Temperature dependent sensitivity of isolated guinea-pig atria to sympathomimetic amines

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.

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 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.

Vascular and metabolic responses to adrenergic stimulation in isolated canine subcutaneous adipose tissue at normal and reduced temperature

1. The circulatory and metabolic effects of temperature reduction were studied in autoperfused canine subcutaneous adipose tissue in situ. 2. Cooling the adipose tissue sufficiently to reduce venous effluent temperature by 5--6 degrees C decreased blood flow from an average of 6.4--4.1 ml. min-1 . 100g-1. 3. Vasoconstrictor responses to sympathetic nerve stimulation (4 HZ) and injected noradrenaline (5 n-mole) were potentiated by cooling while vasodilator components of the vascular responses, such as autoregulatory escape and post-stimulatory hyperaemia, were virtually abolished by this treatment. 4. Oxygen uptake was reduced by cooling without signs of tissue hypoxia. This reduced oxygen demand may partly cause the decrease in adipose tissue blood flow. 5. Cooling inhibited glycerol mobilization from the adipose tissue during sympathetic nerve stimulation. Post-stimulatory lipolysis was, however, not inhibited. In vitro studies with 'perifused' rat fat cells suggest that this may be due to impaired inactivation of the lipolytic process, rather than to changes in transmitter removal, following stimulation at low temperature. 6. Cooling inhibited the mobilization of fatty acids more than that of glycerol, suggesting increased re-esterification of fatty acids within the tissue at low temperature. 7. It is concluded that cooling increases the sensitivity to vasoconstrictor stimuli and that inhibition of metabolic vasodilator mechanisms play a role for this effect. The stimultaneous inhibition of activating and inactivating mechanisms could explain the unchanged vascular and lipolytic responses to brief stimuli. Some possible implications of the present findings for the physiology of adipose tissue during cooling are discussed.

The role of cyclic AMP in temperature-dependent changes of contractile force and sensitivity ot isoprenaline and papaverine in guinea-pig atria

Right and left guinea-pig atria responded to decreasing temperatures (42-27 degrees C) with elevation for force of contraction and concomitant increases in cAMP. When atria were rapidly cooled from 42 to 27 degrees C the increase in cAMP occurred prior to the onset of the inotropic responses. Papaverine (3 X 10(-5) M) potentiated the effects of temperature on cAMP and force of contraction on left atria driven at 0.5 Hz. On right atria beating spontaneously at frequencies above 2 Hz papaverine only potentiated the effect of decreasing temperatures on the response of cAMP but not on that of force of contraction. Time course studies of the effects of isoprenaline (3 X 10(-8) M) on right atria at 27 degrees C showed large inotropic responses to isoprenaline which were accompanied by increases in cAMP. At 42 degrees C the responses of force of contraction and cAMP to isoprenaline occurred faster and were only short-lasting. As with the time courses for isoprenaline, dose-response curves for the effect of isoprenaline and papaverine on cAMP content and force of contraction also appeared to be shifted towards higher levels at hypothemia. However, pD2 values reflected increases in affinity for inotropic, but not for the cAMP responses to isoprenaline and papaverine at hypothermia. These results show that cyclic AMP is involved in the inotropic responses to hypothermia, but not in the supersensitivity of heart to isoprenaline and papaverine as observed at low temperatures.

The influence of temperature upon reserpine-induced supersensitivity of guinea-pig isolated atria to isoprenaline and salbutamol

1. The aim of this investigation was to determine whether supersensitivity of isolated atria to sympathomimetic amines following pretreatment with reserpine was evident at low temperatures, which alone induced supersensitivity. 2. Cumulative dose-response curves for the positive inotropic and chronotropic responses of isolated guinea-pig atria to isoprenaline and the partial agonist salbutamol were plotted as a percentage of the maximum response to isoprenaline. 3. In atria from reserpine-pretreated guinea-pigs set up at 38 degrees C, supersensitivity of both rate and tension responses was observed as a shift of the curves to the left and an increase of the maximum responses to salbutamol. Tension responses were potentiated more than rate responses. At 30 degrees C the supersensitivity became less apparent and at 25 degrees C was virtually absent. 4. The dose-response curves in untreated atria at low temperatures revealed that hypothermia itself produced supersensitivity of rate and tension responses. The dose-response curves were displaced to the left and the salbutamol maxima were raised so that at 25 degrees C it became almost a full agonist. The hypothermia-induced supersensitivity was therefore sufficient to mask any supersensitivity resulting from pretreatment with reserpine. 5. The hypothermia-induced supersensitivity of the rate response was dependent upon the method of plotting. When plotted in absolute units of beats per min no supersensitivity of the rate response was evident. Supersensitivity of the tension response at the lower temperatures and of both rate and tension responses following pretreatment with reserpine were independent of the plotting method.

The effect of temperature on potassium chloride contracture in cat myocardium

1. Contracture was induced in cat myocardium by exposure to 140 mM-KC1 In isotonic Tyrode solution. Force of contracture expressed as mg/mm2 (muscle cross-sectional area) falls with increasing cross-sectional area. 2. The effect of temperature on isometric force developed during contracture was evaluated both in normal (untreated) atrial and ventricular muscle and following treatment with sympatholytic drugs. 3. The force of contracture was not significantly affected by sympatholytic drugs at 36 degrees C. 4. In normal atrial and ventricular muscle, force of contracture decreased when the muscle was cooled from 36 to either 29 or 20 degrees C. 5. In atrial muscle, the effect of temperature was not changed by sympatholytic drugs. In contrast, exposure to sympatholytic drugs increased contracture force developed by ventricular muscle at 20 degrees C. Also, contracture force was significantly greater at 20 than at 36 degrees C in ventricular muscle from reserpine-pretreated cats. 6. It is suggested that ventricular muscle becomes more sensitive to the relaxing effects of endogenous catecholamines at temperature is lowered. 7. The differences shown between atrial and ventricular muscle with respect to the effect of temperature and sympatholytic drugs on contracture force may result from the differing amounts of sarcoplasmic reticulum found in these types of cardiac muscle and also from different mechanisms of "excitation-contracture" coupling in atrial and ventricular muscle.

Studies on the temperature-dependent sensitivity of mouse atria to adrenergic drugs

The sensitivity of mouse atria (chronotropic response) to beta-adrenergic receptor antagonists was examined at 37 and 26 degrees C under various experimental conditions. When isoproterenol was used as agonist, at 37 degrees C, the pA2 value (from pA2 plots) for propranolol was 8.86 and the slope of the regression line was 0.54. At 26 degrees C, the pA2 value for propranolol was 9.2 and the slope was 0.7. In the presence of tropolone, 10-5 M, the values for pA2 and slope of the regression lines were, respectively, 9.0 and 0.90 at 37 degrees C and 9.17 and 0.98 at 26 degrees C. In other words, decreasing antagonism was prevented by low temperature and tropolone. Isoproterenol was potentiated by tropolone and low temperature, but the effects of low temperature were reduced by tropolone. With sotalol as antagonist, there was greated blockade (dose ratios) of the effects of isoproterenol at 26 than at 37 degrees C. When nylidrin, a non-catecholamine, was used as agonist there was no temperature-dependent sensitivity to sotalol. Furthermore, nylidrin was not potentiated by low temperature. The data show that the observed degree of beta-receptor antagonism can be altered by changing the bath temperature and suggest that this effect is related to COMT activity or an influence of agonist disposition in the tissue.