Role of selective alpha and beta adrenergic receptor mechanisms in rat jejunal longitudinal muscle contractility.

Gut motility is modulated by adrenergic mechanisms. The aim of our study was to examine mechanisms of selective adrenergic receptors in rat jejunum. Spontaneous contractile activity of longitudinal muscle strips from rat jejunum was measured in 5-ml tissue chambers. Dose-responses (six doses, 10(-7) -3 x 10(-5)M) to norepinephrine (NE, nonspecific), phenylephrine (PH, alpha1), clonidine (C, alpha2), prenalterol (PR, beta1), ritodrine (RI, beta2), and ZD7714 (ZD, beta3) were evaluated with and without tetrodotoxin (TTX, nerve blocker). NE(3 x 10(-5)M) inhibited 74 +/- 5% (mean +/- SEM) of spontaneous activity. This was the maximum effect. The same dose of RI(beta2), PH(alpha1), or ZD(beta(3)) resulted in an inhibition of only 56 +/- 5, 43 +/- 4, 33 +/- 6, respectively. The calculated concentration to induce 50% inhibition (EC50) of ZD(beta3) was similar to NE, whereas higher concentrations of PH(alpha1) or RI(beta2) were required. C(alpha2) and PR(beta1) had no effect. TTX changed exclusively the EC50 of RI from 4.4 +/- 0.2 to 2.7 +/- 0.8% (p < 0.04). Contractility was inhibited by NE (nonspecific). PH(alpha1), RI(beta2), and ZD(beta3) mimic the effect of NE. TTX reduced the inhibition by RI. Our results suggest that muscular alpha1, beta2, and beta3 receptor mechanisms mediate adrenergic inhibition of contractility in rat jejunum. beta2 mechanisms seem to involve also neural pathways.

Recruitment of functionally active heart beta2-adrenoceptors in the initial phase of endotoxic shock in pithed rats.

A supersensitivity of the beta-adrenoceptor-mediated chronotropic response has been demonstrated in atria isolated from rats subjected to septic shock. Our study was undertaken to investigate whether bacterial endotoxin/LPS affects the increase in heart rate induced by beta-adrenoceptor agonists in the rat also in vivo. In pithed and vagotomized rats, the nonselective beta-adrenoceptor agonist isoprenaline (0.05-0.15 nmol/kg) and agonists at the high- and low-affinity state of beta1-adrenoceptors, that is, prenalterol (0.3-3 nmol/kg) and (+/-)-4-[3-[(1,1-dimethylethyl)amino]-2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazole-2-one (CGP 12177; 3-6 nmol/kg), respectively, and at beta2-adrenoceptors, that is, fenoterol (1-5 nmol/kg), increased heart rate by 50 to 60 beats/min. Administration of LPS (0.4, 1, and 1.5 mg/kg), under continuous infusion of vasopressin, dose-dependently amplified the chronotropic response to isoprenaline, prenalterol, and fenoterol (by 80%, 50%, and 100%, respectively) but not to CGP 12177. The beta2-adrenoceptor antagonist erythro-(+/-)-1-(7-methylindan-4-yloxy)-3-isopropylaminobutan-2-ol (ICI 118551 0.1 mumol/kg) did not affect the chronotropic responses of isoprenaline, fenoterol, and prenalterol under non-endotoxic conditions, but abolished the potentiation of tachycardia produced by LPS (1.5 mg/kg). The beta1-adrenoceptor antagonist (+/-)-2-hydroxy-5-[2-[[2-hydroxy-3-[4-[1-methyl-4-(trifluoromethyl)-1H-imidazol-2-yl]-phenoxy]propyl]-amino]ethoxy]-benzamide CGP 20712A; 0.1 mumol/kg almost completely reduced the chronotropic effects of isoprenaline, fenoterol, and prenalterol both in control rats and in animals exposed to LPS (1.5 mg/kg). We conclude that LPS sensitizes cardiac beta-adrenoceptors by recruiting functionally active beta2-adrenoceptors, but the amplification of tachycardia occurs only when both beta1- and beta2-adrenoceptors are concomitantly activated. The pithed rat may serve as a model to examine the beta-adrenoceptor supersensitivity in vivo.

Positive inotropic and lusitropic effects mediated via the low-affinity state of beta1-adrenoceptors in pithed rats.

1 Activation by CGP 12177 and cyanopindolol of the human and rat low-affinity state of beta(1)-adrenoceptors increases frequency and contractile force and hastens relaxation in isolated cardiac tissues, and probably relaxes isolated vessels. In order to identify the positive inotropic, positive lusitropic and vasodilator effects of both agonists also in vivo, we have determined their effects on the left ventricular systolic pressure (LVSP), the rate of intraventricular pressure rise (+dP dt(-1)(max)) and decline (-dP dt(-1)(max)), the diastolic blood pressure (DBP) and the mesenteric blood flow (MBF) in pithed and vagotomized rats. 2 CGP 12177 (0.1-100 nmol kg(-1)) and cyanopindolol (1-1000 nmol kg(-1)) dose-dependently enhanced all cardiac parameters. The nonselective beta-adrenoceptor antagonist bupranolol 10 micromol kg(-1) diminished the CGP 12177 (100 nmol kg(-1))-stimulated increases in LVSP from 26.3+/-8.2 to 13.1+/-1.8 mmHg (P<0.05), +dP dt(-1)(max) from 5287+/-290 to 2439+/-296 mmHg s(-1) (P<0.001) and -dP dt(-1)(max) from -3836+/-301 to -2187+/-443 mmHg s(-1) (P<0.05), respectively. The beta(1)-adrenoceptor antagonist CGP 20712A 10 micromol kg(-1) (known to block the low-affinity state of beta(1)-adrenoceptors at high doses) inhibited increases in +/-dP dt(-1)(max) elicited by the highest dose of CGP 12177. 3 The highest doses of CGP 12177 and cyanopindolol increased DBP by about 10 mmHg and MBF by 1.4+/-0.3 and 0.6+/-0.3 ml min(-1), respectively. The vascular effects of CGP 12177 were not affected by bupranolol and CGP 20712A. 4 In conclusion, activation of the low-affinity state of beta(1)-adrenoceptors by CGP 12177 and cyanopindolol in pithed rats causes a positive inotropic and lusitropic effect. By contrast, the vascular effects of CGP 12177 and cyanopindolol are not mediated by these receptors and have only marginal influence under in vivo conditions.

Determination of phenolic sympathomimetic drugs in pharmaceutical samples and biological fluids by flow-injection chemiluminescence.

A rapid and highly sensitive flow-injection chemiluminometric method was developed for determination of 3 sympathomimetic drugs, namely, etilefrine hydrochloride, isoxsuprine hydrochloride, and prenalterol hydrochloride. The method is based on chemiluminescence induced by oxidation of drugs with acidified potassium permanganate in the presence of formic acid as a carrier. The calibration graphs were linear over the concentration ranges 0.2-9, 0.2-12.5, and 0.025-1.25 microg/mL for the 3 compounds, respectively. The method was applied successfully in determining the drugs in dosage forms and in biological fluids. A proposal for the reaction pathway is suggested.

Cardiovascular mediation of clonidine-induced ultrasound production in infant rats.

In infant rats, administration of the alpha2 adrenoceptor agonist clonidine simultaneously evokes ultrasound production and bradycardia. In this study the authors examined in 8-day-old rats whether these 2 responses to clonidine are causally related. In Experiment 1 pups were pretreated with saline or prenalterol (0.1 or 1.0 mg/kg), a beta1 adrenoceptor agonist that increases cardiac rate, followed by administration of clonidine (1.0 mg/kg). Prenalterol pretreatment suppressed clonidine-induced ultrasound production at both doses. Prenalterol also increased skin temperature, however, suggesting that suppression of ultrasound was modulated in part by increased body temperature. Consistent with this suggestion, in Experiment 2 mild hyperthermia significantly inhibited clonidine-induced ultrasound production. Finally, in Experiment 3 the authors found that the pretreatments used in Experiments 1 and 2 prevent or dampen the effects of clonidine on cardiac rate. These results suggest that clonidine's effect on ultrasound production is mediated by its effects on the cardiovascular system.

Heart-rate variability effects of beta-adrenoceptor agonists (xamoterol, prenalterol, and salbutamol) assessed nonlinearly with scatterplots and sequence methods.

Full antagonists of the cardiac beta-adrenoceptor improve heart-rate variability (HRV) in humans; however, partial agonism at the beta2-adrenoceptor has been suggested to decrease HRV. We therefore studied the HRV effects of some partial agonists of the beta1- and beta2-adrenoceptors in normal volunteers. Under double-blind and randomised conditions (Latin square design), eight healthy volunteers received placebo; xamoterol, 200 mg (beta1-adrenoceptor partial agonist); prenalterol, 50 mg (beta1- and beta2-adrenoceptor partial agonist); salbutamol, 8 mg (beta2-adrenoceptor partial agonist); ICI 118,551, 25 mg (selective beta2-adrenoceptor antagonist); and combinations of each partial agonist with ICI 118,551. Single oral doses of medication (at weekly intervals) were administered at 22:30 h with HRV assessed from the overnight sleeping heart rates. HRV was determined by using standard time-domain summary statistics and two nonlinear methods, the Poincaré plot (scatterplot) and cardiac sequence analysis. On placebo, the sleeping heart rate decreased significantly, between 2 and 8 h after dosing. The heart rate with ICI 118,551 was unaltered. Xamoterol, prenalterol, and salbutamol increased the sleeping heart rate. ICI 118,551 blocked the heart-rate effects of salbutamol, attenuated those of prenalterol, but did not influence the xamoterol heart rate. The scatterplot (Poincaré) area was reduced by beta1-adrenoceptor (xamoterol), beta2-adrenoceptor (salbutamol), and combined beta1- and beta2-adrenoceptor (prenalterol) agonism. A reduction in scatterplot length followed salbutamol, prenalterol alone, and prenalterol in combination with ICI 118,551. The geometric analysis of the scatterplots allowed width assessment (i.e., dispersion) at fixed RR intervals. At higher heart rates (i.e., 25 and 50% of RR scatterplot length), dispersion was decreased after xamoterol, prenalterol, and prenalterol/ICI 118,551. Cardiac sequence analysis (differences between three adjacent beats; deltaRR vs. deltaRRn+1) assessed the short-term patterns of cardiac acceleration and deceleration; four patterns were identified: +/+ (a lengthening sequencing), +/- or -/+ (balanced sequences), and finally -/- (a shortening sequence). Cardiac acceleration or deceleration episodes (i.e., number of times deltaRR and deltaRRn+1 were altered in the same direction) were increased after salbutamol and prenalterol. In conclusion, partial agonism at either the cardiac beta1-adrenoceptor (xamoterol), beta2-adrenoceptor (salbutamol), and beta1- plus beta2-adrenoceptors (prenalterol) altered the autonomic balance toward sympathetic dominance in healthy volunteers; blockade of the beta2-adrenoceptor with the highly selective beta2-antagonist ICI 118,551 prevented the effects of salbutamol on HRV, attenuated the HRV effects of prenalterol, but had no effect on the actions of xamoterol. Agonism at both the beta1- and beta2-adrenoceptor reduced HRV in healthy subjects; the implications for the preventive use of the beta-adrenoceptor compounds in cardiovascular disease warrant further investigation.

Beta-adrenoceptor agonist stimulation of pulmonary nitric oxide production in the rabbit.

Nitric oxide (NO) is continuously produced in the lung and is present in exhaled air. We examined the effect of beta-adrenoceptor stimulation on the production of pulmonary NO in rabbits. Exhaled NO was measured by chemiluminescence in anaesthetized and mechanically ventilated rabbits and in buffer-perfused rabbit lungs. Intravenous infusions of adrenaline (0.1-10 microg kg(-1) min(-1)) elicited dose-dependent increases in exhaled NO. The increases in exhaled NO comprised an initial peak followed by a lower plateau level. The increase in exhaled NO was inhibited by propranolol (1 mg kg(-1)) but not by phentolamine (1 mg kg(-1)). Prenalterol, a beta1-adrenoceptor agonist, and terbutaline, a beta2-adrenoceptor agonist, also caused dose-dependent increases in exhaled NO. However, prenalterol was >100 times more potent than terbutaline. Infusions of forskolin (0.01-0.03 micromol kg(-1) min(-1)), an adenylate cyclase stimulator, elicited dose-dependent decreases in blood pressure and concomitant increases in heart rate but caused no alterations in exhaled NO. Nimodipine, a L-type calcium channel blocker, antagonized the increases in exhaled NO in response to prenalterol infusions. The increases in exhaled NO in response to adrenaline and prenalterol were also present in blood-free, buffer perfused lungs during constant-flow conditions. These results demonstrate that pulmonary nitric oxide production can be enhanced by beta-adrenoceptor stimulation. Furthermore, the results indicate that the beta-adrenergic stimulation of pulmonary NO production is not critically dependent on cyclic AMP formation but may require intact calcium-channels.

Spectrofluorimetric determination of prenalterol hydrochloride in pharmaceutical preparations and biological fluids.

A simple and highly sensitive fluorimetric method was developed for the routine determination of prenalterol hydrochloride in bulk, in dosage forms and in biological fluids. The method is based on the fluorescence induced by reaction of the nitroso-derivative of prenalterol hydrochloride with 2-cyanoacetamide in the presence of ammonia. The different experimental parameters were carefully studied and incorporated into the procedure. The fluorescence is measured at 440 nm after excitation at 368 nm. Fluorescence intensity is a linear function of prenalterol hydrochloride concentration over the range of 0.1-2.8 microg x ml(-1) in the solution finally measured. A proposal for the reaction pathway was suggested.

Fluorimetric determination of prenalterol hydrochloride in pharmaceuticals and biological fluids based on its oxidation reaction by hexacyanoferrate(III).

A rapid and sensitive fluorimetric method for the determination of prenalterol hydrochloride is presented, based on the oxidation of the analyte with potassium hexacyanoferrate(III) in a slightly alkaline medium (pH 9.23). The different experimental parameters were carefully studied and incorporated into the procedure. The oxidation product exhibits a blue fluorescence with its emission maximum at 427 nm, and excitation maximum at 314 nm. Fluorescence intensity is a linear function of prenalterol hydrochloride concentration over the range of 0.2-3.6 microg/ml(-1) in the solution finally measured. The method was successfully applied to the determination of prenalterol hydrochloride in pharmaceutical formulations and biological fluids. A proposal for the reaction pathway is suggested.

Pharmacological and molecular characterisation of beta-adrenoceptors in adult rat diaphragm muscle.

Using pharmacological and molecular approaches to investigate beta-adrenoceptor (beta-AR) subtype expression in adult rat diaphragm, we found that adenylyl cyclase (AC) was potently stimulated by the beta2-AR-selective agonist fenoterol, weakly stimulated by the beta1-AR-selective agonist prenalterol and unaffected by the beta3-AR agonist CGP12177. AC activity in response to a submaximal isoproterenol concentration was potently inhibited by the beta2-AR-selective antagonist ICI118551, whereas the beta1-AR-selective antagonist CGP20712A was effective only in very high concentrations. (-)-[125I]-cyanopindolol ([125I]-CYP) saturation binding experiments indicated a single affinity component (dissociation constant (Kd) = 22 +/- 2 pM) for beta-AR sites (maximal beta -AR density (Bmax) = 14 +/- 2 fmol/ mg). Eadie-Hofstee analysis of [125I]-CYP displacement curves by beta1-, beta2- or beta3-AR-selective ligands allowed to characterise a homogeneous population of beta2-AR sites. Finally, reverse transcriptase-polymerase chain reaction analysis of beta-AR subtype mRNAs identified beta2-AR transcripts but no beta1- and beta3-AR mRNAs. Our results demonstrate that beta2-AR is the only beta-AR subtype expressed in the diaphragm.

Sino-aortic denervation causes right atrial beta adrenoceptor down-regulation.

Rat isolated right atria obtained 1 wk after sinoaortic denervation were less sensitive to the chronotropic actions of beta-agonists than were tissues obtained from animals that underwent sham surgery or no surgery at all. The potencies, but not the maximal responses for two high efficacy agonists, norepinephrine and isoproterenol, were reduced about 3- to 4-fold. Sino-aortic denervation (SAD) caused about a 3-fold decrease in potency and about a 60% decrease in maximal response for a low efficacy agonist, prenalterol. The changes in the actions of these agonists occurred in the absence of any changes in the subtype of beta receptor mediating the chronotropic response. The results of analyses of the data for prenalterol showed that SAD caused a decrease in the operational efficacy of this agonist without any changes in its KD value for beta-1 adrenoceptors. SAD had no effect on the responses of the tissue to blockade of uptake 1 and uptake 2, suggesting no compensatory changes in the removal processes caused the decreased potency. The results of radioligand binding assays showed that SAD caused a decrease in the maximal binding of 125I-cyanopindolol without altering its KD. Also, the results of competition binding assays confirmed the lack of effect of SAD on the KD for prenalterol. The SAD-induced changes in the actions of agonists acting at right atrial beta-1 receptors were caused by a down-regulation of beta-1 adrenoceptors, which probably occurred in response to SAD-induced increases in sympathetic tone.

Effect of selective beta-adrenoceptor stimulation on UCP synthesis in primary cultures of brown adipocytes.

Given the co-existence of the three beta-adrenoceptor (beta AR) subtypes (beta 1AR, beta 2AR and beta 3AR) in brown adipocytes, the present study was undertaken to determine the relative importance of these in the induction of UCP synthesis in mouse BAT precursor cells in primary culture. Cells at different stages of differentiation were exposed to different beta AR agonists: prenalterol (a selective beta 1AR agonist), salbutamol or clenbuterol (selective beta 2AR agonists), or BRL 37344 (a selective beta 3AR agonist). As with the endogenous agonist, noradrenaline, and the non-selective beta AR agonist, isoprenaline, all four beta AR agonists induced UCP in the confluent stage of the cells, but with different potencies, and with the highest induction being seen after clenbuterol or BRL 37344 treatment. Cells in the confluent stage of development were the most sensitive to the effects of the agonists, although clenbuterol and BRL 37344 induced a weak UCP synthesis in pre-confluent cells. None of these beta AR agonists were able to induce UCP synthesis in the post-confluent period. The responses to prenalterol and salbutamol were inhibited by propranolol at relatively low concentrations, suggesting their effects were mediated by beta 1AR and beta 2AR, respectively. However, propranolol was a particularly weak antagonist of BRL 37344 and, unexpectedly, of the clenbuterol UCP responses, which suggests that both induce UCP synthesis via the beta 3AR. In summary, the beta 3AR is the most important adrenoceptor coupled to the induction of UCP synthesis, although both beta 1AR and beta 2AR activation may make a contribution. However, all three beta AR subtypes do not become fully functional until cultured cells become confluent.

Influence of adrenoceptor agonists and antagonists on fluid secretion in small bowel obstruction.

OBJECTIVES: To investigate the importance of adrenoceptors on fluid losses in small bowel obstruction. DESIGN: Evaluation of the effects of adrenergic agonists and antagonists on in-vivo net fluid secretion in chronic small bowel obstruction in rats. METHODS: Net fluid transport in a jejunal segment was continuously registered in vivo after 18 h of mechanical obstruction of the small bowel in anaesthetized rats. The effect on net fluid transport of adrenoceptor agonists and antagonists and of isotonic saline was quantified. RESULTS: Clonidine, an alpha 2-agonist, had a significant (P < 0.05) anti-secretory effect, while yohimbine, an alpha 2-antagonist, significantly (P < 0.05) increased net fluid secretion. Phenylephrine, an alpha 1-agonist, and prazosin, an alpha 1-antagonist, lacked significant effects on net fluid transport. Similarly, prenalterol, a beta 1-agonist, and metoprolol, a beta 1-antagonist, had no significant effect on the net fluid transport. The beta 2-agonist salbutamol significantly (P < 0.001) decreased net fluid secretion, while the beta-antagonist propranolol significantly (P < 0.001) decreased net fluid secretion. CONCLUSION: Activation of alpha 2-adrenoceptors and blockade of beta 2-adrenoceptors significantly reduce net fluid secretion in small bowel obstruction. Results also demonstrate a continuous stimulatory effect on fluid secretion mediated by beta 2-receptors and a continuous inhibitory effect mediated by alpha 2-receptors.

Polarographic determination of prenalterol hydrochloride through treatment with nitrous acid.

A simple and sensitive polarographic method is described for the determination of prenalterol hydrochloride through treatment with nitrous acid. The different experimental parameters affecting the derivatization process, as well as the polarographic analysis were studied. The derivatization product was found to be reducible at the dropping mercury electrode over the whole pH range in Britton-Robinson buffers. At pH 5, a well defined diffusion-controlled cathodic wave was produced. The limiting current vs. the concentration plot was linear over the range 0.015-0.15 mM in the direct current mode, with a minimum detectability of 0.8 microM. The proposed method was applied successfully to the determination of prenalterol hydrochloride either in the pure form or in its dosage forms.

Facilitation of noradrenaline release from rat brain slices by beta-adrenoceptors.

The present study examined whether stimulation of beta-adrenoceptors facilitated noradrenaline release in the rat brain. Electrical stimulation-evoked overflow of tritium from rat cerebral cortical, hypothalamic and hippocampal slices labelled with 3H-noradrenaline was measured during superfusion for 100 min. Tissue slices were electrically simulated (1 Hz, 20 mA, 2 ms, 2 min), at 20 (S1) and 70 (S2) min after the onset of superfusion. The nonselective beta-adrenoceptor agonist isoproterenol (0.1-10 nM) enhanced stimulation-evoked overflow of tritium from slices of cerebral cortex, hypothalamus and hippocampus in a concentration-dependent manner; mean S2/S1 ratios with 10 nM isoproterenol were 161 +/- 11%, 142 +/- 15% and 143 +/- 12% of control, respectively, in the three brain regions. The facilitatory effect of isoproterenol in cerebral cortical slices was antagonized by propranolol (50 nM), a nonselective beta-adrenoceptor antagonist, and by the beta 1- and beta 2-selective adrenoceptor antagonists ICI 89,406 (1 nM) and ICI 118,551 (1 nM), respectively. The beta 1- and beta 2-selective adrenoceptor agonists prenalterol and albuterol (0.1-10 nM), respectively, also increased stimulation-evoked overflow of tritium from cerebral cortical slices; these effects were antagonized by beta-adrenoceptor antagonists. These findings suggest that stimulation of beta-adrenoceptors enhance noradrenaline release from rat cerebral cortical, hypothalamic and hippocampal slices; this release mechanism appears to involve both beta 1- and beta 2-adrenoceptor subtypes. These facilitating presynaptic receptors may be involved in mediating the antidepressant-like behavioral effects of beta 2-adrenoceptor agonists.

Facilitation of norepinephrine release from cerebral cortex is mediated by beta 2-adrenergic receptors.

Facilitatory effects of prenalterol and albuterol (beta 1- and beta 2-selective adrenergic agonists, respectively) in the absence and presence of propranolol (a nonselective beta-adrenergic antagonist), ICI 89,406 or ICI 118,551 (beta 1- and beta 2-selective adrenergic antagonists, respectively) on electrical stimulation-evoked release of 3H-NE from rat cerebral cortical slices were assessed. Albuterol (0.1-100 nM) increased evoked release of 3H-NE from the cerebral cortical slices with greater potency than prenalterol (1-100 nM). The beta 2-adrenergic antagonist ICI 118,551 (1 nM) and propranolol (50 nM) abolished the facilitatory effects of albuterol (0.1 and 10 nM). In contrast, the beta 1-adrenergic antagonist ICI 89,406 (1 nM) did not alter the release-enhancing effect of albuterol. Prenalterol (10 and 100 nM)-induced facilitation of evoked release of 3H-NE was abolished by ICI 118,551; propranolol reduced the effect of 10 nM prenalterol and abolished that of 100 nM prenalterol. ICI 89,406 inhibited the effect of 100 nM prenalterol without altering that of 10 nM prenalterol. Basal release of 3H-NE was not altered by the drugs used in this study. These results suggest that facilitation of 3H-NE release induced by beta-adrenergic agonists is mediated primarily by beta 2-adrenergic receptors.

The inhibitory GTP-binding protein (Gi) regulates the agonistic property of beta-adrenergic ligands in isolated rat adipocytes. Evidence for a priming effect of cyclic AMP.

Prenalterol, an allegedly beta 1-selective adrenergic agonist with high intrinsic sympathomimetic activity (ISA), was shown to be weakly lipolytic in rat adipocytes. However, in pertussis-toxin-treated adipocytes, the ISA of prenalterol was markedly increased (from 10-20% to approx. 100% of that of isoprenaline). The cellular sensitivity was also increased (EC50 approx. 60 nM and approx. 3 microM in pertussis-toxin-treated and control cells respectively). A similar effect was seen for other partial agonists such as the beta 2-selective agonist terbutaline and for beta-adrenergic antagonists with some intrinsic activity (metoprolol, pindolol). There was no clear change in sensitivity to isoprenaline's ability to stimulate adenylate cyclase in adipocyte membranes from pertussis-toxin-treated animals but the cyclase activity was increased approx. 4-fold in the presence of 1 microM-GTP. Prenalterol stimulated lipolysis by only small increases in intracellular cyclic AMP (cAMP) levels (less than 10% of that seen with isoprenaline). Basal lipolysis was increased in cells from pertussis-toxin-treated rats and the cellular sensitivity to the non-degradable cAMP analogue, N6-monobutyryl-cAMP, was increased. In control cells, a submaximal concentration of prenalterol (0.1 microM) increased the sensitivity to the cAMP analogues, N6-monobutyryl-cAMP and 8-bromo-cAMP. A low concentration (1 mM) of 8-bromo-cAMP also increased the effect of prenalterol. Similar effects were seen when the phosphodiesterase was inhibited. Thus (1) lipolysis is extremely sensitive to small increases in intracellular cAMP; (2) the degree of activation of adenylate cyclase and thus cAMP formation is the rate-limiting step for the biological response of partial agonists; (3) the inhibitory GTP-binding protein, Gi, is an important modulator ('tissue factor') of the beta-adrenergic agonistic property; (4) low levels of cAMP exert a priming effect on protein kinase A.