Vagal activity is increased during intravenous isoprenaline infusion in man

Autonomic receptor systems in the failing and aging human heart: similarities and differences

Changes in autonomic receptor systems (alpha- and beta-adrenoceptors and muscarinic receptors) were compared in the aging and failing human heart. In both settings responsiveness of beta-adrenoceptors and all other receptor systems that evoke their effects via cyclic AMP accumulation was diminished. Muscarinic receptor function, on the other hand, was decreased in the aging, but unchanged in the failing heart; in contrast, G protein-coupled receptor kinase activity was increased in the failing, but unchanged in the aging heart. alpha-Adrenoceptor function was unchanged or slightly decreased in the failing heart. However, nothing is known on alpha-adrenoceptor changes in the aging heart. These results indicate that in the failing human heart all autonomic receptor systems appear to be altered in the direction to attenuate beta-adrenoceptor responses to sympathetic (over)stimulation while in the aging human heart autonomic receptor systems appear to be altered in a direction that protects the heart against too pronounced reduction in beta-adrenoceptor responsiveness.

Effects of atropine on human cardiac beta 1- and/or beta 2-adrenoceptor stimulation

The aim of this study was to find out whether, in humans, the increase in vagal tone accompanying cardiac beta-adrenoceptor (beta-AR) stimulation might be different dependent on beta1- or beta2-AR stimulation. For this purpose we studied, in six male healthy volunteers (aged 28+/-1 years), the effects of atropine infusion (0.15 microg/kg/min continuously) on increase in heart rate (HR) and contractility (determined as shortening of HR-corrected duration of electromechanical systole-QS2c) evoked by infusion of isoprenaline (3.5-35 ng/kg/min, increasing HR and QS2c via beta1-and beta2-AR), terbutaline (25-150 ng/kg/min, increasing HR and QS2c via beta2-AR), adrenaline (20-160 ng/kg/min, increasing HR via beta2-and QS2c via beta1-AR) and bicycle exercise in supine position (increasing HR and QS2c via beta1-AR). The three beta-AR agonists and exercise increased HR and shortened QS2c in a dose- or work-load-dependent manner, respectively. Atropine enhanced HR-increasing effects of all three beta-AR agonists and exercise; increases were larger for beta2-AR (terbutaline, adrenaline) mediated effects than for beta1-AR (exercise) mediated effects. Moreover, atropine enhanced beta-AR agonists-induced QS2c shortening; however, atropine effects on QS2c were markedly less pronounced than on HR. From the results we conclude that, in humans, beta1-and beta2-AR mediated stimulation evoked HR-increases are composed of two components: increases via direct beta-AR stimulation and simultaneously decreases via increase in vagal tone. In addition, beta-AR mediated increases in contractility are also dampened by simultaneous activation of vagal tone but to a lesser extent possibly because human ventricular myocardium is only sparsely parasympathetically innervated.

Propofol-nitrous oxide anesthesia enhances the heart rate response to intravenous isoproterenol infusion

Heart rate (HR) response to IV atropine is attenuated during propofol-nitrous oxide (N(2)O) anesthesia. We studied the effects of propofol-N(2)O anesthesia on isoproterenol-induced HR changes. The control group (n = 15) received no propofol and no N(2)O. Patients in the propofol-N(2)O group (n = 21) received IV propofol 2.5 mg/kg over 1 min followed by a continuous infusion of propofol 10 mg x kg(-1) x h(-1). After tracheal intubation, anesthesia was maintained with propofol 5 mg. kg(-1) x h(-1) and 67% N(2)O in oxygen. All patients in both groups received IV isoproterenol at incremental infusion rates (2.5, 5, 7.5, 10, 12.5, 15, and 17.5 ng x kg(-1) x min(-1) for 2 min at each dose) until HR increased more than 20 bpm from baseline values. At the end of each infusion period, hemodynamic data were collected. The HR response to isoproterenol 7.5 ng. kg(-1) x min(-1) was increased more in the propofol group than in the control group (20 +/- 5 versus 14 +/- 4 bpm; P < 0.05). During the isoproterenol infusion at 10 ng. kg(-1) x min(-1), HR increased by more than 20 bpm in all patients in the propofol group but in only 31% of patients in the control group (P < 0.0001). These results suggest that continuous isoproterenol infusion might be useful when a large dose of atropine is ineffective in restoring normal HR during propofol-N(2)O anesthesia.

IMPLICATIONS

We demonstrated that the heart rate response to IV isoproterenol infusion is enhanced during propofol-nitrous oxide anesthesia. This suggests that continuous isoproterenol infusion may be useful when a large dose of atropine is ineffective for restoration of normal heart rate in patients receiving propofol-nitrous oxide anesthesia.

The influence of Type A behavior pattern on the response to the panicogenic agent CCK-4

OBJECTIVES

Review of the literature equivocally suggests that subjects with Type A behavioral pattern (TABP) compared to subjects with Type B behavioral pattern display an increased sympathetic activity, a condition associated with sudden cardiac death. The objective of this study was to determine whether healthy subjects classified as Type A or Type B differed in their reactivity to the beta 1 and beta 2 receptor agonist isoproterenol and to the panicogenic agent cholecystokinin-tetrapeptide (CCK-4). By comparing reactivity to CCK-4 after pretreatment with placebo or propranolol, a beta 1 and beta 2 receptor antagonist, the role of the beta adrenergic system in the hypothesized increased response of Type A subjects to CCK-4 was also assessed.

METHODS

The study used a randomized, double-blind, placebo-controlled design. Twenty-seven Type A or B subjects were included in the study. The reactivity to isoproterenol was assessed with the CD25 of isoproterenol (i.e., the intravenous dose of isoproterenol necessary to increase the heart rate of 25 bpm). The panic symptom response and the cardiovascular response to bolus injection of 50 microg of CCK-4 was assessed in subjects pretreated with either propranolol or placebo infusions prior to the CCK-4 challenge. An additional group of subjects was recruited and these subjects received a placebo infusion pretreatment before an injection of placebo.

RESULTS

The CD25 was significantly greater in Type A subjects than in Type B subjects. No difference was found among the groups on behavioral sensitivity to the CCK-4 challenge. However, CCK-4-induced maximum increase in heart rate was greater in Type A subjects.

CONCLUSION

Our finding that Type A subjects exhibited greater CD25 of isoproterenol and greater increases in heart rate following CCK-4 administration compared to Type B subjects suggests that peripheral beta-receptor sensitivity may be increased in individuals with TABP.

Age, hypertension, and cardiac responses to beta-agonist in humans

OBJECTIVES

To evaluate the effects of hypertension on heart rate and left ventricular responses to beta-agonist in young and older subjects, as well as the modulating effect of the arterial baroreflex on these responses.

METHODS

Isoproterenol (INN, isoprenaline) alone was infused in 14 young normotensive subjects (mean age, 30 +/- 2 years), 18 older normotensive subjects (mean age, 60 +/- 2 years), 11 young hypertensive subjects (mean age, 36 +/- 1 years), and 17 older hypertensive subjects (mean age, 59 +/- 1 years); isoproterenol combined with ganglionic blockade (trimethaphan [INN, trimetaphan]) was administered to eight young normotensive subjects and eight young hypertensive subjects. Isoproterenol was infused at three to four incremental rates, each rate for 8 minutes. Left ventricular responses were assessed by echocardiography.

RESULTS

Isoproterenol caused similar increases in heart rate in all four groups. With ganglionic blockade, heart rate responses were enhanced but were similar in the young normotensive and hypertensive subjects. In young subjects, hypertension did not affect left ventricular responses to isoproterenol alone, whereas older hypertensive subjects showed some blunting of left ventricular responses compared with older normotensive subjects. With ganglionic blockade, young hypertensive subjects also showed mild blunting of left ventricular responses.

CONCLUSION

These results show that, in humans, hypertension does not lead to a decrease in chronotropic responses to infusion of the beta-agonist isoproterenol and causes only a modest decrease in left ventricular responses.

Isoprenaline and inducibility of atrioventricular nodal re-entrant tachycardia

OBJECTIVES

To examine the effect of isoprenaline on slow and fast pathway properties and tachycardia initiation.

DESIGN

Consecutive patients, prospective study.

SETTING

Referral centre for cardiology, academic hospital.

PATIENTS

24 patients suffering from common type atrioventricular nodal reentrant tachycardia (AVNRT).

INTERVENTIONS

Programmed electrical stimulation and radiofrequency catheter ablation of the slow pathway.

MEASUREMENTS AND MAIN RESULTS

AVNRT was induced before and after the administration of isoprenaline in nine patients (group 1), before isoprenaline only in five (group 2), and after isoprenaline only in 10 (group 3). The anterograde effective refractory period of the fast pathway was prolonged significantly during isoprenaline administration in group 1 (405 (31) v 335 (34) ms, p < 0.001) and shortened in group 2 (308 (57) v 324 (52) ms, p = 0.005). There was also significant shortening in group 3 (346 (85) v 395 (76) ms, p < 0.001). Isoprenaline administration did not result in a significant change of the anterograde effective refractory period of the slow pathway in groups 1 and 3, but eliminated slow pathway conduction in group 2. Isoprenaline significantly shortened the minimal and maximal atrial to His bundle conduction interval recording in response to each extrastimulus of the slow pathway (210 (24) v 267 (25) ms, p < 0.001 and 275 (25) v 328 (25) ms, p < 0.001, respectively) in group 1 and significantly prolonged these intervals (331 (34) v 274 (34) ms and 407 (33) v 351 (33) ms, respectively) in group 3. In all groups only minimal changes in the refractory period of the atrium occurred after isoprenaline administration. The effect of isoprenaline was also measured on the ventricular effective refractory period and on the minimal and maximal length of the ventriculoatrial (V2-A2) interval during ventricular pacing. Isoprenaline did not result in a significant change of the ventricular effective refractory period in groups 1 and 2 nor of the shortest and longest V2-A2 interval. In group 3, however, the ventricular effective refractory period and the shortest and longest V2-A2 interval shortened significantly after isoprenaline administration.

CONCLUSIONS

In group 1 isoprenaline did not affect inducibility of AVNRT because it prolonged the fast pathway refractory period without affecting slow pathway conduction. In group 2 isoprenaline shortened the fast pathway refractory period and appeared to abolish slow pathway conduction. Consequently, isoprenaline prevented induction of AVNRT. In group 3 isoprenaline facilitated induction of AVNRT. This effect seemed primarily to be the result of shortening of retrograde refractoriness of the fast pathway with prolongation of slow pathway anterograde conduction and refractory period.

Cardiac muscarinic receptors decrease with age. In vitro and in vivo studies

The M1 muscarinic receptor antagonist pirenzepine in low doses decreases resting heart rate; this effect declines with age (Poller, U., G. Nedelka, J. Radke, K. Pönicke, and O.-E. Brodde. 1997. J. Am. Coll. Cardiol. 29:187-193). To study possible mechanisms underlying this effect, we assessed (a) in six young (26 yr old) and six older volunteers (61 yr old), pirenzepine effects (0.32 and 0.64 mg intravenous [i.v.] bolus) on isoprenaline-induced heart rate increases; (b) in five heart transplant recipients, pirenzepine effects (0.05-10 mg i.v. bolus) on resting heart rate in the recipient's native and transplanted sinus nodes; and (c) in right atria from 39 patients of different ages (5 d-76 yr) undergoing open heart surgery, M2 muscarinic receptor density (by [3H]N-methyl-scopolamine binding) and adenylyl cyclase activity. (a) Pirenzepine at both doses decreased heart rate in young volunteers significantly more than in older volunteers; (b) pirenzepine (< 1 mg) decreased resting heart rate in the recipient's native but not transplanted sinus node; and (c) M2 receptor density and carbachol-induced inhibition of forskolin-stimulated adenylyl cyclase activity decreased significantly with the age of the patients. We conclude that pirenzepine decreases heart rate via inhibition of presynaptic M1 autoreceptors, thereby releasing endogenous acetylcholine, and that the heart rate-decreasing effect of acetylcholine declines with age because right atrial M2 receptor density and function decrease.

Functional role of beta2-adrenoceptors in the transplanted human heart

In the transplanted human heart, beta-adrenoceptor subtypes change with time after transplantation: beta1-adrenoceptors tend to decline, whereas beta2-adrenoceptors are upregulated. The aim of this study was to determine whether, in the transplanted human heart, stimulation of beta2-adrenoceptors can induce heart-rate increases. For this purpose, we assessed in eight heart-transplant recipients (mean posttransplant time: 932 days) the effects of infusion of graded doses of isoprenaline (3.5-35 ng/kg/min) 120 min after pretreatment with the beta1-adrenoceptor antagonist bisoprolol (10 mg p.o.; beta1-adrenoceptor occupancy approximately 80%; beta2-adrenoceptor occupancy <5%) on heart rate in the recipient's native (innervated) and transplanted (denervated) sinus nodes. Isoprenaline, acting under these conditions predominantly at beta2-adrenoceptors, increased heart rate both in the recipient's transplanted and native sinus nodes in a dose-dependent manner; at each dose, increases were significantly higher in the transplanted than in the native sinus node. ED20 values (dose to increase heart rate by 20 beats/min) in the transplanted sinus node were 22.2 +/- 1.8 ng/kg/min, and in the native, >35 ng/kg/min (p < 0.01). We conclude that in the transplanted human heart, beta2-adrenoceptor stimulation does evoke increases in heart rate. The enhanced response to isoprenaline in the transplanted sinus node could be caused by the upregulated beta2-adrenoceptors or by the fact that during isoprenaline infusion, vagal activity increases, thus blunting the response in the native (innervated) but not in the transplanted (denervated) sinus node.

Characterization of the factors that determine the effect of sympathetic stimulation on heart rate variability

Heart rate variability analysis has been used to derive indices of sympathetic tone. As different sympathetic stimuli may give rise to divergent changes in heart rate variability, this study was designed to characterize the factors responsible for these divergent responses. Twelve healthy subjects (7 males, age 24.8 +/- 3.1 years) were evaluated. Five-minute electrocardiographic recordings were obtained at baseline, following upright tilt, and during isoproterenol infusion (25 ng/kg per min) under control conditions and following parasympathetic blockade. Data were acquired during spontaneous respiration and when breathing was timed with a metronome (15 breaths/min). Under control conditions, both upright tilt and isoproterenol infusion resulted in significant decreases in the SD and MSSD from baseline values of 69 +/- 3 ms and 64 +/- 5 ms to 48 +/- 4 ms and 21 +/- 5 ms during tilt and 44 +/- 4 ms and 20 +/- 5 ms during isoproterenol infusion, respectively. LF power also significantly increased from 0.47 +/- 0.17 ln (beats/min)2 at baseline to 1.90 +/- 0.20. In (beats/min)2 and 1.34 +/- 0.18. In (beats/min)2 during tilt and isoproterenol infusion, respectively. No change in HF power was noted. Following parasympathetic blockade, all heart rate variability parameters were significantly decreased. No significant change from baseline in the SD, MSSD, or HF power was noted with either tilt or isoproterenol infusion. The LF power increased only with tilt from a baseline value of -3.17 +/- 0.17 in (beats/min)2 to -0.41 +/- 0.19 in (beats/min)2. Similar changes were noted during spontaneous respiration and metronome breathing. These findings demonstrate that the response of the sinus node to beta-adrenergic stimulation depends on the mode of stimulation. In addition, the associated level of parasympathetic tone affects the observed changes in heart rate variability that are associated with sympathetic stimulation.

Influence of beta-adrenergic and vagal activity on the effect of exogenous adenosine on supraventricular tachycardia termination

Adenosine, which binds to cell surface receptors and couples with guanosine triphosphate-binding inhibitory proteins (G(i)), is potent in terminating supraventricular tachycardia (SVT). However, whether the differences in autonomic tone will influence this effect remains unknown. This study was designed to investigate the role of beta-adrenergic and vagal activity on the action of adenosine. Forty patients with clinically documented SVT (22 with atrioventricular node reentrant tachycardia and 18 with atrioventricular reciprocating tachycardia) were divided into 4 groups with 10 patients in each group. In groups 1 and 2, adenosine was intravenously injected during the baseline state and during infusion of isoproterenol (2 and 4 microg/min, respectively). Group 2 patients received atropine (0.04 mg/kg) injection before isoproterenol infusion. In groups 3 and 4, intravenous injection of adenosine was given during the baseline state and after injection of atropine (0.02 and 0.04 mg/kg, respectively). Group 4 patients received propranolol (0.2 mg/kg) before atropine injection. The minimal dose of adenosine to terminate tachycardia during isoproterenol infusion of 2 microg/min was greater than that during the baseline state in both groups 1 and 2. The minimal dose of adenosine during isoproterenol infusion with 4 microg/min was higher than that with 2 microg/min in group 2, but not in group 1 patients. In both groups 3 and 4, the minimal dose of adenosine required to terminate tachycardia during atropine injection with 0.02 mg/kg was greater than that during the baseline state. The minimal effective dose of adenosine during atropine injection with 0.04 mg/kg was higher than that with 0.02 mg/kg in group 4, but not in group 3 patients. In conclusion, either limb of the autonomic nervous system may modulate the adenosine dosage required for termination of SVT. Patients taking drugs such as beta blockers or vagolytic agents may need alterations in the dose of adenosine for therapy.

Syncope and the autonomic nervous system

The autonomic nervous system plays a central role in the maintenance of hemodynamic stability. Dysfunction of this complex regulatory system can lead to the development of loss of consciousness. This article summarizes our current understanding of the role of the autonomic nervous system in maintaining a stable blood pressure and heart rate under normal and abnormal physiologic conditions. The role of baroreceptors, mechanoreceptors, chemoreceptors, vascular reactivity, and the interaction of these sensor systems with the central nervous system as a whole are reviewed. Current concepts related to the mechanisms of unexplained syncope and the "state-of-the-art" diagnostic and treatment options are also discussed.

Vagal modulation of RR intervals during head-up tilt and the infusion of isoproterenol

The objective of this study was to characterize the autonomic effects of 2 interventions, head-up tilt and isoproterenol infusion, which alter autonomic balance by different mechanisms but produce the same RR intervals. We compared the effect of head-up tilt with the effect of isoproterenol on autonomic balance as measured by power spectral analysis of RR variability. Fifteen normal subjects had baseline measurements and then underwent head-up tilt. After return to baseline supine values, isoproterenol was infused at a rate of 1 microgram/min (low-dose isoproterenol), which was then increased to 2.1 +/- 0.5 microgram/min (high-dose isoproterenol). All subjects underwent a second tilt during high-dose isoproterenol, and 9 subjects completed this second tilt study. During the experiment, normal RR intervals were recorded and 5-minute segments were used to calculate power spectra. High-frequency (HF) power (0.15 to 0.40 Hz) was used as a measure of vagal activity. The effects of head-up tilt were compared with the effects of low-dose isoproterenol. Despite nearly identical mean RR intervals (784 ms with tilt vs 792 ms with low-dose isoproterenol, p = NS), there was significantly (p < 0.05) less HF power during head-up tilt in the drug-free state (172 ms2) than during low-dose isoproterenol in the supine position (307 ms2). A second head-up tilt was performed during the infusion of high-dose isoproterenol. During high-dose isoproterenol, tilt caused a decrease in RR intervals (from 573 to 491 ms; p < 0.01) and a decrease in HF power (from 68 to 28 ms2; p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of physiologic and pharmacologic adrenergic stimulation on heart rate variability

OBJECTIVES

The aim of this study was to evaluate and compare the effects of physiologic and pharmacologic sympathetic stimulation on time and frequency domain indexes of heart rate variability.

BACKGROUND

Measurements of heart rate variability have been used as indexes of sympathetic tone. To date, the effects of circulating catecholamines on heart rate variability have not been evaluated.

METHODS

Fourteen normal subjects (eight men, six women, mean [+/- SD] age 28.5 +/- 4.8 years) were evaluated. Five-minute electrocardiographic recordings were obtained in triplicate after physiologic and pharmacologic sympathetic stimulation: during upright tilt, after maximal exercise, during epinephrine and isoproterenol infusions at 50 ng/kg body weight per min, during beta-adrenergic blockade and during combined beta-adrenergic and parasympathetic blockade.

RESULTS

Beta-adrenergic stimulation resulted in a significant decrease in time domain measures of heart rate variability. The frequency domain indexes showed variable responses, depending on the individual stimulus. Tilt caused an increase in low frequency power and in the ratio of low to high frequency power. These changes were not necessarily observed with other conditions of beta-adrenergic stimulation. Double blockade suppressed baseline heart rate variability, but beta-adrenergic blockade had no significant effect. Time domain measures of heart rate variability demonstrated excellent reproducibility over the three recordings, but the frequency domain variables demonstrated fair to excellent reproducibility.

CONCLUSIONS

These findings suggest that different modes of beta-adrenergic stimulation may result in divergent heart rate variability responses. Thus, current heart rate variability techniques cannot be used as general indexes of "sympathetic" tone. Studies utilizing heart rate variability to quantify sympathetic tone need to consider this.

Autonomic control of cardiovascular function: clinical evaluation in health and disease

The high- and low-pressure baroreceptor reflexes are integral to the control of blood pressure by the autonomic nervous system. Tests of the integrity of these baroreflexes make it possible to identify the site of autonomic dysfunction in patients with orthostatic hypotension. Clinical characteristics and typical results of autonomic testing in patients with autonomic failure, with carotid sinus hypersensitivity, and with hyperadrenergic autonomic dysfunction are described in this review.

Assessment of effects of autonomic stimulation and blockade on the signal-averaged electrocardiogram

BACKGROUND

Signal-averaged ECG is a noninvasive test designed to detect "late potentials." The effects of alterations in autonomic tone on the signal-averaged ECG have not been evaluated systematically.

METHODS AND RESULTS

The effects of autonomic stimulation and blockade on the signal-averaged ECG were evaluated in 14 healthy subjects (8 men and 6 women; age, 28.5 +/- 4.8 years) on 2 separate days. The signal-averaged ECG was recorded at baseline and after physiological and pharmacologic beta-adrenergic stimulation (tilt, exercise, and epinephrine and isoproterenol infusions), sequential and combined beta-adrenergic and parasympathetic blockade, and alpha-adrenergic stimulation before and after parasympathetic blockade. Analysis was performed with a bidirectional filter (40-Hz high-pass). Significant changes in the signal-averaged QRS duration from baseline (105.1 +/- 12.0 milliseconds) were noted with tilt (96.8 +/- 8.8 milliseconds), tilt after double blockade (97.5 +/- 9.0 milliseconds), epinephrine (110.5 +/- 11.8 milliseconds), and isoproterenol (99.6 +/- 12.6 milliseconds). Changes in the root-mean-square voltage of the terminal 40 milliseconds and the low-amplitude (< 40 microV) signal duration paralleled the changes in the QRS duration.

CONCLUSIONS

The signal-averaged ECG does not measure only "fixed" parameters but rather is altered under a variety of physiological and pharmacologic conditions. Upright tilt leads to shortening of the QRS duration before and after autonomic blockade; thus, the decrease in QRS duration with tilt may be related to factors other than changes in autonomic tone. These findings have implications for interpretation of the results of signal-averaged ECG.

Beta-adrenoceptors in cardiac disease

The human heart contains both beta 1 and beta 2-adrenoceptors; both mediate positive inotropic and chronotropic effects. In chronic heart failure, beta-adrenoceptor number is reduced, presumably, by down-regulation by endogenous noradrenaline which is elevated due to increased sympathetic activity. Since the human heart contains only a few spare receptors for beta-adrenoceptor-mediated positive inotropic effects and the amount of spare receptors declines in chronic heart failure, it is not surprising that the reduced beta-adrenoceptor number is accompanied by decreased contractile responses to beta-adrenoceptor agonists (including endogenous catecholamines), and the extent of decrease in maximal inotropic response is more pronounced as the disease becomes more advanced. Moreover, in chronic heart failure myocardial G(i)-protein, which inhibits cAMP formation, is increased, which might further contribute to the reduction in beta-adrenoceptor-mediated effects. It appears that, at present, the best therapy for severe heart failure is a successful heart transplant, since in the transplanted heart beta-adrenoceptor number and function seems to be normalized. Moreover, the data currently available do not suggest any development of super- or subsensitivity of postsynaptic cardiac beta-adrenoceptors in the transplanted human heart.

Plasma catecholamines--analytical challenges and physiological limitations

Catecholamines in plasma may be measured to assess sympathoadrenal activity. Numerous assay methodologies have been published, illustrating the fact that there are many analytical problems. Different methodologies are discussed briefly. A plea for better validation, especially with regard to specificity (which should not be confused with sensitivity or reproducibility), is made. Plasma NA is a frequently used marker for sympathetic nerve activity in humans, but the data obtained are often misinterpreted due to lack of appreciation of the physiological determinants of the NA concentration measured. NA overflow from an organ gives a good reflection of nerve activity in that organ. However, sympathetic nerve activity is highly differentiated, particularly during stress, and conventional plasma NA levels (usually forearm venous samples) cannot be taken as an indication of 'sympathetic tone' in the whole individual. NA is rapidly removed from plasma, resulting in meaningless net veno-arterial concentration differences over organs unless its removal from arterial plasma is taken into account. In the forearm, for example, 40-50% of catecholamines are removed during one passage; about half of the NA in a venous sample is derived from the arm and half from the rest of the body. Therefore, conventional venous sampling overemphasizes local (mainly skeletal muscle) nerve activity. Whole-body sympathetic nerve activity may be monitored in arterial or mixed venous (i.e. pulmonary arterial) samples, which reflect NA overflow from all organs in the body. NA levels are determined both by overflow to plasma and clearance from plasma. NA turnover studies with 3H-NA infusions may be needed to assess clearance, but the simpler concentration measurements usually yield adequate information if the sampling site is relevant. NA overflow from an organ can be assessed (using 3H-NA or ADR as a marker for NA extraction in the organ) and provides valuable information on local sympathetic activity. Mental stress elicits marked circulatory responses, with mainly cardiorenal sympathetic activation and minor elevations of conventional venous plasma NA levels, thus illustrating the differentiated firing pattern of the sympathetic nerves. Circulating ADR is less important than neurogenic mechanisms in the responses to stress. Concentration-effect studies for infused catecholamines may be used for receptor sensitivity studies in vivo, but reflexogenic contributions to responses need to be determined. However, prejunctional mechanisms cannot be assessed without knowledge of the nerve activity present; for example, ADR infusion leads to increased nerve activity. When correctly sampled, measured and interpreted, plasma catecholamines can yield very valuable information on sympathoadrenal activity.

Beta-receptor responsiveness after desipramine treatment

To examine whether a tricyclic antidepressant affects the functional response to a beta-receptor agonist in man, the response of heart rate, blood pressure, and plasma cAMP to isoproterenol was measured in 14 normal controls taking 75 mg desipramine daily. Desipramine significantly increased the bolus dose of isoproterenol needed to increase heart rate by 25 bpm at 14-30 days but not at 3-8 days. During infusions of isoproterenol, the increase in systolic blood pressure was blunted at both 3-8 days and 14-30 days, while the decrease in diastolic blood pressure was unaffected. Blood pressure findings were not affected by preadministration of bethanechol. In ten controls, isoproterenol infusions increased plasma cAMP, but this was unaffected by desipramine treatment. These findings suggest a decrease in the functional response of beta 1, but not beta 2, receptors after treatment with desipramine.

Accentuated vagal antagonism of beta-adrenergic effects on ventricular repolarization. Evidence of weaker antagonism in hostile type A men

BACKGROUND

Prior research has suggested a weaker parasympathetic antagonism of sympathetic effects on the heart in type A (coronary-prone) men. To confirm this phenomenon and extend our understanding of it, we investigated the effects of prior muscarinic blockade on the electrocardiogram T wave and other cardiovascular and neuroendocrine responses to isoproterenol in type A and type B (non-coronary-prone) men.

METHODS AND RESULTS

Responses to two 5-minute intravenous isoproterenol infusions (0.01 micrograms/kg/min and 0.02 micrograms/kg/min) were evaluated in six type A and six type B men after pretreatment with either dextrose placebo or atropine (1.2 mg). Atropine significantly potentiated T wave attenuation in the recovery period after isoproterenol infusion (0.30 +/- 0.07 mV) compared with placebo (0.54 +/- 0.09 mV, p less than 0.001). Atropine also potentiated the heart rate increase to isoproterenol (39 +/- 3 beats per minute versus 20 +/- 2 beats per minute after placebo). Atropine enhanced decreases in systolic, diastolic, and mean arterial pressures as well as pulse pressure to isoproterenol. Atropine enhancement of many of these responses was increased among subjects with high scores on various hostility/anger scales. Isoproterenol alone produced greater T wave attenuation in type A than in type B men. However, atropine enhancement of T wave attenuation and blood pressure falls by isoproterenol was present only in type B men.

CONCLUSIONS

These findings indicate that there is accentuated parasympathetic antagonism of T wave attenuation and blood pressure responses induced by beta-adrenergic stimulation. Relative weakness of this antagonism of sympathetic effects on the heart in hostile type A individuals may contribute to their higher coronary disease risk.

Beta-adrenoceptors in the transplanted human heart: unaltered beta-adrenoceptor density, but increased proportion of beta 2-adrenoceptors with increasing posttransplant time

To study whether in the (denervated) transplanted human heart beta-adrenoceptors are up-regulated we determined in right ventricular endomyocardial biopsies (weighing 2-7 mg) of 8 heart transplant recipients beta-adrenoceptor density and beta 1:beta 2-adrenoceptor ratio in weekly, later monthly intervals for 6-18 posttransplant months. beta-adrenoceptor density was assessed by 1-2 concentration (-)-[125I]-iodocyanopindolol (ICYP, 200 and 150 pmol/l) binding, the beta 1:beta 2-adrenoceptor ratio by a beta 1-adrenoceptor saturating concentration of the selective beta 1-adrenoceptor antagonist CGP 20712 A (500 nmol/l). In biopsies taken from the donor hearts immediately before transplantation beta-adrenoceptor density was 64.5 +/- 6.3 fmol ICYP bound/mg protein (n = 5), the beta 1:beta 2-adrenoceptor ratio 81.2 +/- 2.2:18.8 +/- 2.2%. beta-Adrenoceptor density did not change during the whole period investigated; it was in the first 9 posttransplant weeks 73.1 +/- 8.3 (n = 22) and after 4-18 months 74.2 +/- 7.6 fmol/mg protein (n = 16). In contrast, with increasing posttransplant time beta 1:beta 2-adrenoceptor ratio was shifted towards beta 2-adrenoceptors: in the first 9 weeks it was 80:20% (n = 17), after 16 weeks 75:25% (n = 10) and after 25-80 weeks 65:35% (n = 7). We conclude that in the first 18 posttransplant months beta-adrenoceptors in the transplanted human heart are not up-regulated. There is, however, a gradual decline in beta 1- and increase in beta 2-adrenoceptors with increasing posttransplant time. As noradrenaline acts in the human heart solely at beta 1-adrenoceptors, while adrenaline activates both beta 1- and beta 2-adrenoceptors with about the same potency, with increasing posttransplant time (and increasing proportion of beta 2-adrenoceptors) adrenaline may play an important role for regulation of contractility and/or heart rate in heart transplant recipients.

Are there beta-adrenergic receptor response differences between racial groups?

Beta-adrenergic receptor (BAR) agonists and antagonists are among the most widely used drugs in America today. This review highlights the importance of cross-racial studies in evaluating BAR effects because heterogeneity in pharmacologic response among humans exists. Clinicians and basic scientists in the drug development industry need to acknowledge racial differences in antihypertensive drug responses and prevent various racial groups from being underrepresented in clinical trials. This article reviews the influence of race on BAR responses with emphasis on blacks and whites. Information from patients with hypertension, healthy volunteers, and in vitro studies is presented.

Effects of isoproterenol on T-wave amplitude and heart rate: a dose-response study

This report examines the hypothesis that electrocardiographic T-wave amplitude is sensitive to graded increases in beta-sympathetic stimulation of the heart. Beta-adrenergic activity was manipulated pharmacologically in 9 healthy men by bolus infusion of isoproterenol in each of six doses: 0.1, 0.25, 0.5, 1.0, 2.0, and 4.0 micrograms. Results indicated that elevations in heart rate above placebo values increased as a linear function of isoproterenol dose. In contrast, the dose-response curve for T-wave amplitude was best described by a quadratic function: an initial reduction in T-wave amplitude at low levels of isoproterenol infusion was followed by a significant reversal of this effect at higher doses. Comparison of the heart rate and T-wave amplitude data points to limitations in the use of the latter as an index of beta-adrenergic activity. One of several possible explanations for the T-wave results would entail a mechanism that preserves ventricular function at high levels of beta-sympathetic stimulation.

Comparison of hypokalaemic, electrocardiographic and haemodynamic responses to inhaled isoprenaline and salbutamol in young and elderly subjects

The purpose of the study was to compare beta-adrenoceptor responsiveness to salbutamol (beta-2 selective agonist) and isoprenaline (non-selective) in young (n = 10, age 23 y) and elderly (n = 7, age 71 y) subjects. Subjects were given cumulative doubling doses of inhaled isoprenaline or salbutamol (500-4000 micrograms), and placebo, in a single-blind randomised cross-over design. Plasma potassium, electrocardiographic (R-R, T-wave, Q-Tc) and blood pressure responses were measured at baseline and at each dose step. There were no difference between baseline values for each of the three study days within each group of subjects. Hypokalaemia was significantly greater in response to salbutamol compared with isoprenaline in both the young (as change from baseline): -0.61 versus -0.10 mmol.l-1: and in the elderly: -0.68 versus -0.20 mmol.l-1. There were no differences between young and elderly responses. T-wave amplitude fell significantly in response to isoprenaline and salbutamol, although this effect was progressively attenuated with increasing doses of isoprenaline. Maximum T-wave response (change from baseline) was greater with salbutamol than isoprenaline in the young: -0.22 versus -0.11 mV: and in the elderly: -0.17 versus -0.08 mV, and there were no differences between the two groups. There were no differences between the effects of isoprenaline and salbutamol on Q-Tc prolongation or heart rate. Chronotropic responses to salbutamol were greater in the elderly: 39 versus 24 beats.min-1. There were larger increases in SBP with isoprenaline in both groups. Falls in DBP in response to isoprenaline and salbutamol were significantly greater in the elderly.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison between isoprenaline infusions and bolus injections to assess beta-adrenoceptor function in man, with special reference to cardiac contractility and the influence of autonomic reflexes

The present study was performed to characterize cardiovascular responses to isoprenaline and the influence of autonomic reflexes on these responses. Nine healthy volunteers received infusions and bolus injections of isoprenaline before and after 'autonomic blockade' produced by intravenous atropine 0.04 mg kg-1 and clonidine 300 micrograms. Heart rate, blood pressures, systolic time intervals and various echocardiographic measures of cardiac contractility were registered. No significant differences in responsiveness to isoprenaline were seen when infusions were repeated on the same day without 'autonomic blockade'. After 'blockade', delta responses at 1 nmol l-1 isoprenaline (infusions) were increased for diastolic blood pressure and decreased for systolic blood pressure and stroke volume. Bolus injections of 2 micrograms isoprenaline caused enhanced delta responses after 'autonomic blockade' of diastolic blood pressure, left ventricular diameter in systole, ventricular circumferential fibre shortening, mean posterior wall velocity (Vmean PW), stroke volume, systemic vascular resistance, electromechanical systole (QS2) and pre-ejection period. Systolic blood pressure decreased, in contrast to a small increase without 'blockade'. These findings are explained by differences in haemodynamic effects of isoprenaline and by the dependence of responses on reflexes when isoprenaline is administered in different ways. When heart rate was increased by bolus doses of atropine, in the presence of beta-blockade (propranolol), pre-ejection period and left ventricular diameter in systole were unaffected, and Vmean PW and ventricular circumferential fibre shortening showed only small increases (compared with alterations induced by isoprenaline). However, left ventricular ejection time, QS2 and ejection time (by echocardiography), were markedly dependent on heart rate alterations. Thus, pre-ejection period, left ventricular diameter in systole Vmean PW and ventricular circumferential fibre shortening are parameters which can be useful in order to evaluate cardiac beta-adrenoceptor sensitivity in vivo in man.

Chronic beta 1-adrenoceptor antagonist treatment sensitizes beta 2-adrenoceptors, but desensitizes M2-muscarinic receptors in the human right atrium

1. In 64 patients undergoing coronary artery bypass grafting the effects of chronic beta 1-adrenoceptor antagonist (metoprolol, atenolol, bisoprolol) treatment on right atrial beta-adrenoceptor and muscarinic M2-receptor number and functional responsiveness were investigated. 2. The beta 1-adrenoceptor antagonists increased right atrial beta 1-adrenoceptor number, did not affect beta 2-adrenoceptor number, and decreased muscarinic M2-receptor number. 3. Concomitantly, activation of right atrial adenylate cyclase by 10 microM GTP, 10 microM isoprenaline and 1 microM forskolin was enhanced and inhibition by 100 microM carbachol was diminished. 4. On isolated, electrically driven right atria the beta 1-adrenoceptor-mediated positive inotropic effect of noradrenaline was - even with beta 1-adrenoceptor number increased - not altered, while the beta 2-adrenoceptor-mediated effect of procaterol was markedly enhanced. However, the carbachol-induced negative inotropic effect was decreased. 5. It is concluded that chronic beta 1-adrenoceptor antagonist treatment increases beta 1-adrenoceptor number and concomitantly sensitizes beta 2-adrenoceptor function, but desensitizes muscarinic M2-receptor function in the human heart.

Comparison of hypersensitivity to adrenergic stimulation after abrupt withdrawal of propranolol and nadolol: influence of half-life differences

To test whether a beta-blocker's pharmacokinetics has an effect on the development of the beta-blocker withdrawal syndrome, we compared response to exercise and isoproterenol after abrupt withdrawal of two beta-blockers, one with a short half-life (propranolol) and one with a long half-life (nadolol). Eight normal males participated in the randomized cross-over trial. They took propranolol, 40 mg every 6 hours for 6 days, and nadolol, 80 mg daily for 7 days. Exercise- and isoproterenol-induced changes in heart rate and pulse pressure were significantly less than control values during treatment with both drugs (p less than 0.05). Significant hypersensitivity of heart rate response was seen on days 2, 3, and 7 after propranolol withdrawal (p less than 0.05) and on day 6 after nadolol withdrawal (p less than 0.005). Hypersensitive responses to isoproterenol were seen after propranolol withdrawal; no evidence of hypersensitivity to isoproterenol was seen after nadolol withdrawal. The area under the heart rate response, time curve after nadolol withdrawal was significantly less than that after propranolol withdrawal (20.3 beats/min.day vs 44.9 beats/min.day, respectively, (p less than 0.05). There was a significant inverse correlation between the degree of hypersensitivity and nadolol pharmacokinetic half-life (r = -0.80, p less than 0.02). We conclude that abrupt withdrawal of a beta-blocker with a long half-life may protect against the development of the beta-blocker withdrawal syndrome.

Effects of isoproterenol in panic disorder patients after antidepressant treatment

Fourteen panic disorder patients were infused with isoproterenol both before and after treatment with tricyclic antidepressants. Thirteen patients had panic attacks before treatment, and only 2 after treatment. There was a significant decrease in anxiety as measured by a Panic Description Scale during posttreatment infusions, and patients were able to tolerate longer infusions posttreatment. Isoproterenol increased systolic blood pressure and decreased diastolic blood pressure in both untreated patients and in 10 normal controls. Tricyclic antidepressant treatment of panic disorder patients abolished isoproterenol-induced increases in systolic blood pressure, but did not affect diastolic blood pressure response. These changes are consistent with a decrease in sensitivity to beta-1 receptor stimulation after treatment with tricyclic antidepressants.

On the physiologic role of beta-2 adrenoceptors in the human heart: in vitro and in vivo studies

To study the physiologic role of human myocardial beta-2 adrenoceptors, the beta adrenoceptor subtype(s) involved in the effects of catecholamines in vitro on the force of contraction and in vivo on heart rate were characterized. In vitro on both isolated electrically driven right and left atrial and left papillary muscle preparations, isoprenaline and adrenaline caused positive inotropic effects via beta-1 and beta-2 adrenoceptor stimulation. In the atria both beta-1 and beta-2 adrenoceptor stimulation increased contractile force to a maximum; in left papillary muscle, however, only beta-1 adrenoceptor stimulation maximally increased contractile force, whereas beta-2 adrenoceptor stimulation caused only submaximal increases. Noradrenaline, on the other hand, caused a positive inotropic effect nearly exclusively via atrial and ventricular beta-1 adrenoceptor stimulation. In vivo in 10 healthy volunteers isoprenaline-induced tachycardia was antagonized with equal potency by the beta-2 adrenoceptor-selective antagonist ICI 118,551 and the beta-1 adrenoceptor-selective antagonist bisoprolol indicating that it is mediated by cardiac beta-1 and beta-2 adrenoceptor stimulation to about the same degree. In contrast, exercise-induced tachycardia (that is mediated mainly by noradrenaline released from the neurons) was antagonized only by bisoprolol but not by ICI 118,551. It is concluded that in humans under normal physiologic conditions contractility and/or heart rate is regulated only by cardiac beta-1 adrenoceptors. In situations of stress, however, when large amounts of adrenaline are released from the adrenal medulla, stimulation of cardiac beta-2 adrenoceptors could contribute to additional increases in contractility, heart rate, or both.

Agonist-induced desensitization of beta-adrenoceptor function in humans. Subtype-selective reduction in beta 1- or beta 2-adrenoceptor-mediated physiological effects by xamoterol or procaterol

We investigated the effects of beta 2- (procaterol 2 x 50 micrograms/day for 9 days) and beta 1- (xamoterol 2 x 200 mg/day for 14 days) adrenoceptor agonists on lymphocyte beta 2-adrenoceptor density and beta 1- and beta 2-adrenoceptor in vivo function (assessed as isoprenaline-infusion-evoked hemodynamic effects and exercise-induced tachycardia) in healthy volunteers. Procaterol decreased lymphocyte beta 2-adrenoceptor density and all beta 2-adrenoceptor-mediated in vivo effects but did not affect beta 1-adrenoceptor-mediated in vivo effects. In contrast, xamoterol neither affected lymphocyte beta 2-adrenoceptors nor beta 2-adrenoceptor-mediated in vivo effects but decreased beta 1-adrenoceptor-mediated in vivo effects. It is concluded that in humans, generally long-term application of beta 1- or beta 2-adrenoceptor agonists causes desensitization of beta-adrenoceptor function but in a beta-adrenoceptor subtype-selective fashion.

Intracoronary injections of salbutamol demonstrate the presence of functional beta 2-adrenoceptors in the human heart

To demonstrate the presence of functional cardiac beta 2-adrenoceptors in man, we studied the responses to intracoronary injections of salbutamol in three groups of six patients. We injected salbutamol, a selective beta 2-adrenoceptor agonist, into the right coronary artery to avoid peripheral vasodilator action and to stimulate the sinoatrial node directly. Salbutamol injections caused a sinus tachycardia. The same doses of salbutamol injected into the aortic root caused no change in heart rate, ruling out a systemic effect. The mean dose required to cause an increase in heart rate of 30 beats/min (IHR30) was 2.6 micrograms in the first group of six patients. In 12 other patients salbutamol was given after beta-blockade to confirm the beta 2-selectivity of the responses. Doses of practolol (beta 1-selective blockade) and of propranolol (beta 1- and beta 2-blockade) that had equal beta 1-blocking activity were used. In six patients who were given practolol, the mean IHR30 dose was 2.1 micrograms. In six patients who were given propranolol, the mean IHR30 dose was significantly greater at 64 micrograms (p less than 0.001, practolol vs. propranolol). This study demonstrates that direct cardiac beta 2-adrenoceptor stimulation in man has a positive chronotropic effect.

Beta-adrenergic receptor responsiveness to isoprenaline in humans: concentration-effect, as compared with dose-effect evaluation and influence of autonomic reflexes

1. Different techniques of assessing beta-adrenoceptor sensitivity in vivo, by use of i.v. infusions or bolus injections of isoprenaline (ISO), were compared in healthy volunteers. The importance of autonomic reflexes for responses to ISO was evaluated by studying the influence of 'autonomic blockade' by atropine and clonidine, which antagonize muscarinic effects and reduce sympathetic activity, respectively. Estimates of in vivo responsiveness to ISO were compared with parameters reflecting beta 2-adrenoceptor function in vitro in lymphocytes. 2. Heart rate responses to infused ISO were not significantly altered by 'autonomic blockade' when evaluated from concentration-effect curves. When related to the infused dose of ISO, however, sensitivity was artefactually increased (P less than 0.05), as the plasma concentrations of ISO were 40% higher after atropine and clonidine. Heart rate responses to bolus injections of ISO were attenuated (P less than 0.05) by 'autonomic blockade', suggesting that facilitatory reflexes contribute to these non-steady state responses. Intersubject variations in heart rate responsiveness to ISO were greater than the intrasubject variability caused by counterregulatory reflexes. 3. 'Autonomic blockade' lowered venous plasma noradrenaline at rest. The noradrenaline response to ISO infusion was attenuated and the diastolic blood pressure response enhanced, indicating that a counterregulatory vasoconstrictor reflex normally is activated by ISO-induced vasodilatation. The plasma cyclic AMP response to ISO, on the other hand, was unaffected by atropine and clonidine and reflects beta 2-adrenoceptor responsiveness in vivo. 4. In vitro data for beta-adrenoceptor binding sites (Bmax;[125I]-IHYP binding) and cyclic AMP responses to ISO in lymphocytes correlated with DBP and noradrenaline responses to infused ISO. No correlations were found between in vitro data and heart rate, plasma cyclic AMP or plasma glycerol responses to infused ISO in vivo. 5. During prolonged ISO infusions (in six other healthy subjects) physiological responses reached greater than 90% of their steady state level after 8 min, but no definite steady state level could be defined for the plasma concentration of ISO during 40 min of infusion. 6. The ISO infusion test showed a good reproducibility, especially when repeated on the same day. Evaluation of plasma concentration-effect relationships increase the precision of the ISO infusion test as confounding inter- and intra-individual variations in ISO concentrations (as caused by e.g. autonomic blockade) will be taken into account.(ABSTRACT TRUNCATED AT 400 WORDS)

Racial differences in drug response: isoproterenol effects on heart rate in healthy males

It was the purpose of this study to investigate racial alterations in beta-adrenoceptor response. Two groups of healthy, male volunteers gave their consent. There were eight black Americans (mean age, 26.1 +/- 2.5 years) and eight white/Caucasian Americans (mean age 24.4 +/- 1.8 years). Each subject underwent an isoproterenol sensitivity test. There was a significant (P less than 0.05) decrease in the ratio of Emax to ED50 in the white group (25.3 +/- 6.4) compared with the black group (37.1 +/- 12.4). Over the dose range of 0.1 to 1.0 micrograms there was a significant increase in response at both the 0.25- and the 0.5-microgram dose (P less than 0.05), with the black American group appearing to respond with a greater rate of rise in heart rate following the initial doses.

Role of beta 1-receptors and vagal tone in cardiac inotropic and chronotropic responses to a beta 2-agonist in humans

To assess the contribution of cardiac beta 2-receptors in the cardiac inotropic and chronotropic responses to a beta 2 agonist, terbutaline was infused (0.2 and 0.4 micrograms/kg/min), alone or after pretreatment with either oral atenolol 50 mg or atropine 0.04 mg/kg i.v. or both in six healthy subjects with a multiple crossover design. Terbutaline 0.2 micrograms/kg/min increased heart rate by 15 +/- 2 beats/min, and this response doubled (to 29 +/- 3 beats/min) when the terbutaline infusion followed atropine pretreatment, whereas atenolol pretreatment had no significant effect. Heart rate increased by 44 +/- 2 beats/min in response to terbutaline 0.4 micrograms/kg/min. This response was not affected by atropine. Pretreatment with atenolol diminished the chronotropic response to the higher dose of terbutaline to 27 +/- 4 beats/min. The inotropic response (i.e., changes in pressure: volume ratio) to terbutaline 0.2 micrograms/kg/min was potentiated by atropine (from 1.6 +/- 0.3 to 3.4 +/- 0.8 mm Hg/ml), whereas atenolol pretreatment had no effect. At the higher dose of terbutaline, atropine pretreatment had no additional effect, whereas atenolol decreased the rise in pressure: volume ratio from 6.0 +/- 1.4 to 2.6 +/- 1.0 mm Hg/ml. The results with atenolol pretreatment indicate that cardiac beta 1 responses are associated with the higher dose of terbutaline, either through direct beta 1 stimulation or indirectly from presynaptic beta 2 activity. The atropine data show that vagal tone actually increased during the terbutaline infusions, blunting the cardiac effects. The results of the present study support the existence of functional chronotropic, as well as inotropic, beta 2 receptors in the healthy human heart.

Cardiac electrophysiologic and hemodynamic correlates of neurally mediated syncope

This study assessed the temporal relation of RR interval, AH interval and systemic blood pressure changes during induced symptomatic bradycardia-hypotension episodes in 14 patients with recurrent syncope suspected of being neurally mediated. Upright tilt with isoproterenol reproduced symptoms in 9 of 14 patients (positive response) and was negative in 5 of 14 (negative response). Isoproterenol alone shortened supine RR intervals in all patients. With tilt, however, isoproterenol prolonged RR intervals in those with positive results (supine 519 +/- 124 ms vs tilt 845 +/- 212 ms, p less than 0.005) while further shortening RR intervals among negative responders (supine 436 +/- 90 ms vs tilt 377 +/- 82 ms, p less than 0.05). Similarly, tilt with isoproterenol prolonged AH intervals in patients with positive responses despite RR prolongation, while shortening AH in negative responders. Additionally, with combined tilt and isoproterenol, systemic arterial pressure decreased significantly in patients with positive responses (systolic 99 +/- 13 vs 57 +/- 13 mm Hg, p less than 0.001, diastolic 62 +/- 17 vs 28 +/- 9 mm Hg, p less than 0.001) but not in patients with negative responses. Further, onset of hypotension (42 +/- 14 seconds after tilt) preceded onset of RR interval prolongation (52 +/- 23 seconds after tilt). Syncope (142 +/- 72 seconds after tilt) coincided closely with nadir of systemic pressure (136 +/- 74 seconds) and both tended to precede maximum RR prolongation (152 +/- 87 seconds). Thus, the bradycardia and hypotension associated with neurally mediated syncope exhibit characteristic but distinctly different time courses, with arterial pressure changes developing earlier and coinciding more closely with symptom development.

Cardioselectivity of cetamolol compared with atenolol and nadolol

The selectivity of the beta-adrenoceptor blockade produced by single oral doses of cetamolol, atenolol, and nadolol was compared in normal male subjects. Study 1 established the dose at which each drug provides equivalent beta-1 blockade. Beta-1 blockade was estimated using the degree of inhibition of the increased heart rate (HR) response to graded exercise. Cetamolol (30 mg), atenolol (100 mg), and nadolol (80 mg) all attenuated the HR response to a comparable extent. This result established that the dose ratio of cetamolol:atenolol:nadolol of 1.00:3.33:2.67 provides equipotent beta-1 blockade. This ratio of doses was used in Studies 2 and 3 to evaluate the antagonism of beta-2-mediated responses to titrated doses of intravenous isoproterenol (ISO) by low and high doses of each drug. Beta-2 blockade was assessed using the attenuation of ISO-induced reductions in diastolic blood pressure (DBP) in Study 2 and ISO-induced increases in specific airway conductance (sGAW) in Study 3. For within drug comparisons, antagonism of the HR increase induced by ISO (a response mediated by both beta-1 and beta-2 receptors) was also examined. Treatments included cetamolol (15 and 60 mg), atenolol (50 and 200 mg), and nadolol (40 and 160 mg in Study 2; 40 mg only in Study 3). All drugs tested suppressed the HR, DBP, and sGAW responses to ISO, and this blockade was dose dependent. Cetamolol and nadolol produced approximately equipotent beta-1 blockade, whereas cetamolol at both doses produced a less potent beta-2 blockade. Atenolol antagonized ISO effects on all parameters less than either cetamolol or nadolol. Quantitative cardioselectivity indices revealed that cetamolol 60 mg was the most cardioselective and nadolol 40 mg the least. Data from the three studies demonstrate that cetamolol is cardioselective relative to nadolol and that, in contrast to atenolol, cardioselectivity appears to increase at the higher dose.

Conditions for increased cardiac output during occlusion of the descending thoracic aorta

Previous studies have demonstrated that occlusion of the descending thoracic aorta (AO) at constant heart rate induces a rise in stroke volume during a continuous intravenous (i.v.) isoproterenol infusion, but no change in stroke volume during a selective inotropic stimulation of the left ventricle as achieved by an intracoronary (i.c.) isoproterenol infusion. To determine the mechanism for this difference in stroke volume response, the haemodynamic adjustments of both the right and the left ventricle to AO during continuous i.v. and i.c. isoproterenol infusion in anesthetized open-chest pigs were compared. The AO induced a similar rise in left ventricular systolic pressure and end-systolic segment length (measured by an ultrasonic technique) in both ventricles whether isoproterenol was infused i.v. or i.c., but stroke volume rose by 26.3 (11.9-38.1%) (median and 95% confidence interval) (P less than 0.01) during i.v., compared with 3.8 (-7.4-14.7)% (n.s.) during i.c. isoproterenol infusion (difference in response: P less than 0.01). End-diastolic segment length increased more by AO during i.v. than i.c. isoproterenol infusion; 7.8 (5.1-16.9)% vs. 5.7 (3.1-8.5)% (difference in response: P less than 0.01) in the left ventricle, and 5.9 (3.5-8.1)% vs. 1.0 (-1.5-3.3)% (difference in response: P less than 0.01) in the right ventricle. Redistribution of blood through the inferior caval vein measured immediately after AO, amounted to 130 (110-172) ml during i.v. and to 77 (52-89) ml during i.c. isoproterenol infusion (n = 3).(ABSTRACT TRUNCATED AT 250 WORDS)

Improvement of the isoprenaline infusion test by plasma concentration measurements

A simple and sensitive method for the determination of isoprenaline (ISO) in plasma by high performance liquid chromatography (HPLC) with electrochemical detection is presented. Blood pressure and heart rate responses to i.v. infusion of ISO (15, 38 and 76 ng/kg/min) were studied in 15 subjects. Blood samples for ISO analyses were drawn after 7.5 min infusions on each dose level. A four- to six-fold interindividual variation in the venous plasma concentrations of ISO was found. Comparisons were made between estimates of the sensitivity to ISO from concentration-effect and dose-effect curves for both heart rate and diastolic blood pressure responses. Despite an overall correlation between the two methods of estimating ISO sensitivity, individual estimates of sensitivity differed markedly due to the differences in the plasma concentrations attained during infusions of standardized doses of ISO. The venous plasma concentration of ISO required to elevate heart rate by 25 beats/min (CC25) varied between 0.3 and 1.7 nM, whereas the corresponding dose of ISO (CD25) varied between 10 and 27 ng/kg/min.

Participation of the autonomic nervous system in the cardiovascular effects of milrinone

The cardiodynamic activity of intravenously administered milrinone was examined in alpha-chloralose anesthetized dogs. Two groups of dogs were used, one pretreated with hexamethonium to block autonomic reflexes, and a second group which received no pretreatment. In the untreated group milrinone produced dose-dependent increases in +dP/dt and heart rate while decreasing both systolic and diastolic blood pressure and left ventricular end diastolic pressure (LVEDP). After treatment with hexamethonium basal heart rate was significantly increased, whereas reflex changes in heart rate in response to i.v. norepinephrine or nitroglycerin were ablated. Systolic, but not diastolic blood pressure was also markedly reduced by hexamethonium. In the presence of hexamethonium responses to milrinone were qualitatively similar to milrinone responses in the absence of hexamethonium. However, the dose-response curves for milrinone were shifted dextrally for changes in +dP/dt and LVEDP, whereas the dose-response curve for blood pressure was shifted sinistrally. Thus, it appears that the autonomic nervous system enhances the effect of milrinone on +dP/dt and LVEDP, but attenuates its effect on blood pressure.

Effects of ICI 141,292 on exercise tachycardia and isoprenaline-induced beta-adrenoceptor responses in man

The beta-adrenoceptor blocking properties and cardioselectivity of ICI 141, 292 were investigated in healthy male subjects. Seven subjects received in random order oral doses of ICI 141,292 20, 50, 100, 200 and 400 mg, atenolol 50 and 100 mg and placebo. ICI 14 292 had no effect on supine heart rate which was reduced by atenolol 100 mg. ICI 141,292 50, 100 and 200 mg had no effect on standing heart rate which was reduced by 400 mg at 2 h. Both doses of atenolol caused greater reductions. The maximum percent reduction of an exercise tachycardia after ICI 141,292 200 mg (23.9 +/- 3.7%) and 400 mg (24.3 +/- 5.2%) were similar to atenolol 50 mg (27.3 +/- 4.7%) but less than atenolol 100 mg (30.8 +/- 2.9%) (P less than 0.02). Six subjects received in random order single oral doses of ICI 141,292 100, 200 and 400 mg, atenolol 50 mg, propranolol 40 mg and placebo. Following each dose each subject received graded infusions of isoprenaline sulphate until heart rate increased by 40 beats min-1. Dose-response curves were constructed for the changes in heart rate, finger tremor, blood pressure and forearm blood flow produced by each infusion. At the 4 micrograms min-1 dose of isoprenaline, ICI 141,292 200 mg caused more attenuation than atenolol 50 mg but less than propranolol 40 mg in the changes of heart rate, diastolic blood pressure and finger tremor (P less than 0.02). ICI 141,292 400 mg caused more attenuation of the changes of all parameters than atenolol 50 mg but less attenuation of the changes in diastolic blood pressure and finger tremor than propranolol 40 mg (P less than 0.02). These results indicate that ICI 141,292 is a cardioselective beta-adrenoceptor antagonist with partial agonist activity.

The physiological and pharmacological role of presynaptic alpha- and beta-adrenoceptors in man

Two studies were performed each in six normal volunteers in order to find evidence of either a physiological or pharmacological role of presynaptic alpha- and presynaptic beta-adrenoceptors in man. In Study 1 subjects received a 60 min infusion of guanfacine 3 mg (alpha 2-adrenoceptor agonist) preceded by either idazoxan (alpha 2-adrenoceptor antagonist) or vehicle. Guanfacine reduced plasma noradrenaline concentration by approximately 30% and this fall was not antagonised by the alpha 2-receptor antagonist. The 30-fold increase in plasma growth hormone, measured as a marker of the central action of guanfacine, was almost completely blocked by idazoxan. A comparison of the drug concentrations of idazoxan and guanfacine, together with their relative affinities for alpha 2-adrenoceptors, suggested that the idazoxan could not block the peripheral actions of guanfacine and that these were responsible for the fall in plasma noradrenaline concentration. In Study 2 adrenaline 0.05 micrograms kg-1 min-1 was infused for 80 min preceded by either idazoxan or vehicle. After vehicle, adrenaline caused no change in plasma noradrenaline concentration whereas it rose approximately 25% after administration of idazoxan. This was probably due to unmasking of presynaptic beta-adrenoceptor stimulation by adrenaline when the opposing inhibitory autoreceptor was blocked.

Effects of the beta 2-adrenoceptor antagonist ICI 118,551 on exercise tachycardia and isoprenaline-induced beta-adrenoceptor responses in man

ICI 118,551, 5 to 80 mg orally, did not significantly alter resting heart rate or blood pressure. In doses less than 40 mg the reduction in exercise tachycardia was under 10 beats/min. ICI 118,551, 10 to 40 mg, did not appear to reduce the maximum rise in systolic pressure with isoprenaline but did attenuate the changes in diastolic pressure, forearm blood flow and finger tremor. It also attenuated the isoprenaline-induced changes in serum glucose, insulin and potassium. On these observed changes, the effect of ICI 118,551 20 mg was similar to that of 40 mg and of propranolol 10 mg, but greater than that of atenolol 25 mg. An isoprenaline tachycardia was attenuated by all doses of ICI 118,551 studied. After atropine (0.04 mg/kg) ICI 118,551 20 mg still significantly reduced the effects of isoprenaline suggesting that functional beta 2-adrenoceptors may be present in the human heart. In doses less than 40 mg, ICI 118,551 appears to be a selective and competitive antagonist of beta 2-adrenoceptors in man.