Influence of temperature on the sensitivity of the adrenoceptors in the isolated atria of guinea pigs and rats

Effects of epinephrine on the myocardial performance and haemodynamics of the isolated rat heart during moderate hypothermia--importance of calcium homeostasis

Controversies exist concerning myocardial performance in hypothermia. We have studied the effects of epinephrine on myocardial function at various calcium concentrations in moderate hypothermia (28 degrees C) and normothermia (37 degrees C) using an isolated antegrade perfused rat heart. The maximum pressure velocity (dP/dt(max)) developed was significantly higher in normothermia compared with hypothermia and was improved by the addition of calcium in both circumstances. Peak negative pressure velocity (dP/dt(min)) was significantly higher at 37 degrees C compared with 28 degrees C, and was increased by the addition of calcium in normothermia; in contrast to hypothermia, in which no change of dP/dt(min) was observed. A reduction in cardiac output and stroke volume by hypercalcaemia was observed in hypothermia. The addition of epinephrine improved dP/dt(max) and dP/dt(min), but had a depressive effect on stroke volume and cardiac output at normal and elevated calcium concentrations. Myocardial efficiency was significantly higher during hypothermia compared with normothermia, but was impaired by epinephrine during hypothermia. The variable or even paradoxical effects of epinephrine suggest the need for careful haemodynamic monitoring and determination of calcium levels in hypothermia. The impairment of myocardial performance may be explained by impaired diastolic relaxation and calcium overload.

Effects of dopamine on porcine myocardial action potentials and contractions at 37 degrees C and 32 degrees C

BACKGROUND

Little information exists on the effects of drugs with cardiovascular action in hypothermia, and some findings have indicated paradoxic effects of dopamine in this setting. As we have not found any data on the electrophysiologic and contractile effects of dopamine on the heart in hypothermia, we decided to study this in pig myocardium, since pigs have a cardiovascular system more similar to that of humans than other animals.

METHODS

Excised muscle strips from pig ventricular septum were mounted in an organ bath. After 45 min of equilibration at 37 degrees C or 32 degrees C, resting and action potentials, time to peak contraction and contractile force were recorded during pacing with a frequency of 60/min. Dopamine at 4 microM or 8 microM was added and new recordings were made after 15 min.

RESULTS

Cooling to 32 degrees C caused a prolongation of contraction by 48% and the contractile force increased by 39%. The membrane action potential duration at 50% and 90% repolarization levels increased at 32 degrees C by 28% and 16% respectively. Dopamine significantly (P<0.05) increased the contractile force and membrane action potential duration at 50% and 90% repolarization levels both in normothermia and in hypothermia, whereas the duration of the contraction was not significantly changed.

CONCLUSION

Cooling to 32 degrees C significantly prolongs the myocardial action potential and the contraction duration. Dopamine increases the contractile force and prolongs the action potential both at 37 degrees C and at 32 degrees C.

Effect of bretylium tosylate on ventricular fibrillation threshold during hypothermia in dogs

How bretylium tosylate affected the ventricular fibrillation threshold, electrophysiological parameters, and plasma catecholamine levels during hypothermia in dogs was studied. Threshold for ventricular fibrillation was determined by programmed electrical stimulation using a stimulation protocol that involved applying a maximum of five extrastimuli at body temperatures 37, 34, 31, 28, and 25 degrees C, and at the same temperatures during rewarming. Electrocardiogram, epicardial monophasic action potentials (MAP), and electrograms were recorded, and ventricular effective refractory period (VERP) was determined at each of the above temperatures. In one group (n = 7), a bolus dosage of bretylium tosylate (BT), 6 mg/kg body wt, was administered at 25 degrees C before rewarming. Another group (n = 4) was exposed to cooling and rewarming without addition of BT. Cooling to 25 degrees C reduced ventricular fibrillation threshold linearly, reduced heart rate, increased VERP and MAP, and slowed myocardial conduction velocity in both groups. There was no overall increase in plasma catecholamine levels during cooling. Addition of BT at 25 degrees C increased ventricular fibrillation threshold during rewarming compared with cooling. Addition of BT at 25 degrees C increased VERP by +/- 32 milliseconds and the corrected JT time by 0.06 +/- 0.02 seconds. VERP and JTc increased during rewarming with BT compared with cooling with no drug. BT had no effect on conduction velocity, and plasma catecholamine levels were not reduced. The antiarrhythmic effect of BT during hypothermia was attributed to an increased wavelength of refractoriness by its increase in the refractory period. This increased wavelength of refractoriness may prevent excitable gaps or increase circuit pathway in the setting of reentry arrhythmias.

Characterization of beta-adrenoceptor subtypes and indications for two cell populations in isolated bovine mesenteric lymphatic vessels

The pharmacological characterization of beta-adrenoceptor subtypes and the identification of two cell populations were investigated in isolated bovine mesenteric lymphatic vessels. The beta-adrenoceptor agonists isoprenaline, dobutamine and salbutamol concentration dependently decreased the amplitude and the frequency of spontaneous contractions and the amplitude of electrically induced contractions. The order of potency was isoprenaline greater than salbutamol greater than dobutamine. These effects were competitively antagonized by atenolol with pA2 values close to 7 with isoprenaline and dobutamine as agonists, and near 5.5 with salbutamol as the agonist. Noradrenaline concentration dependently reduced electrically induced contractions, an effect which was reversed to a slight enhancement after blockade of beta-adrenoceptors with propranolol (10(-6) M). These results confirmed the presence of postjunctional beta 1- and beta 2-adrenoceptor subtypes in lymphatic vessels and provide the first indication of the existence of two pharmacologically and functionally distinct cell populations, one of which exhibits pacemaker activity.

Myocardial effects of adrenaline, isoprenaline and dobutamine at hypothermic conditions

The pharmacodynamic myocardial effects of adrenaline, isoprenaline and dobutamine were studied in isolated, perfused and spontaneously beating rabbit hearts at hypothermic conditions. Cardiac contraction amplitude increased from the control value at 37 degrees set equal to 100% to about 165% at 22 degrees, whereas contraction velocity decreased to 52%, frequency to 30% and oxygen consumption to about 19%. At 22 degrees all drugs produced pronounced positive inotropic and chronotropic effects. Em-values related to contraction velocity, frequency and oxygen consumption were for isoprenaline 152, 98 and 136%, respectively, for adrenaline 127, 100 and 198% and for dobutamine 120, 86 and 165%, respectively. The corresponding EC50-values decreased and a marked left-shift of the log-concentration response curves was observed as an expression of increased myocardial sensitivity to the drugs. Em for contraction velocity for dobutamine was distinctly reduced at 32 and 27 degrees and for adrenaline at 27 degrees in comparison to the increase seen at 22 and 37 degrees. Em for oxygen consumption showed for all drugs an increase at decreasing temperatures. The frequency-corrected QTc-interval decreased slightly to moderately during exposure to the drugs at hypothermic conditions. None of the drugs caused arrhythmias during the experiments. Coronary flow rate decreased only moderately at the higher drug concentrations at decreased temperatures. Dobutamine and adrenaline at 37 degrees and isoprenaline at 37 and 22 degrees caused an increase of Em for oxygen consumption that was slightly less than proportional to the increase of Em for contraction velocity.

Myocardial responsiveness to isoproterenol and calcium: a comparison of SD and F344 rats

1. Inotropic effects of isoproterenol and extracellular Ca2+ were compared in left atrial muscle isolated from F344 and SD rats. Preparations from the F344 strain were more sensitive to the actions of both agents. 2. The chronotropic action of isoproterenol was not different in right atria isolated from the two strains. 3. This suggests that the strain-related difference in responsiveness to the inotropic effect of isoproterenol is not caused by heterogeneity in the beta-adrenoceptor/adenylate cyclase system but rather by variations in excitation-contraction coupling.

Functional alpha-adrenoceptors in human atrial preparations in the presence of beta-receptor blockade

Inotropic effects via cardiac alpha-adrenoceptors were studied in electrically driven auricular strips (1 Hz, 37 degrees C) from patients treated with beta-blockers for months prior to open heart surgery. Marked alpha-mediated positive inotropic effects were demonstrated with adrenaline (A), noradrenaline (NA) and phenylephrine (PHE) in the presence of beta-blocker and with blockers of the muscarinic receptor and of the neuronal and extraneuronal uptake mechanisms for the catecholamines. In the presence of approximately 10(-6) M propranolol the maximal effects as well as the potencies (pD2-values) for A and NA were not significantly different while higher than for PHE. The alpha 1-blocker, prazosin (10(-6) M), markedly reduced the pD2-values but not the intrinsic activities (alpha-values) for A, NA and PHE in the beta-blocked preparations. Methoxamine, however, induced negative inotropic responses at normal and low frequencies (1, 0.5 and 0.1 Hz) of stimulation, suggestive of non-specific, cardiodepressant effects. Other agonists with alpha-effects in other types of tissue (oxymethazoline, xylomethazoline and clonidine) were without effects on the force and velocity of contraction in the auricular strips under the present experimental conditions. The results show alpha 1-type of adrenoceptor-induced inotropic effects for A, NA and PHE during beta-blockade in human auricular strips, indicating that cardiac alpha 1-receptors may have clinical importance by increasing the inotropy of the human myocardium treated with beta-blocking agents.

Examination of cardiac alpha-adrenoceptors from pharmacological responses and radioligand binding. Comparison of rat and guinea pig tissues

The object of this study was to determine suitable experimental conditions for the pharmacological evaluation of cardiac alpha-adrenoceptors. Atrial and ventricular preparations of the guinea pig and rat were employed, and the alpha-adrenoceptor responsiveness was compared with the binding of the alpha-adrenoceptor radioligand [3H]prazosin in membranes prepared from these cardiac regions. The experimental variables examined were the pacing frequency, bath temperature, choice of agonist, and cardiac region. In guinea pig atria the optimum alpha-adrenoceptor-mediated positive inotropic response to phenylephrine was at 32 degrees C and a pacing frequency of 1 Hz. A comparison of phenylephrine with methoxamine showed that the former yielded biphasic concentration-response curves in guinea pig left atria; the lower portion was alpha-adrenoceptor mediated and the upper, more substantial portion, was beta mediated. Methoxamine produced monophasic curves due entirely to alpha-adrenoceptor stimulation and was therefore used for comparisons between rat and guinea pig tissues. Of the guinea pig tissues, only the left atrium produced positive inotropic responses. Negative chronotropy was obtained with right atria and negative inotropy with ventricular strips and papillary muscles. The rat tissues all responded with positive responses, the largest maximum being in the left atrium. Binding data showed a larger number of alpha-adrenoceptors in the rat tissues, of which the ventricles had the greatest number. The lack of response of guinea pig ventricular tissues was therefore reflected in the low binding. From this study, the most appropriate model for characterizing cardiac alpha-adrenoceptors is therefore the rat left atria at 32 degrees C and paced at 1 Hz with methoxamine as the agonist.

[Alpha-adrenoceptors in the myocardium: incidence and functional significance]

Alpha-adrenoceptors mediating positive inotropic effects are well established in the heart of various species including human heart. The mechanism by which alpha-adrenoceptor stimulation increases force of contraction is not known. cAMP is unlikely to be involved as a mediator. Evidence has been presented that an increase in magnitude and duration of the slow Ca++ inward current may be partly responsible for the positive inotropic effect. In addition, stimulation of alpha-adrenoceptors may increase Ca++ sensitivity of the contractile proteins. Stimulation of alpha-adrenoceptors by endogenous catecholamines may serve as a reserve mechanism under various conditions of impaired beta-adrenergic influence, e.g. hypothyroidism, bradycardia or ischemia. Furthermore, alpha-adrenoceptors may be involved in the genesis of reperfusion arrhythmias in ischemic heart.

Functional characteristics and responses to adrenergic stimulation of isolated heart preparations from hypothermic and hibernating subjects

Contemporary studies of isolated hearts from hibernators and nonhibernators are presented. Original experiments with isolated perfused hamster hearts are reported. Such hearts can maintain left ventricular function at temperatures as low as 7 degrees C. Generated left ventricular pressure was 40 +/- 9 mm Hg and heart rate was 7 +/- 1 beats/min. During cooling heart rate dropped dramatically, coronary flow increased, and ventricular pressure decreased initially, plateaued, and then fell as 7 degrees C was approached. Norepinephrine can cause increased heart rate and left ventricular pressure at 22 and 7 degrees C. This positive inotropic and chronotropic response was associated with increased cAMP at 30 sec after stimulation at 22 degrees C but not at 7 degrees C. Furthermore, cAMP was also not changed at peak response at 7 degrees C. Isoproterenol increased cAMP content in 37 degrees C ventricular slices but not at hypothermic temperatures. Possible mechanisms of nonadenylate cyclase mediation of inotropic and chronotropic responses at 7 degrees C are discussed.

Age-dependent differences in the positive inotropic effect of phenylephrine on rat isolated atria

The age-dependent differences in the involvement of alpha-adrenoceptors in the positive inotropic effect of phenylephrine (Phe) were examined in isolated atria of male Wistar rats 6 weeks (6W), 10 weeks and 7 months (7M) of age. The maximal increase in tension development induced by Phe increased with age, whereas the EC50 values for the positive inotropic effect of Phe did not change with age. The inhibitory effect of phentolamine on the response to Phe increased with age. Propranolol caused only slight inhibition of the effect of Phe in both 6W and 7M rats, and the EC50 values for Phe in the presence of propranolol did not change significantly with age. The EC50 values for isoprenaline and 5-hydroxytryptamine in 7M rats were higher than those in 6W rats. In 7M rats, the duration of the tension development was only slightly affected by Phe in the presence or absence of propranolol, but it was markedly decreased by Phe in the presence of phentolamine. The dose-response curve for Phe was markedly shifted to the left by papaverine in 6W rats, but slightly in 7M rats. The dose-response curve for isoprenaline was markedly shifted to the left by papaverine in both groups. These results are consistent with effects of Phe being mediated by both alpha- and beta-adrenoceptors in both 6W and 7M rats, but there is a shift in the balance from rather more beta-receptors in the young animals to more alpha-receptors in the adults.

Temperature-induced changes in dissociation constants (KA) of agonists at cardiac beta-adrenoceptors determined by use of the irreversible antagonist Ro 03-7894

The positive inotropic responses of guinea-pig left atria and papillary muscles and positive chronotropic responses of right atria to sympathomimetic amines were examined at 38 degrees and 30 degrees C. At the lower temperature, supersensitivity to orciprenaline and isoprenaline was exhibited as shifts of the dose-response curves to the left and significant reductions in EC50 values. This supersensitivity could not be attributed to reduced metabolism since the experiments were performed in the presence of metanephrine (10(-5)M) and U-0521 (3',4'-dihydroxy-2-methylpropiophenone) (10(-4)M) as inhibitors of extraneuronal uptake and catechol-O-methyltransferase (COMT) respectively, and the agonists are not susceptible to neuronal uptake. After incubation of the tissues with Ro 03-7894 (1-(5-chloracetylaminobenzfuran-2-yl)-2-isopropylaminoethanol), followed by its prolonged washout (greater than 2h), the maximum responses to isoprenaline and orciprenaline were depressed, confirming the apparently irreversible beta-adrenoceptor antagonism. Dissociation constants (KA) for isoprenaline and orciprenaline were determined from the equiactive concentrations obtained before (A) and after (A') incubation with Ro 03-7894, plotted as 1/A against 1/A' (KA = (slope-1)/intercept). KA values were the same for orciprenaline in the three cardiac preparations and for isoprenaline in the atria. This applied at 38 degrees and 30 degrees C and indicates that the beta-adrenoceptors mediating the inotropic and chronotropic responses of the guinea-pig heart do not differ. The KA values of both agonists were, however, consistently and significantly lower at 30 degrees than at 38 degrees C, indicating an increase in affinity. 8 It is concluded that hypothermia-induced supersensitivity of cardiac tissue to sympathomimetic amines is associated with an increase in their affinity for the B-adrenoceptors.

The effect of temperature elevation on the cerebrovascular response to noradrenaline and 5-hydroxytryptamine

Fever frequently complicates stroke and subarachnoid haemorrhage. A transient rise in transmitter monoamine levels of plasma and cerebrospinal fluid occurs in these diseases. The present study demonstrates an enhanced vasoconstrictor response of cerebral vessels to noradrenaline-but not 5-hydroxytryptamine-following a rise in temperature. The augmented response is more likely due to an impaired inactivation (re-uptake) of the amine than to an altered sensitivity of the post-synaptic alpha-adrenergic receptor, since it could be reproduced by pretreatment with cocaine. The finding indicates that it may be important to combat fever in these patients.

The use of an irreversible beta-adrenoreceptor antagonist to examine reserpine- and hypothermia-induced supersensitivity of guinea-pig atria

1 Positive inotropic and chronotropic responses to guinea-pig isolated left and right atria respectively to the sympathomimetic amine orciprenaline were recorded. 2 Pretreatment of animals with reserpine for 3 days produced supersensitivity of both the inotropic and chronotropic responses. Lowering the bath temperature from 38 degrees to 30 degrees C produced supersensitivity of untreated atria to the inotropic responses only. 3 Irreversible beta-adrenoreceptor antagonism of orciprenaline by Ro 03-7894 was demonstrated by a persistent depression of the maximum inotropic and chronotropic responses after a prolonged washout from the bath (3 h). 4 The depression of the maximum rate and tension responses by Ro 03-7894 was less in atria from reserpine-pretreated animals. 5 The maxima were also depressed less at the lower bath temperature of 30 degrees C. However, when atria were cooled during the incubation with Ro 30-7894, the maxima were still depressed to the 38 degrees C level. 6 That these results suggest either a receptor proliferation or change in efficacy as possible mechanisms for reserpine- and hypothermia-induced supersensitivity is discussed.