The alpha1A-adrenoceptor gene is required for the alpha1L-adrenoceptor-mediated response in isolated preparations of the mouse prostate.

British Journal of Pharmacology (2008) 155, 103–109 & 2008 Macmillan Publishers Limited All rights reserved 0007– 1188/08 $30.00 www.brjpharmacol.org RESEARCH PAPER The a1A-adrenoceptor gene is required for the a1L-adrenoceptor-mediated response in isolated preparations of the mouse prostate KT Gray, JL Short and S Ventura Prostate Research Co-operative, Medicinal Chemistry and Drug Action, Monash Institute of Pharmaceutical Sciences, Monash University, Parkville, Victoria, Australia Background and purpose: This study investigated whether deletion of the a1A-adrenoceptor gene influences contractile responses of mouse prostate to noradrenaline. Responses of mouse prostate to noradrenaline are known to be mediated by a1L-adrenoceptors, which are thought to be a functional phenotype of a1A-adrenoceptor. Experimental approach: Prostate tissues from a1A-adrenoceptor knockout mice which were homozygous (a1A
/
) and heterozygous (a1A þ /
) for the disrupted a1A-adrenoceptor gene, as well as wild-type (a1A þ / þ ) littermates were mounted in glass-isolated organ baths. Electrical field stimulation of nerves and exogenous application of noradrenaline were used to investigate the effects of a1A-adrenoceptor disruption on prostate contractility. Key results: Frequency–response curves to electrical field stimulation (0.5 ms pulse duration, 60 V, 0.1–20 Hz) yielded frequency-dependent contractions. At frequencies of 10 and 20 Hz, prostates from a1A
/
mice elicited an approximately 30% decreased response compared with prostates from a1A þ / þ mice. Prazosin (0.3 mM) attenuated responses to electrical field stimulation in prostates from a1A þ / þ and a1A þ /
mice but not from a1A
/
mice. Increasing concentrations of exogenously administered noradrenaline (10 nM–1 mM) produced mean concentration–response curves in prostates from a1A þ / þ and a1A þ /
mice, which were not different. Maximum responses to noradrenaline were decreased by approximately 80% in prostates from a1A
/
mice compared with a1A þ / þ mice. Prazosin attenuated responses to noradrenaline in all genotypes. Conclusions and implications: a1L-Adrenoceptor-mediated responses in mouse prostate are abolished in a1A
/
mice, demonstrating that the a1A-adrenoceptor gene is essential to the manifestation of the prostatic a1L-adrenoceptor phenotype. This implies that a1L-adrenoceptors are indeed a functional phenotype of a1A-adrenoceptor. British Journal of Pharmacology (2008) 155, 103–109; doi:10.1038/bjp.2008.245; published online 16 June 2008 Keywords: noradrenaline; prazosin; tamsulosin; benign prostatic hyperplasia; BPH; smooth muscle; nerve stimulation Abbreviation: BPH, benign prostatic hyperplasia Introduction Benign prostatic hyperplasia (BPH) is a common condition in ageing men, which is characterized by a non-malignant enlargement of the prostate (McNeal, 1978). BPH has both a static component, arising due to hyperplasia of the glandular and stromal tissue, and a dynamic component, mediated through increased noradrenergic activation of a1-adrenoceptors. Both of these components place pressure on the urethra and bladder, and result in a number of troublesome lower urinary tract symptoms. Correspondence: Dr S Ventura, Medicinal Chemistry and Drug Action, Monash Institute of Pharmaceutical Sciences, Monash University (Parkville Campus), 381 Royal Parade, Parkville, Victoria 3052, Australia. E-mail: Received 22 April 2008; revised 15 May 2008; accepted 20 May 2008; published online 16 June 2008 The dynamic component of BPH is more influential in producing symptoms, as the severity of symptoms is not related to prostate size (Eckhardt et al., 2001a, b, c), and drugs that relax prostatic smooth muscle are faster acting and more effective in relieving symptoms (Rigatti et al., 2003; Hasan et al., 2007). Benign prostatic hyperplasia is associated with an increased expression of a1-adrenoceptors in hyperplastic tissue compared with normal prostate tissue (Walden et al., 1999; Yamada et al., 2001). Furthermore, a1A-adrenoceptor mRNA has been shown to be nine times more abundant in BPH samples than in non-BPH samples (Nasu et al., 1996). Although studies have focused on a1A-adrenoceptor expression, it is the pharmacologically classified a1L-adrenoceptor that is responsible for mediating the contractile responses in the human (Muramatsu et al., 1994; Israilova et al., 2004), rat 104 a1-Adrenoceptors in the mouse prostate KT Gray et al (Hiraoka et al., 1999) and guinea-pig prostrates (Pennefather et al., 1999), as well as in the mouse prostate (Gray and Ventura, 2006). Although the a1L-adrenoceptor is yet to be cloned (Ramsay et al., 2004), functional pharmacological studies have shown it to have a high affinity for tamsulosin and a low affinity for prazosin, RS-17053 and WB 4101 compared with the a1A-adrenoceptor (Muramatsu et al., 1994; Ford et al., 1996, 1997; Noble et al., 1997; Hiraoka et al., 1999). The a1L-adrenoceptor is believed to be a functional phenotype of the a1A-adrenoceptor (Ford et al., 1997), although the exact mechanisms behind the development of the functional a1L-adrenoceptor phenotype are unknown. The a1L-adrenoceptor may result from specific isoforms of the a1A-adrenoceptor; however, cloned a1A-adrenoceptor isoforms have been shown to exhibit functional pharmacology of a1A-adrenoceptors, indicating that no single isoform of the a1A-adrenoceptor is responsible for the generation of the a1L-adrenoceptor (Daniels et al., 1999). The functional profile of the a1A-adrenoceptor is determined by the structure of the C terminus (Suzuki et al., 2000); however, when homo- and hetero-dimers of three C-terminal splice variants of the a1A-adrenoceptor were formed, they did not display the ligand-binding characteristics of the a1L-adrenoceptor (Ramsay et al., 2004). Evidence to support the notion that the a1L-adrenoceptor may be a functional phenotype of the a1A-adrenoceptor comes from studies using the ‘uroselective’ a1A-adrenoceptor antagonist tamsulosin. Tamsulosin was the first a1A-subtypeselective adrenoceptor antagonist and is currently used as the first-line treatment for BPH, as it is effective at selectively relaxing prostatic smooth muscle and relieving the symptoms associated with BPH (Walden et al., 1998; Flannery et al., 2006; Suzuki et al., 2006), without causing the common cardiovascular side effects seen with other a1adrenoceptor antagonists. Gene knockout technology provides us with an invaluable tool for elucidating the role of certain gene products in the mediation of physiological responses. Although it has been hypothesized that the a1L-adrenoceptor is a functional phenotype of the a1A-adrenoceptor, no one has yet examined the a1L-adrenoceptor response in a system where the a1A-adrenoceptor gene is absent. Such a study would determine whether the a1A-adrenoceptor gene plays a role in the manifestation of the a1L-adrenoceptor-mediated response. To our knowledge, this is the first rep (...truncated)