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Yin and Yang in Vasomotor Control
19
Yin and
Yang
in Vasomotor Control is a system of branching elastic
THE arterial tree tubes which conducts a non-newtonian fluid in a pulsatile fashion to metabolising tissues. Energy losses via both inertial and viscous mechanisms should ideally be minimised, and most organs are able to autoregulate their blood supply to maintain constant flow when perfusion pressure varies. It now appears that nature may have found the solution to this complex problem of engineering design by providing mechanisms to sense local flow velocity and transmural pressure at the endothelial interface between the flow and the vessel wall. Endothelial cells manufacture many substances, but two in
particular--endothelium-derived relaxing factor (EDRF) and endothelium-derived contracting factor (EDCF)--are very potent vasoactive agents that are released by the mechanical forces that act on the intima. These compounds, as their names imply, have opposing actions on vascular smooth muscle. We noted in these columns last year1 that EDRF has been chemically identified as nitric oxide or a closely allied substance, so it may be regarded as the endogenous nitrovasodilator.2-4 The biochemical controls of EDRF production have not been firmly established. Inorganic nitrite is a possible precursor; it could be converted by a nitrite reductase.s Interestingly, this enzyme is linked to the respiratory
chain in bacteria, as is EDRF production by endothelium.6 EDCF is a peptide,7.!5endothelin, which has now been sequenced and cloned.9 Endothelin appears to be a highly potent vasoconstrictor; four cysteine residues form two sets of intrachain bonds-a structure new among mammalian peptides but similar to scorpion toxins that block neurotransmission by binding to tetrodotoxin-sensitive sodium channels.9 Endothelial cells contain mRNA which encodes preproendothelin, and mature endothelin is generated by an "endothelin-converting enzyme", whose cellular location is unknown.9 EDRF is labile in biological systems and vascular smooth muscle has a rapid on-off response time (within seconds) to its actions In contrast, the constriction induced by endothelin occurs over minutes and its pressor action (after a large bolus injection) may be sustained in vivo for nearly an hour.9 These observations imply that vascular tone may be modulated phasically by EDRF and tonically by endothelin. EDRF and EDCF may together explain the complementary homoeostatic mechanisms of flow-dependent dilatationll and the myogenic response.12 The former, an endothelium-dependent event whereby the arterial lumen adjusts dynamically and rapidly to alterations in intraluminal flow rate, 13 seems to be mediated by EDRF/4,15 it serves to limit the pressure rise induced by an increase in flow (eg, metabolically mediated) and so limits mechanical energy losses.16,17 It may also help to coordinate the distribution of flow through vascular networks. 16 The myogenic response refers to the mechanism whereby stretch or a rise in transmural pressure induces vasoconstriction, limits flow, and so contributes to autoregulation. In some/819 but not all types of 6 Griffith M, Edwards DH, Newby AC, Lewis MJ, Henderson AH Production of endothelium derived relaxant factor is dependent on oxidative phosphory lation and extracellular calcium Cardiovasc Res 1986, 20: 7-12 7 Hickey KA, Rubanyl GM, Paul RJ, Highsmith R. Characterisation of a coronary vasoconstrictor produced by cultured endothelial cells Am J Physiol 1985, 248: C550-56 8. Gillespie MN, Owasoyo JO, McMurty IF, O’Brien RF. Sustained coronary vasoconstriction provoked by a peptidergic substance released from endothelial cells in culture J Pharmacol Exp Ther 1986, 236: 339-43 9 Yanagisawa M, Kurihara H, Kimura S, et al. A novel potent vasoconstrictor peptide produced by vascular endothelial cells Nature 1988, 332: 411-15 10 Griffith TM, Edwards DH, Lew is MJ, New bury AC, Henderson AH The nature of endothelium derived vascular relaxant factor Nature 1984, 308: 645-47. 11 Schretzenmayr A. Uber Kreislaufregulatorische Vorgange an den Grossen Artenen bei der Muskelarbeit Pflugers Arch 1933; 232: 743-48. 12 Bayliss WM On the local reaction of the arterial wall to changes in internal pressure
14. 1 Editorial EDRF Lancet 1987; ii: 137-38
2 Furchgott RF.
Studies
on
relaxation of rabbit aorta
by
sodium nitrite. the basis for the
proposal that the acid-activatable inhibitory factor from bovine inorganic nitrite and the endothelium-derived
retractor
penis
15
Rubanyl GM, Romero JC, Vanhoutte PM Flow-induced release derived relaxing factor Am J Physiol 1986, 250: H1145-49
16
Griffith TM. Edwards DH, Davies RLI, Harrison TJ, Evans KT
is
relaxing factor is nitric oxide. In Vanhouette PM, ed Mechanisms of vasodilatation, vol IV New York Ravenin press 3. Ignarro LJ, Byrns RE, Buga GM, Wood KS. Endothelium-derived relaxing factor from pulmonary artery and vein possesses pharmacologic and chemical properties identical to those of nitric oxide radical Circ Res 1987, 61: 866-79 4 Palmer RMJ, Ferrige AG, Moncada S Nitric oxide release accounts for the biological activity of endothelium-derived relaxing factor Nature 1987; 327: 524-26 5 Martin W, Smith JA, Lewis MJ, Henderson AH Evidence that inhibitory factor extracted from bovine retractor penis is nitrite, whose acid-activated derivative is stabilised nitric oxide Br J Pharmacol 1988, 93: 579-86.
J Physiol Loud 1902, 28: 220-31 J, Forstermann U, Pohl U, Giesler M, Bassenge E. Flow-dependent, endothelium-mediated dilation of epicardial coronary arteries in conscious dogs effects of cyclo-oxygenase inhibition J Cardiovasc Pharmacol 1984, 6: 1161-69 Pohl U, Basse R, Kuon E, Bassenge E Pulsatile perfusion stimulates the release of endothelial autocoids J Appl Cardiol 1986, 1: 215-35
13 Holtz
of endothelium-
EDRF coordinates
the behaviour of vascular resistance vessels Nature 1987, 329: 442-45 17 Melkumy ants AM, Balashov SA. Veselov a ES, Khayutin VM Continuous control of the lumen of felinc conduit arteries by blood flow rate Cardiovasc Res 1987, 21:
863-70 activation of cat cerebral arteries on intact endothelium Circ Res 1987 60: 102-07. 19. Karusic ZS, Shepherd JT, Vanhoutte PM Endothelium-dependent contration to stretch in canine basilar arteries Am J Physiol 1987, 252: H671-73 20 Hwal JJ, Bevan JA. Stretch-dependent myogenic tone in rabbit ear resistance arteries Am J Physiol 1986, 250: H87-95
18 Harder DR Pressure-induced myogenic
20
arteries
this
response
also
appears to be an process and therefore not
endothelium-dependent always strictly myogenic. Flow-dependent dilatation and pressure-dependent constriction can be regarded as opposing positive-feedback mechanisms, each of which alone is theoreticallv unstable. Together with metabolic mechanisms of vasoregulation they may stabilise vasomotor tone while preserving flexibility in its dynamic control. Under certain experimental conditions, endothelium can also enhance constriction induced by agents such as noradrenaline21 and neuropeptide- Y /2 and bioassay with vessel segments indicates endothelium-dependent release of a diffusible contracting factor in response to hypoxia.2324 It is unclear whether endothelin is the sole mediator of these events: endothelium produces more than one dilator substance (eg, prostacyclin, although this compound does not contribute to flow-dependent dilatation13) and it may similarly produce more than one constrictor agent. Shear stress, by increasing EDRF release, seems to be the key biophysical signal controlling acute adaptation to flow changes. Shear stress may also be a signal contributing to changes in arterial calibre in response to chronic changes in flow rate, since the changes in calibre are such as to return intimal shear stress to normal. 25 Moreover, there is preliminary evidence that shear stress down-regulates the preproendothelin gene,9 so EDRF and endothelin might both participate in the long-term response. How could mechanical forces regulate the release of EDRF and EDCF? The answer may be that both shear stress and mechanical stretch influence the transmembrane flux of various cations into endothelial cells. A potassium-selective ion current, activated by shear stress, has lately been shown to cause endothelial cell hyperpolarisation.26 The maximum dynamic range of the response is found at normal physiological levels of shear stress; it desensitises slowly and recovers rapidly on cessation of flow. A stretch-activated, non-selective (permeable to K-, Na*, Ca , and Cs"*) endothelial channel, which would be expected to conduct an inward, depolarising current, has also been described.2’ Such ionic currents may influence EDRF and/or endothelin release-eg, EDRF release is known to depend on calcium influx, although not via voltageMey JG. Vanhoutte PM Heterogeneous behaviour of the canine arterial and venous wall Circ Res 1982; 51: 439-47 22 Daly RN, Hieble JP Neuropeptide Y modulates adrenergic neurotransmission by an endothelium dependent mechanism Ein J Pharmacol 1987; 138: 445-46 23 Holden WE McCall E Hypoxia-induced contractions of porcine pulmonary artery strips depend on intact endothelium Exp Lung Res 1984, 7: 101-12 24 Rubany t GM, Vanhoutte PM Hypoxia releases a vasoconstrictor substance from the canine vascular endothelium. J Phy siol Lond 1985, 364: 45-54 25 Kamiya A. Togawa T Adaptiv e regulation of wall shear stress to flow change m the canine carotid artery Am J Phy siol 1980, 239: H14-21 26 Olesen S-P, Clapham DE, Davies PF Haemodynamic shear stress activates a K current in vascular endothelial cells Nature 1988, 331: 168-70. 27 Lansman JB Hallam TJ. Rink TJ Single stretch-activated ion channels in vascular endothelial cells as mechanotransducers. Nature 1987, 325: 811-13 28. Colden-Standfield M. Schilling WP, Ritchie AK. Eskin SG, Navarro LT, Kunzee DL. Bradykinin-induced increases in cytosolic calcium and ionic currents in cultured boy me aortic endothelial cells. Cir, Res 1987; 61: 632-40
dependent calcium channels because they do not exist in endothelium.28 The possibility remains that a hyperpolarising current conducted from endothelium through gap junctions could relax smooth muscle directly.26 Whilst stimulation of EDRF release is associated with smooth muscle hyperpolarisation,29 the changes in membrane potential can be dissociated from the accompanying mechanical relaxation.30 This finding suggests the existence of yet another factor, endothelium-derived hyperpolarising factor or EDHF.30,31 From a functional point of view, the effects of shear (EDRF release and hyperpolarisation) are likely to be limited to immediately subjacent smooth muscle because of binding by haemoglobin in the lumen and the electrical cable properties of the arterial wall, respectively. It is unclear whether the effects of endothelin are similarly localised. Considerable variation in EDRF-mediated responses between different species and artery types has
documented.32 There may also be heterogeneity in the case of endothelin :9 preproendothelin mRNA is found in porcine aorta but not in porcine brain, atrium, lung, or kidney and the supernatant from cultured - human omental endothelial cells does not possess vasoconstrictor been
activity. The realisation that endothelium can modulate vascular tone has generated a new concept of circulatory control and clarified several physiological processes. Heterogeneity of endothelium-dependent responses provides rich potential for complexity in the control of the circulation in health and disease.
Need We Poison the So Often?
Elderly
PRESCRIBING rates for elderly people in England and Wales have continued to rise (by 27% between 1977 and 1985) compared with a slight fall (about 6%) for other age groupS.1 This trend is not necessarily bad: elderly individuals have much to gain from judicious drug therapy; and careful evaluation before and after marketing has made drugs safer than ever. Nevertheless, there is enormous concern about iatrogenic illness in elderly people caused by prescribed medication.2-5 Media treatment of this
21 De
29 Bolton TB, Clapp LH Endothelial-dependent relaxant actions of carbachol and substance P in arterial smooth muscle. Br J Pharmacol 1986; 87: 713-23 30 Feletou M. Vanhoutte PM Endothelium-dependent hyperpolarization of canine coronary smooth muscle. Br J Pharmacol 1988, 93: 515-24 31. Komari K, Suzuki H. Heterogeneous distribution of muscarinic receptors in the rabbit saphenous artery Br J Pharmacol 1987; 92: 657-64 32 Collins P. Chappell SP, Griffith TM, Lew is MJ, Henderson AH. Differences in basal
endothelium-deriv ed relaxing factor activity Pharmacol 1986; 8: 1158-62
in
different artery
types J Car diovasc
Cartwright A, Smith C. Elderly people, their medicines and their doctors. LondonRoutledge, 1988 2. World Health Organisation Drugs for the elderly. Copenhagen WHO Regional Office for Europe, 1985. 3 Royal College of Physicians 1984 Medication for the elderly. J R Coll Phys Lond 1.
1984, 18: 8-17.
Yin and
Yang
in Vasomotor Control is a system of branching elastic
THE arterial tree tubes which conducts a non-newtonian fluid in a pulsatile fashion to metabolising tissues. Energy losses via both inertial and viscous mechanisms should ideally be minimised, and most organs are able to autoregulate their blood supply to maintain constant flow when perfusion pressure varies. It now appears that nature may have found the solution to this complex problem of engineering design by providing mechanisms to sense local flow velocity and transmural pressure at the endothelial interface between the flow and the vessel wall. Endothelial cells manufacture many substances, but two in
particular--endothelium-derived relaxing factor (EDRF) and endothelium-derived contracting factor (EDCF)--are very potent vasoactive agents that are released by the mechanical forces that act on the intima. These compounds, as their names imply, have opposing actions on vascular smooth muscle. We noted in these columns last year1 that EDRF has been chemically identified as nitric oxide or a closely allied substance, so it may be regarded as the endogenous nitrovasodilator.2-4 The biochemical controls of EDRF production have not been firmly established. Inorganic nitrite is a possible precursor; it could be converted by a nitrite reductase.s Interestingly, this enzyme is linked to the respiratory
chain in bacteria, as is EDRF production by endothelium.6 EDCF is a peptide,7.!5endothelin, which has now been sequenced and cloned.9 Endothelin appears to be a highly potent vasoconstrictor; four cysteine residues form two sets of intrachain bonds-a structure new among mammalian peptides but similar to scorpion toxins that block neurotransmission by binding to tetrodotoxin-sensitive sodium channels.9 Endothelial cells contain mRNA which encodes preproendothelin, and mature endothelin is generated by an "endothelin-converting enzyme", whose cellular location is unknown.9 EDRF is labile in biological systems and vascular smooth muscle has a rapid on-off response time (within seconds) to its actions In contrast, the constriction induced by endothelin occurs over minutes and its pressor action (after a large bolus injection) may be sustained in vivo for nearly an hour.9 These observations imply that vascular tone may be modulated phasically by EDRF and tonically by endothelin. EDRF and EDCF may together explain the complementary homoeostatic mechanisms of flow-dependent dilatationll and the myogenic response.12 The former, an endothelium-dependent event whereby the arterial lumen adjusts dynamically and rapidly to alterations in intraluminal flow rate, 13 seems to be mediated by EDRF/4,15 it serves to limit the pressure rise induced by an increase in flow (eg, metabolically mediated) and so limits mechanical energy losses.16,17 It may also help to coordinate the distribution of flow through vascular networks. 16 The myogenic response refers to the mechanism whereby stretch or a rise in transmural pressure induces vasoconstriction, limits flow, and so contributes to autoregulation. In some/819 but not all types of 6 Griffith M, Edwards DH, Newby AC, Lewis MJ, Henderson AH Production of endothelium derived relaxant factor is dependent on oxidative phosphory lation and extracellular calcium Cardiovasc Res 1986, 20: 7-12 7 Hickey KA, Rubanyl GM, Paul RJ, Highsmith R. Characterisation of a coronary vasoconstrictor produced by cultured endothelial cells Am J Physiol 1985, 248: C550-56 8. Gillespie MN, Owasoyo JO, McMurty IF, O’Brien RF. Sustained coronary vasoconstriction provoked by a peptidergic substance released from endothelial cells in culture J Pharmacol Exp Ther 1986, 236: 339-43 9 Yanagisawa M, Kurihara H, Kimura S, et al. A novel potent vasoconstrictor peptide produced by vascular endothelial cells Nature 1988, 332: 411-15 10 Griffith TM, Edwards DH, Lew is MJ, New bury AC, Henderson AH The nature of endothelium derived vascular relaxant factor Nature 1984, 308: 645-47. 11 Schretzenmayr A. Uber Kreislaufregulatorische Vorgange an den Grossen Artenen bei der Muskelarbeit Pflugers Arch 1933; 232: 743-48. 12 Bayliss WM On the local reaction of the arterial wall to changes in internal pressure
14. 1 Editorial EDRF Lancet 1987; ii: 137-38
2 Furchgott RF.
Studies
on
relaxation of rabbit aorta
by
sodium nitrite. the basis for the
proposal that the acid-activatable inhibitory factor from bovine inorganic nitrite and the endothelium-derived
retractor
penis
15
Rubanyl GM, Romero JC, Vanhoutte PM Flow-induced release derived relaxing factor Am J Physiol 1986, 250: H1145-49
16
Griffith TM. Edwards DH, Davies RLI, Harrison TJ, Evans KT
is
relaxing factor is nitric oxide. In Vanhouette PM, ed Mechanisms of vasodilatation, vol IV New York Ravenin press 3. Ignarro LJ, Byrns RE, Buga GM, Wood KS. Endothelium-derived relaxing factor from pulmonary artery and vein possesses pharmacologic and chemical properties identical to those of nitric oxide radical Circ Res 1987, 61: 866-79 4 Palmer RMJ, Ferrige AG, Moncada S Nitric oxide release accounts for the biological activity of endothelium-derived relaxing factor Nature 1987; 327: 524-26 5 Martin W, Smith JA, Lewis MJ, Henderson AH Evidence that inhibitory factor extracted from bovine retractor penis is nitrite, whose acid-activated derivative is stabilised nitric oxide Br J Pharmacol 1988, 93: 579-86.
J Physiol Loud 1902, 28: 220-31 J, Forstermann U, Pohl U, Giesler M, Bassenge E. Flow-dependent, endothelium-mediated dilation of epicardial coronary arteries in conscious dogs effects of cyclo-oxygenase inhibition J Cardiovasc Pharmacol 1984, 6: 1161-69 Pohl U, Basse R, Kuon E, Bassenge E Pulsatile perfusion stimulates the release of endothelial autocoids J Appl Cardiol 1986, 1: 215-35
13 Holtz
of endothelium-
EDRF coordinates
the behaviour of vascular resistance vessels Nature 1987, 329: 442-45 17 Melkumy ants AM, Balashov SA. Veselov a ES, Khayutin VM Continuous control of the lumen of felinc conduit arteries by blood flow rate Cardiovasc Res 1987, 21:
863-70 activation of cat cerebral arteries on intact endothelium Circ Res 1987 60: 102-07. 19. Karusic ZS, Shepherd JT, Vanhoutte PM Endothelium-dependent contration to stretch in canine basilar arteries Am J Physiol 1987, 252: H671-73 20 Hwal JJ, Bevan JA. Stretch-dependent myogenic tone in rabbit ear resistance arteries Am J Physiol 1986, 250: H87-95
18 Harder DR Pressure-induced myogenic
20
arteries
this
response
also
appears to be an process and therefore not
endothelium-dependent always strictly myogenic. Flow-dependent dilatation and pressure-dependent constriction can be regarded as opposing positive-feedback mechanisms, each of which alone is theoreticallv unstable. Together with metabolic mechanisms of vasoregulation they may stabilise vasomotor tone while preserving flexibility in its dynamic control. Under certain experimental conditions, endothelium can also enhance constriction induced by agents such as noradrenaline21 and neuropeptide- Y /2 and bioassay with vessel segments indicates endothelium-dependent release of a diffusible contracting factor in response to hypoxia.2324 It is unclear whether endothelin is the sole mediator of these events: endothelium produces more than one dilator substance (eg, prostacyclin, although this compound does not contribute to flow-dependent dilatation13) and it may similarly produce more than one constrictor agent. Shear stress, by increasing EDRF release, seems to be the key biophysical signal controlling acute adaptation to flow changes. Shear stress may also be a signal contributing to changes in arterial calibre in response to chronic changes in flow rate, since the changes in calibre are such as to return intimal shear stress to normal. 25 Moreover, there is preliminary evidence that shear stress down-regulates the preproendothelin gene,9 so EDRF and endothelin might both participate in the long-term response. How could mechanical forces regulate the release of EDRF and EDCF? The answer may be that both shear stress and mechanical stretch influence the transmembrane flux of various cations into endothelial cells. A potassium-selective ion current, activated by shear stress, has lately been shown to cause endothelial cell hyperpolarisation.26 The maximum dynamic range of the response is found at normal physiological levels of shear stress; it desensitises slowly and recovers rapidly on cessation of flow. A stretch-activated, non-selective (permeable to K-, Na*, Ca , and Cs"*) endothelial channel, which would be expected to conduct an inward, depolarising current, has also been described.2’ Such ionic currents may influence EDRF and/or endothelin release-eg, EDRF release is known to depend on calcium influx, although not via voltageMey JG. Vanhoutte PM Heterogeneous behaviour of the canine arterial and venous wall Circ Res 1982; 51: 439-47 22 Daly RN, Hieble JP Neuropeptide Y modulates adrenergic neurotransmission by an endothelium dependent mechanism Ein J Pharmacol 1987; 138: 445-46 23 Holden WE McCall E Hypoxia-induced contractions of porcine pulmonary artery strips depend on intact endothelium Exp Lung Res 1984, 7: 101-12 24 Rubany t GM, Vanhoutte PM Hypoxia releases a vasoconstrictor substance from the canine vascular endothelium. J Phy siol Lond 1985, 364: 45-54 25 Kamiya A. Togawa T Adaptiv e regulation of wall shear stress to flow change m the canine carotid artery Am J Phy siol 1980, 239: H14-21 26 Olesen S-P, Clapham DE, Davies PF Haemodynamic shear stress activates a K current in vascular endothelial cells Nature 1988, 331: 168-70. 27 Lansman JB Hallam TJ. Rink TJ Single stretch-activated ion channels in vascular endothelial cells as mechanotransducers. Nature 1987, 325: 811-13 28. Colden-Standfield M. Schilling WP, Ritchie AK. Eskin SG, Navarro LT, Kunzee DL. Bradykinin-induced increases in cytosolic calcium and ionic currents in cultured boy me aortic endothelial cells. Cir, Res 1987; 61: 632-40
dependent calcium channels because they do not exist in endothelium.28 The possibility remains that a hyperpolarising current conducted from endothelium through gap junctions could relax smooth muscle directly.26 Whilst stimulation of EDRF release is associated with smooth muscle hyperpolarisation,29 the changes in membrane potential can be dissociated from the accompanying mechanical relaxation.30 This finding suggests the existence of yet another factor, endothelium-derived hyperpolarising factor or EDHF.30,31 From a functional point of view, the effects of shear (EDRF release and hyperpolarisation) are likely to be limited to immediately subjacent smooth muscle because of binding by haemoglobin in the lumen and the electrical cable properties of the arterial wall, respectively. It is unclear whether the effects of endothelin are similarly localised. Considerable variation in EDRF-mediated responses between different species and artery types has
documented.32 There may also be heterogeneity in the case of endothelin :9 preproendothelin mRNA is found in porcine aorta but not in porcine brain, atrium, lung, or kidney and the supernatant from cultured - human omental endothelial cells does not possess vasoconstrictor been
activity. The realisation that endothelium can modulate vascular tone has generated a new concept of circulatory control and clarified several physiological processes. Heterogeneity of endothelium-dependent responses provides rich potential for complexity in the control of the circulation in health and disease.
Need We Poison the So Often?
Elderly
PRESCRIBING rates for elderly people in England and Wales have continued to rise (by 27% between 1977 and 1985) compared with a slight fall (about 6%) for other age groupS.1 This trend is not necessarily bad: elderly individuals have much to gain from judicious drug therapy; and careful evaluation before and after marketing has made drugs safer than ever. Nevertheless, there is enormous concern about iatrogenic illness in elderly people caused by prescribed medication.2-5 Media treatment of this
21 De
29 Bolton TB, Clapp LH Endothelial-dependent relaxant actions of carbachol and substance P in arterial smooth muscle. Br J Pharmacol 1986; 87: 713-23 30 Feletou M. Vanhoutte PM Endothelium-dependent hyperpolarization of canine coronary smooth muscle. Br J Pharmacol 1988, 93: 515-24 31. Komari K, Suzuki H. Heterogeneous distribution of muscarinic receptors in the rabbit saphenous artery Br J Pharmacol 1987; 92: 657-64 32 Collins P. Chappell SP, Griffith TM, Lew is MJ, Henderson AH. Differences in basal
endothelium-deriv ed relaxing factor activity Pharmacol 1986; 8: 1158-62
in
different artery
types J Car diovasc
Cartwright A, Smith C. Elderly people, their medicines and their doctors. LondonRoutledge, 1988 2. World Health Organisation Drugs for the elderly. Copenhagen WHO Regional Office for Europe, 1985. 3 Royal College of Physicians 1984 Medication for the elderly. J R Coll Phys Lond 1.
1984, 18: 8-17.