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Cardiac beta-adrenoceptor regulation and the effects of partial agonism
Cardiac Beta-Adrenoceptor Regulation and the Effects of Partial Agonism David B. Barnett MD, and Xianyou Lu, MD
The in vivo effects of xamoterol on the regulation of rat cardiac B adrenoceptors were investigated. Rats were implanted subcutaneousty with osmotic minipumps and exposed to the fotlowing treatment regimens: (1) subcutaneous infusion of saline (control), isoprenaline or xamoterol for 6 days, (2) subcutaneous infusion of lsoprenaline with co-administration of xamoterol for various periods up to 66 hours, and (3) subcutaneous infusion of xamoterol for up to 66 hours after previous treatment with isoprenallne for 72 hours. Xamoterol did not induce /Mdrenoceptor down-regulation after short-term (72~hour) or long-term (6day) infusions. When coadmidstered with isoprenaline xamoterol did not affect the rate or extent of down-regulation induced by isoprenaline alone. In addition, recovery of B adrenoceptors down-regulated by isoprenaline by xamoterol treattreatment was not Iment. In all studies, doses of xamoterol were equivalent to those producing full functional responses to the drug in vivo. (Am J Cardiol1991$7:16C-19C)
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t is generally acceptedthat chronic agonist stimulation of /3i adrenoceptorsproducesdesensitizationand is usually associatedwith a reduction in tissuereceptor density (down-regulation) or uncoupling of the recep tor from adenylate cyclase,or both.’ Converselychronic treatment with /3-adrenoceptorantagonists may induce an increase in receptor density (upregulation) and increaseresponsiveness.2 Thus, attempts at chronic treatment of heart failure with the & adrenoceptor agonist pirbuterol havebeenunsuccessfulbecauseof rapid development of receptor desensitization.3 Furthermore, in chronic heart failure, /3-adrenoceptordown-regulation has been demonstrated in hearts removed from transplant recipientspresumablyrelated to the chronic sympathetic nervous stimulation characteristic of this condition.4 It is possiblethat the ,&adrenoceptorselectivepartial agonist xamoterol may be of particular use in heart failure, being lesslikely to induce receptor down-regulation or reduce responsivenessduring chronic treatment. It may alsoprotect cardiac /3adrenoceptorsfrom “adverse” regulatory influences of excessive noradrenergic discharge present during the developmentof heart failure. Becauseof the clinical interest in xamoterol, we have investigatedthe effectsof this agent on regulation of rat cardiac @adrenoceptorsin vivo. The experiments were designed to answer the following questions: (1) Does chronic treatment with xamoterol induce cardiac @-adrenoceptor down-regulation compared with similar treatment with the full agonist isoprenaline? (2) Does coadministration of xamoterol with isoprenaline influence the rate or extent of down-regulation or recovery of the cardiac /3 adrenoceptorsinduced by the full agonist isoprenaline? METHODS
Frpm the Department of Pharmacologyand TherapeuticsUniversity of L&ester, Leicester, United Kingdom. Address for reprints: David B. Barnett, MD, Department of Pharmacology, Clinical Sciences Building, Leicester Royal Infirmary, L&ester, LE2 7LX, United Kingdom.
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THE AMERICAN JOURNAL OF CARDIOLOGY VOLUME 67
Using osmotic minipumps (Alza, Palo Alto, California) implanted subcutaneouslyin rats the following treatment regimenswereemployed:(1) subcutaneousinfusion of saline (control), isoprenaline (40 pg/kg/hour) or xamoterol (600 pg/kg/hour) for 6 days; (2) subcutaneous infusion of isoprenline (40 pg/kg/hour) with coadministration of xamoterol (600 pg/kg/hour) for various periods up to 96 hours; (3) subcutaneousinfusion of xamoterol (600 pg/kg/hour) for up to 96 hours after prior treatment with isoprenaline (40 pg/kg/hour) for 72 hours. Xamoterol treatments in regimens 2 and 3 were precededby a single intraperitoneal priming injection of 0.1 mg/g of the drug.
At the end of the treatment periods, the rats were killed and blood was sampledfor isoprenalineand xamoterol plasma concentration measurements.Hearts were removed and myocardial membranes prepared using standard methods5 Receptor densities (IB,,,) for total j3 adrenoceptorsand & and /32subtypeswere assessed from saturation analysis of ‘251-pindololbinding as previously described.6
similar to thoseproducing full functional responsesin rat heart in vitro and in vivo, suggestingfull receptor occupancy for the partial agonist during the infusion experiments. CONCLUSIONS In this animal model, xamoterol doesnot induce adrenoceptor down-regulation in short-term (72 hours) or long-term (6 days) infusion studiesusing dosesequivalent to thoseproducing full functional responsesto the drug in vivo and in vitro. Xamoterol, when co-administeredwith isoprenaline,doesnot affect the rate or extent of downregulation induced by the full agonist. In addition, recovery of down-regulated/3adrenoceptorsafter isoprenaline treatment is not influenced by xamoterol. Theseresults have important implications for the use of xamoterol in the therapy of chronic heart failure. In particular, it seemsthat tachyphylaxis in responseto the inotropic effects of xamoterol during long-term treatment would be unlikely. The present experiments,however, do not indicate a “protective” effect of xamoterol when the cardiac /3 adrenoceptorsare exposedto strong stimulation by a full agonist. This may be related to the artificial nature of the stimulus usedin the presentmodel (high-dose isoprenaline infusion) as well as the relative dosesof the 2 agents.Further studiesare currently underway to explore these possibilities.
RESULTS I-line: The infusion of isoprenaline for 72 hours reduced total /3-adrenoceptorB,, by approximately 40 to 50%. No further down-regulation was induced by 6 days’ treatment. The relative proportions of the Badrenoceptor subtypes(65 to 70%) were not affected by chronic isoprenaline treatment. However, during short-term isoprenaline infusions, 82 adenoceptorswere down-regulated more rapidly and to a greater extent (maximal down-regulation 60% at 24 hours) than /3i adrenoceptors (maximal down regulation 45% at 48 hours). Recoveryof down-regulated,f3adrenoceptorswas observedin rats treated initially with isoprenalinefor 72 hours. After removal of the minipumps, the rats were allowed to recover and then killed at various time intervals up to 96 hours. pi adrenoceptorsreturned at a slower rate to the control state than did fl2 adrenoceptors(full recovery at 48 and 96 hours for & and PI subtypes, respectively). Xamoterol: Infusion of xamoterol alone for any time period (72 hours to 6 days) did not affect /3-adrenoceptor density. In addition, the relative proportions of the fl- REFERENCES adrenoceptor subtypes were unchanged by xamoterol 1. Minneman KP, Pittman RN, M&off PB. fl-adrenergic receptor subtypes: treatment. This indicates that the p1 selectivepartial ago- properties,distribution, and regulation. Annu Rev Neurosci 1981;4:419-461. Aarons RD, Molinoff PB. Changesin the density of beta-adrenergicreceptors nist did not cause down-regulation of pi adrenoceptors in2. rat lymphocytes,heart and lung after chronic treatment with propranolol. J associatedwith a compensatoryincreasein fi2 adrenocep Pharmacol Exp Ther 1981;221:439-443. Colucci WS, Alexander RW, Williams GH, Rude RE, Holman BL, Konstam tors accounting for the unchanged overall B,,, value. 3. MA, Wynne J, Mudge GH. Braunwald E. Decreasedlymphocytebeta-adrenergic Xamoterol and isoprenaline: When co-administered receptordensityin patientswith heart failure and toleranceto the beta adrenergic with isoprenaline, xamoterol did not affect the rate or agonist pirbuterol. N Eng/ J Med 1981;305:185-190. Bristow MR. Myocardial beta-adrenergicreceptor downregulation in heart degree of down-regulation induced by isoprenaline. In 4. failure. Inr J Cardiol 1984:5:648-652. addition, recovery of down-regulated receptors was not 5. Baker SP, Boyd HM, Potter LT. Distribution and function of &adrenoceptors in different chambersof the canine heart. Br J Pharmacol 1980,68:57-63. influenced by subsequenttreatment with xamoterol. 6. Cook N, Nahorski SR, Barnett DB. ‘liI(-) pindolol binding to the human Plasma concentrations of xamoterol measuredat the platelet beta-adrenoceptor:characterizationand agonistinteractions.Eur J Pharend of all treatment periods (100 to 300 nmol/liter) were macol 1985;113:247-254.
THE AMERICAN JOURNAL OF CARDIOLOGY MAY 6, 1991
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The in vivo effects of xamoterol on the regulation of rat cardiac B adrenoceptors were investigated. Rats were implanted subcutaneousty with osmotic minipumps and exposed to the fotlowing treatment regimens: (1) subcutaneous infusion of saline (control), isoprenaline or xamoterol for 6 days, (2) subcutaneous infusion of lsoprenaline with co-administration of xamoterol for various periods up to 66 hours, and (3) subcutaneous infusion of xamoterol for up to 66 hours after previous treatment with isoprenallne for 72 hours. Xamoterol did not induce /Mdrenoceptor down-regulation after short-term (72~hour) or long-term (6day) infusions. When coadmidstered with isoprenaline xamoterol did not affect the rate or extent of down-regulation induced by isoprenaline alone. In addition, recovery of B adrenoceptors down-regulated by isoprenaline by xamoterol treattreatment was not Iment. In all studies, doses of xamoterol were equivalent to those producing full functional responses to the drug in vivo. (Am J Cardiol1991$7:16C-19C)
I
t is generally acceptedthat chronic agonist stimulation of /3i adrenoceptorsproducesdesensitizationand is usually associatedwith a reduction in tissuereceptor density (down-regulation) or uncoupling of the recep tor from adenylate cyclase,or both.’ Converselychronic treatment with /3-adrenoceptorantagonists may induce an increase in receptor density (upregulation) and increaseresponsiveness.2 Thus, attempts at chronic treatment of heart failure with the & adrenoceptor agonist pirbuterol havebeenunsuccessfulbecauseof rapid development of receptor desensitization.3 Furthermore, in chronic heart failure, /3-adrenoceptordown-regulation has been demonstrated in hearts removed from transplant recipientspresumablyrelated to the chronic sympathetic nervous stimulation characteristic of this condition.4 It is possiblethat the ,&adrenoceptorselectivepartial agonist xamoterol may be of particular use in heart failure, being lesslikely to induce receptor down-regulation or reduce responsivenessduring chronic treatment. It may alsoprotect cardiac /3adrenoceptorsfrom “adverse” regulatory influences of excessive noradrenergic discharge present during the developmentof heart failure. Becauseof the clinical interest in xamoterol, we have investigatedthe effectsof this agent on regulation of rat cardiac @adrenoceptorsin vivo. The experiments were designed to answer the following questions: (1) Does chronic treatment with xamoterol induce cardiac @-adrenoceptor down-regulation compared with similar treatment with the full agonist isoprenaline? (2) Does coadministration of xamoterol with isoprenaline influence the rate or extent of down-regulation or recovery of the cardiac /3 adrenoceptorsinduced by the full agonist isoprenaline? METHODS
Frpm the Department of Pharmacologyand TherapeuticsUniversity of L&ester, Leicester, United Kingdom. Address for reprints: David B. Barnett, MD, Department of Pharmacology, Clinical Sciences Building, Leicester Royal Infirmary, L&ester, LE2 7LX, United Kingdom.
18C
THE AMERICAN JOURNAL OF CARDIOLOGY VOLUME 67
Using osmotic minipumps (Alza, Palo Alto, California) implanted subcutaneouslyin rats the following treatment regimenswereemployed:(1) subcutaneousinfusion of saline (control), isoprenaline (40 pg/kg/hour) or xamoterol (600 pg/kg/hour) for 6 days; (2) subcutaneous infusion of isoprenline (40 pg/kg/hour) with coadministration of xamoterol (600 pg/kg/hour) for various periods up to 96 hours; (3) subcutaneousinfusion of xamoterol (600 pg/kg/hour) for up to 96 hours after prior treatment with isoprenaline (40 pg/kg/hour) for 72 hours. Xamoterol treatments in regimens 2 and 3 were precededby a single intraperitoneal priming injection of 0.1 mg/g of the drug.
At the end of the treatment periods, the rats were killed and blood was sampledfor isoprenalineand xamoterol plasma concentration measurements.Hearts were removed and myocardial membranes prepared using standard methods5 Receptor densities (IB,,,) for total j3 adrenoceptorsand & and /32subtypeswere assessed from saturation analysis of ‘251-pindololbinding as previously described.6
similar to thoseproducing full functional responsesin rat heart in vitro and in vivo, suggestingfull receptor occupancy for the partial agonist during the infusion experiments. CONCLUSIONS In this animal model, xamoterol doesnot induce adrenoceptor down-regulation in short-term (72 hours) or long-term (6 days) infusion studiesusing dosesequivalent to thoseproducing full functional responsesto the drug in vivo and in vitro. Xamoterol, when co-administeredwith isoprenaline,doesnot affect the rate or extent of downregulation induced by the full agonist. In addition, recovery of down-regulated/3adrenoceptorsafter isoprenaline treatment is not influenced by xamoterol. Theseresults have important implications for the use of xamoterol in the therapy of chronic heart failure. In particular, it seemsthat tachyphylaxis in responseto the inotropic effects of xamoterol during long-term treatment would be unlikely. The present experiments,however, do not indicate a “protective” effect of xamoterol when the cardiac /3 adrenoceptorsare exposedto strong stimulation by a full agonist. This may be related to the artificial nature of the stimulus usedin the presentmodel (high-dose isoprenaline infusion) as well as the relative dosesof the 2 agents.Further studiesare currently underway to explore these possibilities.
RESULTS I-line: The infusion of isoprenaline for 72 hours reduced total /3-adrenoceptorB,, by approximately 40 to 50%. No further down-regulation was induced by 6 days’ treatment. The relative proportions of the Badrenoceptor subtypes(65 to 70%) were not affected by chronic isoprenaline treatment. However, during short-term isoprenaline infusions, 82 adenoceptorswere down-regulated more rapidly and to a greater extent (maximal down-regulation 60% at 24 hours) than /3i adrenoceptors (maximal down regulation 45% at 48 hours). Recoveryof down-regulated,f3adrenoceptorswas observedin rats treated initially with isoprenalinefor 72 hours. After removal of the minipumps, the rats were allowed to recover and then killed at various time intervals up to 96 hours. pi adrenoceptorsreturned at a slower rate to the control state than did fl2 adrenoceptors(full recovery at 48 and 96 hours for & and PI subtypes, respectively). Xamoterol: Infusion of xamoterol alone for any time period (72 hours to 6 days) did not affect /3-adrenoceptor density. In addition, the relative proportions of the fl- REFERENCES adrenoceptor subtypes were unchanged by xamoterol 1. Minneman KP, Pittman RN, M&off PB. fl-adrenergic receptor subtypes: treatment. This indicates that the p1 selectivepartial ago- properties,distribution, and regulation. Annu Rev Neurosci 1981;4:419-461. Aarons RD, Molinoff PB. Changesin the density of beta-adrenergicreceptors nist did not cause down-regulation of pi adrenoceptors in2. rat lymphocytes,heart and lung after chronic treatment with propranolol. J associatedwith a compensatoryincreasein fi2 adrenocep Pharmacol Exp Ther 1981;221:439-443. Colucci WS, Alexander RW, Williams GH, Rude RE, Holman BL, Konstam tors accounting for the unchanged overall B,,, value. 3. MA, Wynne J, Mudge GH. Braunwald E. Decreasedlymphocytebeta-adrenergic Xamoterol and isoprenaline: When co-administered receptordensityin patientswith heart failure and toleranceto the beta adrenergic with isoprenaline, xamoterol did not affect the rate or agonist pirbuterol. N Eng/ J Med 1981;305:185-190. Bristow MR. Myocardial beta-adrenergicreceptor downregulation in heart degree of down-regulation induced by isoprenaline. In 4. failure. Inr J Cardiol 1984:5:648-652. addition, recovery of down-regulated receptors was not 5. Baker SP, Boyd HM, Potter LT. Distribution and function of &adrenoceptors in different chambersof the canine heart. Br J Pharmacol 1980,68:57-63. influenced by subsequenttreatment with xamoterol. 6. Cook N, Nahorski SR, Barnett DB. ‘liI(-) pindolol binding to the human Plasma concentrations of xamoterol measuredat the platelet beta-adrenoceptor:characterizationand agonistinteractions.Eur J Pharend of all treatment periods (100 to 300 nmol/liter) were macol 1985;113:247-254.
THE AMERICAN JOURNAL OF CARDIOLOGY MAY 6, 1991
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