Temperature related effects on the binding characteristics of beta-adrenergic receptor agonists and antagonists by rabbit lung

Comparison of [3H]resiniferatoxin binding by the vanilloid (capsaicin) receptor in dorsal root ganglia, spinal cord, dorsal vagal complex, sciatic and vagal nerve and urinary bladder of the rat

In the present report we compared the properties of [3H]resiniferatoxin (RTX) binding by the vanilloid receptors present at different parts of the primary afferent neurons of the rat. We found no major differences in either the affinity or the cooperativity of [3H]RTX binding by vanilloid receptors on the cell body, central terminals, peripheral terminals or axons. Specific binding of [3H]RTX to dorsal root ganglia, whole spinal cord, dorsal vagal complex, urinary bladder, and sciatic and vagal nerves all followed sigmoidal saturation kinetics indicating positive cooperativity among the binding sites. The cooperativity indexes determined by fitting the data to the Hill equation were 1.82 +/- 0.11, 2.21 +/- 0.04, 2.55 +/- 0.01, 1.91 +/- 0.11, 2.03 +/- 0.09 and 2.27 +/- 0.04, respectively. The dissociation constants in dorsal root ganglia, spinal cord, dorsal vagal complex, urinary bladder, and sciatic and vagal nerve membranes were 46.5 +/- 2.7, 29.3 +/- 5.1, 28.2 +/- 1.2, 60.8 +/- 4.4, 59.9 +/- 1.9 and 45.2 +/- 0.7 pM; the receptor densities were 219 +/- 14, 48 +/- 5, 67 +/- 1, 32 +/- 7, 61 +/- 9, and 100 +/- 20 fmol/mg protein, respectively. We could not show any major differences in the affinities of capsaicin and capsazepine in inhibition of [3H]RTX binding by the different membrane preparations either. In all cases the initial enhancement of [3H]RTX binding by nonradioactive RTX, capsaicin, and capsazepine confirmed the existence of positive cooperativity among the binding sites. We were unable to detect specific [3H]RTX binding sites in membrane preparations of the preoptic area, locus ceruleus, substantia nigra, striatum and paraventricular nuclei of the rat brain under our present conditions. Our results suggest the uniformity of the vanilloid receptors present at different parts of the primary afferent neuron.

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.

Comparison of rat and human left ventricle beta-adrenergic receptors: subtype heterogeneity delineated by direct radioligand binding

Beta adrenergic receptors were identified in rat myocardial left ventricle and human papillary muscle by using the antagonist radioligand 3H-dihydroalprenolol. The number (37.3 and 44.5 fmol/mg of protein, respectively in rat and man), and the KD (1.6 and 2.8 nM, respectively in rat and man) of beta receptors were not significantly different. Adrenergic receptors of both beta 1 and beta 2 subtypes were found to coexist in the left ventricle. The relative proportions of the two beta receptor subtypes were determined by the use of competition radioligand selective binding and computer modelling techniques employing the subtype selective antagonists ICI 118,551 (beta 2 selective) and atenolol (beta 1 selective) in rat or metoprolol (beta 1 selective) in man. The rat left ventricle contained about 74% beta 1 and 26% beta 2 adrenergic receptors, human left ventricle papillary muscles contained about 69% beta 1 and 31% beta 2. Human and rat left ventricles contain both beta 1 and beta 2 adrenergic receptors with similar affinities. Rat might be a model for the study of human myocardial beta adrenergic receptors.

No evidence for temperature-dependent changes in the pharmacological specificity of beta 1- and beta 2-adrenoceptors in rabbit lung membranes

In rabbit lung membranes known to contain both beta 1- and beta 2-adrenoceptors it was studied whether changes in incubation temperature may affect binding characteristics and/or selectivity of beta 1- and beta 2-adrenoceptor drugs. For this purpose inhibition of binding of the highly specific beta-adrenoceptor radioligand (+/-)-125iodocyanopindolol (ICYP) by beta 1-and beta 2-selective as well as non-selective drugs was determined at incubation temperatures of 37 degrees C and 18 degrees C and analyzed by Hofstee-plots. 1. The density of beta-adrenoceptors in rabbit lung membranes was identical independently of the incubation temperature. 2. At both incubation temperatures beta 1- and beta 2-selective drugs showed biphasic displacement curves and non-linear Hofstee-plots, while inhibition of binding by the non-selective drugs resulted in monophasic displacement curves with linear Hofstee-plots. With decreasing temperature affinity of antagonists to beta-adrenoceptors increased only slightly, while affinity of agonists increased markedly. 3. For all beta 1- and beta 2-selective drugs the same ratio beta 1/beta 2-adrenoceptors was calculated from the Hofstee-plots independently of the incubation temperature: It amounted to about 80% beta 1- and 20% beta 2-adrenoceptors in rabbit lung. 4. At an incubation temperature of 37 degrees C the displacement curve of the agonist isoprenaline was biphasic in the absence of GTP with a non-linear Hofstee-plot indicating that at 37 degrees C isoprenaline binds to high and low affinity states of the beta-adrenoceptors in rabbit lung. At 18 degrees C, however, beta-adrenoceptors do not form the high affinity GTP-sensitive complex with agonists, since GTP had no influence on isoprenaline displacement curves. 5. It is concluded that a decrease in the incubation temperature of ICYP binding assay from 37 degrees C to 18 degrees C does neither alter the relative amount of beta 1- and beta 2-adrenoceptors in rabbit lung membranes nor the selectivity of beta 1- and beta 2-selective adrenoceptor drugs.

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.

Uptake and displacement of [3H]dihydroalprenolol, [3H]epinephrine and [3H]clonidine in isolated perfused rabbit lung

Uptake and displacement of three adrenergic receptor ligands, [3H]dihydroalprenolol ([3H]DHA), [3H]epinephrine ([3H]EPI) and [3H]clonidine ([3H]CLON), were examined in isolated rabbit lungs by recirculating perfusion. Removal of [3H]DHA was the most extensive (85% uptake; 6.6 ml/min clearance), [3H]CLON removal was intermediate (50%; 3.8 ml/min), and [3H]EPI removal was the lowest (33%; 1.2 ml/min). Specific displacement of each radioligand from lung was attempted using several competing agents. Both (--)- and (+)-propranolol equally displaced [3H]DHA from lung. Phentolamine, (--)-phenylephrine and (--)-epinephrine were unable to displace 10 nM [3H]EPI from lung, although the latter two agents did produce concentration-dependent increases in perfusion pressure. High concentrations of (--)-epinephrine, which produced near maximal physiological responses, inconsistently displaced 30-4- nM [3H]EPI from lung. [3H]Clonidine was displaced by unlabeled clonidine at concentrations that caused increases in perfusion pressure. Pretreatment of lungs with either 10 microM phentolamine or phenoxybenzamine did not alter the total amount of [3H]CLON displaced by clonidine, suggesting that [3H]CLON was displaced predominantly from non-specific sites, perhaps preventing detection of [3H]CLON displacement from specific (receptor) sites. Alternatively, these results may be interpreted as inhibition of uptake of each radioligand. Thus, both (--)- and (+)-propranolol interfered with [3H]DHA removal, suggesting a common mechanism for uptake and/or retention for these two beta-adrenergic receptor antagonists. Inhibition of [3H]EPI removal was observed only at high concentrations of (--)-epinephrine which indicates that pulmonary removal of epinephrine occurs through a low affinity uptake system. [3H]Clonidine removal was effectively inhibited by the same (microM) concentrations of unlabeled clonidine that produced physiological responses. Neither phentolamine nor phenoxybenzamine was able to interfere with pulmonary removal of [3H]CLON. Therefore, uptake and displacement of these adrenergic receptor radioligands showed no correlation with pharmacological effects produced by these agents in isolated perfused rabbit lung. The results are more closely associated with inhibition of removal and/or non-specific retention of the radioligands examined.