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R J Summers

Publications and source records attributed to R J Summers.

At least 37 records · Page 2Linked to original sources
PubMed1999

Functional analysis of desensitization of the beta-adrenoceptor signalling pathway in rat cardiac tissues following chronic isoprenaline infusion.

1. This study examined beta-adrenoceptor signalling in cardiac tissues following infusion of isoprenaline (400 microg kg(-1) h(-1)) or vehicle to rats for 14 days. 2. Isoprenaline infusion caused marked hypertrophy of atria and ventricles and reduced the resting rate of spontaneously beating right atria and the basal force of left atrial contraction. 3. In spontaneously beating right atria, concentration-response curves to isoprenaline and forskolin were shifted 7.9 and 3.2 fold to the right following treatment whereas responses to the cyclic AMP analogue 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole-3', 5'-cyclic monophosphorothioate were unchanged. 4. In electrically driven left atria, concentration-response curves to isoprenaline and forskolin were shifted 4 fold to the right and maximum responses reduced. Responses to dibutyryl cyclic AMP were shifted 3.2 fold to the right but those to Ca2+ were unchanged. 5. Inotropic responses of left and right ventricular papillary muscles to isoprenaline were abolished and markedly reduced respectively by isoprenaline treatment. Responses to forskolin were shifted 5 fold to the right. Responses to dibutyryl cyclic AMP were shifted to the right 3.2 and 2 fold in left and right ventricular papillary muscles. Responses to isobutyl methyl xanthine were shifted to the right 15.8 and 6.3 fold in left and right papillary muscles whereas those to Ca2+ were not significantly altered. 6. This study indicates differences in beta-adrenoceptor desensitization in different regions of the heart following chronic infusion of isoprenaline. Chronotropic responses showed impaired signalling between the receptor and adenylate cyclase whereas inotropic responses exhibited additional desensitization at the level of cyclic AMP dependent protein kinase.

Adrenergic beta-Agonists↗
PubMed1999

Alternative splicing generates two isoforms of the beta3-adrenoceptor which are differentially expressed in mouse tissues.

The beta3-adrenoceptor (AR) differs from the beta1-AR and beta2-ARs in having introns within and downstream of the coding block. This study demonstrates two splice variants of the mouse beta3-AR which differ within the coding region. Reverse transcription/polymerase chain reaction with intron-spanning primers was used to demonstrate the splice variant of the mouse beta3-adrenoceptor. The novel beta3b-AR has 17 amino acids encoded by exon 2 (SSLLREPRHLYTCLGYP) which differ from the 13 in the known beta3a-AR (RFDGYEGARPFPT). Beta3b-AR mRNA is differentially expressed in mouse tissues, with levels relative to beta3a-AR mRNA highest in hypothalamus, cortex and white adipose tissue, and lower in ileum smooth muscle and brown adipose tissue.

Adipose Tissue↗
PubMed1999

Desensitization and resensitization of beta 1- and putative beta 4-adrenoceptor mediated responses occur in parallel in a rat model of cardiac failure.

1. Cardiostimulant effects of the non-conventional partial agonist, CGP 12177A, are mediated by a receptor distinct from the beta3-adrenoceptor and termed the putative beta4-adrenoceptor. Using a rat model of cardiac failure, induced by myocardial infarction (MI), we compared the desensitization and resensitization of responses to CGP 12177A with those to isoprenaline and RO 363 in left (LA) and right atria (RA). We also examined the ability of beta-adrenoceptor antagonists to block responses to CGP 12177A. 2. MI reduced the maximum inotropic response to isoprenaline by 48% (sham 4.1+/-0.6 mN, n=10; MI 2.1+/-0.4 mN, n=8, P<0.02), RO 363 by 61% (sham 4.2+/-0.5 mN, n=10; MI 1.8+/-0.3 mN, n=8, P<0.005) and CGP 12177A by 49% (sham 1.4+/-0.1 mN, n=5; MI 0.7+/-0.2 mN, n=7, P<0.05) in electrically stimulated LA. MI also reduced the sensitivity to isoprenaline (pEC50: sham 8.79+/-0.08, n=10; MI 8.30+/-0.10, n=8; P=0.001) and RO 363 (pEC50: sham 8.69+/-0.07, n=10; MI 8.33+/-0.10, n=8; P<0.01). The maximum chronotropic responses to isoprenaline, RO 363 and CGP 12177A in RA were unaffected. 3. Pertussis toxin treatment (10 microg kg-1, i.p.) restored the maximum inotropic response and sensitivity to isoprenaline (sham 3.5+/-0.5 mN, n=9; MI 3.2+/-0.6 mN, n=11, P=0.702) and CGP 12177A (sham 1.6+/-0.3 mN, n=6; MI 1.9+/-0.4 mN, n=7, P=0.537) in MI animals to levels similar to those in the sham group. 4. CGP 20712A (pKB: LA 6.7+/-0.2, n=6; RA 7. 1+/-0.1, n=4), ICI 118,551 (pKB: LA 6.4+/-0.1, n=5; RA 6.3+/-0.1, n=6), propranolol (pKB: LA 6.6+/-0.1, n=5; RA 6.8+/-0.1, n=6) and bupranolol (pKB: LA 7.2+/-0.1, n=6; RA 7.7+/-0.1, n=8), showed moderate affinity for the putative beta4-adrenoceptor. 5. Desensitization after MI and resensitization (after pertussis toxin treatment) to isoprenaline and CGP 12177A therefore occur in parallel, suggesting that the beta1- and putative beta4-adrenoceptor use the same signalling pathway. Antagonist affinity studies confirmed that drugs acting at beta1-adrenoceptors also interact with putative beta4-adrenoceptors with approximately 100 times lower affinity. We suggest that CGP 12177A produces its cardiac effects by interacting with a low affinity state of the beta1-adrenoceptor.

Animals↗
PubMed1998

The effects of the beta3-adrenoceptor agonist BRL 35135 on UCP isoform mRNA expression.

The mitochondrial uncoupling protein UCP-1 uncouples respiration from ATP synthesis in brown adipose tissue (BAT) and thus energy is dissipated as heat. Recently two further isoforms have been identified which may play a similar role in other tissues. We have determined the effects of the rodent-selective beta3-adrenoceptor (beta3-AR) agonist BRL 35135, on beta3-AR and UCP mRNA levels in tissues from lean and obese (fa/fa) Zucker rats. beta3-AR mRNA levels were reduced in fa/fa white (WAT) and brown (BAT) adipose tissue relative to levels in lean littermates. BRL 35135 treatment increased expression levels of beta3-AR mRNA in both genotypes. UCP-2 and UCP-3 mRNA levels in BAT, WAT and skeletal muscle were reduced by 2-3 fold in the fa/fa rats relative to the lean rats. We confirm that BRL 35135 increases BAT UCP-1 mRNA in lean rats, and find that BAT UCP-3 mRNA was reduced 3.2 fold, with no changes in UCP-2 expression. In WAT BRL 35135 increased UCP-2 and UCP-3 expression 2-3 fold in both lean and fa/fa rats. In lean rats, skeletal muscle UCP-3 mRNA was increased 2.3 fold by BRL 35135 whereas UCP-2 was reduced by 2.2 fold. BRL 35135 had no effects on UCP-2 and UCP-3 expression in skeletal muscle of the fa/fa rats. Our results demonstrate that mechanisms regulating UCP isoform synthesis in fa/fa rats are impaired and that WAT could be involved in the thermogenic response of BRL 35135.

Adipose Tissue↗
PubMed1998

Cyclic AMP accumulation in rat soleus muscle: stimulation by beta2- but not beta3-adrenoceptors.

The beta-adrenoceptor subtypes involved in cyclic AMP accumulation in rat soleus muscle were studied using beta1- beta2- and beta3-adrenoceptor agonists and antagonists. Responses to (-)-isoprenaline were antagonised by (-)-propranolol (p KB = 8.32 at 0.1 microM) and by erythro-DL-1(7-methylindian-4-yloxy)-3-isopropylaminobuta n-2-ol (+/-)-ICI 118551) (pKB = 9.38 at 10 nM and 9.65 at 100 nM) but not by 2-hydroxy-5(2-((2-hydroxy-3-(4-((1-methyl-4-trifluoromethyl)1H-imidazole -2-yl)-phenoxy)propyl)amino)ethoxy)-benzamide monomethane sulfonate ((+/-)-CGP 20712A at 10 nM or 100 nM). The beta3-adrenoceptor agonist sodium-4-[-2[-2-hydroxy-2-(-3-chlorophenyl)ethylamino]propyl]phenoxya cetate (BRL 37344 at 10 pM or 10 microM) caused no significant change in basal cyclic AMP levels and had no effect on the level of cyclic AMP accumulation stimulated by (-)-isoprenaline, zinterol or forskolin. (-)-Isoprenaline pretreatment (400 microg kg(-1) h(-1), 14 days) abolished responses to (-)-isoprenaline (10 microM) and zinterol (1 microM) while BRL 37344 had no effect in either isoprenaline or vehicle-treated groups. These results show that beta3-adrenoceptor agonists do not stimulate cyclic AMP accumulation in rat soleus muscle and that (-)-isoprenaline induced increases in cyclic AMP levels are mediated predominantly by beta2-adrenoceptors. This suggests that the previously reported increase in glucose uptake by beta3-adrenoceptor agonists in skeletal muscle does not involve direct stimulation of adenylate cyclase.

Adrenergic beta-2 Receptor Agonists↗
PubMed1998

Cardiac binding of [3H]inositol 1,4,5-trisphosphate following chronic stimulation of cyclic AMP signalling in guinea pigs.

Binding of [3H]inositol 1,4,5-trisphosphate ([3H]IP3) to guinea pig heart has been characterised, localised and the effect of the in vivo desensitisation of cardiac beta-adrenoceptorcyclic AMP signalling examined. Quantitative autoradiography showed highest levels of [3H]IP3 binding associated with coronary blood vessels and with the endothelial cells of the aorta and the mitral and tricuspid valves. Moderate levels of [3H]IP3 bound to the atrioventricular conducting system, cardiac valves and aortic smooth muscle. Lower levels of [3H]IP3 binding were detected in atrial and ventricular myocardium. Although the phosphoinositide signalling pathway does not contribute greatly to normal cardiac function, there is evidence of an increased importance in situations of compromised excitation-contraction such as occurs in cardiac failure or with, beta-adrenoceptor desensitisation. We examined whether chronic in vivo stimulation of beta-adrenoceptor-adenylate cyclase signalling affected cardiac binding of [3H]IP3. Infusion of guinea pigs with isoprenaline (400 micrograms kg-1 h-1, 7 days) tended to reduce [3H]IP3 binding in myocardium although not significantly (P > 0.05, n = 4). These data indicate that [3H]IP3 binding has a heterogeneous distribution in guinea pig heart with highest levels of binding discretely localised to endothelial cells. Desensitisation of beta-adrenoceptor-cyclic AMP signalling in heart did not lead to upregulation of [3H]IP3 binding.

Adrenergic beta-Agonists↗
PubMed1998

Comparison of relaxin receptors in rat isolated atria and uterus by use of synthetic and native relaxin analogues.

1. The receptors for relaxin in the rat atria and uterus were investigated and compared by use of a series of synthetic and native relaxin analogues. The assays used were the positive chronotropic and inotropic effects in rat spontaneously beating, isolated right atrium and electrically driven left atrium and the relaxation of K+ precontracted uterine smooth muscle. 2. Relaxin analogues with an intact A- and B-chain were active in producing powerful chronotropic and inotropic effects in the rat isolated atria at nanomolar concentrations. Single-chain analogues and structural homologues of relaxin such as human insulin and sheep insulin-like growth factor I had no agonist action and did not antagonize the effect of the B29 form of human gene 2 relaxin. 3. Shortening the B-chain carboxyl terminal of human gene 1 (B2-29) relaxin to B2-26 reduced the activity of the peptide and removal of another 2 amino acid residues (B2-24) abolished the activity. This suggests that the B-chain length may be important for determination of the activity of relaxin. More detailed studies are needed to determine the effect of progressive amino acid removal on the structure and the bioactivity of relaxin. 4. Porcine prorelaxin was as active as porcine relaxin on a molar basis, suggesting that the presence of the intact C-peptide did not affect the binding of the prorelaxin to the receptor to produce functional responses. 5. Relaxin caused relaxation of uterine longitudinal and circular smooth muscle precontracted with 40 mM K+. The pEC50 values for human gene 2 and porcine relaxins were lower than those in the atrial assay, but rat relaxin had similar pEC50 values in both atrial and uterine assays. Rat relaxin was significantly less potent than either human gene 2 or porcine relaxin in the atrial assay, but in the uterine assay they were equipotent. The results suggest that the relaxin receptor or the signalling pathway in rat atria may differ from that in the uterus.

Animals↗
PubMed1998

Differential regulation of beta3-adrenoceptors in gut and adipose tissue of genetically obese (ob/ob) C57BL/6J-mice.

1. Levels of beta3-adrenoceptor (AR) mRNA were compared using reverse transcription-polymerase chain reaction (RT PCR) in white adipose tissue (WAT), brown adipose tissue (BAT), ileum and colon from genetically obese (ob/ob) and lean (+/+) C57BL/6J mice. Functional responses to the beta3-AR agonist CL 316243 were also characterized in ileal longitudinal smooth muscle from obese and lean mice. 2. Beta3-AR mRNA levels were significantly higher in WAT (100+/-16%) and BAT (100+/-13%) from lean compared to WAT (21.0+/-0.9%; n=4; P<0.005) and BAT (14.1+/-2.2%; n=5; P<0.01) from obese mice. In contrast, beta3-mRNA levels were not significantly different in ileum (100+/-15%) and colon (100+/-22%) from lean mice, compared to ileum (78+/-13%; n=4; P= 0.31) or colon (82+/-15%; n =4; P=0.52) from obese mice. 3. Concentration-response curves to CL 316243 did not differ significantly in slope or position in ileal longitudinal smooth muscle from obese or lean mice. pEC50 (+/-s.e.mean) values were not significantly different (P= 0.59) between obese (7.90+/-0.13, n = 7) and lean (7.77+/-0.20, n = 7) mice. 4. pKB values for the beta1-AR and beta2-AR selective antagonist propranolol or the beta3-AR selective antagonist SR 58894 against relaxations to CL 316243 were similar in ileum of genetically obese (propranolol 6.31+/-0.22 and 6.13+/-0.12; SR 58894 8.22+/-0.06) and lean mice (propranolol 6.40+/-0.08 and 6.60+/-0.13; SR 58894 8.27+/-0.12) and were consistent with values previously found at beta3-AR. 5. Treatment of lean C57BL/6J mice with dexamethasone (1 mg kg(-1), i.p.) significantly reduced beta3-AR mRNA levels after 4 h in WAT (100+/-6.1 to 41.4+/-4.3; n= 16 18; P<0.0001) and BAT (100+/-8.0 to 35.1+/-5.8; n= 17; P<0.0001), but caused no change in ileum (100+/-6.1 to 101+/-17; n= 10-11; P=0.95) or colon (100+/-11 to 101+/-11; n= 11; P= 0.94). Beta3-mRNA levels in ileum and colon also did not change significantly when examined over 24 h or after the administration of a higher dose of dexamethasone (5 mg kg(-1)). 6. In summary, beta3-AR mRNA levels were considerably lower in WAT and BAT of obese compared to lean mice whereas the levels in ileum and colon were not significantly different. The similar beta3-mRNA levels in ileum of obese and lean mice were associated with indistinguishable responses of carbachol-contracted ileum to a beta3-agonist and similar affinity for beta-antagonists. Administration of glucocorticoids to lean mice reduced beta3-AR mRNA levels in WAT and BAT but not in ileum or colon. These studies show that in mice, beta3-ARs are differentially regulated in ileum and colon compared to adipose tissues.

Adipose Tissue↗
PubMed1998

Regulation of beta-adrenoceptors in a rat model of cardiac failure: effect of perindopril.

This study investigated the effects of myocardial infarction (MI)-induced cardiac failure and treatment with an angiotensin-converting enzyme (ACE) inhibitor perindopril (2 mg/kg/day) on rat beta-adrenoceptor (beta-ar) subtypes in anatomically defined regions of infarcted left ventricular (LV) free wall and noninfarcted tissue from right ventricle (RV) by using autoradiography. After 5 weeks of MI, rats with large MI size (>42%) had developed cardiac failure and beta1-ars were significantly decreased (-59%; p < 0.01) in the border region of the infarcted LV and almost abolished in the infarcted area (-90%; p < 0.005) compared with normal LV from sham-operated controls. The beta-ar changes were not found in the noninfarcted area of the same LV or in RV. MI did not significantly alter the number of beta2-ar subtypes in any region of the ventricles. Perindopril treatment for 4 weeks reduced mean cardiac region weights but did not affect beta-ar density in any cardiac region in either sham-operated or MI rats. These results indicate that cardiac failure due to MI causes significant downregulation of beta1-ars only in border and infarcted regions of rat LV and no change in beta2-ar in any area. It also suggests that the improved response of the infarcted rat heart to isoprenaline stimulation after ACE inhibitors does not result from changes in the numbers of cardiac beta-ars.

Angiotensin-Converting Enzyme Inhibitors↗
PubMed1997

Functional and molecular evidence for beta 1-, beta 2- and beta 3- adrenoceptors in human colon.

1. Relaxation of carbachol pre-contracted human colonic muscle to (-)-isoprenaline was examined in circular, longitudinal and taenia coli preparations to determine the beta-adrenoceptor subtypes involved. beta 1-, beta 2- and beta 3-Adrenoceptor mRNAs were also measured in colonic muscle and mucosa. 2. (-)-isoprenaline caused relaxation of longitudinal smooth muscle preparations with pEC50-7.39 +/- 0.12, and this response was inhibited by both propranolol (0.1 microM, pKB 8.55 +/- 0.12) and the selective beta 1-antagonist, CGP 20712A (0.1 microM, pKB 8.80 +/- 0.20), while the selective beta 2-antagonist, ICI 118551 (0.1 microM) failed to inhibit isoprenaline relaxation consistently. 3. (-)-Isoprenaline caused relaxation of taenia coli with a pEC50 of 6.70 +/- 0.17. Propranolol (0.1 microM). CGP 20712A (0.1 microM) and ICI 118551 (0.1 microM) inhibited the isoprenaline response with similar low affinities (pKB values 7.93, 7.71 and 7.54, respectively). Carbachol pre-contracted circular smooth muscle preparations failed to relax consistently to isoprenaline and these responses were not characterized. 4. beta 1- and beta 2-Adrenoceptor mRNAs were present in circular longitudinal muscle samples and taenia coli samples, and lower levels were detected in mucosa. beta 3-mRNA was also present in both muscle preparations but was not detected in human colonic mucosa. 5. In summary, beta 1-adrenoceptors are the predominant subtype mediating isoprenaline-induced relaxation of the thin longitudinal smooth muscle of human colon, while beta 1-receptors do not appear to be involved in these responses. However, beta 3-adrenoceptors may play a role in relaxation of the taenia coli as conventional antagonist affinities are low. beta 3-Adrenoceptor mRNA was present in taenia coli and circular/longitudinal smooth muscle but absent from human colonic mucosa.

Adrenergic beta-Agonists↗
PubMed1996

Chronic (-)-isoprenaline infusion down-regulates beta 1- and beta 2-adrenoceptors but does not transregulate muscarinic cholinoceptors in rat heart.

Regulation of beta-adrenoceptor (beta-ar) subtypes and transregulation of muscarinic cholinoceptors (mAchr) was examined in regions of rat heart after chronic infusion of (-)-isoprenaline (450 micrograms/kg per hour) for 14 days. Following (-)-isoprenaline infusion systolic blood pressure was reduced for 10 days but then gradually returned to control levels, whereas heart rate was increased for 7 days before declining to a level significantly above control. Heart weight to body weight ratio was increased in (-)-isoprenaline treated rats. beta-ar subtype densities were measured by quantitative autoradiography with [125I]-cyanopindolol (CYP) in sinoatrial node (SA), atrioventricular node (AV), bundle of His (BH), left (LB) and right (RB) bundle branches, interventricular (IVS) and interatrial (IAS) septa, right atria (RA), apex (AX) and mitral valve (MV). beta 1-ars were reduced by 59.1-74.2% in the AV conducting regions, 53.4% in the SA node and 43.3-53.4% in myocardial areas, beta 2-ars were markedly reduced in myocardial regions (93.2-98.5%) and in pacemaker and conducting regions (87.7-97.8%). No changes in mAchr densities measured using [3H]-N-methyl scopolamine (NMS) occurred in the AV node, BH, LB, RB, IVS and IAS following (-)-isoprenaline infusion. Densities of beta 1- and beta 2-ars and mAchrs were also measured in ventricular homogenates from control and (-)-isoprenaline treated animals. beta-ar levels were significantly reduced (P < 0.05) in treated animals and the ratio of beta 1- to beta 2-ars increased after treatment. mAchr density in ventricular homogenates measured using either [3H]-NMS or [3H]-quinuclidinyl [phenyl-4-3H]benzilate (QNB) was unchanged. Homogenates of left and right ventricle also showed no change using [3H]-NMS. Organ bath studies were used to investigate the effect of (-)-isoprenaline infusion on negative inotropic and chronotropic effects of the non-selective muscarinic receptor agonist bethanechol in left and right atria, respectively. Lower concentrations of bethanechol (3 x 10(-10) to 10(-6) M) produced a negative inotropic response in isolated electrically driven left atria from (-)-isoprenaline treated rats, but not from control rats, with the slope of the curves being significantly different between groups (ANCOVA, P = 0.037). At concentrations of bethanechol from 10(-6) to 3 x 10(-4) M the negative inotropic response was not changed between (-)-isoprenaline treated and control animals. Bethanechol also produced a negative chronotropic response at lower concentrations (10(-10) to 10(-6) M) in (-)-isoprenaline treated rats, but not in controls. A second, steeper phase of the negative chronotropic response occurred at concentrations of bethanechol greater than 10(-6) M and was also seen in control rats. Expression of M2 (cardiac) mAchrs (m2Achr) in left and right ventricular tissues measured using a quantitative non-competitive polymerase chain reaction (PCR) assay showed a significant (P = 0.001) 28.5% increase in expression in left ventricle and a significant (P = 0.003) 21.5% decrease in expression in right ventricle after (-)-isoprenaline treatment, compared to controls. There was no significant difference in total ventricular m2Achr expression between the two groups of rats. The results suggest that chronic beta-ar stimulation down-regulates both beta 1- and beta 2-ars, and appears to differentially transregulate m2Achr expression, but not mAchr protein. Following (-)-isoprenaline infusion, muscarinic receptor mediated responses were sensitised, with no change in receptor densities, suggesting changes occur in the cell signalling system beyond the level of the receptor.

Animals↗
PubMed1996

Expression of beta 3-adrenoceptor mRNA in rat tissues.

1. This study examines the expression of beta 3-adrenoceptor messenger RNA (beta 3-AR mRNA) in rat tissues to allow comparison with atypical beta-adrenoceptors determined by functional and radioligand binding techniques. 2. A reverse transcription/polymerase chain reaction protocol has been developed for determining the relative amounts of beta 3-AR mRNA in rat tissues. 3. Measurement of adipsin and uncoupling protein (UCP) mRNA was used to examine all tissues for the presence of white and brown adipose tissue which may contribute beta 3-AR mRNA. 4. The beta 3-AR mRNA is expressed at high levels in brown and white adipose tissue, stomach fundus, the longitudinal/circular smooth muscle of both colon and ileum, and colon submucosa. There was substantial expression of adipsin in colon submucosa and moderate expression in fundus, suggesting that in these regions at least some of the beta 3-AR signal may be contributed by fat. Pylorus and colon mucosa showed moderate levels of beta 3-AR mRNA with lower levels of adipsin. Ileum mucosa and submucosa showed low but readily detectable levels of beta 3-AR. 5. Expression of adipsin in rat skeletal muscles coupled to very low levels of beta 3-AR mRNA indicates that the observed beta 3-AR may be due to the presence of intrinsic fat. beta 3-AR mRNA was virtually undetectable in heart, lung and liver. These results raise the possibility that the atypical beta-AR demonstrated by functional and/or binding studies in muscle and in heart is not the beta 3-AR. 6. By use of two different sets of primers for amplification of beta 3-AR cDNA, no evidence was found for differential splicing of the mRNA in any of the tissues examined. 7. The detection of beta 3-AR mRNA in the gut mucosa and submucosa suggests that in addition to its established roles in lipolysis, thermogenesis and regulation of gut motility beta 3-AR may subserve other functions in the gastrointestinal tract. The absence of beta 3-AR mRNA in rat heart or its presence with adipsin in skeletal muscle suggests that atypical beta-adrenoceptor responses in heart and skeletal muscle are unlikely to be mediated by beta 3-AR.

Animals↗
PubMed1995

Beta-adrenoceptor regulation and functional responses in the guinea-pig following chronic administration of the long-acting beta 2-adrenoceptor agonist formoterol.

Formoterol is a long acting beta 2-adrenoceptor agonist designed for the alleviation of the symptoms of asthma. This study examined the effects of 14 day administration of formoterol (200 micrograms/kg/day i.p.) on beta 1- and beta 2-adrenoceptors in guinea-pig cardiac and lung tissue. Quantitative autoradiography was used to measure changes in receptor density and organ bath studies determined alterations in functional response. Formoterol treatment produced marked reductions of between 43% and 77% in beta 2-adrenoceptor density in all regions of the heart (atrioventricular node, bundle of His, right and left bundle branches, interventricular and interatrial septa, right and left atria, ventricles and apex) and lung (bronchial and vascular smooth muscle and parenchyma) (P < 0.01, n = 6). beta 1-Adrenoceptor density remained unchanged in all cardiac and lung regions. In functional studies (-)-isoprenaline was 4 fold less potent at causing relaxation of carbachol (1 microM) precontracted tracheal smooth muscle (pD2: control 8.49 +/- 0.03, formoterol 7.91 +/- 0.10, P < 0.001, n = 4), but formoterol treatment did not change the ability of (-)-isoprenaline to elicit a maximum response. The pKB values for ICI 118,551, 7.33 +/- 0.08 in the control and 7.20 +/- 0.01 in formoterol treated animals, were between those expected for beta 1- and beta 2-adrenoceptors suggesting involvement of both subtypes in the response. In spontaneously beating right atria and electrically paced left atria, tissues in which responses are largely mediated by beta 1-adrenoceptors, there was no significant change in responses to (-)-isoprenaline (right atria pD2: control 8.45 +/- 0.02; formoterol 8.42 +/- 0.11; P = 0.77, n = 4) (left atria pD2: control 8.25 +/- 0.03; formoterol 8.47 +/- 0.08; P = 0.09, n = 4). In the presence of CGP 20712A (100 nM) the pKB values did not change with formoterol treatment (left atria: control 9.59 +/- 0.12, formoterol 9.66 +/- 0.12; P = 0.70, n = 4) (right atria: control 8.93 +/- 0.11, formoterol 9.11 +/- 0.07; P = 0.25, n = 4). The doses and route of administration of formoterol used in this study differs from those used clinically. However, this study demonstrates that chronic formoterol administration produces selective down-regulation of beta 2-adrenoceptors in the lung and heart. The changes in the lung are accompanied by a shift to the right in the concentration-response curve to beta-agonist stimulation with no change in the maximum response.

Adrenergic beta-Agonists↗
PubMed1995

[3H]forskolin binding to cardiac adenylate cyclase in guinea pigs chronically infused with isoproterenol.

Guinea pigs were infused with the beta-adrenoceptor agonist isoproterenol (400 micrograms/kg/hr, 7 days) and cardiac adenylate cyclase was detected using [3H]forskolin. Two populations of [3H]forskolin binding sites were present in heart, the affinities (KD 2 nM and 280 nM) and densities (Bmax 9 and 900 fmol/mg protein) of which were unchanged by isoproterenol infusion compared with vehicle (1 mM HCl). The autoradiographic localisation of [3H]forskolin binding was also unchanged. The G protein activators NaF 10 mM and 5'-guanylylimidodiphosphate (Gpp(NH)p) 10 microM increased [3H]forskolin binding in heart from vehicle-treated animals by 100% and 80% respectively. NaF-stimulated binding was unchanged in isoproterenol-treated animals, however, Gpp(NH)p-stimulated binding was reduced by 35% which may indicate an increased influence of Gi.

Adenylyl Cyclases↗
PubMed1995

Signalling pathways in cardiac failure.

UNLABELLED: 1. Cardiac failure in humans and in animal models is associated with a marked desensitization of the catecholamine signalling pathway. 2. Beta 1- and beta 2- and possibly beta 3-adrenoceptors (beta-AR) are found in the hearts of humans and common laboratory animals such as rats and guinea-pigs. In rats and guinea-pigs chronic stimulation of cardiac beta-AR leads to a rapid loss of beta 2-AR whereas heart failure in humans is associated with a loss of beta 1-AR or beta 1-AR and beta 2-AR. 3. Desensitization is also associated with phosphorylation of beta-AR by beta-AR kinase (beta-ARK) and uncoupling of receptors from the signalling pathway. Beta-ARK but not beta-arrestin activity and mRNA are markedly increased in heart failure. 4. Chronic beta-AR stimulation and heart failure are associated with increases in Gi alpha but little if any change in Gs alpha. 5. The roles of beta gamma subunits of G-proteins, adenylate cyclase subtypes and cAMP dependent protein kinase A in heart failure are unclear at present. ABBREVIATIONS: beta-ARK - beta-adrenoceptor kinase AR - adrenoceptor G-protein - GTP binding protein

Animals↗
PubMed1995

beta-Adrenoceptor regulation in rat heart, lung and skin after chronic treatment with (--)-tertatolol or (--)-propranolol.

1. The effect of long-term treatment with the beta-adrenoceptor antagonists (--)-tertatolol and (--)-propranolol was studied. Sprague-Dawley rats were treated with either (--)-tertatolol (50 micrograms kg-1 hr-1), (--)-propranolol (250 micrograms kg-1 hr-1) or vehicle (1 mM HCl) for 14 days with osmotic minipumps implanted subcutaneously. 2. The mean daily systolic blood pressure and heart rate of rats treated with either (--)-tertatolol (108 +/- 1 mmHg/330 +/- 3 bpm) or (--)-propranolol (103 +/- 1 mmHg/330 +/- 2 bpm) were lower than in the control (126 +/- 1 mmHg/405 +/- 3 bpm, P < 0.001, n = 8-10) indicating the effectiveness of drug delivery. 3. Autoradiographic studies in areas of heart, lung and skin showed that beta-adrenoceptor populations were not significantly affected by the drug treatment (all regions P > 0.05). Nevertheless, the receptor population in the homogenates of (--)-tertatolol treated lung were halved (194 +/- 28 fmol mg protein-1 compared with a control value of 388 +/- 54 fmol mg protein-1, P < 0.01, n = 6). 4. In the presence of CGP 20712A, the left atrial inotropic and right atrial chronotropic responsiveness to (--)-isoprenaline were hypersensitive in both (--)-tertatolol and (--)-propranolol-treated groups (P < 0.005, ANCOVA). 5. (--)-Propranolol treated left ventricular free wall had lower basal [3H]-forskolin binding to adenylate cyclase (14.45 +/- 1.20 fmol mg protein-1 compared with a control value of 18.91 +/- 0.78 fmol mg protein-1, P = 0.01, n = 6). (--)-Tertatolol treatment had no effect on the basal binding. In the presence of the G-protein activators NaF and Gpp(NH)p, the enhancement of [3H]-forskolin binding did not differ between control and the drug treated groups. 6. Chronic (--)-tertatolol or (--)-propranolol treatment therefore did not produce an increase in receptors in heart, lung or skin but the beta-adrenoceptor-mediated responses were enhanced. In addition, [3H]-forskolin binding did not increase suggesting that the hypersensitivity was not due to changes in the number of receptors or adenylate cyclase. Hypersensitivity following beta-adrenoreceptor antagonist administration may therefore involve enhanced coupling of receptors to G-proteins.

Adrenergic beta-Agonists↗