Relationships between preclinical cardiac electrophysiology, clinical QT interval prolongation and torsade de pointes for a broad range of drugs: evidence for a provisional safety margin in drug development

OBJECTIVE

To attempt to determine the relative value of preclinical cardiac electrophysiology data (in vitro and in vivo) for predicting risk of torsade de pointes (TdP) in clinical use.

METHODS

Published data on hERG (or I(Kr)) activity, cardiac action potential duration (at 90% repolarisation; APD(90)), and QT prolongation in dogs were compared against QT effects and reports of TdP in humans for 100 drugs. These data were set against the free plasma concentrations attained during clinical use (effective therapeutic plasma concentrations; ETPC(unbound)). The drugs were divided into five categories: (1) Class Ia and III antiarrhythmics; (2) Withdrawn from market due to TdP; (3) Measurable incidence/numerous reports of TdP in humans; (4) Isolated reports of TdP in humans; (5) No reports of TdP in humans.

RESULTS

Data from hERG (or I(Kr)) assays in addition to ETPC(unbound) data were available for 52 drugs. For Category 1 drugs, data for hERG/I(Kr) IC(50), APD(90), QTc in animals and QTc in humans were generally close to or superimposed on the ETPC(unbound) values. This relationship was uncoupled in the other categories, with more complex relationships between the data. In Category 1 (except amiodarone), the ratios between hERG/I(Kr) IC(50) and ETPC(unbound) (max) ranged from 0.1- to 31-fold. Similar ranges were obtained for drugs in Category 2 (0.31- to 13-fold) and Category 3 (0.03- to 35-fold). A large spread was found for Category 4 drugs (0.13- to 35700-fold); this category embraced an assortment of mechanisms ranging from drugs which may well be affecting I(Kr) currents in clinical use (e.g. sparfloxacin) to others such as nifedipine (35700-fold) where channel block is not involved. Finally, for the majority of Category 5 drugs there was a >30-fold separation between hERG/I(Kr) activity and ETPC(unbound) values, with the notable exception of verapamil (1.7-fold), which is free from QT prolongation in man; this is probably explained by its multiple interactions with cardiac ion channels.

CONCLUSIONS

The dataset confirms the widely-held belief that most drugs associated with TdP in humans are also associated with hERG K(+) channel block at concentrations close to or superimposed upon the free plasma concentrations found in clinical use. A 30-fold margin between C(max) and hERG IC(50) may suffice for drugs currently undergoing clinical evaluation, but for future drug discovery programmes, pharmaceutical companies should consider increasing this margin, particularly for drugs aimed at non-debilitating diseases. However, interactions with multiple cardiac ion channels can either mitigate or exacerbate the prolongation of APD and QT that would ensue from block of I(Kr) currents alone, and delay of repolarisation per se is not necessarily torsadogenic. Clearly, an integrated assessment of in vitro and in vivo data is required in order to predict the torsadogenic risk of a new candidate drug in humans.

Methods of collecting and evaluating non-clinical cardiac electrophysiology data in the pharmaceutical industry: results of an international survey

OBJECTIVE

To assess current practice in the pharmaceutical industry for assessing the potential for QT interval prolongation by non-cardiovascular medicinal products.

METHODS

The survey was based on responses from the Toxicology and (Safety) Pharmacology laboratories (a total of 74 laboratories) of 54 companies based in Europe, Japan/Asia and the USA, received between January and March 1999.

RESULTS

All 54 companies conducted preclinical in vivo electrocardiography (EGG) evaluation of new active substances (NASs). Thirty of these companies also conducted in vitro cardiac electrophysiology studies on their compounds. The majority of in vivo work was done in conscious beagle dogs. There was no consistency within the industry in defining the magnitude of change in QT interval that is considered biologically important. Most companies considered a change greater than 10% to be important, although the design of the studies suggested that group sizes used may not give sufficient statistical power to detect this size of change. Bazett's formula was used by 41% of laboratories to correct QT for changes in heart rate, despite the fact that this formula is generally deemed to be unsuitable for use in dogs. For studies in anaesthetised dogs, the majority of laboratories used barbiturate anaesthesia, but researchers should be aware of the effects of this and some other anaesthetic agents on QT interval. As for in vitro cardiac electrophysiology, there was wide diversity in the testing methodologies, particularly with regard to the test species and tissue type. As with QT prolongation, there was no consensus on the degree of action potential prolongation to cause concern. For both in vitro and in vivo testing, the majority of companies tested a minimum of three dose (or concentration) levels in order to ascertain any dose-response relationship.

CONCLUSIONS

The survey provides a snapshot of the practice in the industry prior to any internationally-agreed consensus on the most effective and efficient approaches to minimising the risk of QT prolongation by new drugs in man. It must be stated that for any given methodology, the 'majority view' in the industry is not necessarily best practice.

A re-evaluation of the role of alpha 2-adrenoceptors in the anxiogenic effects of yohimbine, using the selective antagonist delequamine in the rat

1. The acute behavioural effects of the alpha2-adrenoceptor antagonists, yohimbine, idazoxan and delequamine (RS-15385-197) were compared in two tests of exploratory behaviour in the rat, operated in tandem. These were the elevated X-maze test (5 min) and a modified holeboard test (12 min), which comprised a holeboard arena with a small roof in one corner as a 'refuge'. Rats were first placed into this corner, thus enabling measurements of initial emergence latency and the number of forays. The experiments were always done with a concomitant vehicle control group, with 10-12 rats per group, and with the treatment blinded. 2. In order to validate the tests, the effects of representatives of four classes of psychoactive agents were examined, viz. picrotoxin (anxiogenic), chlordiazepoxide (anxiolytic), (+)-amphetamine (stimulant) and diphenhydramine (sedative). The modified holeboard tended to be more sensitive than the measurement of total arm entries in the elevated X-maze at detecting drug effects on exploratory behaviour, but unlike the X-maze it could not clearly identify each class of agent. Thus, picrotoxin (5 mg kg(-1), i.p.) reduced total arm entries and open arm exploration in the X-maze (P<0.02) and suppressed most measures of activity in the holeboard (P<0.05); chlordiazepoxide (7.5 mg kg(-1), i.p.) increased total arm entries and open arm exploration (P<0.02) in the X-maze, without clear-cut effects in the holeboard; (+)-amphetamine (1 mg kg(-1), i.p.) had no significant effects in the X-maze, but increased most holeboard activities (P<0.05), and diphenhydramine (30 mg kg(-1), i.p.) reduced total arm entries in the X-maze (P<0.002) and hole exploration in the holeboard (P<0.05).

RS-45041-190: a selective, high-affinity ligand for I2 imidazoline receptors

1. RS-45041-190 (4-chloro-2-(imidazolin-2-yl)isoindoline) showed high affinity for I2 imidazoline receptors labelled by [3H]-idazoxan in rat (pKi = 8.66 +/- 0.09), rabbit (pKi = 9.37 +/- 0.07), dog (pKi = 9.32 +/- 0.18) and baboon kidney (pKi = 8.85 +/- 0.12), but had very low affinity for alpha 2-adrenoceptors in rat cerebral cortex (pKi = 5.7 +/- 0.09). 2. RS-45041-190 showed low affinity for other adrenoceptors, dopamine, 5-hydroxytryptamine, and muscarinic receptors and dihydropyridine binding sites (selectivity ratio > 1000). 3. RS-45041-190 showed moderate potency for the inhibition of monoamine oxidase A in vitro (pIC50 = 6.12), but had much lower potency for monoamine oxidase B (pIC50 = 4.47), neither of which equated with its affinity for I2 receptors. 4. RS-45041-190 (0.001 to 3 mg kg-1, i.v. and 1 ng-50 micrograms i.c.v.) had only small, transient effects on blood pressure and heart rate in anaesthetized rats. In conscious rats, RS-45041-190 had no effect on body core temperature or tail skin temperature (1 mg kg-1, s.c.) or on activity or rotarod performance (10 mg kg-1, i.p.). There were also no effects on barbiturate sleeping time in mice after doses of 1-10 mg kg-1, i.p. 5. RS-45041-190 (10 and 25 mg kg-1, i.p.) significantly increased food consumption in rats for up to 4 h after dosing, but unlike idazoxan (10 mg kg-1, i.p.) did not increase water consumption. RS-45041-190 is therefore a selective, high-affinity ligand at I2 imidazoline receptors and its hyperphagic effect may suggest a role for I2 imidazoline receptors in the modulation of appetite.However, in the absence of a selective agonist it is unclear whether this ligand is an agonist or an antagonist at I2 receptors.

[3H]-RS-45041-190: a selective high-affinity radioligand for I2 imidazoline receptors

1. RS-45041-190 (4-chloro-2-(imidazolin-2-yl)isoindoline) is an I2 imidazoline receptor ligand with the highest affinity and selectivity so far described; [3H]-RS-45041-190 has a tritium atom attached to the 7-position on the isoindoline ring. 2. [3H]-RS-45041-190 binding to rat kidney membranes was saturable (Bmax = 223.1 +/- 18.4 fmol mg-1 protein) and of high affinity (Kd = 2.71 +/- 0.59 nM). Kinetic studies revealed that the binding was rapid and reversible, with [3H]-RS-45041-190 interacting with two sites or two affinity states. 3. Competition studies showed that 60-70% of [3H]-RS-45041-190 binding (1 nM) was specifically to imidazoline binding sites of the I2 subtype, characterized by high affinity for idazoxan (pIC50 7.85 +/- 0.03) and cirazoline (pIC50 8.16 +/- 0.05). The remaining 30-40% was displaced specifically by the monoamine oxidase A inhibitors, clorgyline and pargyline. 4. alpha 1- and alpha 2-adrenoceptor, I1 imidazoline, histamine, 5-hydroxytryptamine or dopamine receptor ligands had low affinity suggesting that [3H]-RS-45041-190 did not label receptors of these classes. 5. In autoradiography studies, [3H]-RS-45041-190 labelled discrete regions of rat brain corresponding to the distribution of I2 subtypes, notably the subfornical organ, arcuate nucleus, interpeduncular nucleus, medial habenular nucleus and lateral mammillary nucleus, and additional sites in the locus coeruleus, dorsal raphe and dorsomedial hypothalamic nucleus. 6. [3H]-RS-45041-190 therefore labels I2 receptors with high affinity, and an additional site which has high affinity for some monoamine oxidase inhibitors.

The role of alpha 2-adrenoceptors in the vasculature of the rat tail

1. The effects of alpha 2-adrenoceptor agonists and antagonists on rat tail skin temperature (tts), an indicator of local cutaneous blood flow, were studied in conscious and anaesthetized rats and in the isolated, Krebs perfused, vascular bed of the rat tail. 2. In conscious rats, at an ambient temperature of 18.5-20 degrees C, tts was 21.0 +/- 0.2 degrees C and core (rectal) temperature (tc) was 38.2 +/- 0.04 degrees C (n = 126). The alpha 2-adrenoceptor antagonist, delequamine (RS-15385-197; 1 mg kg-1, s.c., n = 6), produced a rapid elevation in tts to 29.1 +/- 0.7 degrees C (P < 0.001 vs. saline-treated control group), attained 10 min after injection. tc fell slightly, by 1.0 +/- 0.1 degrees C. The tts response was dose-related over the dose-range tested (0.01-1 mg kg-1, s.c.), with an ED50 of 17 micrograms kg-1, s.c. (n = 6 per dose). 3. The maximum increases in tts in response to a dose of 1 mg kg-1, s.c. of alpha 2-adrenoceptor antagonists were as follows (n = 6 per drug): delequamine (+9.6 +/- 0.8 degrees C) > yohimbine (+9.0 +/- 1.0 degrees C) > WY-26703 (+7.9 +/- 1.3 degrees C) > piperoxan (+5.6 +/- 1.7 degrees C) > idazoxan (+4.6 +/- 1.3 degrees C) > imiloxan (+4.1 +/- 1.3 degrees C) > SKF 104078 (+2.0 +/- 1.9 degrees C) > BDF-6143 (+1.3 +/- 0.8 degrees C). 4. Prazosin (0.3 mg kg-1, s.c.), hydralazine (10 mg kg-1, s.c.) and nifedipine (3 mg kg-1, s.c.) did not increase tts, whereas propranolol (10 mg kg-1, s.c.) evoked a small increase in tts (+2.9 +/- 1.0 degrees C). Pentolinium (2-10 mg kg-1, s.c.) elicited a dose-related increase in tts, which was elevated by 4.4 +/- 1.3 degrees C after a dose of 10 mg kg-1; tc was reduced in a dose-related manner. Drug vehicles (1 ml kg-1, s.c.) had no effect on tts or tc. 5. In anaesthetized rats, idazoxan (300 microg, i.v.) produced a rapid increase in tts which was detectable 2 min after beginning the injection, reaching a peak after 7 min. When the same dose was administered i.c.v., tts also rose, but more slowly. The peak response (+ 3.6 +/- 0.70C, n = 5) was significantly smaller than when idazoxan was administered intravenously (+ 6.3 +/- 1.2 C, n = 5), which suggests that the increase in tts following systemic administration of M2-adrenoceptor antagonists is not due to a central effect. The change in tts was not secondary to changes in blood pressure.6. In the isolated, Krebs perfused, tail vascular bed of the rat, at an ambient temperature of 20-21C,under constant flow conditions (3.5-4.0 ml min-1; n = 4), baseline perfusion pressure was 57 +/- 4 mmHg.5-Hydroxytryptamine (5-HT; 70-150 nM) increased perfusion pressure by 56+/- 9 mmHg. The alpha2-adrenoceptor agonist, UK-14,304 (10 nmol), elicited a further increase in perfusion pressure by27.5 +/- 15 mmHg but had no effect in the absence of 5-HT; this response to UK-14,304 was abolished by rauwolscine (1 microM).7. Under constant pressure conditions (-100 mmHg; n = 9), baseline mean perfusion flow was 2.1 +/- 0.2 ml min-1, and mean tail skin temperature was 31.6 +/- 0.6C. 5-HT (119 +/- 28 nM) decreased tts.by 3.3 +/- 2.0 C and reduced flow by 1.2 +/- 0.3 ml min-1. UK-14,304 (10 nmol) further reduced tts by 3.0 +/- 0.3 C without significant effect on flow; this effect was also abolished by 1 microM rauwolscine.8. We conclude that post-junctional M2-adrenoceptors in the vasculature of the rat tail have a major vasoconstrictor role, controlling both the flow and distribution of blood within the tail and thereby thermoregulatory heat loss from its surface.

Modulation of central noradrenergic function by RS-15385-197

1. RS-15385-197, a highly potent and selective alpha 2-adrenoceptor antagonist, was examined in a variety of in vitro and in vivo functional tests to assess the selectivity of its interaction with central noradrenergic neurones in the rat. 2. In hypothalamic slices, RS-15385-197 was potent in augmenting K(+)-evoked release of [3H]-noradrenaline, with an EC50 of 9 nM. Idazoxan and yohimbine showed 100 fold less activity. This was due to its antagonist action at presynaptic alpha 2-adrenoceptors, as RS-15385-197 (10 microM), did not directly release [3H]-noradrenaline from cortical slices unlike reserpine (10 microM), and did not inhibit noradrenaline re-uptake into cortical synaptosomes. 3. In vivo, RS-15385-197 (0.5 mg kg-1, p.o.) increased levels of 3-methoxy-4-hydroxy-phenylglycol (MHPG) in the cerebral cortex without modifying levels of 5-hydroxyindoleacetic acid (5-HIAA). This dose, but not a lower dose (0.1 mg kg-1, p.o.) caused beta-adrenoceptor down-regulation in the cortex when administered once daily for 14 days whereas 5-HT2 receptor number was unaltered, indicating a selective effect on noradrenergic transmission. 4. Selective depletion of cortical 5-HT by administration of p-chlorophenylalanine (PCPA; 100 mg kg-1, i.p. for 14 days) or 5,7-dihydroxytryptamine (5,7-DHT; 150 micrograms i.c.v.) prevented the beta-adrenoceptor down-regulation caused by RS-15385-197, indicating that a tonic 5-hydroxytryptaminergic input was required for it to elicit beta-adrenoceptor down-regulation. It was not possible to prevent the loss of activity of RS-15385-197 in these 5-HT-depleted animals by co-administration with the 5-HT1A partial agonist, 8-hydroxy-n-dipropyl aminotetralin (8-OH-DPAT, 0.3 mg kg-1, i.p. twice daily for final 3 days).5. At a dose (1 mg kg-1, p.o.) which completely prevented the hypoactivity produced by clonidine(0.1 mgkg-1, p.o.), RS-15385-197 did not affect behavioural stereotypy induced by 8-OH-DPAT(0.3 mg kg-1, s.c.). Similarly, following chronic dosing with the racemate, RS-15385-196 (3 mg kg-1,p.o., once daily for 14 days), there was no effect on the behavioural and hypothermic response to 8-OH-DPAT (0.5 mg kg-1, s.c.). Therefore, RS-1 5385-197 was selective for central alpha2-adrenoceptors over 5-HT1A receptors in in vivo functional tests.6. Thus, RS-15385-197 was highly selective in interacting with central noradrenergic neurones in the rat in vitro and in vivo. It is therefore currently the agent of choice for investigations of the role of alpha 2-adrenoceptors in the CNS.

The pharmacology of RS-15385-197, a potent and selective alpha 2-adrenoceptor antagonist

1. RS-15385-197 ((8aR, 12aS, 13aS)-5,8,8a,9,10,11,12,12a,13,13a-decahydro- 3-methoxy-12-(methylsulphonyl)-6H-isoquino [2,1-g][1,6]-naphthyridine) was evaluated in a series of in vitro and in vivo tests as an antagonist at alpha 2-adrenoceptors. 2. RS-15385-197 had a pKi of 9.45 for alpha 2-adrenoceptors in the rat cortex (pA2 in the guinea-pig ileum of 9.72), whereas the 8aS, 12aR, 13aR enantiomer, RS-15385-198, had a pKi of only 6.32 (pA2 6.47) indicating a high degree of stereoselectivity. The racemate RS-15385-196 had a pKi of 9.18. 3. RS-15385-197 showed unprecedented alpha 2 vs. alpha 1 adrenoceptor selectivity in vitro. In the rat cortex, RS-15385-197 had a pKi of 9.45 in displacing [3H]-yohimbine and 5.29 in displacing [3H]-prazosin (alpha 2/alpha 1 selectivity ratio in binding experiments > 14000). The compound had a pA2 of 9.72 as a competitive antagonist of the inhibitory effects of UK-14,304 in transmurally-stimulated guinea-pig ileum and 10.0 against BHT-920-induced contractions in dog saphenous vein (DSV); this latter value was unaltered by phenoxybenzamine. An apparent pKB of 5.9 was obtained against cirazoline-induced contractions in DSV, whilst a pA2 of 6.05 was obtained against phenylephrine-induced contractions in the rabbit aorta (alpha 2/alpha 1 selectivity ratio in functional experiments > 4000). 4. RS-15385-197 was highly selective for alpha 2-adrenoceptors over other receptors: the compound showed low affinity for 5-HT1A (pKi 6.50) and 5-HT1D (pKi 7.00) receptor subtypes, and even lower affinity (pKi < or = 5) for other 5-HT receptor subtypes, dopamine receptors, muscarinic cholinoceptors, beta-adrenoceptors and dihydropyridine binding sites. RS-15385-197 was devoid of affinity for the non-adrenoceptor imidazoline binding site, labelled by [3H]-idazoxan, which provides further evidence that these sites are not related to alpha 2-adrenoceptors. In the DSV, contractile responses to 5-hydroxytryptamine (5-HT) were unaffected by a concentration of 1 microM RS-15385-197. 5. RS-15385-197 was non-selective for the alpha 2A- and alpha 2B-adrenoceptor subtypes in that the pKi for the alpha 2A-adrenoceptor in human platelets was 9.90 and the pKi for the alpha 2B-adrenoceptor in rat neonate lung was 9.70. However, RS-15385-197 showed lower affinity for the alpha 2-adrenoceptor subtype in hamster adipocytes (pKi 8.38). 6. In anaesthetized rats, RS-15385-197 was a potent antagonist of the mydriasis response induced by UK-14,304 or clonidine (AD50 5 and 7 microg kg-1, i.v., respectively; 96 microg kg-1, p.o.) and of UK-14,304-induced pressor responses in pithed rats (AD50 7 microg kg-1, i.v.); the compound therefore is both centrally and orally active. Even at a high dose (10 mg kg-1, i.v.), RS-15385-197 did not antagonize pressor responses to cirazoline in pithed rats, indicating that the selectivity for alpha2 vs. alpha1-adrenoceptors was maintained in vivo.8 RS-15385-197 is therefore a very potent, selective, competitive alpha2-adrenoceptor antagonist, both in vitro and in vivo, is orally active and readily penetrates the brain. It will thus be a powerful pharmacological tool for exploring the various physiological roles of alpha2-adrenoceptors.

The guinea-pig ileum preparation as a model for 5-HT1A receptors: anomalous effects with RS-30199-193

1. Agents that have high and selective affinity for the 5-HT1A site such as 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and N,N-dipropyl-5-carboxamidotryptamine (DP5CT) inhibited the responses to field stimulation in guinea-pig ileum preparations; the inhibitory effects were antagonized by methiothepin and spiperone, consistent with effects at the 5-HT1A site. 2. The inhibitory effects of DP5CT were pronounced in Tyrode solution containing low Ca2+ (0.9 mM), but were much less apparent in Tyrode solution containing 1.8 or 5.4 mM Ca2+. 3. Responses to DP5CT were abolished by pretreatment with phorbol dibutyrate (3 microM), whereas the responses to UK14304 were only slightly inhibited. 4. Buspirone and ipsapirone (1 microM) inhibited the responses to field stimulation, and the effects were resistant to idazoxan, but inhibited by 8-OH-DPAT or spiperone. 5. RS-30199-193 (5-chloro-2-methyl-1,2,3,4,8,9,10,10a-octahydronaphth-[1,8-cd]- aze pine hydrochloride) an azepine with high affinity for the 5-HT1A site in rat cerebral cortex in binding experiments, augmented contractions, but did not antagonize the responses to DP5CT or to 8-OH-DPAT. 6. The hybrid compound of RS-30199-193 with buspirone, RS-64459-193 (5-chloro-2-[4-(8-azaspiro[4,5]decane-7,9-dione)-but-1-yl]- 1,2,3,4,8,9,10,10a-octahydronaphth[1,8-cd]-3-azepine hydrochloride) maintained high affinity for the 5-HT1A binding site in rat brain and both inhibited the response to field stimulation and antagonized the responses to 8-OH-DPAT and DP5CT. Thus the buspirone side chain when added to RS-30199-193 appears either to induce affinity for a distinct subset of receptors in the guinea-pig ileum or is required for functional effectiveness at the 5-HTlA receptor.

Contribution of the periaqueductal grey matter to the development of hypertension in the spontaneously hypertensive rat

The dorsomedial periaqueductal grey matter (PAG) of the mid-brain, an important area for organizing the 'defence reaction', was destroyed using radiofrequency lesions in young, pre-hypertensive spontaneously hypertensive rats (SHRs). During the 12 weeks after surgery, systolic blood pressure (SBP) measured by an indirect tail-cuff technique was consistently lower than that of a sham-operated control group of SHRs (final readings of SBP: sham group (n = 6): 239.1 +/- 4.7 mmHg; lesion group (n = 8): 211.1 +/- 8.2 mmHg; P less than 0.05). Subsequent direct measurement of blood pressure in these rats by cannulation of the ventral tail artery indicated that the SBP in both groups was nearly the same (sham group: 204.7 +/- 5.1 mmHg; lesion group: 203.5 +/- 5.1), lying close to the value obtained by the tail-cuff method in the lesion group. Therefore, lesions of the PAG merely prevented the elevation of blood pressure in the SHR caused by the heating and restraint required for indirect measurement of SBP. Furthermore, ablation of the dorsal PAG had no effect on levels of circulating catecholamines, and did not prevent the cardiovascular response to an alerting stimulus (vibration). It is concluded that this region of the brain is not responsible for the development of hypertension in the SHR.

A search for brain stem cell groups integrating the defence reaction in the rat

The defence areas of the rat brain stem have been extensively explored using electrical and chemical stimulation in an attempt to locate the regions containing the perikarya of neurones which may initiate or integrate the visceral and behavioural components of the defence reaction. In rats anaesthetized with alphaxalone-alphadolone, a cannula electrode was used to compare the responses to electrical stimuli with those evoked by microinjection of the excitatory amino acid D,L-homocysteic acid (DLH), at the same site. A total of 128 sites throughout the brain stem was studied in 75 rats. The pattern of visceral and somatic changes characteristic of the defence reaction, viz. increases in arterial blood pressure and heart rate, vasodilatation in hind-limb muscles and vasoconstriction in the kidney, hyperpnoea and tachypnoea, exophthalmos, mydriasis, twitching of the vibrissae and retroflexion of the tail, was evoked by electrical stimulation within well-defined regions of the brain stem, from the anterior hypothalamus to the pons. Microinjection of DLH into the same regions could evoke the full defence reaction, as defined above, but only from the dorsomedial periaqueductal grey matter. Three other regions were defined from which almost all the autonomic components of the defence reaction were evoked, except that blood pressure fell. These were located: immediately dorsal to the optic chiasma, in the medial tuberal region of the hypothalamus and in the lateral pontine tegmentum. In conscious rats with implanted guide cannulae, darting and flight responses were evoked by microinjections of DLH into the periaqueductal grey matter but not from the hypothalamus or tegmentum. Brisk locomotion followed injections of DLH into the region overlying the optic chiasma. It is concluded that the brain stem neurones involved in integrating the somatic and visceral components of the defence reaction are concentrated within the four regions defined above. Whereas neurones in the periaqueductal grey matter can initiate the fully integrated defence reaction, those concentrated in the three other areas cannot be shown to do so. Of these three cell groups, the suprachiasmatic neurones seem to be closer in function to the periaqueductal group than are the neurones in the tuberal hypothalamus and pontine tegmentum.

The role of catecholamines in the production of ischaemia-induced ventricular arrhythmias in the rat in vivo and in vitro

The role of catecholamines in the production of ischaemia-induced ventricular arrhythmias in vivo and in vitro was studied using coronary artery ligation in the rat. Increases in plasma catecholamine concentrations during coronary artery ligation in pentobarbitone-anaesthetized animals were prevented by either acute adrenalectomy or chronic adrenal demedullation, but these procedures did not protect against the occurrence of ventricular arrhythmias. Thus plasma catecholamines were not obligatory mediators of arrhythmogenesis. Three protocols were used in vitro to evaluate the possible influence of intramyocardial release of noradrenaline, produced by the local conditions of ischaemia, on the production of ventricular arrhythmias. During coronary artery ligation in isolated perfused hearts, no enhanced output of 3H could be detected from [3H]-noradrenaline loaded hearts, even in the presence of inhibitors of catecholamine uptake processes, although washout of lactate from ischaemic regions was readily demonstrable. Both optical isomers of propranolol were equally effective in reducing the incidence of arrhythmias, implying a non-specific effect, since the (+)-isomer possesses considerably less beta-adrenoceptor blocking activity. The equipotency of optical isomers of propranolol combined with a lack of effect of atenolol suggested that arrhythmia production was not a consequence of beta-adrenoceptor stimulation. The alpha-adrenoceptor blockers phentolamine and prazosin, both exerted antiarrhythmic actions of similar potency, but phenoxybenzamine and trimazosin had no significant effects. An evaluation of the pharmacological properties of the alpha-adrenoceptor blockers showed that those drugs which had demonstrable local anaesthetic properties also exerted significant antiarrhythmic effects. No relationship was found between potency of alpha-adrenoceptor blockade and antiarrhythmic efficacy. The overall conclusion from these multifaceted approaches was that catecholamines were not necessary mediators of the early phase of ventricular arrhythmias in the rat.

Components of injury (haemorrhage and tissue ischaemia) affecting cardiovascular reflexes in man and rat

The effects of two components of tissue injury, namely fluid loss from the circulation and tissue ischaemia, on cardiovascular reflex activity have been studied. Moderate blood loss (10-20% blood volume) in the unanaesthetized rat increased the slope of the regression line relating heart period to mean arterial blood pressure and usually displaced it to the left (i.e. towards a relative bradycardia). A blood donation of 500 ml (approximately 10% blood volume) increased the Valsalva ratio in conscious man without a change in resting pulse rate. However, a 15 min period of unilateral limb ischaemia in man reduced the Valsalva ratio. The pattern of change in the pulse rat response to the Valsalva manoeuvre produced by limb ischaemia closely resembles that found previously after limb injury in man. There was no evidence that the endogenous opioids were involved in the interaction between limb ischaemia and cardiovascular reflex activity in man.

Effect of limb ischaemia on blood pressure and the blood pressure-heart rate reflex in the rat

The effects of bilateral hind-limb ischaemia on blood pressure and on the blood pressure-heart rate reflex have been studied in the rat. Limb ischaemia increased blood pressure and decreased the elevation and slope of the regression line describing the relationship between heart period (H.P.) and mean arterial pressure (M.A.P.). Nociceptive afferents from muscle receptors using long fibre tracts in the anterolateral part of the spinal cord seem to be responsible for the changes seen. The changes in the blood pressure-heart rate reflex were mediated by a combination of vagal inhibition and sympathetic activation. The efferent pathway for the pressor effect was in the sympathetic outflow. Central catecholaminergic neurones were involved in the pressor effect of limb ischaemia but not in the changes in the blood pressure-heart rate reflex. Electrolytic lesions in the posterior hypothalamus attenuated the inhibition of the reflex and it is suggested that neurones in the defence area may be activated by limb ischaemia. The interaction between limb ischaemia and the H.P.-M.A.P. relationship was not affected by opioid antagonists. After the period of ischaemia there was an increase in the elevation of the regression line describing the relationship between H.P. and M.A.P. which was secondary to the fall in body temperature characteristic of this phase of the response to injury.