthesia, this obviously cannot account for the anesthetic effects of hypoxia and temperature excursions on the whole organism. We believe that further tests of the pump/leak concept are warranted, particularly if they take into account the predicted relation between anesthetic action and energy metabolism, ion transport and lipid bilayer permeability we have recalled as a milestone here.
|
REFERENCES
Related Articles
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
