New directions in adrenoceptor pharmacology

Discrimination by SZL49 between contractions evoked by noradrenaline in longitudinal and circular muscle of human vas deferens

The effects of irreversible alpha1-adrenoceptor antagonists, SZL-49 (an alkylating analogue of prazosin), dibenamine and benextramine on contractions to noradrenaline (NA) in longitudinal and circular muscle of human epididymal vas deferens were investigated. Competitive alpha1-adrenoceptor antagonists were also used to further characterize the alpha1-adrenoceptor subtype stimulated by NA in longitudinal and circular muscle. NA evoked concentration-dependent contractions of both muscle types (pD2; 5.4 and 5.2 respectively). The contraction of circular muscle was comparatively more sensitive than that of longitudinal muscle to pretreatment with SZL-49. In contrast, dibenamine or benextramine produced comparable effects in both muscle types. The relationship between receptor occupancy and contraction in either longitudinal or circular muscle was nonlinear, with half-maximal response requiring similar receptor occupancy (longitudinal muscle 14%, circular muscle 16%). Maximal response in both muscle types occurred with little or no receptor reserve (<10%). The competitive alpha1-adrenoceptor antagonists produced dextral shifts of the dose-response curves to NA in longitudinal and circular muscle. The inhibitory potencies, estimated from the apparent pKB values were significantly different in longitudinal and circular muscle respectively for either WB 4101 (pKB, 8.6 and 9.5) or RS-17053 (pKB, 7.1 and 9.0) but not for Rec 15/2739 (pKB, 9.2 and 9.8) or HV 723 (pKB, 8.3 and 8.4). In conclusion, the potency profile of the competitive alpha1-adrenoceptor antagonists and the lack of different receptor reserves for NA in the muscle types suggest that the discriminatory effects of SZL-49 is primarily due to a predominance of the alpha1L-adrenoceptor subtype in longitudinal muscle and alpha1A-subtype in circular muscle.

Localization and changes in distribution of brain alpha 2- and beta-adrenoceptors in response to acclimation state in the American bullfrog (Rana catesbeiana)

Alpha (alpha)- and beta (beta)-adrenoceptors regulate physiological processes in vertebrates. This study determined the location of alpha 2- and beta-adrenoceptors in the brain of the American bullfrog, Rana catesbeiana, using autoradiography. As the density of receptors may be affected by environmental temperature, a comparative numerical analysis of adrenoceptors in the areas of localization with respect to warm and cold acclimation was also carried out. Areas of greatest concentration of alpha 2-adrenoceptors were the accessory olfactory bulb, medial pallium, and olfactory bulb. Adrenoceptor numbers were significantly decreased in the accessory olfactory bulb and medial pallium in cold-acclimated animals. beta-adrenoceptors were localized in the thalamus, cerebellum, medial pallium, and amygdala/ striatum. Cold acclimation decreased adrenoceptor density in medial pallium and torus semicircularis and increased adrenoceptor density in the thalamus and hypothalamic preoptic areas. Among the alpha 2- and beta-adrenoceptors, only four regions of overlap existed, the medial pallium, hypothalamic preoptic area, optic tract, and isthmic tegmentum. Otherwise, where there were alpha 2-adrenoceptors, there were few or no beta-adrenoceptors. No alpha 2- or beta-adrenoceptors were found in the pituitary and optic chiasm. The distribution of adrenoceptors in particular areas of the brain may have functional significance with respect to physiological changes which occur in response to hibernation.

[3H]RS79948-197 binding to human, rat, guinea pig and pig alpha2A-, alpha2B- and alpha2C-adrenoceptors. Comparison with MK912, RX821002, rauwolscine and yohimbine

The Kd values of the recently introduced radioligand [3H]RS79948-197 ((8a R,12aS,13a-S)-5,8,8a,9,10,11,12,12a,13,13a-decahydro-3-metho xy-12-(ethylsulphonyl)-6H-isoquino[2,1-g][1,6]naphthyridine) were determined for the recombinant human and rat alpha2A-, alpha2B- and alpha2C- as well as guinea pig alpha2B- and alpha2c-adrenoceptors expressed in COS (CV-1 Origin, SV40) cells. In addition, the Kd values were also determined for [3H]RS79948-197 for the guinea pig spleen alpha2A-adrenoceptor and for pig alpha2A-, alpha2B- and alpha2C-adrenoceptors in membranes obtained from kidney and striatum. Available radioligands for alpha2-adrenoceptors, besides [3H]RS79948-197 are the tritiated forms of MK912 ((2S,12bS)1',3'-dimethylspiro(1,3,4,5',6,6',7,12b-octa hydro-2H-benzo[b]furo[2,3-a]quinazoline)-2,4'-pyrimidin-2'-one), RX821002 (2-methoxy-idazoxan), rauwolscine and yohimbine. In the present article the binding constants of all these substances for the alpha2A-, alpha2B- and alpha2C-adrenoceptor subtypes in human, pig, rat and guinea pig are reviewed. In all species tested MK912 was alpha2C-selective, RX821002 showed a minor alpha2A-selectivity, whereas [3H]RS79948-197 was non-selective among the alpha2-adrenoceptor subtypes, showing high affinity for all three subtypes. Rauwolscine and yohimbine showed relatively low affinities for nmost of the alpha2-adrenoceptor subtypes investigated, the exception being rauwolscine having high affinity for the human and porcine alpha2C-adrenoceptors.

Autoradiographic studies of central alpha 2A- and alpha 2C-adrenoceptors in the rat using [3H]MK912 and subtype-selective drugs

In the present study we examined the distribution of alpha 2A- and alpha 2C-adrenoceptors in tissue slices from the rat cervical spinal cord and from brain slices collected at the level of the striatum. To differentiate between alpha 2A- and alpha 2C-adrenoceptors, the slices were incubated with [3H]MK912 in the presence of graded concentrations of the alpha 2A-selective drug, BRL44408, or the alpha 2C-selective drug, spiroxatrine. Computer analysis of the autoradiograms indicated that 0.4 nM [3H]MK912 plus 185 nM BRL44408 selectively labeled alpha 2C-adrenoceptors, while 0.4 nM [3H]MK912 plus 220 nM spiroxatrine selectively labeled alpha 2A-adrenoceptors. Using this approach, alpha 2C-adrenoceptors were detected in the striatum, while alpha 2A-adrenoceptors predominated in the cortical layers 1-4, the spinal cord distal dorsal horn, the septum and the endopiriform nucleus.

Effects of acidosis or alkalosis on the actions of nifedipine on excitation-contraction coupling in the rat tail artery

1. The clinical success of calcium channel blockers in the management of organ ischaemia is less than theoretically anticipated. Blood gas/pH changes are associated with organ ischaemia; therefore, we studied the possibility that pH changes could alter the pharmacological effects of the calcium channel blocker nifedipine on rat tail artery contracted by either noradrenaline (NA) or potassium. 2. Segments (2-2.5 cm) of the proximal third of the male Sprague-Dawley rat tail ventral artery were initially bathed and perfused with a physiological salt solution (PSS; pH 7.48) for 25-30 min, after which time bathing/perfusion was continued with a nominally calcium-free PSS made acidotic (pH 7.20), alkalotic (pH 7.67) or unaltered (control). After equilibration, the perfusion pressure (PP) responses to increasing concentrations of calcium in the presence of NA (3.0 mumol/L) or potassium (100 mmol/L) with nifedipine or its vehicle were recorded. 3. The calcium sensitivity of potassium- or NA-stimulated rat tail arteries was reduced during acidosis, as was the maximum PP in potassium- but not NA-stimulated tissues. Alkalosis reduced the calcium sensitivity in potassium- but not NA-stimulated contraction and had no effect on maximum PP. 4. The inhibitory effect of nifedipine (0.6 mumol/L) on contraction was enhanced during acidosis in either NA- or potassium-stimulated arteries and also during alkalosis in NA-treated arteries, although it had little effect during normal conditions. 5. The results indicate that changes in pH alter the vascular contractility profile in a manner dependent on the excitation-contraction coupling mode. The calcium antagonistic effect of nifedipine is pH dependent and it is suggested that pH changes associated with ischaemic conditions may alter the therapeutic profile of nifedipine.

Differential regulation of the expression of alpha1-adrenergic receptor subtype genes in brown adipose tissue

The physiological control of the expression of the genes for the alpha1-adrenoceptor subtypes was examined in rat brown adipose tissue by analysing Northern blots of poly(A)-enriched RNA with oligonucleotide probes. In control rats, alpha1B-receptor gene expression was much lower in brown adipose tissue than in liver, but the expression of both alpha1A and alpha1D was higher than in the heart, making brown adipose tissue one of the mammalian tissues with the highest expression of these subtypes. During acute exposure to cold, alpha1B-receptor gene expression was essentially unchanged, alpha1A-receptor gene expression was increased and alpha1D-receptor gene expression was transiently decreased. Noradrenaline injection could mimic these effects of acute cold exposure, indicating that the physiologically induced up- and down-regulations were due to the interaction of noradrenaline with cells within the tissue. In chronically cold-acclimated animals, alpha1B-receptor gene expression was decreased but that of the alpha1A-receptor gene remained at a level twice that of controls. alpha1D-Receptor gene expression was also somewhat decreased. It is suggested that the enhanced expression of the alpha1A-receptor gene explains the increased alpha1-receptor density in recruited brown adipose tissue reported previously. The intricate and differential regulation of alpha1-receptor gene expression and the markedly enhanced expression of the alpha1A-receptor may imply that alpha1-receptors are important for the recruitment process or for maintenance of the recruited state in this tissue.

Interaction between alpha- and beta-adrenergic receptor agonists modulating the slow Ca(2+)-activated K+ current IAHP in hippocampal neurons

Noradrenaline inhibits the Ca(2+)-activated K+ current IAHP, which underlies the slow afterhyperpolarization and spike frequency adaptation in hippocampal and neocortical neurons. The resulting increase in excitability probably contributes to the state control of the forebrain during arousal and attention. The modulation of IAHP by noradrenaline has previously been shown to be mediated by beta 1 receptors, cyclic AMP and protein kinase A, but not by alpha receptors. We have now tested the possibility that alpha receptors also contribute to IAHP modulation through interaction with beta receptors, by the use of whole-cell recordings in CA1 pyramidal cells of rat hippocampal slices. The alpha-receptor agonist 6-fluoro-noradrenaline strongly potentiated the effect of isoproterenol on IAHP. The synergistic effect of 6-fluoro-noradrenaline and isoproterenol was blocked by the beta-receptor antagonist timolol, but the receptor type mediating the effect of 6-fluoro-noradrenaline could not be unequivocally identified by using alpha-receptor antagonists. The effect of high concentrations of noradrenaline on IAHP was only partly blocked by the beta-receptor antagonist timolol, and was further reduced by blocking alpha receptors, again suggesting a contribution from alpha receptors. In contrast, the effect of low concentrations of noradrenaline seemed to be potentiated by the alpha-receptor antagonist phentolamine in 57% of the cells, suggesting concentration-dependent antagonistic interaction between alpha and beta receptors. Further tests indicated that the cross-talk between 6-fluoro-noradrenaline and isoproterenol occurs upstream from cyclic AMP production, and that protein kinase A serves as a final common path for the modulation of IAHP by noradrenaline, and by the combination of 6-fluoro-noradrenaline and isoproterenol.

Influence of pH changes on the actions of verapamil on vascular excitation-contraction coupling

We have previously shown that pH changes alter the cardiovascular responses to verapamil in rat, in vivo and in isolated rat heart. The current study investigated the influence of pH changes on the actions of verapamil on potassium- or noradrenaline-stimulated contraction in rat tail arteries. The proximal 2-2.5 cm of ventral tail artery was bathed in and perfused initially (20-25 min) with physiological salt solution (pH 7.4) which was later made calcium-free at pH 7.4 (control), pH 7.2 (acidosis) or pH 7.67 (alkalosis). After equilibration each artery was exposed to verapamil following which the contractile responses to increasing concentrations of calcium were recorded. The patterns of responses in noradrenaline- or potassium-stimulated arteries were different. In normal conditions, the vasodilator effect of verapamil was predominant in potassium-stimulated arteries but less in the noradrenaline-stimulated preparations. With pH changes the effect of verapamil was enhanced more in noradrenaline- than in potassium-stimulated arteries. It is postulated that pathology-induced changes in the character of calcium channels could alter the effect of calcium channel blockers.

Urinary bladder function and drug development

Disorders of the bladder are extremely common and are becoming more so in an ageing population. Recently, our understanding of lower urinary tract physiology and pathology has also increased. Here, Douglas Ferguson and Nim Christopher summarize this new knowledge of lower urinary tract function, the changes in innervation that occur with age and the common disease states, and discuss how it is being used to develop new drug treatments for bladder disorders.

Regulation of cytosolic calcium levels in vascular smooth muscle

Ca2+ plays an important role in the contraction of skeletal, cardiac, and smooth muscle, as well as in a number of important processes, such as secretion and neuronal activity. In this review, I focus on the various mechanisms by which cytosolic Ca2+ concentration is regulated in vascular smooth muscle, in the resting state and during activation. Particular attention is paid to the calcium pumps of the plasmalemma and the sarcoplasmic reticulum, to the inositol 1,4,5-trisphosphate- and ryanodine-sensitive calcium channels of the sarcoplasmic reticulum, and to voltage-dependent and voltage-independent calcium channels of the plasmalemma.