Mechanical function and gene expression of alpha(1)-adrenoceptor subtypes in dog intravesical ureter

The pharmacology of α1-adrenoceptor subtypes

This review examines the functions of α₁-adrenoceptor subtypes, particularly in terms of contraction of smooth muscle. There are 3 subtypes of α₁-adrenoceptor, α_1A- α_1B- and α_1D-adrenoceptors. Evidence is presented that the postulated α_1L-adrenoceptor is simply the native α_1A-adrenoceptor at which prazosin has low potency. In most isolated tissue studies, smooth muscle contractions to exogenous agonists are mediated particularly by α_1A-, with a lesser role for α_1D-adrenoceptors, but α_1B-adrenoceptors are clearly involved in contractions of some tissues, for example, the spleen. However, nerve-evoked responses are the most crucial physiologically, so that these studies of exogenous agonists may overestimate the importance of α_1A-adrenoceptors. The major α₁-adrenoceptors involved in blood pressure control by sympathetic nerves are the α_1D- and the α_1A-adrenoceptors, mediating peripheral vasoconstrictor actions. As noradrenaline has high potency at α_1D-adrenceptors, these receptors mediate the fastest response and seem to be targets for neurally released noradrenaline especially to low frequency stimulation, with α_1A-adrenoceptors being more important at high frequencies of stimulation. This is true in rodent vas deferens and may be true in vasopressor nerves controlling peripheral resistance and tissue blood flow. The α_lA-adrenoceptor may act mainly through Ca²⁺ entry through L-type channels, whereas the α_1D-adrenoceptor may act mainly through T-type channels and exhaustable Ca²⁺ stores. α₁-Adrenoceptors may also act through non-G-protein linked second messenger systems. In many tissues, multiple subtypes of α-adrenoceptor are present, and this may be regarded as the norm rather than exception, although one receptor subtype is usually predominant.

The modulation of ureteral smooth muscle contractile responses by α1- and α2-adrenoceptor activation

PURPOSE

This study was performed to investigate the influence of α-adrenoceptor subtypes upon ureteral smooth muscle contractile responses.

METHODS

Rat ureters were challenged in vitro with noradrenaline (NA), the α₁-adrenoceptor agonist phenylephrine (PE), and the α₂-adrenoceptor agonist clonidine (CLON). The influences of the agonists on the magnitude and frequency of acetylcholine (ACh)-stimulated phasic contractile responses were recorded.

RESULTS

The magnitude of the phasic contractile responses effected by ACh was not significantly influenced by the adrenoceptor agonists, but the frequency of the response was significantly enhanced by all three agonists (p < 0.05). Idazoxan and prazosin abolished the rise in frequency effected by CLON and PE, respectively, whereas both antagonists in combination were required to abolish the increase in frequency effected by NA.

CONCLUSIONS

It has been demonstrated that α₁- and α₂-adrenoceptors modulate the contractile function of rat ureteral smooth muscle by increasing the frequency, but not the magnitude, of phasic contractile responses. The enhancement of contractile function by NA is mediated by mechanisms dependent upon both α₁- and α₂-adrenoceptors.

Effects by silodosin on the partially obstructed rat ureter in vivo and on human and rat isolated ureters

BACKGROUND AND PURPOSE

α1 -adrenoceptor (-AR) antagonists may facilitate ureter stone passage in humans. We aimed to study effects by the α1 A -AR selective antagonist silodosin (compared to tamsulosin and prazosin) on ureter pressures in a rat model of ureter obstruction, and on contractions of human and rat isolated ureters.

EXPERIMENTAL APPROACH

After ethical approval, ureters of male rats were cannulated beneath the kidney pelvis for in vivo ureteral intraluminal recording of autonomous peristaltic pressure waves. A partial ureter obstruction was applied to the distal ureter. Mean arterial blood pressure (MAP) was recorded. Approximate clinical and triple clinical doses of the α1 -AR antagonists were given intravenously. Effects by the α1 -AR antagonists on isolated human and rat ureters were studied in organ baths.

KEY RESULTS

Intravenous silodosin (0.1-0.3 mg kg(-1) ) or prazosin (0.03-0.1 mg kg(-1) ) reduced obstruction-induced increases in intraluminal ureter pressures by 21-37% or 18-40% respectively. Corresponding effects by tamsulosin (0.01 or 0.03 mg kg(-1) ) were 9-20%. Silodosin, prazosin and tamsulosin reduced MAP by 10-12%, 25-26% (P < 0.05), or 18-25% (P < 0.05) respectively. When effects by the α1 A -AR antagonists on obstruction-induced ureter pressures were expressed as a function of MAP, silodosin had six- to eightfold and 2.5- to eightfold better efficacy than tamsulosin or prazosin respectively. Silodosin effectively reduced contractions of both human and rat isolated ureters.

CONCLUSIONS AND IMPLICATIONS

Silodosin inhibits contractions of the rat and human isolated ureters and has excellent functional selectivity in vivo to relieve pressure-load of the rat obstructed ureter. Silodosin as pharmacological ureter stone expulsive therapy should be clinically further explored.

Symptoms Predictive for Efficacy of Naftopidil in Patients with Benign Prostate Hyperplasia

OBJECTIVES

To evaluate the lower urinary tract symptoms predicting the efficacy of the α1-adrenoreceptor (AR) antagonist naftopidil in patients with benign prostate hyperplasia.

METHODS

The efficacy of naftopidil was examined on the basis of changes in the international prostate symptom score (IPSS). All patients received naftopidil (50 mg/day) for 12 weeks. We defined a "responder" as a patient whose total IPSS improved by five or more points and assessed the lower urinary tract symptoms predicting the efficacy of treatment by performing multivariate and probit analyses.

RESULTS

Among 132 patients whose data could be analyzed, the efficacy rate was 50.8%. All IPSS items except the urgency score were significantly higher in the responders than the non-responders before treatment, and all IPSS items were lower in the responders after treatment. In the responder group, significant improvements were observed in the total IPSS score, quality of life (QOL) index, maximum flow rate (Qmax ), residual urine volume, and all IPSS items after treatment. In contrast, in the non-responder group, no parameter except the QOL index improved significantly. The probit analysis demonstrated that the score for weak stream (≥3) or nocturia (≥4) in the IPSS were factors predicting an effective response to naftopidil treatment.

CONCLUSIONS

Weak stream and/or nocturia are the key symptoms that predict the efficacy of naftopidil treatment in patients with benign prostatic hyperplasia. Those with a score of ≥3 for weak stream or of ≥4 for nocturia are expected to achieve a good response in the subjective symptoms with administration of naftopidil.

α(1)-Adrenoceptors in the urinary tract

α(1)-Adrenoceptors have been identified and characterized extensively by functional, radioligand-binding, and molecular biological techniques. Molecular clones have been isolated for three α(1)-subtypes (α(1a), α(1b), and α(1d)), and these subtypes are also functionally characterized. α(1)-Adrenoceptors are present in the prostate, urethra, bladder (urothelium, smooth muscle, and afferent nerves), ureter, vas deferens, peripheral ganglia, nerve terminals, vascular tissues, and central nervous system (CNS), and they could all potentially influence overall urinary function and contribute to both the therapeutic and adverse effects of α(1)-adrenoceptor antagonists in patients with benign prostatic hyperplasia (BPH) and lower urinary tract symptoms (LUTS). This review aimed to discuss the relevant physiological and pharmacological roles and molecular biology of α(1)-adrenoceptor subtypes in the prostate, urethra, bladder, ureter, and CNS.

Clinical efficacy of a loading dose of naftopidil for patients with benign prostate hyperplasia

PURPOSE

The efficacy of a loading dose of α(1)-adrenoreceptor antagonist for patients with benign prostate hyperplasia who did not improve at a low dose has not been determined. We performed a prospective study to estimate the efficacy of incremental naftopidil administration.

METHODS

The efficacy of naftopidil was examined based on changes in the International Prostate Symptom Score (IPSS). We defined a "responder" as a patient who improved by ≥5 points in IPSS total score. All patients were administered naftopidil at 50 mg/day for 12 weeks, and nonresponders at 50 mg/day were increased to 75 mg/day. Efficacy was finally evaluated after an additional 12 weeks of administration at 75 mg/day.

RESULTS

Among 122 patients whose data could be analyzed, the efficacy rate after administration at 50 mg/day was 52.5%. In all IPSS items except urgency score, the responders had significantly higher (poorer) values than nonresponders before the start of treatment. Of 40 patients whose dose was increased to 75 mg/day and whose data could be analyzed, prostate volume in the responder group (9 patients) was significantly smaller than that in the nonresponder group (31 patients). Multivariate analysis showed that patients with improved IPSS total score, voiding symptoms, urgency, and weak stream after administration of 50 mg/day naftopidil were more likely to improve after a dose increase.

CONCLUSIONS

A dose increment of naftopidil to 75 mg/day may be useful in patients with BPH who did not improve at 50 mg/day.