Regulation of beta-adrenoceptor density and function in rat vas deferens

Resistance of beta 2-adrenoceptor-mediated responses of lung strips to desensitization by long-term agonist exposure--comparison with atrial beta 1-adrenoceptor-mediated responses

In vitro desensitization of beta 2-adrenoceptor-mediated relaxation of guinea-pig isolated parenchymal strips was examined. Concentration-response curves for isoprenaline were obtained and after long-term incubation with isoprenaline, followed by washout, a second curve was obtained. Correction for time-dependent loss of sensitivity was made from time-matched controls. After incubation with 10(-5) M isoprenaline for 0.5, 1, 2, 4 and 8 h, loss of responsiveness of carbachol-contracted lung strips was observed after 4h as a reduced post-incubation maximum response. When the concentration was reduced to 10(-6) M, a 4 h incubation with 1 h washout no longer induced a shift of the post-incubation curve in carbachol-contracted lung strips. In contrast, lung strips with intrinsic tone displayed reduced responsiveness to isoprenaline after 4 h incubation with 10(-6) M isoprenaline. Incubation of the tissue for 4 h with lanthanum (1.4 x 10(-6) M), a relaxant not operating through beta 2-adrenoceptors or their effector coupling, had the same effect upon isoprenaline concentration-response curves as incubation with isoprenaline. This was irrespective of whether intrinsic tone (10(-6) M isoprenaline) or carbachol-contracted (10(-5) M isoprenaline) lung strips were used. It was concluded that the loss of beta 2-adrenoceptor responsiveness after incubation with 10(-6) M isoprenaline was due to the prolonged maximal relaxation of the tissue for 4 h rather than desensitization of the beta 2-adrenoceptor. Indeed, after correction for maximal relaxation and for time, no significant change in beta 2-adrenoceptor sensitivity of the lung occurred after incubation with 10(-6) M isoprenaline for 4 h. This contrasts with significant rightwards shifts of the concentration-response curves for the beta 1-adrenoceptor-mediated increases in rate and tension of guinea-pig right and left atria after identical incubation conditions. Thus whereas beta 1-adrenoceptor-mediated responses displayed desensitization after long-term in vitro agonist exposure, the beta 2-adrenoceptor-mediated responses were resistant.

Effect of temperature reduction on the reactivity of the mouse vas deferens to adrenergic drugs

1. Dose-response curves for noradrenaline, phenylephrine and clonidine were determined isometrically on the mouse vas deferens at 26 degrees C, 15 degrees C and compared to the one obtained at 37 degrees C. 2. In the presence of noradrenaline, reducing temperature induced an increase of both maximal developed tension and sensitivity to the drug. Reduction by 50% of the extracellular calcium concentration abolished the maximal contraction potentiation. 3. When reducing temperature to 26 degrees C, the maximal contraction was increased and depressed in the presence of phenylephrine and clonidine respectively. 4. The results suggest (a) that cooling increases the reactivity of mouse vas deferens by activation of alpha 1 adrenoceptors and depresses it by activation of alpha 2 adrenoceptors (b) that calcium ions could play an important role in the potentiation of the maximal contraction.

Evidence from lack of decentralization-induced supersensitivity that beta 2-adrenoceptors of the cat nictitating membrane are non-innervated

The right superior of cervical sympathetic trunk of cats was sectioned preganglionically under anaesthesia. Six days later the blood pressure, heart rate and contractions of the left (control) and right (decentralized) nictitating membranes were recorded under chloralose anaesthesia (80 mg kg-1). The alpha-adrenoceptor-mediated contractile responses of the nictitating membrane to intravenous adrenaline were greater on the decentralized side than the control side, with a significant shift of the dose-response curve to the left. After phentolamine (8 mg kg-1 i.v.), adrenaline administered intra-arterially exerted beta-adrenoceptor-mediated relaxation of the nictitating membranes. However, there was no difference in the sensitivity or magnitude of responses between decentralized and control sides. In a separate series of experiments, the alpha-adrenoceptor-mediated contractile responses of the nictitating membrane to intra-arterial noradrenaline displayed supersensitivity on the decentralized side, the dose-response curve being significantly shifted to the left. In the same animals, the beta-adrenoceptor-mediated relaxation responses to intra-arterial isoprenaline were non-significantly greater on the decentralized side, presumably because of raised tone. However, when expressed as a percentage of the maximum relaxation, there was no difference in sensitivity. This study shows that the alpha-adrenoceptor-mediated contractile response of the nictitating membrane displays supersensitivity after preganglionic section of the sympathetic innervation. This is presumably because of an up-regulation arising from loss of sympathetic traffic onto the receptor. The relaxation response is mediated via adrenoceptors of the beta 2-subtype and shows no supersensitivity. This suggests that these receptors are not under the influence of the sympathetic innervation.

Regulation of beta-adrenoceptors in the guinea pig left ventricle and skeletal muscle following chronic agonist treatment

Pretreatment of guinea pigs with adrenaline, isoprenaline or terbutaline for 7 days significantly reduced the Bmax for the radioligand [125I]cyanopindolol (ICYP) in the gastrocnemius muscle (beta 2-adrenoceptors). Pretreatment of guinea pigs with terbutaline reduced the responsiveness of gastrocnemius muscle slices adenylate cyclase to isoprenaline (10(-4) M). In the left ventricle (predominantly beta 1-adrenoceptors) pretreatment of guinea pigs with isoprenaline or adrenaline for 7 days did not alter the Bmax for ICYP. The responsiveness of adenylate cyclase to isoprenaline (10(-4) M) in left ventricle slices was significantly reduced following isoprenaline pretreatment of the guinea pigs. Thus desensitisation of beta-adrenoceptors in left ventricle and skeletal muscle developed following chronic agonist pretreatment. Reduction of beta 2-adrenoceptors in the skeletal muscle could be responsible for the desensitisation of adenylate cyclase. In the left ventricle the receptors were resistant to agonist induced down-regulation. It is proposed that other mechanisms which are tissue- and species-specific independent of the receptor subtype can be responsible for agonist-induced desensitisation in the left ventricle of the guinea pig in vivo.

Regulation of beta-adrenoceptors by catecholamines in the rabbit skeletal muscle

(-)-[125I]Cyanopindolol (ICYP) was used to characterise beta-adrenoceptors in the gastrocnemius muscle of the rabbit. The binding of ICYP was saturable. The KI value for the selective beta 2-adrenoceptor antagonist, ICI 118551, was 0.16 +/- 0.01 nM (mean +/- S.E.M) and for the selective beta 1-adrenoceptor antagonist, metoprolol, was 300.1 +/- 33.3 nM. Hofstee plots for both antagonists were linear indicating the presence of homogeneous beta 2-adrenoceptors in the skeletal muscle of the rabbit. In gastrocnemius muscles obtained from rabbits chronically pretreated in vivo with vehicle (0.1% ascorbic acid), adrenaline (40-50 nmol.kg-1.h-1) or noradrenaline (80-100 nmol.kg-1.h-1) via osmotic mini-pumps implanted for 10 days, the Bmax values were 44 +/- 3, 25 +/- 4 (P less than 0.05) and 41 +/- 7 (fmol.mg-1 protein) and KD values were 7.3 +/- 0.9, 6.3 +/- 0.8 and 9.3 +/- 2.5 (pM), respectively. Thus pretreatment with the circulating hormone adrenaline but not noradrenaline down-regulated the number of skeletal muscle beta 2-adrenoceptors but did not influence the affinity.