The effect of arterial wall shear stress on the incremental elasticity of a conduit artery

AIMS

The purpose of this investigation was to determine the effects of flow mediated dilatation on arterial incremental elasticity (E(inc) ).

METHODS

  In four female anaesthetized pigs, the iliac artery and vein were connected by a shunt with a variable resistance which allowed blood flow and therefore shear stress to be regulated. E(inc) was calculated from simultaneous records of diameter and pressure throughout a minimum of four cardiac cycles.

RESULTS

Passive increases in diameter (∼1-2%) throughout a cardiac cycle, brought about by pressure, resulted in a two- to threefold increase in E(inc) . In contrast, increases in shear stress caused active smooth muscle relaxation and a significant increase in diameter from 3.663 ± 0.215 mm to 4.488 ± 0.163 mm (mean ± SEM, P < 0.05) equivalent to a fractional increase in diameter (fD) of 1.5 with no significant change in mean arterial pressure, 108 ± 2 mmHg to 106 ± 1 mmHg (mean ± SEM). The average value of E(inc) per cardiac cycle at baseline was 2.17 ± 0.10 × 10(3) kPa and remained relatively constant until fD exceeded 1.3 thereafter increasing to a maximum of 9.23 ± 1.0 × 10(3) kPa.

CONCLUSION

These results show that in a conduit artery during the dilatory response to shear stress, the interaction between smooth muscle and collagen operates so as to maintain E(inc) relatively constant over much of the working range of dilatation. This is consistent with a model of the arterial wall in which collagen is recruited both by passive stretch, in response to an increase in pressure and therefore wall stress, and also by active contraction of smooth muscle.

Dilatation in the femoral vascular bed does not cause retrograde relaxation of the iliac artery in the anaesthetized pig

AIM

We tested the hypothesis that dilatation of a feeding artery may be elicited by transmission of a signal through the tissue of the arterial wall from a vasodilated peripheral vascular bed.

METHODS

In eight pentobarbital anaesthetized pigs, acetylcholine (ACh, an endothelium-dependent vasodilator) was injected intra-arterially above (upstream) and below (downstream) a test segment of the left iliac artery, the diameter of which was measured continuously by sonomicrometry.

RESULTS

Under control conditions, ACh injections upstream and downstream of the test segment caused dilatation. Downstream injection dilated the peripheral arterioles, resulting in increased blood flow and proximal dilatation. This is a shear stress, nitric oxide (NO)-dependent response. The experiment was then repeated after applying a stenosis to prevent the increased flow caused by downstream injection of ACh; the stenosis was placed either above the site of diameter measurement to allow retrograde conduction, or below that site to prevent distally injected ACh reaching the measurement site. Under these conditions, downstream injection of ACh had a minimal effect on the shear stress of the test segment with no increase in test segment diameter. This was not due to endothelial damage or dysfunction as injection of ACh upstream still caused a large increase in test segment diameter.

CONCLUSIONS

Our results indicate that dilatation of the feeding artery of a vasodilated bed is caused by increased shear stress within the feeding artery and not via a signal transmitted through the arterial wall from below.

Characteristics of the response of the iliac artery to wall shear stress in the anaesthetized pig

The functional significance of shear stress-induced vasodilatation in large conduit arteries is unclear since changes in the diameter have little effect on the resistance to blood flow. However, changes in diameter have a relatively large effect on wall shear stress which suggests that the function of flow-mediated dilatation is to reduce wall shear stress. The mean and pulsatile components of shear stress vary widely throughout the arterial system and areas of low mean and high amplitude of wall shear stress are prone to the development of atheroma. In this study, using an in vivo model with the ability to control flow rate and amplitude of flow independently, we investigated the characteristics of the response of the iliac artery to variations in both the mean and amplitude of wall shear stress. The results of this study confirm that increases in mean wall shear stress are an important stimulus for the release of nitric oxide by the endothelium as indicated by changes in arterial diameter and show for the first time, in vivo, that increases in the amplitude of the pulsatile component of shear stress have a small but significant inhibitory effect on this response. A negative feedback mechanism was identified whereby increases in shear stress brought about by increases in blood flow are reduced by the release of nitric oxide from the endothelium causing dilatation of the artery, thus decreasing the stimulus to cell adhesion and, through a direct action of nitric oxide, inhibiting the process of cell adhesion. The results also provide an explanation for the uneven distribution of atheroma throughout the arterial system, which is related to the ratio of pulsatile to mean shear stress and consequent variability in the production of NO.

Vagal postganglionic origin of vasoactive intestinal polypeptide (VIP) mediating the vagal tachycardia

Our aim was to confirm the role of postganglionic vagal fibres and vasoactive intestinal polypeptide (VIP) in mediating the vagal tachycardia in anaesthetised dogs. Vagal postganglionic stimulation after atenolol (1 mg/kg) and hexamethonium (10 mg/kg) caused a bradycardia (40 beats/min, n = 2), after atropine (0.5 mg/kg i.v.) the resulting tachycardia (37 beats/min) was attenuated by VIP receptor antagonism with VIP (6-28) (100 mug i.c.) by approximately 50%. VIP release from vagal postganglionic fibres mediates the vagal tachycardia.

Differential inhibition by hyperglycaemia of shear stress- but not acetylcholine-mediated dilatation in the iliac artery of the anaesthetized pig

Clinical hyperglycaemia affects vascular endothelial function, but the effect on shear stress-induced arterial dilatation has not yet been established. We hypothesized that hyperglycaemia would inhibit this response via impaired glycocalyx mechanotransduction. Experiments were carried out in the anaesthetized pig in which pressure, blood flow and diameter of the left iliac artery were measured at two sites: proximal (d1) and distal (d2). Infusion of glucose, sufficient to raise blood glucose to 16-30 mm along the whole length of the artery, attenuated the shear stress-dependent dilatation in both sections of the artery with preservation of the responses to acetylcholine. The distal site was then isolated using snares and the lumen exposed to blood containing 25-35 mm glucose for 20 min. In the control situation, after exposure of both sections to normoglycaemia (5.7 mm glucose), both sections of artery showed increases in diameter in response to shear stress and acetylcholine. Hyperglycaemia attenuated the shear stress-dependent dilatation in the distal section only (P < 0.25), but not the response to acetylcholine. It is concluded from these results that the hyperglycaemia-impaired dilatation is consistent with loss of mechanotransducing properties of the endothelial glycocalyx by hyperglycaemia. These findings offer a possible explanation for the increased incidence of vascular disease in diabetic patients.

An investigation into the physiological relevance of the vagal tachycardia in the anaesthetized dog

AIM

Our aim was primarily to assess whether or not a vagal tachycardia can be elicited in vivo without administration of atropine, and secondly to evaluate whether the dose of atropine, a muscarinic antagonist, determines the magnitude of the tachycardia.

METHODS

Experiments were carried out in the presence of atenolol (2 mg kg(-1)). The vagal tachycardia requires high vagal activity which was induced by noradrenaline infusion (20 microg min(-1)). Two techniques were then used to elicit a tachycardia, vagal section and atropine administration.

RESULTS

The increase in blood pressure caused heart rate to fall to 60 +/- 7 beats min(-1) (mean +/- SEM). When the vagi were sectioned (n = 5) heart rate increased by 9 +/- 2 beats min(-1) above the intrinsic rate which was 108 beats min(-1), this increase was not significant. In contrast atropine given (9-20 microg kg(-1)) (n = 5) during high vagal activity increased heart rate by 81 +/- 22 beats min(-1) above the intrinsic rate (P < 0.05). To assess if the dose of atropine affects the magnitude of the vagal tachycardia, the right vagus was stimulated electrically at increasing frequencies (2, 4, 8, 16, 32 Hz) before and after increasing doses of atropine (0.02, 0.05, 1 mg kg(-1)). This reduced the magnitude of the bradycardia; however, the magnitude of the vagal tachycardia was unaffected.

CONCLUSION

The vagal tachycardia cannot be elicited without atropine suggesting that it does not play a significant physiological role.

The efficacy of nicorandil, calcium chloride and nitroglycerin in treatment of ropivacaine-induced cardiotoxicity

BACKGROUND AND OBJECTIVE

The amide-linked local anaesthetics, bupivacaine and ropivacaine, can cause depression of cardiac contractility and dysrhythmias. In a previous study, we observed decreased contractility and ST segment depression following ropivacaine administration in anaesthetized dogs. The efficacy of intravenous (i.v.) and intracoronary nicorandil (30 and 100 microg kg(-1)), i.v. nitroglycerin (glyceryl trinitrate) (5 microg kg(-1)) and calcium chloride (1, 2 and 4 mmol) in reversing the cardiotoxic effects of intracoronary ropivacaine were studied following the administration of intracoronary ropivacaine.

METHODS

Six dogs were studied. The dogs were anaesthetized with i.v. pentobarbital (30 mg kg(-1)). A left-sided thoracotomy was performed and the left circumflex coronary was cannulated. For each dog, the dose of ropivacaine was identified, which produced measurable cardiotoxicity. In each case, ropivacaine was followed by one of the three resuscitation drugs. The effects of each resuscitation drug on ST segments and left ventricular contractility (dP/dt) produced by ropivacaine alone were compared with those produced by ropivacaine followed by each of the three resuscitation drugs using Fisher's exact test.

RESULTS

The doses of ropivacaine required to produce depression of left ventricular dP/dt and ST segments ranged from 1 to 8 mg. Ropivacaine-induced depression of left ventricular contractility (dP/dt) was more rapidly and completely reversed by calcium chloride than by either nitroglycerin or nicorandil (P = 0.008).

CONCLUSIONS

Calcium chloride may be effective in the treatment of inadvertent intravascular administration of amide local anaesthetic agents.

Effects of fenoldopam on renal blood flow and its function in a canine model of rhabdomyolysis

BACKGROUND AND OBJECTIVE

Our hypothesis was that fenoldopam, a selective DA1 agonist, would protect against rhabdomyolysis-induced renal injury.

METHODS

We studied the effects of intravenous fenoldopam (0.1-1.0 microg kg(-1) min(-1)) or saline on renal blood flow and function in 10 anaesthetized Labrador dogs in whom rhabdomyolysis and myoglobinuric acute renal failure had been induced by administration of glycerol 50% (10mL kg(-1)) intramuscularly. Haemodynamic measurements including renal blood flow and derived parameters of renal function including creatinine clearance were recorded before and for the 30 min following glycerol injection, and during the 3 h following commencement of each infusion. Serum malondialdehyde concentrations were measured before and 15 min after glycerol intramuscularly, and 30 and 150 min after commencement of the infusion.

RESULTS

In the fenoldopam group, creatinine clearance was less than placebo at 1 and 2 h after commencing the infusion (12.7 +/- 11.5 versus 31.3 +/- 9.9 mL min(-1), P = 0.04; 8.5 +/- 5.3 versus 20.1 +/- 7.4 mL min(-1), P = 0.03). A 140-fold increase in serum malondialdehyde concentration occurred in one dog (fenoldopam group).

CONCLUSION

Fenoldopam increased the severity of the renal injury in this canine model of myoglobinuric acute renal failure.

An evaluation of the efficacy of vasoactive intestinal polypeptide antagonists in vivo in the anaesthetized dog

The effectiveness of competitive peptide vasoactive intestinal polypeptide (VIP) receptor antagonists was evaluated on heart rate in the anaesthetized dog. Two specific antagonists, VIP (6-28) and [D-p-Cl-Phe(6), Leu(17)]-VIP, and a nonspecific antagonist, pituitary adenylate cyclase activating peptide fragment (6-27) (PACAP), were studied. VIP (6-28) and [D-p-Cl-Phe(6), Leu(17)]-VIP (100 microg i.c.) increased the heart rate, whereas PACAP (100 microg i.c.) reduced the baseline heart rate. All three shifted the VIP dose-response curve to the right by two- to threefold for 30 min. In conclusion, PACAP, VIP (6-28), and [D-p-Cl-Phe(6), Leu(17)]-VIP have a direct effect on the heart rate, are equally effective, and the effects last approximately 30 min in vivo.

The effect of tezosentan, a non-selective endothelin receptor antagonist, on shear stress-induced changes in arterial diameter of the anaesthetized dog

The effects of changes in the mean (S(m)) and pulsatile (S(p)) components of arterial wall shear stress on arterial dilatation of the iliac artery of the anaesthetized dog were examined in the absence and presence of the endothelin receptor antagonist tezosentan (10 mg kg(-1) I.V.; Ro 61-0612; [5-isopropyl-pyridine-2-sulphonic acid 6-(2-hydroxy-ethoxy)-5-(2-methoxy-phenoxy)-2-(2-1H-tetrazol-5-yl-pyridin-4-yl)-pyrimidin-4-ylamide]). Changes in shear stress were brought about by varying local peripheral resistance and stroke volume using a distal infusion of acetylcholine and stimulation of the left ansa subclavia. An increase in S(m) from 1.81 +/- 0.3 to 7.29 +/- 0.7 N m(-2) (means +/- S.E.M.) before tezosentan caused an endothelium-dependent arterial dilatation which was unaffected by administration of tezosentan for a similar increase in S(m) from 1.34 +/- 0.6 to 5.76 +/- 1.4 N m(-2) (means +/- S.E.M.). In contrast, increasing the S(p) from 7.1 +/- 0.8 to a maximum of 11.5 +/- 1.1 N m(-2) (means +/- S.E.M.) before tezosentan reduced arterial diameter significantly. Importantly, after administration of tezosentan subsequent increases in S(p) caused arterial dilatation for the same increase in S(p) achieved prior to tezosentan, increasing from a baseline of 4.23 +/- 0.4 to a maximum of 9.03 +/- 0.9 N m(-2) (means +/- S.E.M.; P < 0.001). In conclusion, the results of this study provide the first in vivo evidence that pulsatile shear stress is a stimulus for the release of endothelin from the vascular endothelium.

Reverse arterial wall shear stress causes nitric oxide-dependent vasodilatation in the anaesthetised dog

The effects of a maintained increase in mean arterial wall shear stress (SS(m)) caused by blood flow in the normal and reverse direction on dilatation of the iliac artery were examined in the anaesthetised dog. Blood flow in the left iliac artery was varied in both the forwards and reverse directions by a perfusion pump connecting the right and left femoral arteries. An increase in blood flow, and therefore SS(m) in either direction, caused an increase in arterial diameter. However, an increase in forwards SS(m) (control 4.1+/-0.11 mm) caused a significantly greater change in arterial diameter than an equivalent increase in the reverse direction (control 4.3+/-0.08), 0.198+/-0.02 mm vs. 0.132+/-0.02 mm (mean+/-SEM) respectively, for the same increase in SS(m) (3.23 N/m(2)). The increase in arterial diameter in response to an increase in forwards or reverse SS(m) was attenuated by L-NAME (80 mg/kg i.v.), indicating that the arterial dilatation was mediated by nitric oxide (NO). These findings confirm that endothelial NO release is dependent on the steady-state SS(m) and that the response occurs irrespective of the direction in which this force is applied, but is attenuated in the reverse direction.

Nitric oxide facilitates vagal control of heart rate via actions in the cardiac parasympathetic ganglia of the anaesthetised dog

The effects of neuronal inhibition of nitric oxide (NO) production on the bradycardia resulting from stimulation of preganglionic and postganglionic parasympathetic fibres were investigated in an anaesthetised dog preparation following transection of the cervical vagi and in the presence of a beta-adrenoreceptor antagonist. Injection of 1-(2-trifluoromethylphenyl) imidazole (TRIM), an inhibitor of neuronally released NO, into the sinus node artery reduced the bradycardia evoked by right cervical vagal stimulation. In contrast, when the response to preganglionic stimulation had been abolished by hexamethonium (10 mg kg(-1)), the bradycardia following stimulation of postganglionic parasympathetic fibres on the atrial epicardium was unaffected by TRIM. First, these results confirm the facilitatory actions of neuronally released NO on vagal heart responses in the dog. Second, they indicate that this modulatory and facilitatory role of NO is likely to be exerted at vagal preganglionic-postganglionic synaptic mechanisms in the cardiac parasympathetic ganglia and not at the postganglionic-sinoatrial node synapse.

The potentiation of sinus arrhythmia by vasoactive intestinal polypeptide (VIP) in the anaesthetized dog

Vasoactive intestinal polypeptide (VIP) is a neuropeptide released from the vagus, which in contrast to acetylcholine has a long-acting positive chronotropic effect on the heart. The aim of this study, in the anaesthetized dog, was to examine the effects of VIP and a VIP antagonist when injected into the sinus node artery of a vagally intact heart in sinus arrhythmia. The response was compared to that produced by noradrenaline (NAD) infusion and stimulation of the sympathetic nerves to the heart. Mean +/- S.D. of 30 R-R intervals was used to describe mean heart rate interval and heart rate variability. VIP, a VIP antagonist, NAD and sympathetic nerve stimulation all caused increases in heart rate without significant increases in blood pressure. However, only VIP caused an increase in heart rate variability; VIP antagonism and NAD caused a decrease and sympathetic nerve stimulation had no effect. These results suggest that VIP and acetylcholine when released from the vagus act synergistically to increase sinus arrhythmia.

Inhibition of neuronal nitric oxide reduces heart rate variability in the anaesthetised dog

In the vagally intact anaesthetised dog, we have investigated the role of nitric oxide (NO) on a normal sinus arrhythmia using an inhibitor of neuronally released NO, 1-(2-trifluoromethylphenyl) imidazole (TRIM). The mean and S.D. of the R-R interval was used to describe mean heart rate and heart rate variability, respectively. TRIM (0.8 mg I.C.) injected into the sinus node artery increased the mean heart rate slightly but reduced heart rate variability 3-fold from a control of 790 +/- 124 ms (mean +/- S.D.; n = 5) to 666 +/- 36 ms (P < 0.01 Student's paired t test, n = 5). These results suggest that neuronally released NO may have a vagal facilitatory role in the maintenance of sinus arrhythmia in the normal heart.

Effects of prophylactic fenoldopam infusion on renal blood flow and renal tubular function during acute hypovolemia in anesthetized dogs

OBJECTIVE

It was hypothesized that fenoldopam mesylate, a selective dopamine agonist, may preserve renal perfusion and decrease tubular oxygen consumption during states of hypoperfusion, such as hypovolemic shock. The objective of this study was to quantify the effects of fenoldopam (0.1 microg x kg(-1) x min(-1)) on renal blood flow, urine output, creatinine clearance, and sodium clearance in pentobarbital anesthetized dogs that had undergone partial exsanguination to acutely decrease cardiac output.

DESIGN

Prospective, randomized, controlled experiment.

SETTING

University-based animal laboratory and research unit.

SUBJECTS

Eight female beagle dogs.

INTERVENTIONS

Arterial blood pressure, heart rate, cardiac output, renal blood flow, urine output, creatinine clearance, and fractional excretion of sodium were measured and calculated at four times: a) before infusion of fenoldopam or normal saline; b) during infusion of fenoldopam or normal saline (1 hr); c) during a 90-min period of hypovolemia (induced by acute partial exsanguination), with concurrent infusion of fenoldopam or normal saline; and d) during a 1-hr period after retransfusing the dogs.

MEASUREMENTS AND MAIN RESULTS

Administration of fenoldopam (0.1 microg x kg(-1) x min(-1)) was not associated with hemodynamic instability. Renal blood flow and urine output decreased significantly from baseline (p <.01) during the hypovolemic period in the placebo group (72 +/- 20 to 47 +/- 6 mL/min and 0.26 +/- 0.15 to 0.08 +/- 0.05 mL/min, respectively) but not in the fenoldopam group (75 +/- 14 to 73 +/- 17 mL/min and 0.3 +/- 0.19 to 0.14 +/- 0.05 mL/min, respectively). Creatinine clearance and fractional excretion of sodium decreased significantly from baseline (p <.01) in the placebo group during the hypovolemic period (3.0 +/- 0.4 to 1.8 +/- 0.8 mL x kg(-1) x min(-1) and 1.7% +/- 0.9% to 0.4% +/- 0.2%, respectively) but not in the dogs that received fenoldopam (3.0 +/- 1.0 to 2.9 +/- 0.5 mL x kg(-1) x min(-1) and 1.9% +/- 1.1% to 1.7% +/- 2.7%, respectively).

CONCLUSIONS

Fenoldopam ablated the tubular prerenal response to profound hypovolemia and maintained renal blood flow, glomerular filtration rate, and natriuresis without causing hypotension. This suggests that fenoldopam may have a renoprotective effect in acute ischemic injury.

Characteristics of arterial wall shear stress which cause endothelium-dependent vasodilatation in the anaesthetized dog

1. The effects of changes in the mean and amplitude of arterial wall shear stress on endothelium-dependent arterial dilatation of the iliac artery of the anaesthetized dog were examined. 2. Changes in the mean and amplitude of blood flow and wall shear stress were brought about by varying local peripheral resistance and stroke volume using a distal infusion of acetylcholine and the stimulation of the left ansa subclavia. Changes in the diameter of a segment of the iliac artery with the endothelium intact, relative to a segment with no endothelium, were used as an index of the release of nitric oxide. 3. The increase in mean blood flow was from 84 +/- 12 to 527 +/- 53 ml min-1 and in amplitude was from 365 +/- 18 to 695 +/- 38 ml min-1 (means +/- S.E.M.). The increase in mean wall shear stress was from 1.78 +/- 0.30 to 7.66 +/- 1.01 N m-2 and in amplitude was from 7.37 +/- 0.46 to 13.9 +/- 2.00 N m-2 (means +/- S.E.M.). 4. Increases in mean shear stress caused an increase in the diameter only of the section of artery with endothelium; the slope of the relationship was 0.064 +/- 0.006 mm N-1 m2 (mean +/- S.E.M., P < 0.001); changes in the amplitude of shear stress did not cause an increase in diameter. Changes in both the mean and amplitude of shear stress had no significant effect on the diameter of the section of artery with no endothelium. 5. These findings coupled with the known anti-atheroma effects of nitric oxide and the effect of shear stress on cell adhesion and platelet aggregation offer a possible explanation for the disposition of atheroma in those parts of the arterial system which have low mean and high amplitude of wall shear stress.

The effects of fenoldopam on renal blood flow and tubular function during aortic cross-clamping in anaesthetized dogs

Postoperative renal impairment is a recognized complication of infrarenal aortic cross-clamping. Fenoldopam, a selective dopamine agonist, may increase renal blood flow and decrease tubular oxygen consumption. The objective of this study was to quantify the effects of fenoldopam (0.1 microg kg-1 min-1) on renal blood flow and renal tubular function in anaesthetized dogs that have undergone aortic cross clamping. Eight labrador dogs were selected to receive either saline or fenoldopam (0.1 microg kg-1 min-1) intravenously. Arterial pressure, heart rate, renal blood flow, urinary output, fractional excretion of sodium, creatinine clearance and lithium clearance were measured (a) prior to infusions of saline or fenoldopam (b) 1 h after commencing the infusion (c) during a 90-min period of infrarenal aortic cross-clamping with concurrent infusion of fenoldopam or saline and (d) for 1 h after simultaneous aortic declamping and discontinuation of the infusions. There was no haemodynamic instability upon commencing the infusion of fenoldopam (0.1 microg kg-1 min-1). Creatinine clearance (2.03 +/- 0.5-2.45 +/- 0.3 mL min-1 kg-1 (mean +/- SD)), urine output (0.23 +/- 0.16-0.35 +/- 0.23 mL min-1 (mean +/- SD)), and fractional excretion of sodium (0.7 +/- 0.52-1.3 +/- 0.73% (mean +/- SD)) increased (P < 0.05), following commencement of the fenoldopam infusion. Fractional excretion of sodium (1.2 +/- 0.7% (mean +/- SD)) and urine output (0. 36 +/- 0.21 mL min-1 (mean +/- SD)) were maintained during the aortic cross-clamp period (P < 0.05). Renal blood flow increased when the fenoldopam infusion was commenced (145 +/- 43.3-161 +/- 39. 2 mL min-1 (mean +/- SD)) and remained greater than baseline during the aortic cross-clamping period (152 +/- 44 mL min-1 (mean +/- SD)), although these increases did not reach statistical significance. The most striking abnormalities observed by electron microscopy were marked disruption of the microvillus brush border in proximal tubules, vacuolation and separation of epithelial cells on basolateral infolds. The changes were similar in the two groups. In conclusion fenoldopam (0.1 microg kg-1 min-1) may have renoprotective effects which persist during infrarenal aortic cross clamping.

Effects of respiratory and metabolic pH changes and hypoxia on ropivacaine-induced cardiotoxicity in dogs

We have studied the effects of acute changes in acid-base status and hypoxia on the cardiotoxic effects of intracoronary injection of ropivacaine in anaesthetized dogs. The effects of intracoronary ropivacaine were compared when ropivacaine was administered during eucapnia and during each of another nine states in random order: hypocapnia, hypercapnia, hypoxia, metabolic alkalosis, metabolic acidosis, combined metabolic acidosis and hypocapnia, combined metabolic alkalosis and hypercapnia, combined hypoxia and hypercapnia, and combined metabolic acidosis and hypoxia. Hypocapnic alkalosis consistently reduced the cardiotoxic effects of intracoronary ropivacaine (P < 0.01). Our findings indicate that induction of hypocapnic alkalosis may provide a useful adjunct to standard resuscitative measure after inadvertent administration of amide local anaesthetic agents.

The relationship between blood flow and diameter in the iliac artery of the anaesthetized dog: the role of endothelium-derived relaxing factor and shear stress

The quantitative relationship between, increase in blood flow and arterial diameter was determined in an anaesthetized dog preparation (pentobarbitone, induction 30 mg kg-1 i.v., maintenance 3 mg kg-1 i.v. every 30 min). Changes in external iliac artery diameter were measured using piezoelectric ultrasound transducers capable of measuring diameters within the range of 2-20 mm with a resolution of +/- 0.005 mm. The diameter of the artery was measured at two sites, at one of which the endothelium was damaged using a balloon angioplasty catheter. Increases in blood flow were brought about by a combination of vasodilatation and cardiac stimulation (intra-arterial administration of acetylcholine, downstream to the sites of diameter measurement, and electrical stimulation of the left ansa subclavia), thereby preventing large changes in blood pressure. The effects of both transient and maintained increases in blood flow on mean arterial diameter in the section of artery with intact endothelium were measured. Transient increases in mean flow from 147 +/- 0.21 to 611 +/- 80.0 ml min-1 caused increases in diameter of 0.12 +/- 0.02 mm from a control of 5.42 +/- 0.19 mm. The mean delay between maximum flow and maximum diameter was 24.51 +/- 1.1 s and the half-time for the return to control diameter was 82.0 +/- 9.6 s, compared with 12.1 +/- 1.5 s for the return to control flow. Maintained (3-4 min) increases in mean blood flow (from 104.7 +/- 15.1 to 694.7 +/- 135.1 ml min-1) produced larger increases in diameter of 0.48 +/- 0.30 mm from a control diameter of 4.89 +/- 0.12 mm. These changes in diameter were abolished by N omega-nitro-L-arginine methyl ester (L-NAME. 10-100 mg kg-1 i.v.). In the section of artery with damaged endothelium, changes in diameter were relatively small and associated with small changes in blood pressure. This effect of a nearly 7-fold increase in flow on arterial diameter is dependent upon the integrity of the endothelium and the release of endothelium-derived relaxing factor and causes a 29% reduction in calculated boundary wall shear stress.

Interaction between the effects of 5-hydroxytryptamine and adrenaline on the growth of platelet thrombi in the coronary artery of the anaesthetized dog

1. The interaction between adrenaline and 5-hydroxytryptamine (5-HT) has been quantitated on the rate of thrombus formation, in the stenosed coronary artery with damaged endothelium of the anaesthetized dog. 2. Changes in the plasma concentration of adrenaline were produced by varying the rate of an intravenous infusion of adrenaline and in the effects of 5-HT, by intravenous injections of the selective 5-HT2 receptor antagonist, ICI 170809. 3. Increases in the plasma concentration of adrenaline, which did not cause significant changes in blood pressure and heart rate, increased the rate of thrombus formation. 4. Antagonism of the 5-HT2 receptor by ICI 170809, in the absence of an infusion of adrenaline, abolished thrombus formation (mean ED50 0.41 microgram kg-1, i.v.). 5. The effects of adrenaline were non-competitively antagonized by ICI 170809; maximum effects were obtained in the dose-range 50-200 micrograms kg-1, i.v., when the mean dose-ratio increase in adrenaline required to restore equivalent rates of thrombus formation was 39 fold. 6. These results are consistent with a synergism between adrenaline and 5-HT and emphasize the importance of both on thrombus formation.

Redirection of arachidonic acid metabolism by ICI D1542: effects on thrombus formation in the coronary artery of the anaesthetized dog

1. The effects of simultaneous redirection of arachidonic acid metabolism, by inhibition of thromboxane A2 (TXA2) synthase and blockade of the platelet thromboxane A2 receptor (TP-receptor), was examined on the rate of thrombus formation in a stenosed coronary artery with damaged endothelium in an anaesthetized dog. 2. Redirection of arachidonic acid metabolism was achieved by intravenous doses of ICI D1542, a selective and potent inhibitor of TXA2 synthase and the TP-receptor. 3. Redirection of arachidonic acid metabolism was demonstrated in whole blood, stimulated ex vivo by collagen. The ED50 for inhibition of TXB2 production was 7.1 micrograms kg-1, i.v.; there were corresponding increases in the production of the eicosanoids prostaglandin D2 (PGD2), PGE2 and PGF2 alpha. 4. Thrombus formation was inhibited by D1542 (ED50 0.55 micrograms kg-1, i.v.), but could be restarted by an intravenous infusion of adrenaline (0.2-38 micrograms kg-1 min-1, i.v.). In the presence of the maximum effective dose of D1542 (1 mg kg-1, i.v.) a 190 fold increase in the infusion rate of adrenaline was required to restore thrombus formation. 5. In the presence of D1542, removal of endoperoxide metabolites by inhibition of cyclo-oxygenase with aspirin (5 mg kg-1, i.v.) caused thrombus formation to restart, indicating the ability of the endoperoxide metabolites to inhibit thrombus formation in vivo. 6. These results indicate that, in the stenosed and damaged coronary artery of the dog, redirection of arachidonic acid metabolism by D1542 is more effective at preventing thrombus formation than inhibition of cyclo-oxygenase by aspirin.

Comparative effects of beta-adrenoceptor partial agonists on isolated rat atrium

The chronotropic effect of three beta-1-adrenoceptor partial agonists prenalterol, xamoterol and epanolol has been compared on the right atria of the rat in order to evaluate their intrinsic activity and to place them in rank order of effectiveness. The results show that prenalterol, xamoterol and epanolol are all partial agonists. The intrinsic activities relative to that of isoprenaline are 0.84 for prenalterol, 0.59 for xamoterol and 0.29 for epanolol. This rank order of intrinsic activities should remain the same in different species and in man. Both atenolol and propranolol reversed the chronotropic effects of the three agonists. The KB of the two blockers was similar against prenalterol and xamoterol, which indicates that the two partial agonists are probably competing for the same population of receptors. The EC50 is twice as large than KB for xamoterol, which is consistent with isoprenaline working through both beta-1- and beta-2- receptors and xamoterol finds it more difficult to block the beta-2-receptors.

Relationship between positive inotropic responses and plasma concentrations of xamoterol in middle-aged and elderly patients

1. We examined the relationship between the contractile state of the left ventricle and the plasma concentration of xamoterol in patients with ischaemic heart failure. 2. Identical studies were conducted in 14 middle-aged (all male; mean age 51.3 years, range 42-61) and 10 elderly patients (six male, four female; mean age 67.7 years, range 64-72). 3. Patients received seven cumulative doses (0.0005-0.2 mg kg-1) of xamoterol. After each dose the rate of change of pressure in the left ventricle at a developed pressure of 40 mm Hg and normalised for this pressure, (dP/dt)/DP40, and plasma concentrations of xamoterol were measured. 4. There were dose-related increases in (dP/dt)/DP40. Curves relating changes in (dP/dt)/DP40, expressed as a percent of the maximum observed response, to changes in xamoterol plasma concentrations were constructed for the middle-aged and elderly patients. From these curves the mean effective concentration (EC) value to produce a particular response could be calculated. In the sample sizes studied, the difference between the EC values over a range of responses for the middle-aged and elderly patients did not reach statistical significance, indicating that cardiac responsiveness to xamoterol was similar in the two groups of patients. 5. Plasma concentrations of xamoterol over the range of 39 to 150 ng ml-1 produced positive inotropic responses which varied between 70% and 90% of the maximum observed effect of xamoterol.

Changes in exercise tolerance and resting haemodynamics during long-term treatment of heart failure with xamoterol

The long-term efficacy of xamoterol in improving effort capacity and haemodynamics was investigated in a study lasting 1 year in heart failure patients. Following a 3 month, double-blind period of treatment with xamoterol, patients continued therapy for a further 9 months. Exercise duration rose by 27% at 3 months and this improvement was maintained at 12 months. Similarly, the statistically significant increase in cardiac index and reduction in pulmonary wedge pressure and exercise heart rate persisted for 12 months.

Modulation of the autonomic control of the failing heart

The failing heart operates with an abnormal combination of heart rate, stroke volume, and enddiastolic volume. This mismatch becomes more evident during exercise of patients with heart failure, when an increase in cardiac output is achieved with higher heart rate, a lower stroke volume and a higher enddiastolic volume. Using the beta 1-adrenoceptor partial agonist xamoterol which lacks beta 2-adrenoceptor agonism the response of the heart to sympathetic stimulation can be modulated. At rest and low levels of exercise xamoterol provides an inotropic support of the heart, whereas it reduces inappropriate tachycardia at higher levels. Thereby, xamoterol tends to normalize the balance of the inotropic and chronotropic control of the failing heart, because cardiac output is increased with a more normal combination of heart rate, stroke volume, and filling pressure. The beneficial effects of xamoterol are discussed as being especially important for failing ischemic hearts, because the balance between energy supply and energy demand may be improved by xamoterol.

Effects of xamoterol, a beta 1 adrenoceptor partial agonist, in patients with ischaemic dysfunction of the left ventricle

The effects of xamoterol (200 mg twice a day) in 21 patients with left ventricular dysfunction were studied in a double blind, randomised, crossover, placebo controlled trial with treatment periods of four weeks. Most patients had moderate heart failure (New York Heart Association class II), all had ischaemic heart disease, a history of a myocardial infarction, and symptoms of dyspnoea on exertion. Patients were assessed in terms of exercise duration (bicycle ergometer), clinical signs of heart failure, symptoms and activities, and ejection fraction. Xamoterol increased exercise duration (mean (SD] (from 445 (8) seconds to 484 (8) seconds) and ejection fraction (from 41.9 (1.3)% to 46.6 (1.3)%) and reduced the signs and symptoms of heart failure. The results of this study show that xamoterol is a safe and effective treatment for left ventricular dysfunction resulting from ischaemic heart disease.

The pharmacology of xamoterol: a basis for modulation of the autonomic control of the heart

1. Xamoterol (Corwin, Carwin, Corwil, Xamtol, ICI 118,587) is a beta-adrenoceptor partial agonist which is of benefit in the chronic treatment of heart failure (e.g. The German and Austrian Xamoterol Study Group, 1988). These results contrast with those obtained with other beta-adrenoceptor drugs, prenalterol and pirbuterol which were unsuccessful on chronic dosing (Currie et al., 1984; Glover et al., 1985). 2. Unlike prenalterol and pirbuterol, xamoterol has no significant agonist activity at the beta 2-adrenoceptor and has shown no tachyphylaxis in animals or man. 3. The overall action of xamoterol is to modulate sympathetic control of the heart such that at rest and at low levels of exercise the heart receives inotropic support whilst during more severe exercise, heart rate is reduced. In patients with left ventricular dysfunction these effects lead to an improvement in the relation between filling pressure and cardiac output at all levels of activity such that a given cardiac output is achieved with a lower filling pressure. It is suggested that this alteration in the pattern of cardiac activity over a long period of time results in a beneficial adaptation of the myocardium and is a possible explanation of the observed clinical improvement.

The cardiovascular pharmacology of xamoterol, cicloprolol, prenalterol and pindolol in the anaesthetised dog

An anaesthetised dog preparation was used to examine the pharmacological profiles of xamoterol, cicloprolol, prenalterol and pindolol. The effects of these agents on heart rate and hind limb perfusion pressure revealed that xamoterol is highly cardioselective and has no significant agonist activity at beta 2-receptors and no membrane stabilising effect. These pharmacological properties are not complicated by the formation of active metabolites and may explain the clinical benefits in the long-term treatment of heart failure.

Effects of xamoterol on sodium excretion in volunteers

Xamoterol has been shown to reduce the frequency of oedema and lung crepitations in heart failure. We examined its effects on blood pressure and renal function in healthy volunteers. Systolic blood pressure rose, sodium and chloride excretion increased and there was a strong correlation in individual subjects between rises in systolic blood pressure and in sodium excretion. Although no changes in glomerular filtration rates were seen, changes sufficient to explain the observed rise in sodium excretion are well within the experimental error of this study. Xamoterol may increase sodium excretion by an action on renal haemodynamics.

Long-term efficacy of xamoterol (a beta 1-adrenoceptor partial agonist) in patients with mild to moderate heart failure

This study was designed to assess the efficacy of xamoterol in 14 patients with mild to moderate heart failure over a period of 18 months. A beneficial effect on exercise capacity and a lowering of heart rate on exercise was sustained, and ejection fraction did not change, although some deterioration may be expected over this length of time in patients with heart failure. Xamoterol is safe and effective, and its benefits are maintained over at least 18 months.

The effects of age, sex and treatment with xamoterol on exercise capacity in patients with heart failure

The influence of age, sex and xamoterol treatment on exercise capacity has been investigated in 705 heart failure patients who took part in a multicentre, placebo-controlled study. Regression analysis suggests that although less than 20% of the total variation in exercise capacity between patients is explained by these factors, useful information may be gained by taking them into account. Exercise capacity declines with age, more rapidly in males than in females; the benefits of xamoterol are independent of age and sex. This approach demonstrates that exercise testing is a sensitive indicator of physical performance and allows the effects of treatment to be seen in context.

The effects and dose-response relationship of xamoterol in patients with ischaemic heart disease

1. In a double-blind placebo controlled four-way crossover study the effects and dose response relationships of xamoterol were studied in nine patients with angina and dyspnoea secondary to chronic left ventricular dysfunction. The duration of exercise on a treadmill and heart rate were measured at the end of each phase of the study at 2 h and 24 h after dosing. 2. Xamoterol at 200 mg and 400 mg orally once daily had no effect on the mean resting heart rate but there was a small (5.7 beats min-1) but significant reduction in resting heart rate on 600 mg at 2-2.5 h after dosing. All three doses of xamoterol significantly reduced the maximum exercise heart rate at 2-2.5 h after dosing. 3. Xamoterol at all three doses significantly increased exercise duration at 2-2.5 h after dosing but not at 24 h. 4. Mean plasma xamoterol concentration at both 2-2.5 h and 24 h after dosing were dose related. The EC50 for xamoterol is 33.5 ng ml-1, where EC50 is the effective plasma concentration required to produce 50% of the maximum effect on exercise heart rate.

Facilitation of calcium blocking and membrane effects by intrinsic sympathomimetic activity

The interactions between ICI 118, 587 (Corwin) a beta 1-selective partial adrenergic agonist, and atenolol (Tenormin), propranolol (Inderal) and verapamil were examined first in anaesthetised dogs pretreated with syrosingopine and vagotomised. ICI 118, 587 was administered iv in cumulative doses of 0.1 to 1000 micrograms X kg-1. In four animals, heart rate increased from 102 +/- 4 to 188 +/- 12 beats X min-1 with a KA of 2.5 +/- 0.9 micrograms X kg-1. ICI 118, 587 was then administered to five groups each of four animals pretreated with atenolol (250 micrograms X kg-1 and 500 micrograms X kg-1iv), propranolol (250 micrograms X kg-1 and 500 micrograms X kg-1) or verapamil (200 micrograms X kg-1 plus 5 micrograms X kg-1 X min-1 iv). Both atenolol and propranolol shifted the dose response curve to the right but verapamil did not. Atenolol did not lower the maximal heart rate response to ICI 118,587. Both propranolol and verapamil slowed heart rate significantly (P less than 0.05) after, but not before, ICI 118,587. Further studies were then carried out in vitro. Cat papillary muscles were superfused with physiological saline at pH 7.4 and with either atenolol or ICI 118,587 at 5 X 10(-5) mol X litre-1. Tension was recorded at Lmax over a [Ca2+] range of 0.5 to 8 X 10(-3)mol X litre-1. The pA2 for verapamil against Ca2+ in the presence of atenolol and ICI 118,587 was 5.25 +/- 0.10 and 6.57 +/- 0.22 respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

The cardiovascular effects of ICI 118,587: A beta 1-adrenoceptor partial agonist

1 In a preparation in which cardiovascular reflexes were prevented from occurring, ICI 118,587 (1-(p-hydroxyphenoxy)-3-beta-(morpholinocarbonamido) ethylamino-2-propranol fumarate) caused dose-dependent positive chronotropic and inotropic effects upon the dog heart. 2 The increase in heart rate brought about by ICI 118,587 was about 43% of the maximum increase produced by isoprenaline. 3 For a given chronotropic effect produced by either ICI 118,587 or isoprenaline, each compound produced a similar inotropic effect as indicated by an increase in LV dp/dtmax. 4 In contrast to the direct stimulant action of ICI 118,587 on the heart no direct effects on vascular smooth muscle were observed. 5 ICI 118,587 was shown to be a competitive antagonist of the chronotropic and vasodilator effects of isoprenaline on the heart and blood vessels and of the chronotropic effects of noradrenaline on the heart. 6 It is concluded that ICI 118,587 is a selective beta 1-adrenoceptor partial agonist.

Beta-adrenoceptor stimulant properties of amidoalkylamino-substituted 1-aryl-2-ethanols and 1-(aryloxy)-2-propanols

Parallel series of 2-[(2-amidoethyl)amino]-1-arylethanols and 1-[(2-amidoethyl)amino]-3-(aryloxy)-2-propanols have been prepared, and the compounds were tested as beta-adrenoceptor stimulants on the heart and circulation of the dog. The corresponding 2-(alkylamino)-1-arylethanols and 3-(alkylamino)-2-propanols have been tested for comparison and the structure-activity relationships (SAR) examined. The arylethanols are potent full agonists, showing selectivity for the heart relative to blood vessels, while the (aryloxy)propanols are even more cardioselective and are partial agonists. Within a narrow series of 1-[(amidoethyl)amino]-3-(4-hydroxyphenoxy)-2-propanols, careful examination of the SAR of the amide group showed that great variation in cardioselectivity and degree of agonism may be produce. From this study ICI 118587, N-[20[[2-hydroxy-3-(4-hydroxyphenoxy)propyl]amino]ethyl]-4-morpholinecarboxamide, was selected for its high cardioselectivity and 50% agonist properties. This compound in under clinical evaluation as a cardiac stimulant.

Atrial receptors and heart rate: the efferent pathway

1. Stimulation of left atrial receptors by distension of the junctions between the pulmonary veins and the left atrium is known to cause a reflex increase in heart rate. It was suggested that the efferent path of this reflex was solely in the sympathetic nerves to the heart but more recently the existence of a vagal efferent component has been postulated by Albrook, Bennion & Ledsome (1972). 3. The junctions between the pulmonary veins and the levt atrium were distended before and after the administration of I.C.I. 66082 and bretylium tosylate. The response of an increase in heart rate was significantly decreased after the administration of I.C.I. 66082 (5 mg/kg) and abolished after the administration of bretylium tosylate (10 mg/kg). 3. It is concluded that the efferent pathway of the reflex is solely in the sympathetic nerves to the heart.

The assessment of acid-base disturbance in man by the use of carbon dioxide titration curves

1. Carbon dioxide titration curves were determined in vivo in dog and man at various degrees of acute non-respiratory acidaemia and alkalaemia. 2. The slope of the CO2 titration curve (delta log Pco2/delta pH) was found to increase with the severity of the acute non-respiratory alkalaemia the slope (delta log Pco2/delta pH) tended towards unity. 3. A simple scheme based on the CO2 titration curves determined in vivo has been proposed for the assessment of acute acid-base disturbances in man. 4. Carbon dioxide titration curves were also determined in vivo in patients with chronic respiratory and non-respiratory acidaemia and it was found that these curves were not significantly different from those obtained in states of acute acid-base disturbances. It is therefore suggested that the scheme described in this paper is applicable to all acid-base disturbances.

Effect of destruction of the posterior pituitary on the diuresis from left atrial receptors

1. In anaesthetized dogs, stimulation of atrial receptors after destruction of the pituitary gland results in a diuresis. This response was not abolished by the administration of bretylium tosylate and was also observed in a surgically denervated kidney. 2. The diuresis is qualitatively similar to that observed in anaesthetized dogs with intact pituitary glands. 3. It is concluded that the diuresis which results from stimulation of the left atrial receptors is mediated by a blood-borne agent which is not the antidiuretic hormone.

The effects of three beta-adrenoceptor blocking drugs on isolated preparations of skeletal and cardiac muscle

1 The effects of propranolol, oxprenolol and practolol on the isometric twitch responses to electrical stimulation of isolated diaphragm muscles from the rat and of isolated papillary muscles from the rabbit are described.2 Depression of the twitch responses of the diaphragm muscle was produced by propranolol (20 mug/ml), by oxprenolol (100 mug/ml) and by practolol (500 mug/ml).3 Depression of the twitch responses of the papillary muscles was produced by propranolol (20 mug/ml) by oxprenolol (100 mug/ml) and by practolol (200 mug/ml).4 No increase of twitch tension was produced by oxprenolol or practolol on either tissue.5 It is concluded that propranolol, oxprenolol and practolol produce negative inotropic actions on isolated cardiac muscle by a mechanism unrelated to blockade of beta-adrenoceptors and which occurs at doses which are well in excess of those doses required to produce beta-blockade.