- Email: [email protected]
May the force be with you
TlPS - April 1988 [Vol. 91
124
’
--
rl
Fig.2. The&urcb at San Giorgb:whereE&ski bagsmeet-
sandwiches, field samples, plants and assorted camping gear. It is
stained with the stains of a dozen misadventures.
AMERICANS, WE ARE told, drink 33 million gallons of coffee a day. There has been a brisk, but inconclusive, debate as to whether the habit of imbibing coffee is associated with de~ment~ consequences to one% health. But what about the ingredients we add to the coffee? Ask for cream for your coffee in America, and you will be given a small container on which appears to be printed a
microfilmed page from a chemical catalog. Inside is an ingenious concoction, a whitish powder known as creamer. Creamers typically contain vegetable oils. Of 25 creamers analysed, 22 contained coconut oil, which has a high proportion of saturated fag. The authors estimate that a teaspoon of such creamers yields 10 kcal of saturated fat. Several cups a day, with a spoonful or two of creamer in each, can add sub-
A plea for more gravity in reporting muscle research
May the force be with you I read, and occasionally referee, papers reporting findings from experiments on muscle; all kinds of muscle. I have been dismayed by an abuse of units common in this field. The pharmacological fraternity in particular seems enamoured of the view that its scraps of blood vessel and digestive system generate grams when suitably provoked with adrenaline, potassium, electricity and the like. Now grams, like kilograms and milligrams, are units of mass. (If the butchering trade ever dis-
covers that one apparentiy need only apply stimulating de&odes to best beef to increase its m&ss, the economy could take an interesting turn.) In my well meaning attempts to disabuse colleagues of this error, I encounter a mixture of resistance, ignorance and complacency that one might expect in a faculty meeting rather than in Science. I believe that my plea for a general insistence upon calibrating force (tension) records in appropriate units is justified on these clear
The bags wander the world. At any gathering of pharmacologists, they can be seen, serviceable yet carrying a mystic aura of the elect, a campaign medal identifying the wearer as a participant in that most memorable of IUPHAR meetings, which opened with a feast of baccanalian pleasures, satisfying gourmets and gourmands alike, and closed to a serenade of a thousand violinaceous five year olds. What could better symbolize our discipline than these useful objects, macroreceptors slung on pharmacological shoulders around the world. I propose that IUPI-IAR adopt the Ebashi bag as its official symbol. The Heralds would have fun describing it: ‘Bag rampant on field vert, voided and gorged with Goodman and Gilman piled. . .’
stantially to one’s intake of these currently undesirable fatty acids. These, it has been established, lead to an increase in total circulating cholesterol, and to an undesirable shift in the HDL/LDL ratio. B. MAX References 1 Engelberg, H.
(1984) P~UYRJQCO~. Rev. 36, 91-110 2 Vanness, A. F. and McManus, B. M. (1986) N. Engl. J_ Med. 314,651
[Irounds: (1) it is misleading to vou, me and others to confuse mass and force; (2) those of us .involved in teaching as well as research have our task made harder when indefensible examples are set in the professional literature; and (3) imparting a clear grasp of the distinction between mass and force is a useful didactic tool in teaching ‘muscle’. I had thought to apologize for the small lesson in junior level physics which follows, but experience assures me that for some, at least, no apology is necessary! Measurements of mass are notoriously difficult, though approximations are easy by the balance principle. We all slip easily into equating objects of identical weigh! with equal MUSS, Numerical errors thus created are generally small, though by no means always insignificant.
TIPS - April 1988 [Vol. 91 However, the close numerical approximation of mass and weight in no way allows an equation of the dimensions these numbers have (mass and mass x acceleration, respectively). Thus, the tension transducer is readily calibrated with known weights (never, I believe, referred to as known masses). The weight of an object is, of course, the net force exerted on it by the earth’s gravitational attraction. Weight takes account of the flotation effect of air, or any other fluid in which the object finds itself when weighed. Objects literally weigh less in water than air, indeed slightly less in damp air than dry air, but it is easy to accept that their mass has not changed. Thus weight is a convenient measure of force. Provided the force transducer can be suitably arranged, it is easy to apply known forces .to it by the expedient of hanging known weights from it. Thus, a muscle when contracting (in the physiologists’/ pharmacologists’ sense, i.e. not necessarily getting shorter!) generates a force which can be expressed in equivalent weight units. These units will be g wt, mg wt, kg wt. [Continental Europeans have the advantage of the colloquial unit the kilopond jiiterally kiiogram weight) though whether general appreciation of the real distinction between weight and ma.98 is better than in the UK I have not been able to establish.] Some workers escape by using newtons, quite appropriately, to express force. These units have not found widespread acceptance, as a glance through the literature will show you. [The useful aid to memory is that a newton (1 kg m 6, dimensions of mass X acceleration, i.e. force) is exactly the force exerted by a typical Cox’s Orange Pippin or Bramley apple in free fall towa.rds the earth when i Ipinging upon the head of Sir Isaac or other firm surface.] In most labs, 1 suspect, the subterfuge adopted is to convert by ‘981’ (a magic ‘number!) from a g wt calibration d the transducer to a graph ordinate in newtons. This may be apprcnpriate in Paris (where ‘981’ was established) but does it apply in Paris, Texas? I leave that to you for homework. DAVID J. MILLER
125
More evidence for presynaptic control of neurotransmitter release
Neurotraztsmission at the NMJ The recent review article entitled ‘Feedback control of transmitter release at the neuromuscular junction’ by Bowman et al. (TiPS 9, 16-20, january 1988) puts forward the concept that transmitter release from motor nerves is under the local control of presynaptic facilitatory nicotine receptors. Indirect evidence for this concept comes from electrophysiological and functional studies (measurement of neurally evoked endplate currents or muscle contractions)s3. In these studies, tubocurarine and hexamethonium produced a marked fade of the end-organ responses during repetitive stimulation without a substantial depression of peak response evoked by a single pulse. A different pattern was observed with (Ybungarotoxin and erabutoxin-b which produced depression of peak response and faded simultaneously. The authors exclude a postsynaptic origin of fade and are strongly of the opinion that fade reflects a presynaptic event, a reduction of transmitter release by a blockade of facilitatory nicotine receptors. Indeed, clear evidence for this concept has previously been found in release studiesqd. Incubation of endplate preparations of the rat phrenic nerve with [3H]choline leads to the formation of neuronal [3H]acetylcholine which can be released upon electrical nerve stimulation and measured in the absence of cholinesterase inhibition’. Using this radiolabelling method, it could be demonstrated that nicotine receptor agonists (nicotine, cytisine, dimethyl-4-phenylpiperazinium; pM concentrations) enhanced and antagonists (tubocurarine, hexamethonium) reduced the neurally evoked transmitter output4-6e8. The enhancing effect of the agonists could be prevented by a pretreatment with tubocurarine. Hexamethonium markedly differed in its potency to reduce transmitter release (presynaptic effect) and to reduce muscle contraction (postsynaptic effect). All these results clearly support the concepts that: (1) two populations of nicotine receptors with different pharmacological propertics are
present within the endplate region; (2) stimulation of presynaptic nicotine receptors enhances transmitter release from the motor nerve; and (3) transmitter release is regulated by a positive nicotinic feedback. The authors claimed that the use of hemicholinium (to block re-uptake of rH]choline originating from hydrolysis of released [3H]acetycholine) might have artificially modified the results by inhibition of acetylcholine synthesis. This possibility, however, appears unlikely, because hemicholinium was added after the labelling period, i.e. after synthesis of [3H]acetylcholine had occurred. [Little further synthesis of [‘Hlacetylcholine occurs after the addition of hemicholinium (Ref. 9).] Consequently, neurally evoked output of [3H]acetylcholine reflects transmitter release from a preformed pool(s) of radiolabelled transmitter. Inhibition of neurally evoked [3H]acetylcholine output can only be explained by a reduced probability of preformed transmitter being released (i.e. reduction or impaired refilling of the readily releasable vesicle pool, reduction of release sites, or reduction of vesicle refilling). Impaired synthesis of radiolabelled transmitter cannot be involved. One should also consider that modulation of neurally evoked [3H]acetylcholine output is observed when a total of only 100 pulses (5-50 Hz) is applied5 - a stimulation condition which releases only about 3% of the tissue store of radiolabelled transmitter present within the phrenic nerve’. Moreover, neurally evoked [3H]acetylcholine output can be increased by nicotine receptor stimulatio&‘* ‘, which makes it unlikely that blockade of synthesis (by hemicholinium) terminates [3H]acetylcholine output during short stimulation periods (5 I-I-I; 10-20, s). studies biochemical strongly support the concept of a positive nicotinic feedback mechanism put forward by BOwman and co!lragues. The facilitatory mechanism is activated in a frequency-dependent marine?
124
’
--
rl
Fig.2. The&urcb at San Giorgb:whereE&ski bagsmeet-
sandwiches, field samples, plants and assorted camping gear. It is
stained with the stains of a dozen misadventures.
AMERICANS, WE ARE told, drink 33 million gallons of coffee a day. There has been a brisk, but inconclusive, debate as to whether the habit of imbibing coffee is associated with de~ment~ consequences to one% health. But what about the ingredients we add to the coffee? Ask for cream for your coffee in America, and you will be given a small container on which appears to be printed a
microfilmed page from a chemical catalog. Inside is an ingenious concoction, a whitish powder known as creamer. Creamers typically contain vegetable oils. Of 25 creamers analysed, 22 contained coconut oil, which has a high proportion of saturated fag. The authors estimate that a teaspoon of such creamers yields 10 kcal of saturated fat. Several cups a day, with a spoonful or two of creamer in each, can add sub-
A plea for more gravity in reporting muscle research
May the force be with you I read, and occasionally referee, papers reporting findings from experiments on muscle; all kinds of muscle. I have been dismayed by an abuse of units common in this field. The pharmacological fraternity in particular seems enamoured of the view that its scraps of blood vessel and digestive system generate grams when suitably provoked with adrenaline, potassium, electricity and the like. Now grams, like kilograms and milligrams, are units of mass. (If the butchering trade ever dis-
covers that one apparentiy need only apply stimulating de&odes to best beef to increase its m&ss, the economy could take an interesting turn.) In my well meaning attempts to disabuse colleagues of this error, I encounter a mixture of resistance, ignorance and complacency that one might expect in a faculty meeting rather than in Science. I believe that my plea for a general insistence upon calibrating force (tension) records in appropriate units is justified on these clear
The bags wander the world. At any gathering of pharmacologists, they can be seen, serviceable yet carrying a mystic aura of the elect, a campaign medal identifying the wearer as a participant in that most memorable of IUPHAR meetings, which opened with a feast of baccanalian pleasures, satisfying gourmets and gourmands alike, and closed to a serenade of a thousand violinaceous five year olds. What could better symbolize our discipline than these useful objects, macroreceptors slung on pharmacological shoulders around the world. I propose that IUPI-IAR adopt the Ebashi bag as its official symbol. The Heralds would have fun describing it: ‘Bag rampant on field vert, voided and gorged with Goodman and Gilman piled. . .’
stantially to one’s intake of these currently undesirable fatty acids. These, it has been established, lead to an increase in total circulating cholesterol, and to an undesirable shift in the HDL/LDL ratio. B. MAX References 1 Engelberg, H.
(1984) P~UYRJQCO~. Rev. 36, 91-110 2 Vanness, A. F. and McManus, B. M. (1986) N. Engl. J_ Med. 314,651
[Irounds: (1) it is misleading to vou, me and others to confuse mass and force; (2) those of us .involved in teaching as well as research have our task made harder when indefensible examples are set in the professional literature; and (3) imparting a clear grasp of the distinction between mass and force is a useful didactic tool in teaching ‘muscle’. I had thought to apologize for the small lesson in junior level physics which follows, but experience assures me that for some, at least, no apology is necessary! Measurements of mass are notoriously difficult, though approximations are easy by the balance principle. We all slip easily into equating objects of identical weigh! with equal MUSS, Numerical errors thus created are generally small, though by no means always insignificant.
TIPS - April 1988 [Vol. 91 However, the close numerical approximation of mass and weight in no way allows an equation of the dimensions these numbers have (mass and mass x acceleration, respectively). Thus, the tension transducer is readily calibrated with known weights (never, I believe, referred to as known masses). The weight of an object is, of course, the net force exerted on it by the earth’s gravitational attraction. Weight takes account of the flotation effect of air, or any other fluid in which the object finds itself when weighed. Objects literally weigh less in water than air, indeed slightly less in damp air than dry air, but it is easy to accept that their mass has not changed. Thus weight is a convenient measure of force. Provided the force transducer can be suitably arranged, it is easy to apply known forces .to it by the expedient of hanging known weights from it. Thus, a muscle when contracting (in the physiologists’/ pharmacologists’ sense, i.e. not necessarily getting shorter!) generates a force which can be expressed in equivalent weight units. These units will be g wt, mg wt, kg wt. [Continental Europeans have the advantage of the colloquial unit the kilopond jiiterally kiiogram weight) though whether general appreciation of the real distinction between weight and ma.98 is better than in the UK I have not been able to establish.] Some workers escape by using newtons, quite appropriately, to express force. These units have not found widespread acceptance, as a glance through the literature will show you. [The useful aid to memory is that a newton (1 kg m 6, dimensions of mass X acceleration, i.e. force) is exactly the force exerted by a typical Cox’s Orange Pippin or Bramley apple in free fall towa.rds the earth when i Ipinging upon the head of Sir Isaac or other firm surface.] In most labs, 1 suspect, the subterfuge adopted is to convert by ‘981’ (a magic ‘number!) from a g wt calibration d the transducer to a graph ordinate in newtons. This may be apprcnpriate in Paris (where ‘981’ was established) but does it apply in Paris, Texas? I leave that to you for homework. DAVID J. MILLER
125
More evidence for presynaptic control of neurotransmitter release
Neurotraztsmission at the NMJ The recent review article entitled ‘Feedback control of transmitter release at the neuromuscular junction’ by Bowman et al. (TiPS 9, 16-20, january 1988) puts forward the concept that transmitter release from motor nerves is under the local control of presynaptic facilitatory nicotine receptors. Indirect evidence for this concept comes from electrophysiological and functional studies (measurement of neurally evoked endplate currents or muscle contractions)s3. In these studies, tubocurarine and hexamethonium produced a marked fade of the end-organ responses during repetitive stimulation without a substantial depression of peak response evoked by a single pulse. A different pattern was observed with (Ybungarotoxin and erabutoxin-b which produced depression of peak response and faded simultaneously. The authors exclude a postsynaptic origin of fade and are strongly of the opinion that fade reflects a presynaptic event, a reduction of transmitter release by a blockade of facilitatory nicotine receptors. Indeed, clear evidence for this concept has previously been found in release studiesqd. Incubation of endplate preparations of the rat phrenic nerve with [3H]choline leads to the formation of neuronal [3H]acetylcholine which can be released upon electrical nerve stimulation and measured in the absence of cholinesterase inhibition’. Using this radiolabelling method, it could be demonstrated that nicotine receptor agonists (nicotine, cytisine, dimethyl-4-phenylpiperazinium; pM concentrations) enhanced and antagonists (tubocurarine, hexamethonium) reduced the neurally evoked transmitter output4-6e8. The enhancing effect of the agonists could be prevented by a pretreatment with tubocurarine. Hexamethonium markedly differed in its potency to reduce transmitter release (presynaptic effect) and to reduce muscle contraction (postsynaptic effect). All these results clearly support the concepts that: (1) two populations of nicotine receptors with different pharmacological propertics are
present within the endplate region; (2) stimulation of presynaptic nicotine receptors enhances transmitter release from the motor nerve; and (3) transmitter release is regulated by a positive nicotinic feedback. The authors claimed that the use of hemicholinium (to block re-uptake of rH]choline originating from hydrolysis of released [3H]acetycholine) might have artificially modified the results by inhibition of acetylcholine synthesis. This possibility, however, appears unlikely, because hemicholinium was added after the labelling period, i.e. after synthesis of [3H]acetylcholine had occurred. [Little further synthesis of [‘Hlacetylcholine occurs after the addition of hemicholinium (Ref. 9).] Consequently, neurally evoked output of [3H]acetylcholine reflects transmitter release from a preformed pool(s) of radiolabelled transmitter. Inhibition of neurally evoked [3H]acetylcholine output can only be explained by a reduced probability of preformed transmitter being released (i.e. reduction or impaired refilling of the readily releasable vesicle pool, reduction of release sites, or reduction of vesicle refilling). Impaired synthesis of radiolabelled transmitter cannot be involved. One should also consider that modulation of neurally evoked [3H]acetylcholine output is observed when a total of only 100 pulses (5-50 Hz) is applied5 - a stimulation condition which releases only about 3% of the tissue store of radiolabelled transmitter present within the phrenic nerve’. Moreover, neurally evoked [3H]acetylcholine output can be increased by nicotine receptor stimulatio&‘* ‘, which makes it unlikely that blockade of synthesis (by hemicholinium) terminates [3H]acetylcholine output during short stimulation periods (5 I-I-I; 10-20, s). studies biochemical strongly support the concept of a positive nicotinic feedback mechanism put forward by BOwman and co!lragues. The facilitatory mechanism is activated in a frequency-dependent marine?