Functional and molecular biological evidence for a possible beta3-adrenoceptor in the human detrusor muscle.
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The possible existence of a beta3-adrenergic receptor (beta3-AR) in the human detrusor muscle was investigated by in vitro functional studies and analysis of mRNA expression. Isoprenaline, noradrenaline and adrenaline each produced a concentration-dependent relaxation of the human detrusor. The rank order for their relaxing potencies was isoprenaline (pD2 6.37+/-0.07) > or = noradrenaline (pD2 6.07+/-0.12) > or = adrenaline (pD2 5.88< or =0.11). Neither dobutamine (beta1- and beta2-AR agonist) nor procaterol (beta2-AR agonist) produced any significant relaxation at concentrations up to 10(-5) M. BRL37344A, CL316243 and CGP-12177A (beta3-AR agonists), relaxed the preparations significantly at concentrations higher than 10(-6) M. The pD2 values for BRL37344A, CL316243 and CGP-12177A were 6.42+/-0.25, 5.53+/-0.09 and 5.74+/-0.14, respectively. CGP-20712A (10(-7) - 10(-5) M), a beta1-AR antagonist, did not affect the isoprenaline-induced relaxation. On the other hand, ICI-118,551, a beta2-AR antagonist, produced a rightward parallel shift of the concentration-relaxation curve for isoprenaline only at the highest concentration used (10(-5) > M) and its pKB value was 5.71+/-0.19. Moreover, SR58894A (10(-7) - 10(-5) M), a beta3-AR antagonist, caused a rightward shift of the concentration-relaxation curve for isoprenaline in a concentration-dependent manner. The pA2 value and slope obtained from Schild plots were 6.24+/-0.20 and 0.68+/-0.31. The beta1-, beta2- and beta3-AR mRNAs were all positively expressed in detrusor smooth muscle preparations in a reverse transcription polymerase chain reaction assay. In conclusion, the present results provide the first evidence for the existence of the beta3-AR subtype in the human detrusor. They also suggest that the relaxation induced by adrenergic stimulation of the human detrusor is mediated mainly through beta3-AR activation. (+info)
Evaluation of the effect on heart rate variability of a beta2-adrenoceptor agonist and antagonist using non-linear scatterplot and sequence methods.
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AIMS: To examine the impact on heart rate variability (HRV), of agonism or antagonism at the cardiac beta2-adrenoceptor in healthy volunteers, using standard time-domain summary statistics and non-linear methods (scatterplot and quadrant analysis). METHODS: Under double-blind and randomised conditions (Latin square design), 17 normal volunteers received placebo, salbutamol (beta2-adrenoceptor partial agonist), ICI 118,551 (specific beta2-adrenoceptor antagonist), or salbutamol plus ICI 118,551. Single oral doses of medication (at weekly intervals) were administered at 22.30 h, with HRV assessed from the sleeping heart rates. RESULTS: Salbutamol reduced the long-term (SDNN: 135 ms [120, 156], SDANN: 107 ms [89, 124]) time-domain indicators of HRV compared with placebo (SDNN: 39 [24, 55], SDANN 42 [29, 56], [mean difference [95% confidence intervals of difference]]). Alone, ICI 118,551 did not effect HRV, but in combination blocked the actions of salbutamol. Scatterplot length (944 ms [869, 1019]) and area (222*10(3) ms2 [191, 253]) were reduced by salbutamol compared with placebo; (length difference (164 [98, 230]) and area difference 59 [36, 83]). Scatterplot width (dispersion) was lower at both low (width RR-1 25% salbutamol 277 ms [261, 293]: salbutamol minus placebo 14 ms [0, 28]) and high (width 75% salbutamol 417 [391, 443]: salbutamol minus placebo 41 [20, 62]) heart rates. ICI 118,551 alone did not alter scatterplot parameters but in combination blocked the effect of salbutamol. Cardiac acceleration episodes (i.e. consecutive deltaRR and deltaRRn+1 shorten) were increased following salbutamol 7288 [6089, 8486] compared with placebo -1890 [-2600, -1179]; the beat-to beat difference (deltaRRn+1) was reduced after salbutamol compared with the other treatments. ICI 118,551 did not effect acceleration episodes but reduced the effect of salbutamol when used in combination. CONCLUSIONS: Agonism at the cardiac beta2-adrenoceptor in healthy volunteers with salbutamol altered autonomic balance towards sympathetic dominance; this re-balancing was blocked by ICI 118,551 given in combination with salbutamol. However antagonism at the beta2-adrenoceptor with ICI 118,551 alone did not significantly alter the HRV. The beta2-adrenoceptor modulates HRV in healthy volunteers; the implications of agonism and antagonism at the beta2-adrenoceptor in cardiovascular disease states warrants further investigation. (+info)
Aryl propanolamines: comparison of activity at human beta3 receptors, rat beta3 receptors and rat atrial receptors mediating tachycardia.
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1. The in vitro activity of four aryl propanolamines was compared to two prototypic beta3 receptor agonists, CGP 12177 and CL316243 at the human beta3 receptor, the rat beta3 receptor in the stomach fundus and receptors mediating atrial tachycardia. 2. L-739,574 was the most potent (EC50 = 9 nM) and selective agonist at the human beta3 receptor with high maximal response (74% of the maximal response to isoproterenol). 3. A phenol-biaryl ether analogue possessed modest affinity for the human beta3 receptor (EC50 = 246 nM), but was highly efficacious with a maximal response 82% of the maximal response to isoproterenol. The other derivatives were intermediate in potency with low maximal responses. 4. These agonists at the human beta3 receptor did not activate the rat beta3 receptor in the rat stomach fundus. In fact, the aryl propanolamines (10(-6) M) inhibited CL316243-induced activation of the rat beta3 receptor. Thus, agonist activity at the human beta3 receptor translated into antagonist activity at the rat beta3 receptor. 5. L739,574 and the phenol biaryl ether increased heart rate via beta1 receptors. 6. Although CGP12177 produced atrial tachycardia, neither the indole sulphonamide nor biphenyl biaryl ether did, although both had high affinity for the human beta3 receptor. Thus, the atrial tachycardic receptor was not identical to the human beta3 receptor. 7. These studies (a) characterized four aryl propanolamines with high affinity at the human beta3 receptor, (b) found that they were antagonists at the rat beta3 receptor, an observation with profound implications for in vivo rat data, and (c) established that the rodent atrial non-beta1, beta2 or beta3 tachycardic receptor was also unrelated to the human beta3 receptor. (+info)
Actions of vasoactive intestinal peptide on the rat adrenal zona glomerulosa.
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Previous studies, by this group and others, have shown that vasoactive intestinal peptide (VIP) stimulates aldosterone secretion, and that the actions of VIP on aldosterone secretion by the rat adrenal cortex are blocked by beta adrenergic antagonists, suggesting that VIP may act by the local release of catecholamines. The present studies were designed to test this hypothesis further, by measuring catecholamine release by adrenal capsular tissue in response to VIP stimulation. Using intact capsular tissue it was found that VIP caused a dose-dependent increase in aldosterone secretion, with a concomitant increase in both adrenaline and noradrenaline release. The effects of VIP on aldosterone secretion were inhibited by atenolol, a beta1 adrenergic antagonist, but not by ICI-118,551, a beta2 adrenergic antagonist. Binding studies were carried out to investigate VIP receptors. It was found that adrenal zona glomerulosa tissue from control rats contained specific VIP binding sites (Bmax 853+/-101 fmol/mg protein; Kd 2.26+/-0.45 nmol/l). VIP binding was not displaced by ACTH, angiotensin II or by either of the beta adrenergic antagonists. The response to VIP in adrenals obtained from rats fed a low sodium diet was also investigated. Previous studies have found that adrenals from animals on a low sodium diet exhibit increased responsiveness to VIP. Specific VIP binding sites were identified, although the concentration or affinity of binding sites in the low sodium group was not significantly different from the controls. In the low sodium group VIP was found to increase catecholamine release to the same extent as in the control group, however, in contrast to the control group, the adrenal response to VIP was not altered by adrenergic antagonists in the low sodium group. These data provide strong support for the hypothesis that VIP acts by the local release of catecholamines in adrenal zona glomerulosa tissue in normal animals. It does not appear that VIP acts through the same mechanism in animals maintained on a low sodium diet. The mechanism by which VIP stimulates aldosterone in this group remains to be determined. (+info)
Recent advances in the drug treatment of heart failure.
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A number of important questions surrounding the treatment of systolic congestive heart failure have been answered by randomised clinical trials completed within the past 2 years. In particular, these studies have established that high-dose angiotensin-converting enzyme (ACE) inhibition is more beneficial than low dose therapy, and that angiotensin II receptor antagonists are an acceptable alternative in patients unable to tolerate ACE inhibitors. Digoxin has been shown to be the only inotropic agent not associated with increased mortality, while amiodarone exerts a modest survival benefit in arrhythmiaprone patients. Beta-blockers appear to be beneficial for selected patients although their precise role remains to be defined by ongoing studies. (+info)
Altered airway and cardiac responses in mice lacking G protein-coupled receptor kinase 3.
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Contraction and relaxation of airway smooth muscles is mediated, in part, by G protein-coupled receptors (GPCRs) and dysfunction of these receptors has been implicated in asthma. Phosphorylation of GPCRs, by G protein-coupled receptor kinase (GRK), is an important mechanism involved in the dampening of GPCR signaling. To determine whether this mechanism might play a role in airway smooth muscle physiology, we examined the airway pressure time index and heart rate (HR) responses to intravenous administration of the cholinergic agonist methacholine (MCh) in genetically altered mice lacking one copy of GRK2 (GRK2 +/-), homozygous GRK3 knockout (GRK3 -/-), and wild-type littermates. (GRK2 -/- mice die in utero.) GRK3 -/- mice demonstrated a significant enhancement in the airway response to 100 and 250 microgram/kg doses of MCh compared with wild-type and GRK2 +/- mice. GRK3 -/- mice also displayed an enhanced sensitivity of the airway smooth muscle response to MCh. In addition, GRK3 -/- mice displayed an altered HR recovery from MCh-induced bradycardia. Although direct stimulation of cardiac muscarinic receptors measured as vagal stimulation-induced bradycardia was similar in GRK3 -/- and wild-type mice, the baroreflex increase in HR associated with sodium nitroprusside-induced hypotension was significantly greater in GRK3 -/- than wild-type mice. Therefore, these data demonstrate that in the mouse, GRK3 may be involved in modulating the cholinergic response of airway smooth muscle and in regulating the chronotropic component of the baroreceptor reflex. (+info)
Sympathovagal balance: how should we measure it?
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There are complex interactions between the sympathetic and parasympathetic nervous system inputs to the sinus node. The concept of "sympathovagal balance" reflects the autonomic state resulting from the sympathetic and parasympathetic influences. Despite widespread usage of a variety of heart rate (HR) variability parameters as indexes of sympathovagal balance, no index has been validated as a measure of sympathovagal balance. This study evaluated the utility of HR, HR variability, and a new parameter termed the vagal-sympathetic effect (VSE) as indexes of sympathovagal balance. The ideal parameter had to satisfy the following criteria: 1) the index should vary similarly among subjects in response to different autonomic conditions; 2) the variability in the index among subjects exposed to the same autonomic conditions should be small; and 3) the response of the index to various autonomic conditions should reflect the underlying changes in physiological state and have a meaningful interpretation. Volunteers [8 men, 6 women; mean age 28.5 +/- 4.8 (SD) yr] were evaluated for the effects of sympathetic and parasympathetic stimulation and blockade on HR and HR variability. VSE was defined as the ratio of the R-R interval to the intrinsic R-R interval. VSE and R-R interval consistently changed in the expected directions with parasympathetic and sympathetic stimulation and blockade. A general linearized model was used to evaluate the response of each parameter. VSE and R-R interval had r2 values of 0.847 and 0.852, respectively. Natural logarithm of the low-frequency power had an r2 value of 0.781 with lower r2 values for all the other HR variability parameters. The coefficient of variation was also lowest for each condition tested for the VSE and the R-R interval. VSE and R-R interval best satisfy the criteria for the ideal index of sympathovagal balance. Because it is impractical under most conditions to measure the VSE as the index of sympathovagal balance, the most suitable index is the R-R interval. (+info)
How does beta-adrenergic stimulation increase the heart rate? The role of intracellular Ca2+ release in amphibian pacemaker cells.
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1. The mechanism by which sympathetic transmitters increase the firing rate of pacemaker cells was explored in isolated cells from the sinus venosus of the cane toad Bufo marinus. Intracellular calcium concentration ([Ca2+]i) was measured with indo-1 and membrane potential and currents were recorded with the nystatin perforated-patch technique. 2. Adrenaline or isoprenaline (2 microM) increased the transient rise in [Ca2+]i and increased the firing rate; these effects were blocked by propranolol (2 microM). 3. To determine whether the changes in [Ca2+]i might influence the firing rate we studied agents which affect either the loading or the release of Ca2+ from the sarcoplasmic reticulum (SR). Rapid application of caffeine (10 mM) to spontaneously firing cells caused a large Ca2+ release from the SR and the cells were then quiescent for 24 s. In the presence of beta-adrenergic stimulation the caffeine-induced [Ca2+]i was 14 % larger but the period of quiescence after application was reduced to 12 s. 4. Ryanodine, at either low (1 microM) or high (> 10 microM) concentration, stopped firing. However, when the SR store content of Ca2+ was tested with caffeine, at low ryanodine concentration the SR Ca2+ store was empty whereas at the high concentration the SR store was still loaded with Ca2+. beta-Adrenergic stimulation was not able to restore firing at the low concentration of ryanodine but did restore firing at the high ryanodine concentration. 5. An SR Ca2+ pump blocker, 2, 5-di(tert-butyl)-1,4-hydroquinone (TBQ) which depletes the SR store of Ca2+, also rapidly and reversibly stopped spontaneous firing. 6. The relation between the amplitude of the [Ca2+]i transient and firing rate established in the presence of ryanodine was similar when firing was restored by beta-stimulation. 7. In both spontaneously firing and voltage-clamped cells, depleting the SR store with either ryanodine or TBQ suggested that about half of the Ca2+ which contributes to the calcium transient is released from the SR. 8. These results show that the amplitude of the [Ca2+]i transient is an important factor in the firing rate of toad pacemaker cells and consequently agents which modify SR Ca2+ release influence firing rate. The effects of beta-stimulation on firing rate seem to be largely mediated by changes in amplitude of the [Ca2+]i transient. (+info)