The contribution of metabolism to the hypothermia-induced supersensitivity of guinea-pig isolated atria; selective supersensitivity for beta-adrenoceptor agonists and their positive inotropic responses

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.

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.

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.

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.

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

The beta-adrenoceptor-mediated responses of various isolated tissues from the guinea-pig were examined at bath temperatures of 38 and 30 degrees C. The positive inotropic responses to orciprenaline of paced left atria and papillary muscles were potentiated at the lower bath temperature, as indicated by a significant shift of the dose-response curve to the left. A similar hypothermia-induced supersensitivity was observed for the positive chronotropic response of right atria. The beta-adrenoceptor-mediated inhibition of coaxially stimulated ileum in response to orciprenaline also exhibited supersensitivity at 30 degrees C. In contrast, the relaxation of lung strips was not altered at the lower bath temperature. The relaxant responses of potassium-contracted uterine strips from untreated guinea-pigs or progesterone-dominated guinea-pigs and rats also failed to reveal any hypothermia-induced supersensitivity. The responses of lung and uterine strips are mediated via beta-adrenoceptors of the beta 2-type, whereas they are of the beta 1-type in the cardiac preparations and ileum. Therefore, these results suggest that hypothermia-induced supersensitivity occurs only at the beta 1-adrenoceptors, indicating a fundamental temperature-dependent difference between the two receptor types.

Dissociation constants of isoprenaline and orciprenaline and their relative efficacies on guinea-pig isolated atria determined by use of an irreversible beta-adrenoceptor antagonist

1. Dissociation constant (KA) of isoprenaline and orciprenaline were determined for the positive inotropic and chronotropic responses of guinea-pig isolated atria. Cumulative dose-response curves to the agonists were constructed before and after incubation with and washout of the irreversible beta-adrenoreceptor antagonist, Ro 03-7894 1-(5-chloracetylaminobenzfuran-2-yl)-2-isopropylaminoethanol). 2. After 20 min washout, the curves were displaced to the right with depression of the maxima. After 3 h washout, there was only depression of the maxima. 3. Dissociation constants were determined by plotting reciprocals of molar concentrations before Ro 03-7894 (1/A) against reciprocals of the equiactive concentrations after Ro 03-7894 (1/A'). KA = (slope - 1)/ intercept. 4. Isoprenaline had a greater affinity (KA) than orciprenaline on both rate and tension. The affinity for rate and tension was identical for both agonists, indicating that the beta-adrenoceptors were identical. 5. Isoprenaline and orciprenaline produced identical rate response maxima when compared in the same preparation but the orciprenaline tension maximum was only 92.0 +/- 0.3% that of isoprenaline. 6. These dose-response curves were replotted as response against -log RA/Rt (fraction of receptors occupied) for each agonist concentration, calculated from the equation RA/Rt = (A)/(KA + (A)). The antilogarithm of the distance along the -log RA/Rt axis gave the efficacy of orciprenaline relative to isoprenaline. It had a greater efficacy (2.24) for rate but a lower efficacy for tension responses (0.5).

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.

Functional antagonism as a means of determining dissociation constants and relative efficacies of sympathomimetic amines in guinea-pig isolated atria

1 The positive inotropic and chronotropic responses to sympathomimetic amines were examined in guinea-pig isolated atria. 2 The order of potency measured from EC50 values was isoprenaline greater than orciprenaline greater than salbutamol greater than or equal to fenoterol greater than terbutaline. Terbutaline and salbutamol were partial agonists on rate and together with orciprenaline and fenoterol also on tension responses. 3 Functional antagonism by carbachol caused a rightwards shift of the dose-response curve and depression of the maximum response. The rate maxima for orciprenaline, fenoterol and terbutaline were above that of isoprenaline. All the tension maxima were below isoprenaline. 4 Dissociation constants (KA) and relative efficacies (er) were determined by analogy with irreversible antagonism. 5 The relative orders of affinity (KA) were isoprenaline greater than orciprenaline greater than fenoterol greater than salbutamol greater than terbutaline. Affinities were identical on rate and tension. 6 The relative efficacies were all greater than isoprenaline for rate responses. On tension they were the same or less than isoprenaline. 7 The implications of these results are discussed, in particular the fact that a partial agonist has a greater efficacy than a full agonist.