Responses mediated via beta 1, but not beta 2-adrenoceptors, exhibit hypothermia-induced supersensitivity

β-adrenergic stimulation after rewarming does not mitigate hypothermia-induced contractile dysfunction in rat cardiomyocytes

Victims of severe accidental hypothermia are frequently treated with catecholamines to counteract the hemodynamic instability associated with hypothermia-induced cardiac contractile dysfunction. However, we previously reported that the inotropic effects of epinephrine are diminished after hypothermia and rewarming (H/R) in an intact animal model. Thus, the goal of this study was to investigate the effects of Epi treatment on excitation-contraction coupling in isolated rat cardiomyocytes after H/R. In adult male rats, cardiomyocytes isolated from the left ventricle were electrically stimulated at 0.5 Hz and evoked cytosolic [Ca2+] and contractile responses (sarcomere length shortening) were measured. In initial experiments, the effects of varying concentrations of epinephrine on evoked cytosolic [Ca2+] and contractile responses at 37 °C were measured. In a second series of experiments, cardiomyocytes were cooled from 37 °C to 15 °C, maintained at 15 °C for 2 h, then rewarmed to 37 °C (H/R protocol). Immediately after rewarming, the effects of epinephrine treatment on evoked cytosolic [Ca2+] and contractile responses of cardiomyocytes were determined. At 37 °C, epinephrine treatment increased both cytosolic [Ca2+] and contractile responses of cardiomyocytes in a concentration-dependent manner peaking at 25-50 nM. The evoked contractile response of cardiomyocytes after H/R was reduced while the cytosolic [Ca2+] response was slightly elevated. The diminished contractile response of cardiomyocytes after H/R was not mitigated by epinephrine (25 nM) and epinephrine treatment reduced the exponential time decay constant (Tau), but did not increase the cytosolic [Ca2+] response. We conclude that epinephrine treatment does not mitigate H/R-induced contractile dysfunction in cardiomyocytes.

Decreased cAMP Level and Decreased Downregulation of β1-Adrenoceptor Expression in Therapeutic Hypothermia-Resuscitated Myocardium Are Associated With Improved Post-Resuscitation Myocardial Function

Background

Epinephrine administered during cardiopulmonary resuscitation (CPR) is associated with severe post‐resuscitation myocardial dysfunction. We previously demonstrated that therapeutic hypothermia reduced the severity of post‐resuscitation myocardial dysfunction caused by epinephrine; however, the relationship between myocardial adrenoceptor expression and myocardial protective effects by hypothermia remains unclear.

Methods and Results

Rats weighing between 450 and 550 g were randomized into 5 groups: (1) normothermic placebo, (2) normothermic epinephrine, (3) hypothermic placebo, (4) hypothermic epinephrine, and (5) sham (not subject to cardiac arrest and resuscitation). Ventricular fibrillation was induced and untreated for 8 minutes for all other groups. Hypothermia was initiated coincident with the start of CPR and maintained at 33±0.2°C for 4 hours. Placebo or epinephrine was administered 5 minutes after the start of CPR and 3 minutes before defibrillation. Post‐resuscitation ejection fraction was measured hourly for 4 hours then hearts were harvested. Epinephrine increased coronary perfusion pressure during CPR (27±6 mm Hg versus 21±2 mm Hg P<0.05). Post‐resuscitation myocardial function was impaired in the normothermic epinephrine group compared with other groups. The concentration of myocardial cAMP doubled in the normothermic epinephrine group (655.06±447.63 μmol/L) compared with the hypothermic epinephrine group (302.51±97.98 μmol/L; P<0.05). Myocardial β₁‐adrenoceptor expression decreased with normothermia cardiac arrest but not with hypothermia regardless of epinephrine.

Conclusions

Epinephrine, administered during normothermic CPR, increased the severity of post‐resuscitation myocardial dysfunction. This adverse effect was inhibited by intra‐arrest hypothermia resuscitation. Declined cAMP with more preserved β₁‐adrenoceptors in hypothermia‐resuscitated myocardium is associated with improved post‐resuscitated myocardial function in vivo.

Altered pharmacological effects of adrenergic agonists during hypothermia

Rewarming from accidental hypothermia is often complicated by hypothermia-induced cardiac dysfunction, calling for immediate pharmacologic intervention. Studies show that although cardiac pharmacologic support is applied when rewarming these patients, a lack of updated treatment recommendations exist. Mainly due to lack of clinical and experimental data, neither of the international guidelines includes information about pharmacologic cardiac support at temperatures below 30 °C. However, core temperature of accidental hypothermia patients is often reduced below 30 °C. Few human studies exploring effects of adrenergic drugs during hypothermia have been published, and therefore prevailing information is collected from pre-clinical studies. The most prominent finding in these studies is an apparent depressive effect of adrenaline on cardiac function when used in doses which elevate cardiac output during normothermia. Also noradrenaline and isoprenaline largely lacked positive cardiac effects during hypothermia, while dopamine is a more promising drug for supporting cardiac function during rewarming. Data and information from these studies are in support of the prevailing notion; not to use adrenergic drugs at core temperatures below 30 °C.

Negative inotropic effects of epinephrine in the presence of increased β-adrenoceptor sensitivity during hypothermia in a rat model

BACKGROUND

Animal studies show reduced inotropic effects of cardiac β-adrenoceptor agonists like epinephrine (Epi) during hypothermia and rewarming, while drugs targeting other pharmacological mechanisms have positive effects. This study therefore aimed to determine β-adrenoceptor sensitivity in isolated cardiomyocytes and investigate hemodynamic effects of Epi and its ability to stimulate cardiac β-adrenoceptors at different temperatures in vivo.

METHODS

Isolated rat myocardial cells were incubated with the radioactive β-adrenoceptor ligand [(3)H]-CGP12177 and propranolol, used as a displacer. Cells were subjected to normothermia (37 °C) or hypothermia (15 °C). After incubation, radioactivity was measured to estimate β-adrenoceptor affinity for propranolol (IC50), as a measure of β-adrenoceptor sensitivity. In separate in vivo experiments, Epi (1.25 μg/min) was administered the last 5min of experiments in normothermic (37 °C, 5h), hypothermic (4h at 15 °C) and rewarmed rats (4h at 15 °C, and subsequently rewarmed to 37 °C). Hemodynamic parameters were monitored during infusion. Hearts were thereafter freeze-clamped and tissue cAMP was measured.

RESULTS

In vitro measurements of IC50 for propranolol showed a hypothermia-induced increase in β-adrenoceptor sensitivity at 15 °C. Corresponding in vivo experiments at 15 °C showed decreased cardiac output and stroke volume, whereas total peripheral resistance (TPR) increased during Epi infusion, simultaneous with a 4-fold cAMP increase.

CONCLUSIONS

This experiment shows a hypothermia-induced in vivo and in vitro increase of cardiac β-adrenoceptor sensitivity, and simultaneous lack of inotropic effects of Epi in the presence of increased TPR. Our findings therefore indicate that hypothermia-induced reduction in inotropic effects of Epi is due to substantial elevation of TPR, rather than β-adrenoceptor dysfunction.

Cardiovascular effects of levosimendan during rewarming from hypothermia in rat

BACKGROUND

Previous research aimed at ameliorating hypothermia-induced cardiac dysfunction has shown that inotropic drugs, that stimulate the cAMP, - PKA pathway via the sarcolemmal β-receptor, have a decreased inotropic effect during hypothermia. We therefore wanted to test whether levosimendan, a calcium sensitizer and dose-dependent phosphodiesterase 3 (PDE3) inhibitor, is able to elevate stroke volume during rewarming from experimental hypothermia.

METHODS

A rat model designed for circulatory studies during experimental hypothermia (4h at 15°C) and rewarming was used. The following three groups were included: (1) A normothermic group receiving levosimendan, (2) a hypothermic group receiving levosimendan the last hour of stable hypothermia and during rewarming, and (3) a hypothermic placebo control group. Hemodynamic variables were monitored using a Millar conductance catheter in the left ventricle (LV), and a pressure transducer connected to the left femoral artery. In order to investigate the level of PKA stimulation by PDE3 inhibition, myocardial Ser23/24-cTnI phosphorylation was measured using Western-blot.

RESULTS

After rewarming, stroke volume (SV), cardiac output (CO) and preload recruitable stroke work (PRSW) were restored to within pre-hypothermic values in the levosimendan-treated animals. Compared to the placebo group after rewarming, SV, CO, PRSW, as well as levels of Ser23/24-cTnI phosphorylation, were significantly higher in the levosimendan-treated animals.

CONCLUSION

The present data shows that levosimendan ameliorates hypothermia-induced systolic dysfunction by elevating SV during rewarming from 15°C. Inotropic treatment during rewarming from hypothermia in the present rat model is therefore better achieved through calcium sensitizing and PDE3 inhibition, than β-receptor stimulation.

The effect of therapeutic hypothermia on drug metabolism and response: cellular mechanisms to organ function

INTRODUCTION

Therapeutic hypothermia is being employed clinically due to its neuro-protective benefits. Both critical illness and therapeutic hypothermia significantly affect drug disposition, potentially contributing to drug-therapy and drug-disease interactions. Currently, there is limited information on the known alterations in drug concentration and response during mild hypothermia treatment, and there is a limited understanding of the specific mechanisms that underlie alterations in drug concentrations and the potential clinical importance of these changes.

AREAS COVERED

A systemic review of the effect of therapeutic hypothermia on drug metabolism, disposition and response is provided. Specifically, the clinical and preclinical evidence of the effects of therapeutic hypothermia on blood flow, specific hepatic metabolism pathways, transporter function, renal excretion, pharmacodynamics and the effects during rewarming are reviewed.

EXPERT OPINION

Available evidence demonstrates that mild hypothermia decreases the clearance of a variety of drugs with apparently little change in drug-protein binding. Recent evidence suggests that the magnitude of the change is elimination route specific. Further research is needed to determine the impact of these alterations on both drug concentration and response in order to optimize the therapeutic hypothermia in this vulnerable patient population.

Changes in cardiovascular beta-adrenoceptor responses during hypothermia

The purpose of this study was to determine cardiovascular beta-adrenergic responses during hypothermia. In the present study, we used isoproterenol (Iso), a nonselective, potent beta-adrenoceptor agonist, well known for its positive chronotropic and inotropic pharmacologic actions at normothermia. Rats were instrumented to measure mean arterial pressure (MAP) and left ventricular (LV) pressure-volume changes using a Millar pressure-volume conductance catheter. Core temperature was manipulated from 37 (normothermia) to 24 degrees C (hypothermia) and back to 37 degrees C (rewarming) using both internal and external heat exchangers. During cooling at each temperature (33, 30, 27, and 24 degrees C), central hemodynamic variables and MAP were measured while intravenously infusing Iso (doses of 1.7, 5, 10, and 20 ng/min). Seven animals underwent all phases of the protocol. At normothermia Iso infusion resulted in a significant, dose-dependent increase in heart rate (HR), stroke volume (SV), cardiac output (CO), LV dP/dt(max) (left ventricular maximum derivative of systolic pressure over time) but no change in MAP. During cooling Iso infusion caused no dose-dependent change in any of the hemodynamic variables. After rewarming, baseline HR and LV dP/dt(max) were increased, whereas SV was significantly reduced when compared with their pre-hypothermic baseline values. This study shows that physiological cardiovascular responses mediated by the beta-adrenoceptor are significantly diminished during core hypothermia.

Moderate hypothermia induces a preferential increase in pancreatic islet blood flow in anesthetized rats

The aim of the study was to characterize the effects of induced moderate hypothermia on splanchnic blood flow, with particular reference to that of the pancreas and the islets of Langerhans. We also investigated how interference with the autonomic nervous system at different levels influenced the blood perfusion during hypothermia. For this purpose, hypothermia (body temperature of 28°C) was induced by external cooling, whereas normothermic (37.5°C) anesthetized Sprague-Dawley rats were used as controls. Some rats were pretreated with either propranolol, yohimbine, atropine, hexamethonium, or a bilateral abdominal vagotomy. Our findings suggest that moderate hypothermia elicits complex, organ-specific circulatory changes, with increased perfusion noted in the pylorus, as well as the whole pancreas and the pancreatic islets. The pancreatic islets maintain their high blood perfusion through mechanisms involving both sympathetic and parasympathetic mediators, whereas the increased pyloric blood flow is mediated through parasympathetic mechanisms. Renal blood flow was decreased, and this can be prevented by ganglionic blockade and is also influenced by β-adrenoceptors.

Effects of sympathetic stimulation during cooling on hypothermic as well as posthypothermic hemodynamic function

This experimental study was performed to explore hemodynamic effects of a moderate dose epinephrine (Epi) during hypothermia and to test the hypothesis whether sympathetic stimulation during cooling affects myocardial function following rewarming. Two groups of male Wistar rats (each, n = 7) were cooled to 15 °C, maintained at this temperature for 1 h, and then rewarmed. Group 1 received 1 μg/min Epi, i.v., for 1 h during cooling to 28 °C, a dose known to elevate cardiac output (CO) by approximately 25% at 37 °C. Group 2 served a saline solution control. At 37 °C, Epi infusion elevated CO, left ventricular systolic pressure, maximum rate of left ventricle pressure rise, and mean arterial pressure. During cooling to 28 °C, these variables, with the exception of mean arterial pressure, decreased in parallel to those in the saline solution group. In contrast, in the Epi group, mean arterial pressure remained increased and total peripheral resistance was significantly elevated at 28 °C. Compared with corresponding prehypothermic values, most hemodynamic variables were lowered after 1 h at 15 °C in both groups (except for stroke volume). After rewarming, alterations in hemodynamic variables in the Epi-treated group were more prominent than in saline solution controls. Thus, before cooling, continuous Epi infusion predominantly stimulates myocardial mechanical function, materialized as elevation of CO, left ventricular systolic pressure, and maximum rate of left ventricle pressure rise. Cooling, on the other hand, apparently eradicates central hemodynamic effects of Epi and during stable hypothermia, elevation of peripheral vascular vasopressor effects seem to take over. In contrast to temperature-matched, non-Epi stimulated control rats, a significant depression of myocardial mechanical function occurs during rewarming following a moderate sympathetic stimulus during initial cooling.

Affinities of full agonists for cardiac beta-adrenoceptors calculated by use of in vitro desensitization

Positive chronotropic and inotropic responses of guinea-pig isolated spontaneously beating right atria and paced left atria to beta-adrenoceptor agonists were recorded. Cumulative concentration-response curves for the rate and tension responses to isoprenaline, orciprenaline, terbutaline and fenoterol were obtained before incubation of the tissues with isoprenaline (10(-6) M), the tissues were then washed during 1 h to remove isoprenaline before constructing a post-incubation curve to the same agonist. Incubation with isoprenaline for 4 h caused parallel rightwards shifts to the curves, but extending the incubation to 8 h caused additional depression of the rate (85.2 +/- 5.4%) and tension (68.9 +/- 2.3%) maxima. Incubation with isoprenaline for 4 h only shifted the curves for orciprenaline to the right but depressed the post-incubation maximum rate and tension responses to terbutaline (74.8 +/- 1.5 and 33.8 +/- 2.5%) and fenoterol (85.6 +/- 5.5 and 76.0 +/- 5.1%). Thus incubation with isoprenaline for 4 h induced desensitization of the cardiac beta-adrenoceptor which was revealed as a loss in sensitivity to both isoprenaline and other beta-adrenoceptor agonists. The reduced maxima were attributed to a loss of beta-adrenoceptors and exhaustion of the receptor reserve. Those experiments displaying the reduced maxima were used for calculation of dissociation constants (KA) values by the method of Furchgott (1966) for irreversible antagonism. The values for the right atria were 57 (19-170)nM, 29 (8.4-100)microM and 13 (1.9-8.4)microM and for the left atria, 89 (15-510)nM, 25 (9.9-64)microM and 18 (7.7-42)microM for isoprenaline (8 h incubation), terbutaline and fenoterol respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparison of the properties of prenalterol and corwin at beta 1- and beta 2-adrenoreceptors in vitro

1. The affinities and efficacies (relative to isoprenaline) of prenalterol and corwin at beta 1- and beta 2-adrenoreceptors, have been determined in isolated cardiac and vascular tissues respectively. 2. Prenalterol and corwin have similar affinities for cardiac beta 1-adrenoreceptors. The affinity of prenalterol for beta 2-adrenoreceptors is approximately 10 times lower than for beta 1-adrenoreceptors; that for corwin is approximately 100 times lower than for beta 1-adrenoreceptors. 3. The efficacies of prenalterol and corwin, relative to isoprenaline, at beta 1 and beta 2-adrenoreceptors, are similar. 4. The greater selectivity of corwin compared with prenalterol, as an agonist at beta 1-adrenoreceptors, is a reflection of its lower affinity for beta 2-adrenoreceptors.

The use of forskolin to investigate the site of cardiac beta-adrenoreceptor supersensitivity

The positive inotropic responses of left atria and papillary muscles and the positive chronotropic responses of right atria of guinea-pigs to isoprenaline and forskolin were examined. An increase in sensitivity of the three preparations to isoprenaline was observed by lowering the bath temperature from 38 to 30 degrees C as demonstrated by a leftwards shift of the concentration-response curves. A similar degree of supersensitivity was observed for forskolin. Since forskolin is reputed to stimulate adenylate cyclase directly, whereas isoprenaline stimulates via the regulatory nucleotide Ns protein, this would suggest a common site for the supersensitivity at adenylate cyclase. However, the possibility that forskolin also stimulates via the Ns protein in producing cardiac stimulation and that this is the site of hypothermia-induced supersensitivity is discussed. Supersensitivity to isoprenaline was also observed in left atria and papillary muscles from guinea-pigs chronically pretreated with reserpine for 3 days (5 mg/kg at 72 h, 3 mg/kg at 48 and 24 h) or 7 days (0.1 mg/kg daily). In the same tissues, there was no change in the sensitivity to forskolin. The site of the supersensitivity can therefore be concluded to occur before the level of adenylate cyclase activation either directly or via the regulatory Ns protein; possibly at the beta-adrenoreceptor itself.

Hypothermia-induced supersensitivity to adenosine for responses mediated via A1-receptors but not A2-receptors

Four isolated tissues were examined in which the responses to adenosine are mediated via either A1- or A2-receptors. The responses examined were the inhibition of cholinergic transmission of field-stimulated guinea-pig ileum (A1), inhibition of noradrenergic transmission of field-stimulated rat vas deferens (A1), inhibition of developed tension of rat paced left atria (A1) and relaxation of carbachol-contracted guinea-pig trachea (A2). Cumulative concentration-response curves for adenosine and 2-chloroadenosine were constructed at 37, 30 or 27 degrees C. When plotted as a percentage of the maximum response, the concentration-response curves were displaced to the left by cooling in the ileum, vas deferens and atria, indicative of supersensitivity. This increase in sensitivity does not arise from inhibition of uptake or deamination by cooling, since it occurs equally for adenosine and 2-chloroadenosine, the latter being immune to these processes. In contrast, the sensitivity of the trachea was not affected (2-chloroadenosine) or reduced (adenosine) by cooling. Thus responses mediated via adenosine receptors of the A1 subtype exhibit hypothermia-induced supersensitivity, whereas those mediated via A2-receptors do not. This suggests a fundamental temperature-dependent difference between the two adenosine receptor subtypes.

Temperature dependence of beta 1-adrenoceptor-mediated responses examined by use of partial agonists

The inotropic and chronotropic responses of guinea pig atria, and the relaxation responses of guinea pig intestine, trachea, lung, uterus and vas deferens to catecholamines have been examined at bath temperatures of 38 degrees C and 30 degrees C. Hypothermia resulted in a supersensitivity of cardiac tissues with a decrease in isoprenaline EC50 and an increase in the maximum response to the partial agonist, salbutamol. Ileum responses to isoprenaline were potentiated at 30 degrees C but no partial agonist could be found on this tissue. Responses of the lung and vas deferens to partial agonists were not affected by temperature, while uterine responses were inhibited by hypothermia. The trachea was supersensitive to isoprenaline at 30 degrees C, however this was not due to a change in beta-adrenoceptor sensitivity but an inhibition of COMT. Partial agonist responses of trachea were similar at both temperatures. beta-Adrenoceptor supersensitivity was therefore observed only where responses are mediated primarily by beta 1-adrenoceptors and supports the concept that beta 1- but not beta 2-adrenoceptors exhibit hypothermia-induced supersensitivity.

A fundamental temperature-dependent difference between beta-adrenoceptor agonists and antagonists

Positive inotropic and chronotropic responses of guinea-pig isolated left and right atria to sympathomimetic amines were examined at bath temperatures of 38, 30 or 25 degrees C. The concentration-response curves to isoproterenol and orciprenaline were displaced to the left by cooling, indicating hypothermia-induced supersensitivity. The affinities of isoproterenol and orciprenaline were determined as their dissociation constants (pKA) from antagonism of their responses by either the functional antagonist carbachol or Ro 03-7894 which is reported to be an irreversible beta-adrenoceptor antagonist. By both methods of calculation, the affinities of isoproterenol and orciprenaline for the beta-adrenoceptors mediating inotropic and chronotropic responses were increased by lowering the temperature. In contrast, the affinity of practolol, measured as the pA2 for competitive antagonism of the isoproterenol- and orciprenaline-induced inotropic and chronotropic responses, did not increase with cooling. Thus hypothermia-induced supersensitivity is associated with an increase in agonist affinity only, which indicates a fundamental temperature-dependent difference between agonist and antagonist interactions with the beta-adrenoceptor.

Characterization of beta-adrenoceptors mediating relaxation of the guinea-pig ileum

Relaxation responses to sympathomimetic amines were recorded in potassium-contracted segments of guinea-pig ileum. Experiments were performed in the presence of phentolamine (5 X 10(-6) M) to eliminate beta-adrenoceptor-mediated effects. Metanephrine (10(-5) M) and desmethylimipramine (5 X 10(-7) M) were also present to prevent extraneuronal and neuronal uptake respectively. A potency order (-)-isoprenaline greater than (-)-noradrenaline greater than (-)-adrenaline was established, indicating a beta 1-adrenoceptor involvement for this relaxation. The potency of salbutamol (beta 2-selective) relative to isoprenaline in the ileum compared closely with its relative potency in isolated cardiac tissues (beta 1) but differed significantly from the value in lung parenchymal strips and vas deferens (beta 2). The pA2 values for antagonism of selective agonists (-)-noradrenaline (beta 1-selective) and fenoterol (beta 2-selective) by practolol (beta 1-selective) were identical, indicating a single beta 1-adrenoceptor population. The pA2 values for antagonism of these agonists by ICI 118,551 (beta 2-selective) were also identical and compatible with a beta 1-adrenoceptor population. Relaxation of the guinea-pig ileum is therefore mediated via a homogeneous population of beta 1-adrenoceptors.

Is Ro 03-7894 an irreversible antagonist at beta-adrenoceptor sites?

Ro 03-7894 (0.6 mM) produced a non-parallel shift to the right of dose-response curves to (-)-isoprenaline in K+ depolarized uterine preparations from the guinea-pig. The displacement of the curves was readily reversed by washing. A rightward shift of similar magnitude was also produced by Ro 03-7894 in transmurally stimulated ileal preparations. The relaxant effects of fenoterol in carbachol-contracted guinea-pig tracheal preparations (in the presence of 2 microM atenolol) were not altered by 0.6 mM Ro 03-7894. In the three tissues there was no evidence of a reduction in the maximal inhibitory response to the agonists. In uterine and tracheal preparations, Ro 03-7894 (0.6 mM) depressed contractile responses to exogenous calcium. The depression of responses was enhanced after washout of Ro 03-7894 for 80 min. Contractile responses of ileal preparations to transmural stimulation were also depressed by Ro 03-7894. Concentration-effect curves for the positive inotropic effects of (-)-isoprenaline in guinea-pig left atrial preparations were markedly shifted to the right and the maximum response depressed by 0.6 mM Ro 03-7894. Although the rightward shift of the curves was fully reversed during the 120 min washout period, the maximal responses remained depressed. In similar experiments, Ro 03-7894 produced a washout-resistant depression of inotropic responses to histamine and calcium. The results of radioligand binding studies in left atria using (-)-[125I]-iodocyanopindolol indicated that, when compared to the untreated atria, there was no reduction in the maximal density of binding sites 120 min after washout of 0.6 mM Ro 03-7894. 5 On the basis of the present results it is concluded that Ro 03-7894 induces a non-specific depressant effect on smooth and cardiac muscle preparations during exposure to the drug. This depressant effect persists following washout of the drug. There is no evidence for an irreversible effect of Ro 03-7894 at beta-adrenoceptor sites.

Effect of hypothermia on beta 1-adrenoceptor-mediated relaxation of pig bronchus

The relaxant potencies of (+/-)-isoprenaline and of (-)-noradrenaline (NA) in the pig isolated bronchus were increased 5.4 and 3.1 fold respectively by lowering the organ bath temperature from 37 degrees C to 27 degrees C, whereas the potencies of the non-catecholamine beta-adrenoceptor agonists fenoterol and orciprenaline were not significantly changed. At 37 degrees C, the catechol-O-methyl transferase (COMT) inhibitor U-0521 (30 microM), caused a 7.2 fold increase in the potency of isoprenaline but had no effect on the potency of fenoterol. At 27 degrees C the potency of isoprenaline was similar in the absence or presence of U-0521 (30 microM). Furthermore, in bronchi where extraneuronal uptake was inhibited by phenoxybenzamine, the potency of NA was not significantly altered by reducing bathing temperature from 37 degrees C to 27 degrees C. These results suggest that the hypothermic potentiation of isoprenaline in pig bronchus resulted from inhibition of COMT or of access to COMT, rather than from sensitization of beta 1-adrenoceptors.